Nadège KAINA from Institut Langevin will give a seminar "Revisiting locally resonant metamaterials beyond homogenization : The importance of structure at deep sub-wavelength scales" **on friday, june 17 at 11:00**, Meeting room of 3rd floor.

**Abstract :**

Since early 20th, researchers got interested in composite materials that can mimic the properties of natural materials but at larger scales and for broader range of frequencies. As those properties arise either from the material structuration or from its atomic composition, two kinds of composite materials have been developed to echo each characteristic, namely photonic/phononic crystals (PC) and metamaterials (MM). The first are wavelength scaled and governed by interferences based on multiple scattering on their periodic structure, so that the wave manipulation is mostly achieved by engineering the spatial organization of the constitutive scatterers. Metamaterials on the other hand are organized at deep subwavelength scales, so that they are generally considered as homogenized materials with macroscopic effective properties that stem from the response of the meta-atoms, their constitutive unit-cells, regardless of the spatial organization. This apparent fundamental difference of physical behavior prevents transposing concepts from PC to MM and especially limits the degrees of freedom for the wave manipulation at subwavelength scales in metamaterials. In previous works however, we demonstrated, at the light of a microscopic approach, that the properties of many resonant MM can be understood from a combined effect of Fano interferences (based on the resonant unit cell) and multiple scattering, hence underlining analogies with PC. Taking advantage of the fact that multiple scattering partly drives the properties of metamaterials, we here highlight the importance of their spatial subwavelength structuration. For instance, we numerically prove that a so-called single negative metamaterial (presenting only one negative effective property) can be turned into a double negative one (hence presenting a negative index of refraction), simply by smartly organizing the building blocks of the metamaterial, at scales much smaller than the wavelength. We finally experimentally confirm this result by demonstrating, using an array of soda cans, the first acoustic superlens based on a negative index that effectively beats the diffraction limit.