Goal of META-MAT
The purpose of this website (META-MAT.ORG) is to disseminate research results in acoustic, thermal and mechanical metamaterials through the organization of online seminars, symposia, conferences, summer schools and workshops with industry and the academic world.
What Are Metamaterials?
In 2001, R. M. Walser coined the term “metamaterial,” defining it as a macroscopic composite with a manmade, three-dimensional, periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation [WALSER 01]. Since then, the definition has evolved. M. Wegener and others have proposed a broader definition: metamaterials are rationally designed composites made from tailored building blocks that result in properties beyond those of their individual materials [KADIC 19].
Foundations of Metamaterials: Key Theories and Researchers
The concept of metamaterials is deeply rooted in nano-scale science and electromagnetism. Pioneering work by V. Veselago and J. Pendry revolutionized how we understand light propagation in complex media, where the refraction index can take any value, real or complex [VESELAGO 68, PENDRY 99, PENDRY 00].
Metamaterials can exhibit both positive and negative refractive indices, allowing for unprecedented control over wave propagation. These properties align with passive and active media concepts, as described by the Kramers-Kronig relations [GRALAK 10]. For more complex cases, the refraction index is expressed as a tensor representing anisotropic media with sign-shifting phases [MILTON 02, SMITH 04].
Negative Refraction and Anisotropic Media
A hallmark of negatively refracting media is the periodic arrangement of elements much smaller than the wavelength in question. These arrangements create materials with effective properties such as negative optical index or hyperbolic metamaterials [IOR 13, PODDUBNY 13]. This allows for revolutionary applications, such as invisibility cloaks, which work based on the form invariance of Maxwell’s equations [PEN 06].
Applications: From Electromagnetics to Acoustics
The transition from electromagnetic to acoustic metamaterials became feasible through the study of phononic crystals, which range in size from meters to nanometers. At these scales, the laws of classical mechanics still apply, enabling researchers to create structures with complete phononic band gaps, as demonstrated by Sigalas and Economou in 1992 [ECONOMOU 93].
One breakthrough in acoustic metamaterials was the discovery that high-density rods and spheres exhibit properties such as negative effective density at resonance [LIU 00]. The effective density, like the refractive index, can be represented as a complex-valued matrix, which is also seen in mechanical metamaterials [CHRISTENSEN 15].
Expanding Beyond Electromagnetism: New Horizons
Interestingly, the principles behind metamaterials extend beyond electromagnetism and acoustics to include heat, mass, and light diffusion phenomena. Researchers have uncovered surprising analogies between these fields and wave propagation, expanding the potential applications of metamaterials [GUENNEAU 13, SCHITTNY 13].
References: https://en.wikipedia.org/wiki/Metamaterial
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