**Previous Webinars:**

Seminar 51:** Silence is bliss in a Platonic relationship**

Speaker:** William J. Parnell **(Department of Mathematics, University of Manchester, UK)

**Q & A:**

**Abstract: **In the context of active control we work in three dimensions and describe an effective cloaking strategy for the scalar Helmholtz equation. Building on the approach of Guevara-Vasquez et al, and Norris and Parnell, we employ multipole active sources. We choose to locate them at the vertices of the Platonic solids, thus creating a silent zone interior to the imaginary Platonic solid and ensuring that only the incident field remains exterior to the active source configuration. The Platonic distribution of the sources ensures an extremely efficient implementation of the cloaking strategy. In particular, once the multipole source amplitudes required at a single source location are determined, the other source amplitudes can be calculated by a simple post-processing involving multiplication of the source vector by a rotation matrix, exploiting the Platonic source distribution. Depending on time we may also briefly describe some recent work on elastic wave control via resonant metaclusters.

**Biography:** William Parnell is Professor of Applied Mathematics in the School of Mathematics at the University of Manchester (UK) and holds an EPSRC Research Fellowship. He received a First Class degree in Applied Mathematics from the University of Bristol (UK) in 1999, before moving to the University of Oxford (UK) to study for a Masters in Mathematical Modelling and Scientific Computing, graduating with distinction in 2000. After a year travelling he began a PhD in 2001 at the University of Manchester under the supervision of I. David Abrahams (now Director of the Isaac Newton Institute at the University of Cambridge), completing this in 2004.

Parnell’s research interests reside principally in the development of new mathematical techniques to understand the mechanical properties of inhomogeneous materials and the dynamic behaviour of particulate media. More recently his work has involved linking theory with experiments in order to develop new composites and metamaterials. He has a particular interest in understanding the constitutive behaviour of complex soft solids and tuning this via novel fillers. He leads the Mathematics of Waves and Materials (MWM) research group at Manchester, which consists of five Faculty and a thriving group of Postdocs, PhD students and Masters students.

Parnell has held visiting positions at Universite Paris 6 and 12 (France), University of Trento (Italy), University of Oxford (UK) and Colorado School of Mines and Rutgers (USA). He has published more than 60 research papers and 2 book chapters. He is a Fellow of the Institute of Mathematics and its Applications (UK), is the founding director of the *Manchester Materials Modelling Centre* at Manchester and became Editor in Chief of the journal *Wave Motion* in 2017. In 2019 he was awarded one of the London Mathematical Society’s Whitehead Prizes for highly novel and extensive research contributions in the fields of acoustic and elastodynamics metamaterials and theoretical solid mechanics, as well as excellence in the promotion of mathematics in industry.

Seminar 50:** Ultrasound self-assembly of small particle patterns in a fluid**

Speaker:** Fernando Guevara Vasquez **(Department of Mathematics, University of Utah, USA)

**Q & A:**

**Abstract: **Standing ultrasound waves can be used to steer small particles suspended in a fluid into user defined patterns. If the fluid is e.g. a resin that can be cured, this technique can be used to fabricate materials with a custom microstructure. We start with the idealized case of infinitely many ultrasound transducers in the far-field to establish that the particle patterns are expected to be diffraction-limited. Then we move on to the patterns that can be obtained by N parallel pairs of transducers in the far-field. If N is equal to the dimension, the particles arrange in periodic patterns, with the caveat that only a subset of the possible crystallographic symmetries are achievable with this method. If N is larger than the dimension, the particles arrange in quasiperiodic patterns that can be rotated, translated, etc. by adjusting the amplitudes and phases of the ultrasound transducers.

**Biography:** Fernando Guevara Vasquez received his PhD in Computational and Applied Mathematics in 2006 from Rice University. He is currently an Associate Professor at the University of Utah Mathematics Department. His research interests include optics, inverse scattering, inverse problems, wave control and cloaking.

Home – Prof. Fernando Guevara Vasquez

Seminar 49:** Time-domain investigation of wave propagation in effective metamaterial models**

Speaker:** Bruno Lombard** (Laboratoire de Mécanique et d’Acoustique LMA, CNRS – Centrale Marseille, France)

**Q & A:**

**Abstract: **Wave propagation in microstructured media is often expressed in the frequency domain. This is particularly the case for resonant media, where resonance frequencies are explicitly involved in the PDE (e.g. Helmholtz resonators, high contrast media, etc.). However, it is possible to describe these phenomena based on the auxiliary field formalism. It then becomes possible to use the powerful mathematical and numerical tools specific to hyperbolic systems. The temporal approach allows to study the time of the establishment of periodic phenomena with a single simulation, and is particularly useful when the studied system presents nonlinearities.

We will illustrate the subject by reviewing different cases of interest for the “metamaterials” community:

- High contrast volume homogenization (coll: Cédric Bellis, link)
- Resonant outer cloak (coll: Sébastien Guenneau and Cédric Bellis, link)
- Resonant metasurfaces (Marie Touboul’s PhD thesis, coll: Cédric Bellis, Agnès Maurel, Kim Pham and Jean-Jacques Marigo, link1 and link2)
- Dynamics of solids with nonlinear cracks (coll. Raphaël Assier, Cédric Bellis, Marie Touboul, link)

**Biography:** Bruno Lombard received his PhD in acoustics in 2002 from the University of Aix-Marseille. He is a CNRS Research Director at the Laboratoire de Mécanique et d’Acoustique (Marseille). His work is at the interface of scientific computing, waves and continuum mechanics. He leads the French research group MecaWave (link) and is in charge of the Erasmus international master in acoustics WAVES (link).

Seminar 48:** Acoustic metasurfaces and their application to turbulent flow**

Speaker:** Tim Starkey **(Physics and Astronomy, University of Exeter, UK)

**Q & A:**

**Abstract: **The use of surface structures can have a significant impact on the pressure fluctuations that manifest above them. Acoustic metamaterials that comprise structured surfaces have been designed to influence the acoustic pressure field for a variety of applications such as acoustic filtering, focusing, and absorption, whilst in the aerodynamics of turbulent boundary layer flows, surface treatments can cause significant changes to the sound generated from, and pressure fluctuations present within, the flow. So, the question arises, can we apply acoustic metamaterial concepts as a strategy to influence a turbulent flow?

In principle, metasurfaces used to control acoustic fields should also respond to evanescent pressure fluctuations generated by turbulence. However, interfacing a metasurface and boundary layer poses some challenges, not least in the large disparity between acoustic and flow scales at low Mach number, and the need to avoid metasurface configurations that generate drag or other undesirable flow effects.

In this talk I will discuss our recent work on acoustic surface waves and their application to a turbulent fluid flow. First I will talk about the characteristics and structures that support acoustic surface wave propagation in a quiescent ‘wholly acoustics’ environment. I will then present the strategy developed with collaborators at Virginia Tech, that interfaces a cavity-type metasurface with a turbulent flow using a hydrodynamically smooth but acoustically transparent membrane. Our experiments demonstrate the excitation of a bound acoustic mode using the stochastic near-field pressure fluctuations of a turbulent flow. The tested metasurfaces demonstrate a route for acoustic surface mode dispersions with reduced phase velocities to match to the convective velocity of the turbulent structures.We view this as an important first step in interfacing flows and metamaterials, with potential applications to the control of flow-generated edge noise or energy harvesting.

**Biography:** Tim is a research fellow at the University of Exeter whose research interests span, acoustic metamaterials, fluid flows, and photonics. He completed his PhD at Exeter in 2014 investigating Bio-inspired photonic sensors, then worked at Thales UK in the underwater acoustics domain before returning to Exeter in 2016 to work on fundamental acoustic metamaterials research. Tim’s research is sponsored by DSTL.

Seminar 47:** Quasiperiodic composites: Forward and inverse homogenization**

Speaker:** Elena Cherkaev **(Department of Mathematics, University of Utah, USA)

**Q & A:**

Acoustic radiation potential creating quasiperiodic patterns with 10-fold and 12-fold symmetries (E. Cherkaev, F. Guevara Vasquez, C. Mauck, M. Prisbrey, B. Raeymaekers, PRL, 2021, 126, 145501)

**Abstract: **Quasiperiodic materials present a novel class of metamaterials that possess extraordinary mechanical, thermal, and electromagnetic properties, such as superconductivity, extremely low thermal conductivity, unusual mechanical properties, and diffraction patterns forbidden by crystallographic symmetries. The properties of such materials critically depend on the microstructure of the media. The talk will discuss the effective properties and homogenized equations governing the effective behavior of quasiperiodic composites. I will also address an inverse homogenization problem of reconstructing information about microstructural parameters from known effective properties. An approach to the inverse homogenization problem is based on the Stieltjes analytic representation which involves a spectral measure of a self-adjoint operator. The spectral measure contains all information about the geometry of the composite and can be uniquely recovered from effective measurements given in an interval of frequency. Stieltjes analytic representation also allows us to determine spectral characteristics and establish an analogy between Anderson transition in quantum transport and transition from ordered to disordered behavior in classical transport in composites without scattering or wave interference effects.

**Biography:** Elena Cherkaev is a Professor in the Department of Mathematics at the University of Utah. After receiving a Ph.D. from the St. Petersburg State University, she worked at the Russian Academy of Sciences and the New York University before moving to the University of Utah. She is a specialist in optimization, inverse problems, wave propagation, mathematical and numerical modeling of heterogeneous materials. Her current research interests include quasiperiodic composites, homogenization, inverse problems for materials with microstructure, robust optimal design of elastic and viscoelastic composites, dispersion and dissipation of waves propagating in heterogeneous media. She is an author or co-author of more than 100 publications, has given more than 150 plenary, keynote, and invited talks, and served as a member of multiple Scientific Committees and session organizer for national and international conferences.

Seminar 46:** Multistable inflatable origami – from deployable structures to robots**

Speaker:** Katia Bertoldi **(Harvard John A. Paulson School of Engineering & Applied Sciences, Harvard University, USA)

**Q & A:**

**Abstract: **Multistable structures can reversibly change between multiple stable configurations when a sufficient energetic input is provided. While originally the field focused on understanding what governs the snapping, more recently it has been shown that these systems also provide a powerful platform to design a wide range of smart structures. In this talk, I will first show that pressure-deployable origami structures characterized by two stable configurations provide opportunities for a new generation of large-scale inflatable structures that lock in place after deployment and provide a robust enclosure through their rigid faces. Then, I will demonstrate that the bistable origami modules provide an ideal platform to design actuators that can switch between different configurations, reach multiple, pre-defined targets in space, and move along complex trajectories. Unlike previously proposed robots that require complex input control of multiple actuators, a single input signal suffices to activate our robot, as all features required for functionality are embedded into the architecture of the building blocks.

**Biography:** Wikipedia

Seminar 45:** Bubbly Acoustic Metamaterials**

Speaker:** John Page **(Distinguished Professor Emeritus, University of Manitoba, Canada)

**Q & A:**

**Abstract: **Bubbly metamaterials are created by exploiting the low-frequency Minnaert resonance of bubbles, and can radically modify acoustic wave behaviour. In this presentation, I will first review the properties of bubble metascreens, which consist of a single layer of bubbles in a soft solid, and can be very efficient absorbers of waterborne acoustic waves. Two different metalayer configurations will be considered that allow almost perfect compensation of the radiative scattering losses, so that virtually all of the acoustic energy can be absorbed. I will show how optimization of bubble metascreens’ performance is facilitated by a relatively simple analytical model, and that, despite being resonance-based, near-perfect absorption is possible over a very wide frequency range even when the metalayer is ultrathin. In the second part of the talk, I will describe three-dimensional structures with pair-wise spatial correlations between the bubbles. Such structures exhibit doubly negative behaviour even though the low-frequency resonance of a single bubble is purely monopolar. Furthermore, this doubly negative behaviour can occur when the bubble pairs are arranged in either random or periodic configurations. Predictions for both types of structure will be presented and the influence of dissipation on doubly negative behaviour discussed. For the 3D crystalline metamaterial case, the focusing and imaging capabilities of a flat metalens will be demonstrated, showing the imaging of a point source by a slab with a relative refractive index* n* = -1, evidence of super-resolved focusing, and the imaging of an extended object.

**Biography:** John Page leads the internationally recognized Ultrasonics Research Laboratory in the Department of Physics and Astronomy at the University of Manitoba, where he holds the title of Distinguished Professor Emeritus. He came to the University of Manitoba in 1985 as a NSERC University Research Fellow, following doctoral studies at the University of Oxford as a Rhodes Scholar, and postdoctoral research at Université Paris VI and Queen’s University. He specializes in the study of novel wave phenomena in strongly scattering media, and the development of new ultrasonic scattering techniques to probe the structure and dynamics of heterogeneous materials. His pioneering research on phononic crystals has been widely acclaimed in the academic research community, and extensively reported in the media. More recently, he and his collaborators have made significant contributions to the study of acoustic metamaterials. Backed by strong support from Canada’s Natural Sciences and Engineering Research Council, Dr. Page has also made key contributions to understanding the wave physics of disordered mesoscopic materials. His group’s unambiguous experimental demonstration of the Anderson localization of ultrasound in three dimensions is especially notable, as 3D classical wave localization is often considered the most challenging, fascinating and elusive aspect of wave transport in disordered media. His ultrasonic research is impacting other areas of science, from optics to seismology, and underpins his strong interdisciplinary collaboration investigating the physical properties of foods and other industrially relevant biomaterials.

Seminar 44:** New Trends Towards Seismic Metamaterials**

Speaker:** Philippe Roux **(Director of ISTerre, University Grenoble Alpes, France)

**Q & A:**

**Abstract: **We report on a seismic metamaterial experiment in a pine-tree forest environment where the dense collection of trees behaves as subwavelength coupled resonators for surface seismic waves. For the METAFORET experiment, more than 1000 seismic sensors were deployed over a 120 m × 120 m area to study the properties of the ambient and induced seismic wavefield that propagates in the ground and in trees. The goal of the experiment was to establish a link between seismic-relevant scales and microscale and mesoscale studies that pioneered the development of metamaterial physics in optics and acoustics. The first results of the METAFORET experiment show the presence of frequency band gaps for Rayleigh waves associated with compressional and flexural resonances of the trees, which confirms the strong influence that a dense collection of trees can have on the propagation of seismic waves.

**Biography**: Philippe Roux is a physicist with a strong background in ultrasonics, underwater acoustics and geophysics. He received his Ph.D. degree on ultrasonic time reversal from the University of Paris, France, in 1997. Between 2002 and 2005, he was a Research Associate at the Marine Physical Laboratory, Scripps Institution of Oceanography, San Diego, USA. Since July 2005, He is a Research Director at ISTerre, Grenoble, France, where he develops small-scale laboratory experiments in wave physics and geophysics. His research topics range from seismic tomography performed with noise correlation to metamaterial physics applied to seismic waves.

Seminar 43:** Acoustic Metamaterials with Broadband Tunable Impedance Matching**

Speaker:** Nicholas X. Fang **(Department of Mechanical Engineering, MIT, Cambridge, USA)

**Q & A:**

**Abstract: **

Recent development of acoustic metamaterials opens a door to an unprecedented large design space for acoustic properties such as negative bulk modulus, negative density, and refractive index. Such novel concept paves the way for the design of a new class of acoustic materials and devices with great promise for diverse applications, such as broadband noise insulation, sub-wavelength imaging and acoustic cloak from sonar detection.

In this talk, I will present our research progress on advanced design and micro/nanofabrication techniques, to enable exploration and rapid prototyping of architectured metastructures for acoustic waves. These structures show promise on focusing and rerouting ultrasound through broadband metamaterials. As an example, we report a class of impedance transformers to overcome the fundamental limit of narrowband transmission. We experimentally show that the transformer device offers efficient implementation in broadband underwater ultrasound detection with the benefit of being soft and tunable. The broadband impedance matched nonreflecting acoustic metamaterial can also robustly prohibit reflection and reverberation of airborne sound waves over a wide range of incident angles. I will also discuss the acoustic labyrinthine metamaterials which can exhibit extreme constitutive parameters and an exceptional ability to control the phase of sound at deep-subwavelength scale.

**Biography**: Nicholas X. Fang received his BS and MS in physics from Nanjing University, and his PhD in mechanical engineering from University of California Los Angeles. He is currently Professor of Mechanical Engineering. Prior to MIT, he worked as an assistant professor at the University of Illinois Urbana-Champaign until 2010. Professor Fang’s areas of research look at nanophotonics and nanofabrication. His research on nanoarchitectured metamaterials was highlighted among the top 10 Emerging breakthrough technologies of the year 2015. His recognitions also include the OSA Fellow (2021); ASME Chao and Trigger Young Manufacturing Engineer Award (2013); the ICO prize from the International Commission of Optics (2011); the NSF CAREER Award (2009) and MIT Technology Review Magazine’s 35 Young Innovators Award (2008).

Seminar 42:** Longitudinal electromagnetic waves with extremely short wavelength**

Speaker:** Pavel Belov **(Research Center of Nanophotonics and Metamaterials, Physics and Engineering Department, ITMO University, St. Petersburg, Russia)

**Q & A:**

**Abstract: **

Elastic waves may have either longitudinal or transverse polarization: the acoustic (compression) waves are longitudinal while the sheer stress waves are usually transverse. The electromagnetic waves are in many aspects similar to the elastic ones. However, the electromagnetic waves in vacuum and most materials have transverse polarization. Longitudinal electromagnetic waves with electric field parallel to wave vector are very rare and appear under special conditions in a limited class of media, for example in plasma.

In this work, we study the dispersion properties of an easy-to-manufacture metamaterial consisting of two three-dimensional cubic lattices of connected metallic wires inserted one into another, also known as an interlaced wire medium. It is shown that the metamaterial supports longitudinal waves at extremely wide frequency band from very low frequencies up to the Bragg resonances of the structure.

The waves feature unprecedentedly short wavelengths comparable to the period of the material.

The revealed effects highlight spatially dispersive response of interlaced wire medium and provide a route toward generating electromagnetic fields with strong spatial variation.

For more details see https://arxiv.org/abs/2103.10205

**Biography:**

Pavel Belov is a Head of Physics and Engineering School, ITMO Univeristy, St. Petersburg, Russia

He defended two PhD theses: one in ITMO University in Russia in 2003 and the second one in Finland in 2006 at the Helsinki University of Technology.

From 2007 to 2012 he was EPSRC research fellow at Queen Mary University of London, UK. Also, he has extensive experience of working with such industrial giants as Nokia, Samsung Electronics and Bosch.

He is a holder of the Russian Federation President’s Prize in Science and Innovation for Young Scientists (2009), IET Achievement Medal (IET, UK, 2006), International Dennis Gabor Award (NOVOFER Foundation, Hungary, 2003)

Pavel Belov is co-author of more than 300 scientific articles in refereed journals and 13 book chapters.

Seminar 41:** Designs of acoustic metamaterials: From effective medium to deep learning**

Speaker:** Ying Wu **(King Abdullah University of Science and Technology, Saudi Arabia)

**Q & A:**

**Abstract: **Acoustic metamaterials have offered great opportunities to control acoustic wave propagation as desired. The intriguing property of an acoustic metamaterial is attributed to its unique effective property. Therefore, effective medium theory, a bridge connecting the response of the medium to the incident wave and the medium’s microstructure, provides design principles of new acoustic metamaterials. Recently, the blossoming of artificial intelligence enabled another efficient way of designing acoustic metamaterials.

In this talk, I will start with a review our early contributions in deriving effective medium theories followed by showcases of our recent designs of acoustic metamaterials and their properties. Finally, I will introduce our most recent work on deep learning enabled design of acoustic cloak of invisibility.

[1] C Xu, G Ma, ZG Chen, J Luo, J Shi, Y Lai, Y Wu “Three-dimensional acoustic double-zero-index medium with a fourfold degenerate Dirac-like point” Phys. Rev. Lett. 124 (7), 074501, 2020

[2] M Landi, J Zhao, WE Prather, Y Wu, L Zhang “Acoustic Purcell effect for enhanced emission”

Phys. Rev. Lett. 120 (11), 114301, 2018

[3] M Farhat, S Guenneau, A Alù, Y Wu “Scattering cancellation technique for acoustic spinning objects” Phys. Rev. B 101 (17), 174111, 2020

[4] WW Ahmed, M Farhat, X Zhang, Y Wu “Deterministic and probabilistic deep learning models for inverse design of broadband acoustic cloak” Phys. Rev. Res. 3 (1), 013142, 2021[5] Y Wu, Y Lai, ZQ Zhang “Effective medium theory for elastic metamaterials in two dimensions” Phys. Rev. B 76 (20), 205313, 2007

**Biography:** Dr. Ying Wu is currently an associate professor of Applied Mathematics and Computational Sciences (AMCS) with secondary affiliations in Electrical and Computer Engineering (ECE) and Applied Physics (AP) at King Abdullah University of Science and Technology (KAUST). She obtained her BSc and PhD degrees from Nanjing University and the Hong Kong University of Science and Technology, respectively, both in Physics. Dr. Wu works on development of new models and computational tools to describe wave propagation in complex systems, with a particular focus on homogenization schemes for complex heterogeneous media, acoustic and elastic metamaterials, and Dirac and Dirac-like cones in phononic systems. She serves as an associate editor for Europhysics Letters (EPL) and Wave Motion. In the year 2017, she was awarded the Young Investigator Award by the International Phononics Society.

Seminar 40:** Conceptual-based design of ultra-broadband microwave metamaterial absorber**

Speaker:** Ping Sheng **(Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China)

**Q & A:**

**Abstract: **By introducing metallic-ring structural resonances in the microwave regime, we have designed and realized a metamaterial absorber with hierarchical structures that can display an averaged -19.4 dB reflection loss from 3-40 GHz. The overall thickness of the sample is 14.2 mm, only 5% over the minimum thickness dictated by the causality limit. The measured performance is polarization-independent at normal incidence, while absorption at oblique incidence remains considerably effective up to 45 degrees. We provide a conceptual basis for our absorber design, based on the magnetically-excited, two-dimensional capacitive-coupled (electrical) dipolar resonances in the lateral plane, coupled to the standing wave along the incident wave direction. To realize broadband impedance matching, we use a metallic boundary to split the dipolar resonance into two resonances, and add resistors so as to maximize dissipation over the frequency range in-between the two split resonances. To further extend the absorption spectrum to an ultra-broadband range, we employ a double-layer self-similar structure, in conjunction with the absorption of the diffracted waves at the higher end of the frequency spectrum. The resulting microwave absorber pushes the overall performance close to the causality limit, while simultaneously possesses a large relative absorption bandwidth.

**Biography:**

Ping Sheng is the William Mong Professor of Nanoscience and Chair Professor of Physics at HKUST. He obtained his BSc in Physics from the California Institute of Technology, and PhD in Physics from Princeton University in 1971. After a stay at the Institute for Advanced Study, Ping joined RCA David Sarnoff Research Center in 1973. In 1979 he joined the Exxon Corporate Research Lab, where he served as the head of the theory group during 1982-86. In 1994 Ping joined the HKUST as a professor of physics and served as the head of the physics department from 1999 to 2008.

Prof. Sheng is a Fellow of the American Physical Society and a Member of the Asia Pacific Academy of Materials. He served as the Executive Editor of Solid State Communications, a Division Associate Editor of Physical Review Letters and a member of the editorial board of New Journal of Physics. He was awarded Technology Leader of the Year by the Sing Tao Group in 2002, the Brillouin Medal by the International Phononics Society in 2013, the National Natural Science Award (second class) by the State Council of the People’s Republic of China in 2014, and the Rolf Landauer Medal by the ETOPIM Society in 2018.

Prof. Sheng has published more than 480 papers with a total of over 42,500 citations and an h-index of 93 (by Google Scholar). He has presented over 340 keynote, plenary or invited talks at international meetings and conferences. His current research interests include acoustic metamaterials, superconductivity in carbon nanotubes, nanostructured graphene, giant electrorheological fluids, fluid-solid interfacial phenomena, and effective medium theory of composites.

Seminar 39:** Parity-Time (PT) symmetry in chiral electromagnetic metamaterials**

Speaker:** Maria Kafesaki **(Department of Materials Science and Technology, Crete, Greece)

**Q & A:**

**Abstract: **Combining chirality and Parity-Time-symmetry in a single metamaterial structure one can achieve a variety of novel propagation and scattering characteristics. These characteristics include mixed PT-related phases, multiple exceptional points, asymmetric reflection, asymmetric optical activity, and others. In the talk, besides discussing the possibility and conditions for the simultaneous coexistence of PT-symmetry and chirality, I will discuss all the above characteristics along with the potential to control them and exploit in a variety of applications related with electromagnetic wave polarization control.

**Biography:** Maria Kafesaki is Associate Professor in the Dept. of Materials Science and Technology of the University of Crete and Adjunct Researcher at the Institute of Electronic Structure and Laser (IESL) of Foundation for Research and Technology Hellas (FORTH). She obtained her Ph.D. in 1997, at the Physics Department of the University of Crete, Greece, on elastic wave propagation in complex media. She has worked as a post-doctoral researcher in the Consejo Superior de Investigaciones Cientificas in Madrid, Spain, and in IESL of FORTH (1997-2001). Her current research is on the area of electromagnetic wave propagation in periodic and random media, with emphasis on photonic crystals and metamaterials, where she has large theoretical and computational experience. She has more than 110 publications in refereed journals (with more than 6500 citations and h-index=42, according to Web of Science), and more than 70 invited talks at international conferences and schools. She has participated in many European projects as well as in the organization of many international conferences and schools. She is a Fellow of the Optical Society of America. Figure: A simple PT-symmetric chiral bi-layer. Under oblique incidence of circularly polarized (CP) waves the bi-layer eigenvalues (σ) indicate the existence of a mixed phase and two exceptional points, highly tunable by the incidence angle.

Seminar 38:** Space-time metamaterials: dragging and amplifying light**

Speaker:** Paloma Arroyo Huidobro** (Instituto de Telecomunicações, Insituto Superior Técnico-University of Lisbon)

**Q & A:**

**Abstract: **In this talk I will consider space-time metamaterials of travelling-wave type [1] and present a theory of homogenisation of these modulated media [2]. This framework provides analytical expressions for the effective permittivity, permeability and magnetoelectric coupling of these media in the long wavelength limit. From the derived parameters we will see how it is possible to achieve nonreciprocal effects away from the asymmetric band gaps, and even down to the quasistatic limit if both the permittivity and permeability are modulated. This way, the synthetic motion present in these systems allows a new and tunable form of Fresnel drag effect of light in moving media [3]. The theory unveils a regime where the modulation speed approaches that of waves in the background medium where homogenisation breaks down and a new mechanism for gain emerges [4], enabling a form of nonreciprocal broadband amplification that could be realised in graphene [5]. On the other hand, temporal only modulations enable novel ways to excite surface waves from the far field [6].

[1] Galiffi, Huidobro, and Pendry, *“Space-time metamaterials” *in *Roadmap on multimode light shaping *coordinated by Piccardo and Ginis, arXiv:2104.03550 (2021)

[2] Huidobro, Silveirinha, Galiffi and Pendry, *“Homogenisation theory of space-time metamaterials” *arXiv:2009.10479 (2020).

[3] Huidobro, Galiffi, Guenneau, Craster, and Pendry, *“Fresnel drag in space-time modulated metamaterials” *Proceedings of the National Academy of Sciences 116 (50) 24943-24948 (2019).

[4] Pendry, Galiffi and Huidobro, *“A new mechanism for gain in time dependent media” *arXiv:2009.12077 (2020).

[5] Galiffi, Huidobro, and Pendry, “*Broadband Nonreciprocal Amplification in Luminal Metamaterials”, *Physical Review Letters, 123, 206101 (2019).

[6] Galiffi, Wang, Lim, Pendry, Alù, and Huidobro, “*Temporal Wood Anomalies: Smoothing the Path to the Near-Field” *Physical Review letters 125, 127403 (2020).

**Biography:** Paloma Arroyo Huidobro received her PhD from Universidad Autónoma de Madrid (Spain) in 2013 working under the supervision of Francisco J. García Vidal. In 2014 she joined Imperial College where she held a Marie Sklodowska-Curie Fellowship and worked with John Pendry and Stefan Maier. In 2019 she took up an FCT Researcher Fellowship in Lisbon, where she is currently a researcher at Instituto de Telecomunicações, based in Instituto Superior Técnico – University of Lisbon. Her work is devoted to developing theory of nanoscale light-matter interactions, metamaterials and waves in time-varying media.

Seminar 37:** Machine Learning Assisted Photonics: From Metasurface Design and Materials to Quantum Photonics**

Speaker:** Alexandra Boltasseva **(School of Electrical and Computer Engineering, Birck Nanotechnology Center and Purdue Quantum Science and Engineering Institute, Purdue University, West Lafayette, IN 47906, USA and The Quantum Science Center (QSC), a National Quantum Information Science Research Center of the U.S. Department of Energy (DOE), Oak Ridge, TN 37931)

**Q & A:**

**Abstract: **Discovering unconventional optical designs via machine-learning promises to advance on-chip circuitry, imaging, sensing, energy, and quantum information technology. In this talk, photonic design approaches and emerging material platforms will be discussed showcasting emerging photonic materials, machine-learning-assisted topology optimization for thermophotovoltaic metasurface designs and machine-learning-enabled quantum optical measurements.

**Biography:** Alexandra Boltasseva is a Ron and Dotty Garvin Tonjes Professor of Electrical and Computer Engineering with courtesy appointment in Materials Engineering at Purdue University. She received her PhD in electrical engineering at Technical University of Denmark, DTU in 2004. Boltasseva specializes in nanophotonics, nanofabrication, optical materials, and quantum photonics. She is 2018 Blavatnik National Award for Young Scientists Finalist and received the 2013 IEEE Photonics Society Young Investigator Award, 2013 Materials Research Society (MRS) Outstanding Young Investigator Award, the MIT Technology Review Top Young Innovator (TR35), the Young Researcher Award in Advanced Optical Technologies from the University of Erlangen-Nuremberg, Germany, and the Young Elite-Researcher Award from the Danish Council for Independent Research. She is a Fellow of the National Academy of Inventors (NAI), Fellow of the IEEE, Fellow of Optical Society of America (OSA), MRS and SPIE. She served on MRS Board of Directors and is currently Editor-in-Chief for OSA’s Optical Materials Express journal.

Home – Prof. Alexandra Boltasseva

Seminar 36: **Nanophononic metamaterials: Thermal conductivity reduction by “millions” of local resonances**

Speaker:** Mahmoud I. Hussein **(Ann and H.J. Smead Department of Aerospace Engineering Sciences Department of Physics, University of Colorado Boulder, USA)

**Q & A: **

**Abstract: **The concept of a metamaterial has been introduced in electromagnetics and acoustics as an artificially structured locally resonant material that exhibits unique, and often extreme, effective properties in the subwavelength regime. Here we introduce the notion of a locally resonant metamaterial to the area of heat transfer at the nanoscale. In contrast to electromagnetic and acoustic metamaterials, the underlying phenomenon is not limited to the subwavelength regime and does not depend on an effective properties characterization. This new type of metamaterial–termed *nanophononic metamaterial* (NPM) [1-3]–is also fundamentally different from what is known as thermal metamaterials, which are primarily concerned with transformation heat diffusion and not wave-based phenomena.

One realization of an NPM is a freestanding silicon membrane (thin film) with a periodic array of nanoscale pillars erected on one or both free surfaces. Heat is transported along the membrane portion of this nanostructured material as a succession of propagating vibrational waves, *phonons*. The atoms making up the minuscule pillars on their part generate resonant vibrational waves, which we describe as *vibrons*. These two types of waves linearly interact causing a mode coupling for each pair which appears as an avoided crossing in the pillared membrane’s phonon band structure. This in turn (1) enables the generation of new modes localized in the nanopillar portion(s) and (2) reduces the base membrane phonon group velocities around the coupling regions. In addition, the phonon lifetimes drop due to changes in the scattering environment, including both phonon-phonon scattering and boundary scattering. These three effects inhibit the in-plane thermal transport. Given that the number of vibrons scales with the number of degrees of freedom of a nanopillar, these effects intensify as the size of the nanopillar(s) increases–possibly reaching millions of vibrons–and in principle may be tuned to influence the entire phonon spectrum (which for silicon extends up to over 17 THz). This novel phenomenon thus provides an opportunity for achieving exceptionally strong reductions in the thermal conductivity, with excellent potential for utilization in thermoelectric energy conversion.

[1] Davis, B.L. and Hussein, M.I., “Nanophononic metamaterial: Thermal conductivity reduction by local resonance,” *Physical Review Letters* **112**, 055505, 2014.

[2] Honarvar, H. and Hussein, M.I., “Two orders of magnitude thermal conductivity reduction in silicon membranes by resonance hybridizations,” *Physical Review B* **97**,** **195413, 2018.

[3] Hussein, M.I., Tsai, C.N. and Honarvar, H., “Thermal conductivity reduction in a nanophononic metamaterial versus a nanophononic crystal: Review and Comparative Analysis,” *Advanced Functional Mater*ials, **30**, 1906718, 2020.

**Biography:**

Mahmoud I. Hussein is the Alvah and Harriet Hovlid Professor at the Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder. He holds a courtesy faculty appointment in the Department of Physics and an affiliate faculty appointment in the Department of Applied Mathematics, and he serves as the Faculty Director of the Pre-Engineering Program at the College of Engineering and Applied Science. He received a BS degree from the American University in Cairo (1994) and MS degrees from Imperial College, London (1995) and the University of Michigan‒Ann Arbor (1999, 2002). In 2004, he received a PhD degree from the University of Michigan, after which he spent two years at the University of Cambridge as a postdoctoral research associate.

Dr. Hussein’s research focuses on the dynamics of materials and structures, especially phononic crystals and locally resonant phononic metamaterials, at both the continuum and atomistic scales. His approach to phononics is rather broad, considering applications that range from vibrations of aerospace structures and passive flow control to lattice dynamics and thermal transport in semiconductor-based nanostructured materials. His studies are concerned with physical phenomena governing these systems, relevant theoretical and computational treatments, and analysis of the effects of dispersion, resonance, dissipation and nonlinearity. Recently he has also been conducting experiments to support portions of the theoretical work. Dr. Hussein received a DARPA Young Faculty Award in 2011, an NSF CAREER award in 2013, and in 2017 was honored with a Provost’s Faculty Achievement Award for Tenured Faculty at CU Boulder. He has co-edited a book titled *Dynamics of Lattice Materials* published by Wiley. He is a Fellow of ASME and a former associate editor for the *ASME Journal of Vibration and Acoustics*. In addition, he is the founding vice president of the International Phononics Society and has co-established the Phononics 20xx conference series which was launded in 2011 and has been organized biennially since then.

Home – Prof. Mahmoud I. Hussein

Seminar 35**: Waves in elastic chiral lattice systems**

Speakers:** Natasha Movchan **(Department of Mathematical Sciences, University of Liverpool, UK)

**Q & A:**

**Abstract: **The lecture gives an overview of the recent work on a new class of chiral elastic waves in lattice systems. It is based on the recent joint work with G. Carta, I. Jones and A. Movchan.

One can distinguish between geometrically chiral structures and physically chiral structures, and our emphasis here is on physical chirality whose presence is reflected in the governing equations, whatever the geometry of the overall structure is.

The analysis of the dispersion properties reveals a new class of vortex waveforms that characterise the dynamic response of the chiral elastic system and shows the dynamic anisotropy. Special attention is given to the defect modes and chiral Green’s function for discrete systems.

**Biography:** Professor Natasha Movchan is based at the Department of Mathematical Sciences, University of Liverpool.

Her main research interests are in mathematical modelling of dynamic fracture, wave propagation in heterogeneous media, passive and active cloaking, dynamic anisotropy and wave localisation in structured media.

Seminar 34**: Homogenization of resonant metafilms**

Speakers:** Agnes Maurel **(Langevin Institute, ESPCI Paris, CNRS) **& Kim Pham** (IMSIA, ENSTA Paris, Institut Polytechnique de Paris, France)

**Q & A:**

**Abstract: **In this talk, we will illustrate with three different examples the modeling of resonant metasurfaces using asymptotic homogenization techniques. We will first present the method on a periodic array of Helmholtz resonators (open cavities) in acoustics. In this case, the combination of bulk homogenization and interface homogenization allows to derive an approximated model in the time domain where the array of resonators is replaced by an anisotropic effective medium with effective jump conditions at boundaries. These conditions encapsulate all the evanescent contribution of the fields near the opening of the resonators and account for the transition between a quarter wavelength resonator to a Helmholtz resonator as the opening shrinks. In a second example we will focus on Mie resonance in elastodynamic for anti-plane shear waves. We will consider an array of sub-wavelength soft inclusions embedded in an elastic matrix. Due to the contrast of material properties between the constituents, the wavelength inside the soft inclusions become comparable to its size, hence leading to Mie resonances. We will show how asymptotic homogenization allows to capture the resonant behavior in an effective model consisting in frequency-dependent jump conditions across the array. Finally we will extend these results to a non-linear example in acoustic consisting in a screen of air bubbles inside water subjected to large pressure variations which is known to exhibit subwavelength Minnaert resonance. Homogenization theory applied to Navier-Stokes equations allows in this case to derive non-linear jump conditions where the behavior of the bubbles radii is governed by a non-linear equation of the Rayleigh-Plesset’s type.

**Biography:** Dr Agnès Maurel has received her PhD in 1994 from the Sorbonne-Université in Paris, France. She is currently Director of Research Scientist at CNRS, Institut Langevin.

Her research focuses on wave physics (water waves, elastodynamics, acoustics and electromagnetism) and metamaterials.

Dr Kim Pham has received his PhD in 2010 from the Sorbonne-Université in Paris, France. He is currently Assistant Professor at ENSTA Paris, IMSIA.

His research focuses on homogenization techniques for wave propagation in metamaterials and variational modeling of damage mechanics.

Seminar 33**: Surprising acoustic propagation in foams : from liquid to solid**

Speaker:** Juliette Pierre **(Institut Jean Le Rond D’Alembert, Paris, France)

**Q & A:**

**Abstract: **Liquid and solid foams present surprising acoustic properties.

A liquid foam is a complex gas-liquid system with a low liquid fraction (<30%). Its rigidity is due to a liquid skeleton, containing the most important part of the liquid and linked by thin liquid films. The bubble size distribution is strongly polydisperse.

Over the past years we investigated, in a large frequency range, how a liquid foam behaves when insonified. We showed that liquid foams act as natural and 3D isotropic acoustic metamaterials. Moreover, this work confirmed that liquid foams have important soundproofing properties. First, I will show you how an acoustic wave propagates and dissipates when it travels through a collection of thin liquid films loaded by the surrounding air and a massive liquid skeleton.

More recently, we investigated the case of solid foams elaborated by solidification of liquids foams and where the entire liquid skeleton (even films) has been preserved. In this second part, I will show you that the presence of very thin membranes is far from being a handicap to blocking sound.

**Biography:** Juliette Pierre has received her PhD in physical acoustics in 2011 from Sorbonne University at Paris and is currently a research scientist at CNRS – d’Alembert Institute of Sorbonne University. She is interested in experiments in acoustic wave propagation and liquid-gas systems (bubbles, films and foams).

Seminar 32**: Elastic Metamaterials and Their Applications to Wave Control**

Speaker:** Gengkai Hu **(School of Aerospace Engineering, Beijing Institute of Technology, Beijing, China)

**Q & A:**

**Abstract: **Wave property of materials is intimately related to their microstructures. By carefully designing the microstructure, different wave functions not available for traditional materials can be envisaged. In this talk, I will demonstrated through three examples the elastic wave control by the design of elastic metamaterials. The first is elastic negative refraction by designing an elastic metamaterial with both negative effective mass and modulus. The second example focuses on elastic wave filtering, we show that solids with only single polarization mode (either transverse wave or longitudinal wave) can be designed with pentamode materials. Finally I will explain the principle to make an asymmetric metamaterials which breaks the property of symmetric stress, and apply it to design elastic wave cloak. These results demonstrate a great capacity of elastic wave tailoring by elastic metamaterials and will surely find engineering applications in the near future.

**Biography:** Gengkai Hu is currently the Chair Professor of solid mechanics at Beijing Institute of Technology (BIT). He received his Ph.D. in mechanics and materials from Ecole Centrale de Paris (France) in 1991 and spent two years at the same university as a postdoctoral research associate before joining BIT. He has written or coauthored more than 170 papers in refereed journals, and received the Award of National Outstanding Youth Scientist by National Natural Science Foundation of China in 2003, and National Outstanding Teacher Award in 2004. He and his research group led some of the early works on elastic metamaterials. His current research interests include: dynamic homogenization of composite materials, metamaterials for controlling elastic wave propagation

Seminar 31**: On the compact wave dynamics of tensegrity metamaterials**

Speakers:** Fernando Fraternali **(Department of Civil Engineering University of Salerno, Italy)

**Q & A:**

**Abstract: **A research subject attracting remarkable attention from many areas of engineering concerns the use of noninvasive tools to target defects in materials, as well as devices for monitoring structural health in materials and structures. Most of current methods for the focusing and defocusing of acoustic waves (based mainly on exploiting linear acoustic effects) present the problem of having essentially little to no tunability ranges and poorly scalable dimensions. The use of highly nonlinear systems, allowing for a high level of control over the acoustic speed may enable the creation of revolutionary types of devices with far more advanced wave-focusing methodologies.This talk illustrates the employment of tensegrity metamaterials featuring elastically stiffening/hardening response for the fabrication of tunable focus acoustic lenses or innovative devices for monitoring structural health and damage detection in materials and structures. These devices support extremely compact compression waves in multiple dimensions. Previous research on 1D systems is generalized to 2D and 3D tensegrity beams and plates with stiffening-type response (acting as phononic crystals). The presented results reveal that the dynamics of such systems is characterized by the thermalization of the lattice nearby the impacted regions of the boundary. The portion of the absorbed energy moving along the longitudinal direction is transported by compression waves with compact support. Such waves emerge with nearly constant speed, and slight modifications of their spatial shape and amplitude, after collisions with compression waves traveling in opposite direction.

**References**

Fraternali, F.; Senatore, L.; Daraio, C. Solitary waves on tensegrity lattices. J. Mech. Phys. Solids 2012, 60, 1137–1144.

Fraternali, F.; Carpentieri, G.; Amendola, A.; Skelton, R.E.; Nesterenko, V.F. Multiscale tunability of solitary wave dynamics in tensegrity metamaterials. Appl. Phys. Lett. 2014, 105, 201903.

Davini, C.; Micheletti, A.; Podio-Guidugli, P. On the impulsive dynamics of T3 tensegrity chains. Mecc. 2016, 51, 2763–2776.Micheletti, A.; Ruscica, G.; Fraternali, F. On the compact wave dynamics of tensegrity beams in multiple dimensions. Nonlinear Dyn. 2019, 98, 2737–2753.

**Biography:** Fernando Fraternali is Professor of Structural Mechanics in the Department of Civil Engineering at the University of Salerno (Diciv), Italy. He received his B.Sc. and M.Sc. degrees in Civil and Environmental Engineering from the University of Salerno, and a Ph.D. in Multiscale Mechanics from King’s College London. F. Fraternali has participated as a PI or co-PI in various research projects funded by the Italian National Research Council, the Ministry of Education, the Ministry of Foreign Affairs (Italy-USA scientific cooperation), and US research agencies. He currently serves as PI of the Research Project of National Relevance “Multiscale Innovative Materials and Structures” granted by the Italian Ministry of Education, University and Research for the years 2019 –2023. Prof. Fernando Fraternali is Delegate to Research and Doctorate Affairs and serves as Coordinator of the PhD Course on “Risk and Sustainability in Civil, Architecture and Environmental Engineering Systems” (cycles XXXII-XXXVI) at Diciv. Most of his research work concerns multiscale modeling and simulation of solids and structures, the nonlinear dynamics of innovative materials and structures, and the design and engineering of sustainable materials at multiple scales. Prof. Fraternali was awarded a Fulbright Research Scholarship for the academic year 2005/06 and has been Visiting Professor at the Graduate Aerospace Laboratories of the California Institute of Technology since September 2005 (several periods), and the Department of Mechanical and Aerospace Engineering, University of California, San Diego, USA, from August 2012 through to the present. F. Fraternali is Associate Editor of Mechanics Research Communications (ISSN: 0093-6413), Frontiers in Materials (ISSN: 2296-8016) and Ingegneria Sismica – International Journal of Earthquake Engineering (ISSN: 0393-1420).

Home – Prof. Fernando Fraternali

Seminar 30**: Flexural Wave Localization in Structured and Disordered Thin Plates**

Speaker: **Patrick Sebbah **(Department of Physics, The Jack and Pearl Resnick Institute for Advanced Technology, Bar Ilan University, Israel)

**Q & A: **

**Abstract: **Predicting the spatial pattern of vibrational modes in complex systems remains a key scientific and engineering challenge with strong repercussions in various domains such as Anderson localization prediction, laser cavity design or musical instrument architecture. A recent theoretical breakthrough brought a new tool, called the “localization landscape”, for retrieving crucial information on the spatial and frequency properties of localized waves [1]. In this talk, we first investigate the predictive power of the “localization landscape” function for wave localization of elastic waves in a structured thin plate and show how regions of wave confinement can be predicted from the knowledge of the static deformation of the plate [2]. We then report the first observations of disorder-induced Anderson localization of flexural waves in thin plates. In a first system, we measure modes of a randomly-pinned plate. Strongly confined modes at low frequencies progressively couple and extend over the disordered structure, as predicted by the landscape theory. A radically different behavior is found for surface acoustic waves propagating at the surface of a silicon wafer with randomly-distributed resonant blind holes. The localized modes are found at frequencies around the hybridization gap opened at the resonance frequency of the scatterers. In this regime, an adequate landscape theory is still to be developed for predicting the regions of localization.

**Biography:** Patrick Sebbah is currently a professor at the Physics Department, The Jack and Pearl Resnick Institute for Advanced Technology, at Bar Ilan University, Israel. He graduated from the École Nationale Superieure des Telecommunications de Paris in 1988 and from the University of Paris XI – Orsay in 1989. He received his Ph.D in Physics (1993) as well as his Habilitation to Lead Research (2003) from the University of Nice, France. he did his postdoctoral work at the Physics Department of Queens College, CUNY, NY, USA, in the group of Prof. Azriel GENACK. He entered the CNRS in 1994. He became a Director of Research of the CNRS when in moved to the Langevin Institute, ESPCI-ParisTech, in 2011. He is now leading the Mesoscopic Optics and Complex Media research group at Bar Ilan University.

His research focuses mainly on Experimental Physics of Wave Propagation in Complex Media. His research interests are mainly Light-Matter Interaction, Multiple Scattering in Disordered Systems, Nonlinear and Active Random Media. He made significant contributions in the field of Random Lasers, Anderson localization and Elastic Metamaterials.His research is currently funded by the Israeli Science Foundation, the Bi-Science Foundation (BSF-NSF), the US Air Force and the MAFAT.

[1] M. Filoche, and S. Mayboroda, Proceedings of the National Academy of Sciences 109, 14761-14766 (2012).

[2] G. Lefebvre, A. Gondel, M. Dubois, M. Atlan, F. Feppon, A. Labbé, C. Gillot, A. Garelli, M. Ernoult, S. Mayboroda, M. Filoche, and P. Sebbah, Phys. Rev. Lett. 117, 074301 (2016).

[3] K. Tang and P. Sebbah, “Anderson localization of flexural waves in randomly pinned plate”, in preparation.[4] G. Lefebvre, M. Dubois, P. Sebbah, “Strong localization induced by resonating scatterers in plates”, in preparation.

Seminar 29**: Towards the engineering design of metamaterials’ structures through micromorphic enriched continuum modeling**

Speaker: **Angela Madeo **(GEOMAS Laboratory of the Institut National des Sciences Appliquées de Lyon, France)

**Q & A:**

**Abstract: **In this talk, I will show how enriched continuum models of the micromorphic type can be used to describe the dynamical behavior of anisotropic mechanical metamaterials. I will show to which extent one of the the proposed models, that I contributed to pioneer, is able to capture all the main microscopic and macroscopic characteristics of the targeted metamaterials, namely, stiffness, anisotropy, dispersion and band-gaps. The simple structure of our material model, which simultaneously lives on a micro-, a meso- and a macroscopic scale, requires only the identification of a limited number of frequency-independent parameters thus allowing the introduction of pertinent boundary conditions to be imposed at metamaterials’ boundaries when the model is framed in the context of Variational Principles. I will show how this model can be applied to the study of the scattering properties of finite-size metamaterials’ structures thus opening new perspectives for metastructural engineering design.

**Biography:** Angela Madeo is currently Full Professor at the GEOMAS Laboratory of the Institut National des Sciences Appliquées de Lyon. She obtained a Master of Science in Civil Engineering at the University of Rome “La Sapienza” (Italy) in 2005 and a second one in Engineering Science and Mechanichs at the Virginia Polytechnic Institute and State University (USA) in 2006. She obtained her PhD in Theoretical and Applied Mechanics at the University of Rome “La Sapienza” in 2009. She has been Associate Professor at INSA Lyon from 2010 to 2017. Her research expertise seats on the study of Enriched Models in Continuum Mechanics and their applications to mechanical metamaterials, as well as to other materials with heterogeneous microstructures. She is recipient of the ERC Consolidator Grant META-LEGO (2021-2026). She is member of the prestigious Institut Universitaire de France since 2016, when she was nominated asjunior IUF member for her ground-breaking research on enriched continuum modeling of metamaterials. She was recipient of the CNRS Bronze medal in 2015. She coordinates several research projects funded with National French grants (ANR), as well as European grants (ERC, RIA, Horizon 2020). She gives lectures in the Civil Engineering Departement of INSA-Lyon, mainly in the field of Applied Math- ematics, Continuum Mechanics and Mechanical Behavior of Materials with Microstructure. She co-authored more than 60 papers in high-level international journals, she is author of a book on Generalized Continuum Mechanics and Engineering Applications, edited by ISTE Editions in 2015 and she has an H-index of 30 (according to Google Scholar). She is member of the Editorial board of 3 high level international journals in the field of Theorethical and Applied Mechanics.

Seminar 28**: Nonlinear Waves and Flexible Elastic Metamaterials**

Speaker: **Vincent Tournat **(Laboratoire d’acoustique de l’Université du Mans, UMR CNRS 6613, Institut d’Acoustique – Graduate School, France)

**Q & A:**

**Abstract: **Flexible elastic metamaterials can be described as artificial structures, designed to be highly compliant and capable of withstanding large elastic deformations. Their current significance lies in the fact that they possess a number of unusual properties that can be controlled and, in addition, they belong to an extremely large design space. While their static character has been widely studied, their dynamic properties are still in their early stages, especially with regard to their non-linear dynamics. Nevertheless, we recall here that these non-linear properties can be designed in a rational way, allowing the development of metamaterials for the control of large amplitude elastic waves [1-5]. In this context, I synthesize in this talk a set of our recent results on the propagation of nonlinear waves in these flexible elastic metamaterials [1-10], and I will draw up possible perspectives from these initiated directions, ranging from vibration control, toy models for fundamental wave physics, to possible practical applications.

[1] B. Deng, J.R. Raney, V. Tournat, K. Bertoldi, **Elastic vector solitons in soft architected materials**, Phys. Rev. Lett. 118(20), 204102 (2017). **PDF** **Supp. Mat.**

[2] B. Deng, P. Wang. Q. He, V. Tournat, K. Bertoldi, **Metamaterials with amplitude gaps for elastic solitons**, Nature Communications 9, 3410 (2018). **PDF** **Supp. Mat.**

[3] B. Deng, V. Tournat, K. Bertoldi, **Effect of predeformation on the propagation of vector solitons in flexible mechanical metamaterials**, Phys. Rev. E 98, 053001 (2018). **PDF**

[4] B. Deng, V. Tournat, P. Wang, K. Bertoldi, **Anomalous collisions of elastic vector solitons in mechanical metamaterial**, Phys. Rev. Lett. 122, 044101 (2019). **PDF** **Supp. Mat.**

[5] X. Guo, V.E. Gusev, V. Tournat, B. Deng, K. Bertoldi, **Frequency-doubling effect in acoustic reflection by a nonlinear, architected rotating-square metasurface**, Phys. Rev. E 99, 052209 (2019). **PDF**

[6] B. Deng, C. Mo, V. Tournat, K. Bertoldi, J.R. Raney, **Focusing and Mode Separation of Elastic Vector Solitons in a 2D Soft Mechanical Metamaterial**, Phys. Rev. Lett. 123(2), 024101 (2019). **PDF** **Supp. Mat.**

[7] L. Jin, R. Khajehtourian, J. Mueller, A. Rafsanjani, V. Tournat, K. Bertoldi, D. M. Kochmann, **Guided transition waves in multistable mechanical metamaterials**, Proc. Natl. Acad. Sci. 201913228 (2020). **PDF**

[8] B. Deng, P. Wang, V. Tournat, K. Bertoldi, **Nonlinear Transition Waves in Free-standing Bistable Chains**, J. Mech. Phys. Sol. 136, 103661 (2020). **PDF**

[9] B. Deng, S. Yu, A. E. Forte, V. Tournat, K. Bertoldi, **Characterization, stability, and application of domain walls in flexible mechanical metamaterials**, Proc. Natl. Acad. Sci. 202015847 (2020). **PDF** **Supp. Mat**.

[10] B. Deng, J. Li, V. Tournat, P. Purohit, K. Bertoldi, **Dynamics of mechanical metamaterials: A framework to connect phonons, nonlinear periodic waves and solitons**, J. Mech. Phys. Sol. 147, 104233 (2021). **PDF**

**Biography:** Vincent TOURNAT (VT) is currently a Research Director at CNRS and is the director of Institut d’Acoustique – Graduate School. VT conducts research activities in the field of acoustics and nonlinear waves at the Laboratoire d’Acoustique de l’Université du Mans in France (LAUM UMR CNRS 6613). He graduated with a major in fundamental physics and acoustics and defended his PhD thesis on nonlinear acoustics in granular materials in 2003. His research ranges from elastic waves in complex solids (granular media and crystals, cracked solids, metamaterials) to audible range acoustics of metamaterials, nonlinear ultrasonics in liquids, nonlinear ultrasonic NDT of solids and laser ultrasonics. His main current research activities combine theoretical, computational and experimental methods for the study of nonlinear elastic and acoustic waves in metamaterials.

Seminar 27**: 4D Metastructures for Wave and Diffusion Phenomena**

Speaker: **Nader Engheta **(Department of Electrical and Systems Engineering, University of Pennsylvania, USA)

**Q & A:**

**Abstract: **Materials are often used to manipulate waves and fields. Metamaterials have provided far-reaching possibilities in achieving “extremes” in such wave-matter interaction. Various exciting functionalities have been achieved in exploiting metamaterials and metasurfaces in nanophotonics and nano-optics. We have been exploring how spatiotemporal metamaterials can give us new platforms in structuring light for exploiting waves to do certain useful functions for us. Several scenarios are being investigated in my group. As one scenario, we have been developing metastructure platforms that can perform analog computation, such as solving integral and differential equations and inverting matrices, with waves as waves interact with them. Such “metamaterial machines” can function as wave-based analog computing machines, suitable for micro- and nanoscale integration. Another scenario deals with 4-dimensional (4D) metamaterials, in which temporal variation of material parameters is added to the tools of spatial inhomogeneities for manipulating light-matter interaction with spatiotemporal platforms. These 4D structures can also be used for manipulation of diffusion to achieve asymmetric diffusion and trapping. The third category of structured waves is achieved in the near-zero-index materials and associated photonic doping that exhibit unique features in light-matter interaction, opening doors to exciting new wave-based and quantum optical features. In this talk, I will present some of our ongoing work in the above topics, and will forecast possible future research directions in these paradigms.

**Biography:** Nader Engheta is the H. Nedwill Ramsey Professor at the University of Pennsylvania in Philadelphia, with affiliations in the Departments of Electrical and Systems Engineering, Bioengineering, Materials Science and Engineering, and Physics and Astronomy. He received his BS degree from the University of Tehran, and his MS and Ph.D. degrees from Caltech.

He has received several awards for his research including the *Isaac Newton Medal and Prize *from the Institute of Physics (UK)*, Max Born Award *from the Optical Society*,* *Ellis Island Medal of Honor,* the *IEEE Pioneer Award in Nanotechnology*, the *Gold Medal from SPIE, the Balthasar van der Pol Gold Medal *from the International Union of Radio Science (URSI)*, *the *William Streifer Scientific Achievement Award*, induction to the *Canadian Academy of Engineering* as an International Fellow, *the Fellow of US National Academy of Inventors (NAI), *the *IEEE Electromagnetics Award*, *the Vannevar Bush Faculty Fellowship Award *from US Department of Defense*, the Wheatstone Lecture in King’s College London, 2006 Scientific American Magazine 50 Leaders in Science and Technology*, the *Guggenheim Fellowship*, and the *IEEE Third Millennium Medal.*

He is a Fellow of seven international scientific and technical organizations, i.e.,* *IEEE, OSA, APS, MRS, SPIE, URSI, and AAAS. He has received the honorary doctoral degrees from the Aalto University in Finland in 2016, the University of Stuttgart, Germany in 2016, and Ukraine’s National Technical University Kharkov Polytechnic Institute in 2017.His current research activities span a broad range of areas including photonics, metamaterials, electrodynamics, nano-optics, graphene photonics, imaging and sensing inspired by eyes of animal species, microwave and optical antennas, and physics and engineering of fields and waves.

Seminar 26**: Phason Engineering for Topological Wave Steering**

Speaker: **Emil V Prodan **(Physics Department, Yeshiva University, New York, USA)

**Q & A:**

**Abstract: **A general principle discovered by Jean Bellissard says that every aperiodic pattern has an intrinsic global degree of freedom that lives on a topological space called the hull of a pattern. This concept generalizes the notion of phason found in the physics literature on quasicrystals. In this talk, I will describe various ways to characterize and compute the hull of quasi-periodic patterns (i.e. aperiodic perturbations of periodic lattices) and then I will turn to problem of generating patterns with prescribed hulls, a process that I call phason engineering. Specifically, I will describe the main philosophy and supply a very general algorithm that produces phason spaces that are d-tori with d arbitrarily large. If discrete resonators are placed according to such a pattern, I will show that the hull augments the physical space, hence opening a door to the physic of the Integer Quantum Hall Effect in arbitrary dimensions. In the second part, I will demonstrate laboratory implementations of the ideas with mechanical and acoustic metamaterials that exhibit 2D and 4D IQHE physics. Among these examples, is the first un-assisted dynamical edge-to-edge Thouless pumping achieved in a laboratory.

**Biography:** Emil received BS and MS degrees in theoretical and mathematical physics from University of Bucharest. His advisor for the MS degree was mathematical physicists Gheorghe Nenciu. He received another MS degree in theoretical physics from University of Houston and then he graduated with a PhD from Rice University in theoretical physics. His advisor at Rice University was Peter Nordlander and his doctoral thesis was on large-scale quantum simulations of the plasmon response of nano-particles. Emil received further postdoctoral training at University of California Santa Barbara under the direction of Walter Kohn, a Nobel Laureate in theoretical chemistry. He was a fellow of the Princeton Center for Complex Materials at Princeton University, where he was sponsored by Roberto Car (2009 Dirac Medal) and Duncan Haldane (2016 Nobel Prize in Physics). Emil joined the Physics Department of Yeshiva University in 2007 and now he is a full professor of physics. While at Princeton, Emil witnessed Haldane’s effort on extending the concept of topological modes to photonic crystals, and he start working on the mechanical analog of the concept. The outcome, published in 2009 and co-authored with Camelia Prodan, was that Lagrangian systems containing q-dot-p terms can host Chern physics, a prediction confirmed experimentally in 2015. In about the same time, Emil’s research was profoundly influenced by the works of Jean Bellissard, who pioneered methods of analysis based on operator algebras, K-theory and non-commutative geometry. These days, together with many other enthusiasts, Emil is using these methods for search-and-discovery in materials science. He is also advertising what he calls Mathematical Engineering, where deep and utterly abstract concepts from pure mathematics are made concrete and applied, with a focus on advancing the technological progress rather than on just explaining observed phenomena.

Seminar 25:** Resonant States for Scattering Problems: Killing Mie Softly**

Speaker: **Ross McPhedran **(School of Physics, University of Sydney, Australia)

**Q & A:**

**Abstract:** This talk deals with issues common to those addressed by David Bergman, in that the topic is certain difficulties associated with complex resonant states encountered in scattering problems. These difficulties arise when one wishes to evaluate normalisation integrals and inner products for these states, which have rapid oscillations and diverging amplitudes at large dis- tances from the scatterer. Rather than using numerical treatments, the method presented here is analytic in nature, and relies on results of distri- bution theory. The method can yield closed form expressions for integrals over an infinite range involving the product of two Bessel functions of the same or different types, combined with a power of the radial distance. The essential point is that for rapidly oscillating functions with mean zero, the contribution to the integrals from the infinite upper limit is zero, despite the diverging amplitudes (equivocation trumps exaggeration).

** Biography: **Ross McPhedran is an Emeritus Professor at the University of Sydney, a Fellow of the Australian Academy of Science and the Optical Society of America, the Institute of Physics UK and the Australian Institute of Physics and doctor honoris causa of Aix-Marseille University.He works on problems in mathematical physics and wave science. Notably, he has many contributions to the theory of composite materials, the theory of diffraction gratings, and the theory of photonic, phononic, and platonic crystals, the latter name being chosen by Ross, and he codiscovered anomalous localized resonance. He has published over 300 articles in offered scientific journals, has an h index of 64 and around 17,000 citations on Google Scholar. He is the founding president of the ETOPIM association. The topic will include plasmonic resonances of particles, and the mathematical background to effective medium theories (particularly Bergman-Milton bounds and multipole theories).

Seminar 24:** Scattering electromagnetic eigenstates of a two-constituent composite and their exploitation for calculating a physical field**

Speaker: **David Bergman **(School of Physics and Astronomy, Tel Aviv University, Israel)

**Q & A:**

**Abstract: **The spectral representation of an electric field in a two-constituent composite medium is revisited. A theory is developed for calculating the electromagnetic (EM) eigenstates of Maxwell’s equations for such a composite where the magnetic permeability as well as the electric permittivity have different uniform values in the two constituents. The physical electric field **E(r)** produced in the system either by a given incident field or by a given source current density is expanded in this set of biorthogonal eigenstates for any position **r**. If the microstructure consists of a cluster of separate inclusions in a uniform host medium then the EM eigenstates of all the isolated inclusions can also be used to calculate **E(r)**. Once all these eigenstates are known for a given host and a given microstructure then calculation of **E(r) **only involves performing three-dimensional integrals of known functions and solving sets of linear algebraic equations.

**Biography:** I was born in 1940 in Tel Aviv, then in Palestine which was governed by a British mandate from the League of Nations. I got my BSc and MSc degrees from the Hebrew University of Jerusalem, and my PhD in 1968 from the Wezmann Institute of Rehovot, Israel. After two years as a post-doc at UCSD in La-Jolla, CA I joined the academic faculty of Tel Aviv University in 1969. During 2003–2011 I held the Joseph & Rebecca Meyerhoff Chair in Solid State Theory & Thermodynamics, Tel Aviv University (TAU). In 2008 I became Emeritus Professor at the School of Physics & Astronomy of TAU. Since then I have continued my research in theoretical physics at TAU. In 2015 I was awarded a Landauer Medal by the International ETOPIM Society. I am currently president of that society since 2018. I am Fellow of the Israel Physical Society (IPS) and of the American Physical Society (APS).

Here is a brief description of some of my major discoveries in physics: In 1977 I introduced a new approach to the analysis of composite materials. This resulted from the observation that the macroscopic moduli of such a material are analytic functions of the constituent moduli. As a result of this, new bounds for the macroscopic moduli of such materials, where the microstructure is disordered and not precisely known, were evolved. The poles of these analytic functions were shown to be the eigenvalues of the homogeneous Maxwell Equations. An approach for calculating the physical electromag- netic fields based upon the eigenstates of those equations was developed and is known in the community as the “Bergman-Milton spectral representation”. More recently I discovered surprising new properties of transport and optical response of a composite medium which are brought about by the presence of a strong magnetic field **B**. For example, in a mixture of metals with different but comparable resistivities, the macro- scopic Ohmic resistivity ρe can sometimes keep increasing as **B ^{2}** without any saturation when the Hall resistivities of the constituents are greater than their Ohmic resistivities and keep increasing as |

**B**|. This occurs when the microstructure is periodic. Moreover, ρe then depends very strongly on the precise direction of

**B**with respect to the periodic microstructure. These properties can be exploited for developing new types of magnetic field sensors.

Seminar 23:** New results on the range of responses that two-phase composites can have to time varying fields**

Speaker: **Graeme Milton **(Department of Mathematics, University of Utah, USA)

**Q & A:**

**Abstract: **It is now slightly more than 40 years since David Bergman and myself obtained bounds on the effective complex dielectric constant of isotropic composites of two isotropic phases mixed in given proportions. This constant governs the quasistatic response to electromagnetic waves of fixed frequency. Hence it also controls the absorption and refraction of radiation by these composites. There has been renewed interest in these old bounds, in part because they also give shape independent bounds in quasistatics on the amount of energy dielectric or metallic particles of given volume can absorb. Our bounds on the effective complex dielectric constant consisted of a lens shaped region in the complex plane bounded by circular arcs. We show that one bound can be tightened and that the corresponding circular arc corresponds to an assemblage of doubly coated spheres. New hierarchical laminate geometries that attain additional points on the remaining arc are described. Consequently, we see that this bound is very tight, almost optimal. We also bound the quasistatic response to applied electromagnetic fields, or antiplane elastic fields, that can have any variation in time, not necessarily a fixed frequency one. Curiously, judicious choices of the applied field can directly yield the volume fractions of the phases from measurements at specific times. We show how applied fields can be tailored to have this property. The work is joint with my wonderful and energetic collaborators: Christian Kern, Ornella Mattei, Owen Miller, and Mihai Putinar.

**Biography:** Graeme Milton is a distinguished (or maybe extinguished) professor of mathematics at the University of Utah, well known in the field of metamaterials for his 2002 book on the theory of composites, his contributions to bounds on the response of composites, realizability of those bounds, and for co-developing a systematic theory of exact relations for composites which are microstructure independent formulae satisfied by effective moduli. Notable contributions, often with coauthors, include: the first rigorous proof that isotropic elastic materials could have a negative Poisson ratio; the conception of pentamode materials useful for guiding stress; the construction of microgeometries that have reversed Hall coefficient compared to the phases; the discovery in 1994 of anomalous resonance essential to the understanding of superlens imaging; the finding in 2005 of cloaking due to anomalous resonance; and later developments on active cloaking. Additionally, he has obtained bounds on the response of inhomogeneous bodies, not necessarily with microstructure, and used these bounds to estimate the volume fractions of the phases. Other interests include the responses of electric, electromagnetic, and spring-mass networks, and their role in understanding the unusual responses of metamaterials. He is the recipient of Sloan and Packard fellowships, the Prager Medal, the Ralph Kleinman prize, the Landauer Medal, and the Levi-Civita prize among other awards.

Seminar 22**: Wave Control for Wireless Communications: From Time-Reversal to Reconfigurable Intelligent Metasurfaces**

Speaker: **Mathias Fink **(Langevin Institute, ESPCI Paris, PSL University, CNRS, France)

**Q & A:**

**Abstract: **In this talk, I will show how the works performed at Langevin Institute have led 8 years ago to the seminal concept behind large reconfigurable intelligent surfaces (RIS) that is currently a topic of great interest in the wireless communication community and that is proposed as a new paradigm for the sixth generation (6G) of communication networks. Starting with the first demonstrations of ultrasonic time-reversal focusing in complex media, I will underline how these ideas were transposed later, for electromagnetic waves, into the concept of massive multi-user multiple-input multiple-output (MIMO) for the 5G communication networks. Then, I will explain how, working on the concept of wavefront shaping using spatial light modulators in optics, we propose an equivalent for the microwave domain, using tunable metasurfaces, a concept that would solve the drastic price problem of active techniques like time reversal MIMO. By smartly tuning the signal reflection of large metasurfaces, reconfigurable tunable metasurfaces are capable of dynamically altering wireless channels in disordered environments to enhance the communication performance. Using reconfigurable metasurfaces placed inside any complex environment, we tune the medium complexity to obtain the best communication performance. Compared to the well-known massive multiple-input multiple-output (MIMO) technique, RIS can provide excellent system performance without requiring power-hungry components such as power amplifiers.

**Biography:** Mathias Fink is the George Charpak Professor at the Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI Paris). He is member of the French Academy of Science. His area of research is concerned with the propagation of waves in complex media and the development of numerous instruments based on this basic research. 6 start-up companies with more than 300 employees have been created from his research (Echosens, Sensitive Object, Supersonic Imagine, Time Reversal Communications, Cardiawave and Greenerwave).

Seminar 21**: Tailoring thermal emission with metasurfaces**

Speaker: **Jean-Jacques Greffet** (Université Paris-Saclay, CNRS, France)

**Q & A:**

**Abstract: **With the advent of fabrication and imaging at the nanoscale, many unexpected properties of thermal radiation have been explored in the last twenty years. The first part of the talk will be devoted to a review of key features including spatial and temporal coherence of incandescent sources. In the second part of the talk, recent results on an incandescent source that can be modulated beyond 10MHz, six orders of magnitude faster than commercially available hot membranes, will be presented.

**Biography:** Jean-Jacques obtained his PhD from university Paris-Sud Orsay in 1988 in solid state physics and the Habilitation in 1992. Jean-Jacques Greffet was a professor at Ecole Centrale Paris between 1994 and 2008. He is currently a professor at Institut d’Optique, Université Paris Sud and a senior member of Institut Universitaire de France. He has made a number of contributions in light scattering by random systems. Between 1994 and 2005, he worked on the theory of image formation in near-field optics. Since 1998, he investigated thermal radiation at the nanoscale and demonstrated spatial coherent thermal sources and a giant radiative heat transfer at the nanoscale. His current research interests include nanophotonics (nanoantennas, quantum plasmonics, metasurfaces for light emission) and the design of smart IR incandescent sources. He has coauthored 200 refereed papers with more than 17000 citations. He is an OSA fellow and the recipient of the Ixcore foundation prize and the Servant prize of the french Academy of Science.

Seminar 20**: Acoustic Metamaterial Wizardry**

Speaker: **Oliver B. Wright **(Division of Applied Physics, Faculty of Engineering, Hokkaido University, Sapporo, Japan)

**Q & A:**

**Abstract:** Acoustic metamaterials are non-naturally occurring structures consisting of arrays of sub-wavelength resonators designed to manipulate the propagation of sound, exhibiting effective negative density or modulus [1,2]. Counterintuitive metamaterial properties such as acoustic negative refraction, superlensing and cloaking have been demonstrated.

Another example is extraordinary acoustic transmission, i.e. the passage of more acoustic energy than expected through a small sub-acoustic-wavelength aperture acting as an acoustic meta-atom. This has been be demonstrated in a variety of systems, having first been shown in optics. I will first present our experimental work in this field using kHz airborne acoustics [3], in which case we find giant transmission, with enhancements up to ~60, by use of an aperture closed with a membrane. I will then show how gigantic transmission enhancement, by more than a factor of 500, can be achieved in solid acoustics [4,5].

Closely related to extraordinary acoustic transmission is the phenomenon of enhanced transmission between acoustically mismatched media. I will review experimental work on the enormously enhanced passage of acoustic waves from water to air based on the use of a kHz acoustic metasurface [6].

Finally, I will present recent kHz experiments on acoustic metabeams [7] and metarods [8] with perfect bandgaps that prevent all vibrations from passing along them at certain frequencies. The era of metawands that do not vibrate is upon us.

[1] Z. Liu, X. Zhang, Y. Mao, Y. Zhu, Z. Yang, C. T. Chan, and P. Sheng, Science 289, 1734 (2000).

[2] S. H. Lee and O. B. Wright, Phys. Rev. B 93, 024302 (2016).

[3] J. J. Park, K. J. B. Lee, O. B. Wright, M. K. Jung and S. H. Lee, Phys. Rev. Lett. 110, 244302 (2013).

[4] S. Mezil, K. Chonan, P. H. Otsuka, M. Tomoda, O. Matsuda, S. H. Lee and O. B. Wright, Sci. Rep. 6, 33380 (2016).

[5] T. Devaux, H. Tozawa, P. H. Otsuka, S. Mezil, M. Tomoda, O. Matsuda, E. Bok, S. H. Lee and O. B. Wright, Sci. Adv. 6, 8507 (2020).

[6] E. Bok, J. J. Park, H. Choi, C. K. Han, O. B. Wright and S. H. Lee, Phys. Rev. Lett. 120, 044302 (2018).

[7] K. Fujita, M. Tomoda, O. B. Wright and O. Matsuda, Appl. Phys. Lett. 115, 081905 (2019).

[8] A. Ogasawara, K. Fujita, M. Tomoda, O. Matsuda,, O. B. Wright, Appl. Phys. Lett. 116, 241904 (2020).

**Biography:** Oliver B. Wright received his B.A. in physics at University College, Oxford and his Ph.D. in low-temperature solid-state physics at the Cavendish Laboratory, Cambridge. In 1982 he continued this research at CRTBT, C.N.R.S. in Grenoble, France. In 1984 he joined Schlumberger to work on optical sensors in Montrouge, Paris. In 1986 he moved to Nippon Steel Corporation, working as a Senior Researcher at their Electronics Research Laboratories, Sagamihara, Japan, mainly in the field of non-destructive characterization of materials using laser acoustic techniques and electromagnetic acoustic transducers. In 1994 he worked on related topics at C.N.R. Istituto di Acustica in Rome and in 1995 at C.N.R.S. in Besançon, France. Since 1996 he has been working as a professor in the Faculty of Engineering at Hokkaido University in Sapporo, Japan, specializing in particular on laser picosecond ultrasonics, surface acoustic wave imaging and acoustic metamaterials. In 2013 he founded the company Plum Science that makes novel waveguide-based desk lights and stand lights, on sale in many countries.

Seminar 19**: Reflectionless Modes: an alternative spectrum for wave scattering**

Speaker: **Vincent Pagneux **(Laboratoire d’Acoustique de l’Université du Maine, CNRS, France)

**Q & A:**

Figure: two examples of reflectionless modes with real frequency, with an incident wave from the left and perfect transmission to the right (a case of “PT-symmetry” for the reflectionless operator)

**Abstract: **To characterize scattering resonances, a useful tool is the complex resonance spectrum corresponding to eigenmodes (QNM) able to leak energy. However, for reflection/transmission problems (e.g. waveguides, gratings or screens), the complex resonance spectrum does not directly quantify transmission efficiency, and the question of good or perfect transmission is of tremendous importance in many topics of intense study in wave physics: extraordinary optical transmission, topological states immune to backscattering, perfect transmission resonances, transmission eigenchannels through disordered media, reflectionless metamaterials or metasurfaces.

We present here an alternative spectrum allowing one to identify situations where perfect transmissions occur [1]. The operator yielding the spectrum of reflectionless modes is non-selfadjoint, and it is PT-symmetric for systems with spatial mirror symmetry. Eigenmodes (reflectionless modes) and eigenvalues (reflectionless complex frequencies) will be presented in various scattering situation, from the simplest 1D setup to several 2D waveguide geometries.

[1] A.-S. Bonnet-Ben Dhia, L. Chesnel, V. Pagneux. Trapped modes and reflectionless modes as eigenfunctions of the same spectral problem. Proc. R. Soc. A, 474(2213), 20180050 (2018)

**Biography:** Dr. Vincent Pagneux has received his PhD in 1996 from the Université du Maine in France. He is currently Director of Research Scientist at CNRS-LAUM. His research focuses on wave physics, acoustics and metamaterials.

Seminar 18**: Wave Interaction with Subwavelength Resonators**

Speaker: **Habib Ammari** (ETH Zürich)

**Q & A:**

**Abstract: **In this lecture, the speaker reviews recent results on subwavelength resonances. His main focus is on developing a mathematical and computational framework for their analysis. By characterizing and exploiting subwavelength resonances in a variety of situations, he proposes a mathematical explanation for super-focusing of waves, double-negative metamaterials, Dirac singularities in honeycomb subwavelength structures, and topologically protected defect modes at the subwavelength scale. He also describes a new resonance approach for modelling the cochlea which predicts the existence of a travelling wave in the acoustic pressure in the cochlea fluid and offers a basis for the tonotopic map.

**Biography:** Habib Ammari is a Professor of Applied Mathematics at ETH Zürich. Before moving to ETH, he was a Director of Research at the Department of Mathematics and Applications at Ecole Normale Supérieure in Paris. He received a Bachelor’s degree in 1992, a Master’s degree in 1993, and a Ph.D. in applied mathematics in 1995, all from the Ecole Polytechnique, France. Following this, he received a Habilitation degree in Mathematics from the University of Pierre & Marie Curie in Paris three years later. Habib Ammari is a world leading expert in wave propagation phenomena in complex media, mathematical modelling in photonics and phononics, and mathematical biomedical imaging. He has published more than two hundred research papers, eight high profile research-oriented books and edited eight books on contemporary issues in applied mathematics. He has advised thirty four PhD students and twenty three postdoctoral researchers. Habib Ammari was awarded with several international prizes.

Seminar 17**: Metasurfaces for light shaping: a look into the past to better appreciate the present and future**

Speaker: **Philippe Lalanne** (The Photonics, Numerical and Nanosciences Laboratory, CNRS, France)

**Q & A:**

**Abstract: **In integrated circuits, an important prerequisite for success is that every transistor can be driven or read independently, and nanotechnology is used to push the integration limit subject to that essential requirement. In dielectric metasurfaces for wavefront shaping, one aims to imprint spatially varying phases (or amplitudes, polarizations) on an incoming optical beam, and nanotechnology is used to spatially control the phase down to the subwavelength scale, as required for high numerical aperture focusing or imaging, beam steering over large angles, and computer-generated holography. Just like for integrated circuits, it is important that the phase can independently be controlled by adjacent nanostructures (or meta-atoms).

This demanding requirement has marked the history of diffractive optics. I will review this history, evidence some fundamental limitations and introduce recent prospects in my group in relation to disordered metasurfaces.

.

**Biography:** Philippe Lalanne is Research Director at CNRS and is an international expert in computational and nanoscale electrodynamics. He was first involved in the group of Pierre Chavel at l’Institut d’Optique at Orsay. In 1995, he spent a sabbatical year with G.M. Morris at the Institute of Optics in Rochester.

With his colleagues, he has launched new modal theories and improved numerical tools for gratings, waveguides and nanoresonators. He has used these tools to provide deep insight into the physical mechanisms involved in key nanoscale optical phenomena and devices, e.g. light confinement in photonic-crystal cavities, the extraordinary optical transmission, light interaction with plasmonic nanoresonators. He has pioneered the development of large-NA metalenses and has designed and demonstrated novel nanostructures with record or completely novel performance in their time.He has supervised 17 PhD candidates and co-supervised 6 PhD candidates. He is an Associate Editor of Optica, a member of the editorial board of *Laser & Photonics Reviews*, and is director of GDR Ondes, a broad virtual laboratory that gathers the French community working on acoustic and electromagnetic waves. He is a recipient of the Bronze medal of CNRS and the prix Fabry de Gramont of the Société Française d’Optique. He is a fellow of the IOP, OSA and SPIE.

Seminar 16**: Mechanics and dynamics of two-dimensional quasiperiodic composites**

Speaker** **: **Massimo Ruzzene** (University of Colorado Boulder, USA)

**Q & A:**

**Abstract: **Periodic configurations have dominated the design of phononic and elastic-acoustic metamaterial structures for the past decades. Unlike periodic crystals, quasicrystals lack translational symmetry but are unrestricted in rotational symmetry. This provides the opportunity to investigate novel classes of quasicrystal inspired elastic composites whose mechanical static and dynamic properties are largely unexplored. This presentation illustrates the performance of continuous elastic quasicrystals composites, here denoted as *quasiperiodic* (QP) composites, characterized by different rotational symmetry orders which is directly enforced through a design procedure in reciprocal space. Static mechanical properties are investigated as a function of symmetry order and filling fraction. Results indicate that higher order symmetries, such as 8-, 10- and 14-fold, lead to equivalent stiffness characteristics that interpolate those of the constituent materials while maintaining high levels of isotropy for all filling fractions. Thus, QP composites exhibit more uniform strain energy distributions when compared to periodic 4-fold and 6-fold symmetric configurations. Similarly, nearly-isotropic wave propagation is observed over a broader range of frequencies. The spectral dynamic properties are also investigated by enforcing rotational symmetry constraints in a wedge-type unit cell, which allows for the estimation of bandgaps, whose presence is confirmed in frequency response computations. Wave directionality and bandgaps are also estimated through parallel studies conducted on plate structures characterized by QP patterns of surface stubs. These experiments show clear bandgaps, illustrate how wave fronts reflect the rotational symmetry of the domains, and demonstrate that higher order geometries lead to isotropic propagation over a broader range of frequencies. The investigations presented herein open avenues for the general exploration of the properties of quasiperiodic media, with potentials for novel architectured material designs that expand the opportunities provided by periodic media.

**Biography:** Massimo Ruzzene is the Slade Professor of Mechanical Engineering and holds a joint appointment in the Smead Aerospace Engineering Sciences Department of CU Boulder. M. Ruzzene currently serves as the Associate Dean for Research of the College of Engineering and Applied Science. He joined CU in the summer of 2019, after serving as the Pratt and Whitney Professor in the Schools of Aerospace and Mechanical Engineering at Georgia Institute of Technology. M. Ruzzene received a PhD in Mechanical Engineering from the Politecnico di Torino (Italy) in 1999. He is author of 2 books, more than 180 journal papers and 250 conference papers. He has participated as a PI or co-PI in various research projects funded by the Air Force Office of Scientific Research (AFOSR), the Army Research Office (ARO), the Office of Naval Research (ONR), NASA, the US Army, US Navy, DARPA, the National Science Foundation (NSF), as well as companies such as Boeing, Eurocopter, Raytheon, Corning and TRW. Most of his current and past research work has focused on solid mechanics, structural dynamics and wave propagation with application to structural health monitoring, metamaterials, and vibration and noise control. M. Ruzzene is a Fellow of ASME and SES, an Associate Fellow of AIAA, and a member of AHS, and ASA. He served as Program Director for the Dynamics, Control and System Diagnostics Program of CMMI at the National Science Foundation between 2014 and 2016.

Seminar 15**: Dynamics of waves in continuum honeycomb structures**

Speaker** **: **Michael I Weinstein** (Dept of Applied Physics and Applied Mathematics, and Dept of Mathematics, Columbia University)

**Q & A:**

**Abstract: **We overview results on the dynamics of waves (e.g. Schroedinger and Maxwell equations) in honeycomb structures and their deformations.

We study phenomena which arise from the presence of Dirac (conical) points in the bulk band structure.

These include the existence of robust edge (interface) states, which localize along certain sharp terminations, and along domain walls.

We then discuss recent work on the emergence of pseudo-magnetic fields in non-uniformly deformed honeycomb structures.

We apply these results to predict Landau-like energy levels in photonic crystals, and present numerical confirmation of the theory.

**Biography:** Michael I. Weinstein works on the mathematical modeling, analysis, and applications of wave phenomena across many areas of physical science. A recent focus has been on PDE (Partial Differential Equations) models which describe optical and quantum waves in novel media such as topological insulators and metamaterials. Such physical media have applications to technologies which could potentially revolutionize robust information transfer in computing and communication systems. Weinstein received a B.S. in Mathematics from Union College, summa cum laude, in 1977 and a Ph.D. in Mathematics from the Courant Institute at NYU in 1982. He is a Professor of Applied Mathematics in the Department of Applied Physics and Applied Mathematics and a Professor of Mathematics in the Department of Mathematics at Columbia University. Weinstein is a Fellow of the American Mathematical Society (AMS) and a Fellow of the Society for Industrial and Applied Mathematics (SIAM). In 2015, he received a Math + X Investigator Award from the Simons Foundation.

Home – Prof Michael I Weinstein

Seminar 14**: Black-hole waves at corners of a metallic particle**

Speaker: **Anne-Sophie Bonnet-BenDhia** (The Applied Mathematics department of l’ENSTA Paris)

**Q & A:**

**Abstract:** In this talk, we consider electromagnetic waves in presence of a metallic inclusion with corners, assuming that the dielectric permittivity has a very small imaginary part, that will be neglected, and a negative real part. Due to the sign-change of the dielectric permittivity, very unusual singular phenomena take place at corners. In particular, for some configurations, a part of the energy may be trapped by the corners: this is the so-called blackhole effect [1].

In this presentation, we first give a mathematical analysis of this blackhole phenomenon, based on a detailed description of the corner’s singularities, in the 2D case.

This phenomenon leads to numerical instabilities of finite element simulations. The solution that we have found and validated is to introduce a complex scaling at the corners.

Finally, we compute the plasmonic eigenvalues of a 2D subwavelength particle with a corner [2]. While a smooth particle has a discrete sequence of eigenvalues, blackhole waves at the corner lead to the presence of an essential spectrum filling an interval. Numerical results show that the complex scaling deforms this essential spectrum, so as to unveil both embedded eigenvalues and complex plasmonic resonances. The latter are analogous to well-known complex scattering resonances, with the local behaviour at the corner playing the role of the behaviour at infinity.

[1] A-S Bonnet-BenDhia et al, J. Comput. Phys., vol. 322, pp. 224-247, 2016

[2] A-S Bonnet-BenDhia et al, https://hal.archives-ouvertes.fr/hal-02923259

**Biography:** Anne-Sophie Bonnet-Ben Dhia is a former student of the Ecole Normale Supérieure de Jeunes Filles. She received the PhD degree in Applied Mathematics in 1988 and the Habilitation à Diriger les Recherches in 1995 from the University Pierre et Marie Curie. She is presently Directeur de Recherche at CNRS.

She is the head of the research team POEMS (associated to CNRS, INRIA and the Ecole Nationale Supérieure de Techniques Avancées) whose activities are devoted to the mathematical and numerical analysis of wave phenomena. She is a specialist of spectral theory and scattering theory, with a particular interest for waveguides configurations. Her theoretical and numerical contributions apply to various physical domains covering optics and electromagnetism, water waves, acoustics, aeroacoustics and ultrasonics.

Home – Prof Anne-Sophie Bonnet-Ben Dhia

Seminar 13:** Reflections on constitutive relations for acoustic metamaterials**

Speaker** **: **John Willis, FRS **(DAMTP, University of Cambridge)

**Q & A:**

**Abstract: **I shall introduce a particular methodology for deriving effective constitutive relations for metamaterials including (because I was asked to do so) a brief account of how it evolved. It applies to composite materials (and metamaterials in particular) whose microstructure is random and is based on the calculation of ensemble averages. Periodic media are included as a special case, by regarding the exact location of a chosen point in any one cell as being distributed randomly over a cell fixed in space. Within that general framework, all definitions are exact. In practice, except in the case of periodic media, some approximation is required. The approximations developed here are derived systematically from a stochastic variational principle and can at least be regarded as “honest” even though not exact. Some general observations will be supported by examples which illustrate both the utility and the limitations of any formulation involving effective constitutive relations. In particular, except in the “homogenization” or “long-wavelength” limit, boundary layers present near any interface between random media, or between one random medium and one with uniform properties, seriously compromise the use of effective properties. The variational approach can, however, be applied directly to this configuration and has been employed to obtain one of only two solutions for reflection and transmission of acoustic waves at the boundary of a two-component random medium.

**Biography:** John Willis is Emeritus Professor of Theoretical Solid Mechanics in the University of Cambridge. He undertook his undergraduate and graduate studies in Mathematics at Imperial College. During his career, he has held the following positions: Assistant Lecturer, Department of Mathematics, Imperial College (1962-64), Research Associate, Courant Institute (1964-65), Senior Assistant/Assistant Director of Research, DAMTP Cambridge (1965-72), Professor of Applied Mathematics, Bath (1972-94 and 2000-01), Professor of Theoretical Solid Mechanics, DAMTP Cambridge (1994-2000 and 2001-07), Professeur de Mécanique, Ecole Polytechnique (part-time, 1998-2004). He was Editor of the Journal of Mechanics and Physics of Solids (1982-2006). Major research interests have included static and dynamic problems for anisotropic media, problems of irradiation damage of materials, structural integrity, effective properties of composite materials (both static and dynamic), mechanics of nonlinear composites, stability of strained-layer semiconductor devices. Recent work has been on strain-gradient plasticity and the dynamics of composites (as applied to acoustic metamaterials). He was elected FRS (1992), Foreign Member, US National Academy of Engineering (2004) and Associé Etranger, Académie des Sciences (2009). Awards include the Timoshenko Medal, ASME (1997), Prager Medal, SES (1998), Euromech Solid Mechanics Prize (2012).

Seminar 12 : **Moth wings as metamaterial sound absorbers**

Speaker** **: **Marc Holderied** (University of Bristol)

**Q & A:**

**Abstract: **Providing healthy living and working environments in our ever-noisier world necessitates increasing use of noise control technology. The rapidly expanding field of acoustic metamaterials has led to the emergence of ultrathin subwavelength sound absorbing methods, allowing sound insulation to be thinner and lighter than ever before. We will present our discovery of a natural acoustic metamaterial and show our ongoing work towards bioinspired metamaterial absorbers. Our bioinspiration arises from the 65-million-year acoustic arms race between echolocating bats and their moth prey, which has turned the wing scales of moths into an omnidirectional, ultrathin (1/100 λ) and broadband absorber of ultrasound (peak α = 0.71) that provides acoustic camouflage against bats. Our lithographically produced up-sized scale replicas absorb sound and can resonate at the most important frequencies for human communication. This research paves the way for bioinspired broadband sound absorbers that are thinner and lighter than existing solutions.

**Biography:** Dr Marc Holderied is a sensory ecologist and bioacoustician with strong links to bio-inspired engineering. He finds bio-inspired acoustic solutions in three areas. Bat avionics: biosonar place recognition, flow perception and obstacle avoidance, in-flight group coordination and bio-inspired radar. Predator-prey acoustics: bat stealth hawking , the acoustic ecology of scorpion-eating bats, and a new insect ear. Echo evolution: Using airborne ultrasound tomography he discovered retroreflectors in bat-pollinated plants, echo shadows of insects, and identified moth fur as enhanced porous sound absorber, and moth scales as broadband resonant metamaterial absorbers. His research regularly features on the BBC (eg ‘Planet Earth II’, ‘Super Senses’). He heads the Behavioural Acoustics and Sensory Biology lab at the University of Bristol. He is a fellow of the German National Academic Foundation and an international consultant on ultrasonic vision for the automotive sector.

Seminar 11 : **Multipurpose metamaterials: designing for static, acoustic and elastic properties in a single architecture**

Speaker :** Chiara Daraio** (California Institute of Technology)

**Q & A:**

**Abstract: **Mechanical metamaterials are materials with a tailored, architected structure, designed to achieve properties that depart from those found in natural or more “conventional” engineering materials. Common realizations of metamaterials are periodic and derive their properties from an interplay of the constitutive material responses and their architected geometry. Most of these designs to date have been developed to address a single functionality (e.g., for supporting a certain band gap, a low-frequency resonant response, a topological waveguide). However, many engineering applications have to satisfy simultaneously multiple constraints and demand specific static and dynamic responses. Increasing the design complexity allows the realization of materials with desirable effective properties, and added functionalities. In this talk, I will highlight some of our recent work in the design, fabrication and characterization of structured, multipurpose metamaterials and their possible application to engineering problems. For example, materials that can control acoustic and elastic waves simultaneously, which can function as ultimate protectors, or materials that are designed with specific acoustic impedance and load-bearing properties, which can function as acoustic windows, or flexible, resonant metamaterials for haptic interfaces.

**Biography:** Professor Daraio received her undergraduate degree in Mechanical Engineering from the Universita’ Politecnica delle Marche, Italy (2001). She received her M.S. (2003) and Ph.D. degrees (2006) in Materials Science and Engineering from the University of California, San Diego. She joined the Aeronautics and Applied Physics departments of the California Institute of Technology (Caltech) in fall of 2006 and was promoted to full professor in 2010. From January 2013 to August 2016, she joined the department of Mechanical and Process Engineering at ETH Zürich, with a chair in Mechanics and Materials. She returned to Caltech in August 2016, as a Professor of Mechanical Engineering and Applied Physics. She received a Presidential Early Career Award from President Obama (PECASE) in 2012, was elected as a Sloan Research Fellow in 2011 and received an ONR Young Investigator Award in 2010. She is also a winner of the NSF CAREER award (2009), of the Richard Von Mises Prize (2008) and received the Hetenyi Award from the Society for Experimental Mechanics (2015). She was selected by Popular Science magazine among the “Brilliant 10” (2010). She serves as a Board Editor for Science (AAAS) and as an Associate Editor for the journals Matter (Cell Press), Multifunctional Materials (IOP) and Frontiers in Materials (Frontiers). A complete list of publications and research information can be found at:

Seminar 10 **: Theoretical thermotics and thermal metamaterials: A brief review**

Speaker :** Jiping**

**(Department of Physics, Fudan University, Shanghai, China)**

**Huang****Q & A:**

**Abstract: **Transformation thermotics [1-5] is a kind of theoretical method which is based on the coordinate transformation between two different spaces, which can precisely couple geometric structure parameters into physical quantities such as thermal conductivity. This method makes it possible to artificially tailor the geometric structure to accurately control the flow of heat. Free control of heat flux is always a dream of human beings. For this purpose, thermal metamaterials come to appear [6,7], which just originate from the theory of transformation thermotics. Here, I review the crucial theoretical and experimental research progresses in this field after the birth of transformation thermotics for steady-state thermal conduction in 2008 [1,2]. They mainly include the following novel thermal phenomena or functional metamaterial devices: thermal cloak, thermal concentrator, thermal rotator, macroscopic thermal diode, thermal camouflage, thermal transparency, thermal imitator, thermal golden touch, ultra-low thermal conductivity, thermocrystal, zero-energy consumption and negative-energy consumption in ambient temperature difference, thermal anti-parity-time symmetry, thermal convection cloak/concentrator/camouflage, and daily radiation cooling. I will present the relevant microscopic or macroscopic heat-transfer mechanisms and make some prospects for the future.

[1] C. Z. Fan, Y. Gao, and J. P. Huang, Shaped graded materials with an apparent negative thermal conductivity, Applied Physics Letters 92, 251907 (2008).

[2] T. Y. Chen, C. N. Weng, and J. S. Chen, Cloak for curvilinearly anisotropic media in conduction, Applied Physics Letters 93, 114103 (2008).

[3] J. Y. Li, Y. Gao, and J. P. Huang, A bifunctional cloak using transformation media, Journal of Applied Physics 108, 074504 (2010).

[4] G. X. Yu, Y. F. Lin, G. Q. Zhang, Z. Yu, L. L. Yu, and J. Su, Design of square-shaped heat flux cloaks and concentrators using method of coordinate transformation, Frontiers of Physics 6, 70 (2011).

[5] S. Guenneau, C. Amra, and D. Veynante, Transformation thermodynamics: cloaking and concentrating heat flux, Optics Express 20, 8207 (2012).

[6] M. Maldovan, Sound and heat revolutions in phononics, Nature 503, 209 (2013).[7] M. Wegener, Metamaterials beyond optics, Science 342, 939–940 (2013).

**Biography:** Jiping Huang is a professor in the Department of Physics, Fudan University, from 2005-now. He is Vice-Dean of Department of Physics, Fudan University, from 2007-2009 and from 2020-now. He was a Humboldt Research Fellow in Max Planck Institute for Polymer Research from 2004-2005. His research interests: theoretical thermotics (transformation thermotics and extended theory) and thermal metamaterials. Inspired by transformation optics, he proposed the theory of transformation thermotics for steady-state thermal conduction [Appl. Phys. Lett. 92, 251907 (2008)], and predicted the concepts of thermal cloak and apparently negative thermal conductivity (thermal rotator). His work has triggered the upsurge of theoretical and experimental design of artificial structures to control heat transfer, and promoted the formation of a hot research direction, thermal metamaterials. His pioneering contribution has been recognized by peers. In this direction, he has published the first book on thermal metamaterials, namely, “Theoretical Thermotics: Transformation Thermotics and Extended Theories for Thermal Metamaterials (Springer, 2020)”, together with tens of journal papers in Phys. Rev. Lett., Phys. Rev. E, Phys. Rev. Appl., Appl. Phys. Lett., and Int. J. Heat & Mass Transfer.

Seminar 9 **: Localisation of light in ordered time dependent systems**

Speaker** **: **Sir John Pendry, FRS **(Imperial College London)

**Q & A:**

**Abstract: **Electromagnetic waves in periodically ordered materials are delocalised into Bloch states, just like electrons in ordered solids. Recently interest has grown in systems ordered in both space and time the simplest example of which might be a permittivity with a 1D grating,

We stress that this is not a moving medium but a modulation of the medium’s properties phased in time. Hence the velocity of propagation, *c _{g }*= W/g can take any value

*-∞< c*and is not constrained by relativity. Here too light can propagate in extended Bloch states but there is a range of values [1],

_{g}<+∞for which the Bloch picture fails. In this so called luminal region any incident radiation will be localised in a series of wells defined by the grating and travelling at the velocity of the grating. We present analytical solutions to this model and in particular explore the localised/delocalised transition which has an exponent of 1/2. Light incident on a grating with *c _{g}*in the luminal range is compressed and amplified by a novel mechanism into a series of ever more localised pulses which emerge from the far side of the grating as shown in the figure below. For values of

*c*outside the luminal region a plane electromagnetic wave enters the grating as a linear combination of Bloch waves which oscillate in amplitude as they propagate. Approaching the critical velocity the oscillation period and amplitude diverge. The same exponent controls the diverging period as controls the exponential growth inside the luminal regime: a familiar result from localisation in disordered systems.

_{g}[1] Oliner, A. A., and A. Hessel, IRE Transactions on Microwave Theory and Techniques **9** 337-343. (1961).

**Biography:** John Pendry is a condensed matter theorist and has worked at Imperial College since 1981. He has worked extensively on electronic and structural properties of surfaces developing the theory of low energy diffraction and of electronic surface states. More recently he turned his attention to photonic materials this interest led to his present research into the remarkable electromagnetic properties of materials where the normal response to electromagnetic fields is reversed, leading to negative values for the refractive index. In collaboration with scientists at Marconi he designed a series of metamaterials, completely novel materials with properties not found in nature. These designs were subsequently the basis for new concepts with radical consequences, such as the first material with a negative refractive index, the concept of a perfect lens, and a prototype cloaking device, which have both caught the imagination of the world’s media.

Seminar 8 :** Topological metamaterials for robust signal and energy manipulation**

Speaker** **: **Romain Fleury** (Swiss Federal Institute of Technology in Lausanne, EPFL, Switzerland)

**Q & A:**

**Abstract: **Topological metamaterials are artificially engineered structures that confer to the waves that propagate through them a property, for example the presence of a given mode, that is robust to continuous modifications of the metamaterial, such as geometrical deformations, local defects, or irregularities in the wave path. In this talk, I will first explore how robust topological modes can be leveraged as building blocks to engineer resilient scattering responses^{1,2}. Such topological scattering, which enables filtering signals without stringent requirements on the system’s geometry^{3}, is experimentally demonstrated with acoustic waves. We will then discuss the relevance of topology to robust energy concentration or manipulation, in particular using second order topological wave insulators leveraging non-linearity^{4}, or combining topological concepts with classical metamaterial effects such as near-zero-index supercoupling^{5}. Altogether, this talk demonstrates that the field of topology is now sufficiently mature to move from purely physics-driven explorations towards exciting application-oriented developments^{6}.

- Topological analog signal processing,
*Nat. Commun.*, vol. 10, no. 1, pp. 1–10, Dec. 2019. - Topological Fano Resonances,
*Phys. Rev. Lett.*, vol. 122, no. 1, p. 014301, Jan. 2019. - Disorder‐Induced Signal Filtering with Topological Metamaterials,
*Adv. Mater.*, vol. 32, no. 28, p. 2001034, Jul. 2020. - Nonlinear Second-Order Topological Insulators,
*Phys. Rev. Lett.*, vol. 123, no. 5, p. 053902, Aug. 2019. - Zero-Index Weyl Metamaterials,
*Phys. Rev. Lett.*, vol. 125, no. 5, p. 054301, Jul. 2020. - Active times for acoustic metamaterials,
*Reviews in Physics*, vol. 4. Elsevier B.V., p. 100031, 01-Nov-2019.

**Biography:** Romain Fleury is a professor in the School of Engineering at EPFL, the Swiss Federal Institute of Technology in Lausanne, where he leads the Laboratory of Wave Engineering. His research directions include wave-based information processing leveraging disorder, topology, non-Hermiticity, and non-reciprocity, acoustic imaging techniques, and technological applications of locally-resonant metamaterials. He is the 2017 F.V. Hunt fellow of the Acoustical Society of America, an MSCA fellow, and a Campus France Prestige fellow. He has received an Eccellenza award from the Swiss National Science Foundation, a STI Polysphere teaching award from EPFL students, and the best teacher award from the Institute of Electrical Engineering at EPFL.

Seminar 7: **Acoustic metamaterials with digitally virtualized resonance**

Speaker: **Jensen Li** (The Hong Kong University of Science and Technology)

**Q & A:**

**Abstract: **We present a platform in constructing acoustic metamaterials with digital electronics feedback, which allows us to realize arbitrary resonating response, as if they are acoustic metamaterials with physical resonance, but now with more engineering degrees of freedom. In the first part of this webinar, we will introduce the essential steps to virtualize an impulse response of acoustic metamaterial to a digital signal processing code, demonstrating hugely tunable resonating frequency, strength and linewidth. It also allows arbitrary specifications of bulk modulus, density and Willis parameters with controllable bandwidth. In the second part, we will investigate impact of these arbitrary material parameters to explore active time-varying systems. Specifically, high efficiency on frequency conversion is experimentally demonstrated by tailor-made time-varying response function. Its application with material gain and loss being alternating in time is also investigated.

**Biography:** Jensen Li is currently a professor in the Department of Physics at Hong Kong University of Science and Technology. His research directions include optical metasurfaces, non-Hermitian systems, transformation optics and acoustic metamaterials. He has published more than 80 peer-reviewed articles and his research group in Hong Kong and previously in UK has been supported by research grants from EU and Hong Kong RGC. Currently, he is leading a collaborative research project on “Non-Hermitian Systems in Optics and Acoustics” across several universities in Hong Kong.

Seminar 6**: Elastic metasurfaces**

Speaker** **: **Richard Craster** (Imperial College London, Imperial-CNRS Abraham de Moivre IRL)

**Abstract: **Elastic waves guided along surfaces dominate applications in geophysics, ultrasonic inspection, mechanical vibration, and surface acoustic wave devices; precise manipulation of surface Rayleigh waves and their coupling with polarized body waves presents a challenge that offers to unlock the flexibility in wave transport required for efficient energy harvesting and vibration mitigation devices. In this talk I will describe various designs of elastic metasurfaces, based around a graded array of rod resonators attached to an elastic substrate that, together with critical insight from Umklapp scattering in phonon-electron systems, allow us to leverage the transfer of crystal momentum; we mode-convert Rayleigh surface waves into bulk waves that form tunable beams. Experiments, theory and simulation verify that these tailored Umklapp mechanisms play a key role in coupling surface Rayleigh waves to reversed bulk shear and compressional waves independently, thereby creating passive self-phased arrays allowing for tunable redirection and wave focusing within the bulk medium. We have also looked recently at piezoelectric coupling and the use of these devices in energy harvesting and, if time allows, I will discuss this too.

**Biography: **Richard Craster is the Dean of the Faculty of Natural Sciences at Imperial College and formerly Head of Department of Mathematics at Imperial College a role he held for six years (2011-2017). He is also Director of the CNRS-Imperial “Abraham de Moivre” International Research Laboratory in Mathematics. He has been at Imperial College as an academic since 1998 apart from holding a distinguished professorship in Alberta, Canada, 2008-2010 returning to become Head of the Mathematics Department at Imperial. In addition to being a Professor of Applied Mathematics he is also a member of the Mechanical Engineering Department at Imperial.

His research lies in the field of applied mathematics in particular wave mechanics, metamaterials, fluid mechanics and elasticity. He is the co-editor of one of first books on Acoustic Metamaterials, PI of an EPSRC Programme grant on the Mathematical fundamentals of Metamaterials, co-organiser of the UK Acoustics Network (www.acoustics.ac.uk), and recently co-editor of the volume on Elastic Metamaterials for the Handbook of Metamaterials.

Seminar 5**: Metamaterials, shear bands and invisibility**

Speaker: **Davide Bigoni **(University of Trento)

**Q & A:**

**Abstract: **Architected materials are preconized to yield extreme mechanical properties such as foldability, channeled response, and surface effects. These features are expected to lead to the production of artificial materials of unchallenged mechanical properties, in other words, metamaterials. Homogenization theory and Floquet-Bloch techniques will be applied to grids of elastic rods, subject to various conditions, including axial pre-stress, to demonstrate strain localization, wave channeling, total reflection, and negative refraction. Finally, invisibility through cloaking of voids in a flexurally vibrating plate will be presented.

**Biography: **Davide Bigoni is a mechanician working in solid and structural mechanics and material modeling, wave propagation, fracture mechanics. His approach to research is the employment of a broad vision of mechanics, with a combination of mathematical modelling, numerical simulation, and experimental validation. From 2001 Davide Bigoni holds a professor position at the University of Trento, where he is leading a group of excellent researchers in the field of Solid and Structural Mechanics. He has authored or co-authored more than 100 journal papers. He was elected in 2009 Euromech Fellow (of the European Mechanics Society), has received in 2014 the Doctor Honoris Causa degree at the Ovidius University of Constanta and in 2016 the Panetti and Ferrari Award for Applied Mechanics (from Accademia delle Scienze di Torino). He has been awarded an ERC advanced grant in 2013. He is co-editor of the Journal of Mechanics of Materials and Structures and associate Editor of Mechanics Research Communications and in the editorial board of 8 international journals.

Seminar 4:** Broken Symmetries in Acoustic and Mechanical Metamaterials **

Speaker: ** Andrea Alù **(City University of New York)

**Q & A:**

**Abstract: **In this seminar, I will discuss our recent activity in acoustics and mechanics, showing how suitably tailored broken symmetries in *meta-atoms* and their arrangements open exciting venues for new technology. I will focus in particular on the opportunities offered by time modulation and switching, reciprocal and non-reciprocal Willis coupling, and topological order in metamaterials, which provide unique opportunities for exotic acoustic and mechanical responses. Physical insights into the underlying phenomena, and new devices based on these concepts will be presented.

**Biography: **Andrea Alù is the Founding Director and Einstein Professor at the Photonics Initiative, CUNY Advanced Science Research Center. He received his Laurea (2001) and PhD (2007) from the University of Roma Tre, Italy, and, after a postdoc at the University of Pennsylvania, he joined the faculty of the University of Texas at Austin in 2009, where he was the Temple Foundation Endowed Professor until Jan. 2018. Dr. Alù is a Fellow of AAAS, IEEE, OSA, SPIE and APS, a Simons Investigator, a Highly Cited Researcher, a DoD Vannevar Bush Faculty Fellow, and has received several scientific awards, including the IEEE Kiyo Tomiyasu Award (2019), the ICO Prize in Optics (2016), the NSF Alan T. Waterman award (2015), the OSA Adolph Lomb Medal (2013), and the URSI Issac Koga Gold Medal (2011).

Seminar 3: **3D chiral mechanical metamaterials**

Speaker: **Martin Wegener** (Karlsruhe Institute of Technology)

**Q & A:**

**Abstract: **Calling an object or a metamaterial handed or “chiral” requires it to exhibit (i) broken space-inversion symmetry, (ii) no mirror planes, and (iii) no rotation-reflection symmetries. The absence of these symmetries allows for certain effective material behaviour that would otherwise be forbidden by symmetry. Here, we review our recent progress on three-dimensional (3D) chiral mechanical metamaterials in the linear elastic regime. This includes tailored characteristic length scales of push-to-twist conversion behaviour, ultrasound experiments on acoustical activity with transient nanometer-precision displacement-detection, the mapping of this dynamic behaviour (as well as of the static behaviour) onto effective-medium descriptions beyond Cauchy elasticity, and the possibility of isotropic acoustical activity in rationally designed 3D chiral quasi-crystalline mechanical metamaterials.

**Biography: **Martin Wegener is professor of physics at Institute of Applied Physics at Karlsruhe Institute of Technology (KIT) and Scientific Director of Institute of Nanotechnology at KIT. He has published more than 300 papers in peer-reviewed journals, with more than 33,000 citations and a Hirsch index of h=86 according to google scholar.

Seminar 2: **Bio-inspired hierarchical metamaterials**

Speaker** **: **Marco Miniaci** (Institute of Electronics, Microelectronics and Nanotechnology of Lille)

**Q & A:**

**Abstract:** Nature has engineered complex designs to achieve advanced properties and functionalities over hundreds of thousands of years. For instance, a hierarchical organization over multiple length scales allows enhanced quasistatic mechanical properties (such as high specific strength, stiffness, and toughness). While largely investigated in the quasistatic domain, the role of hierarchy in the dynamic behaviour of metamaterials remains largely unexplored. In this webinar a numerical and experimental investigation of the influence of bio-inspired hierarchical organization on the wave dispersion diagram in metamaterials with self-similar structures at various spatial scales is presented. The advantages (and limitations) that the hierarchical architectures provide for the dynamic performance with respect to conventional metamaterials will be discussed.

**Biography: **Marco Miniaci is a researcher at the French National Scientific Research Center (CNRS) within the UMR 8520 — IEMN (Institute of Electronics, Microelectronics and Nanotechnology) of Lille. He specializes in wave dynamics, periodic structures and metamaterials. His actual research activity mainly concerns topological protection in patterned elastic waveguides and bio-inspired hierarchical elastic metamaterials.

Seminar 1: **Topological mode steering across wave physics**

Speaker: **Mehul Makwana **(Imperial College London)

**Abstract: **Steering waves around sharp bends and splitting their energy between different channels, in a robust and lossless manner, is of interest across many areas of engineering and physics. The seminal work of Mekis et. al [Phys. Rev. Lett. 18, 3787, 1996] in photonic crystals using cavity waveguides to perform this is now used widely in devices. A very active area of current research involves trying to use ideas originating in quantum mechanics, and from topological insulators, for similar purposes, but with the added potential of broadband performance, and enhanced robustness. In this presentation, I will relay the main principles of topological modes in classical wave systems and elucidate their benefits and shortcomings. Theoretical, computation and experimental results will be used to highlight a particular class of these insulators that contain passive elements and are solely reliant upon geometrical properties of the structured media.

**Biography: **Mehul Makwana is an EPSRC Research Associate in Mathematical Physics at Imperial College London and a Scientific Consultant to Multiwave Technologies AG. He has experience in in the mathematical modelling of wave phenomena, in particular, that of structured media in elasticity and phononics.