Control of the acoustic waves in the subwavelength regime: metamaterials for perfect absorption and metadiffusers

The ability to control an incoming wave field by a sub-wavelength material is advantageous for several applications in wave physics as energy conversion, time reversal technology, coherent perfect absorbers, or sound- proofing among others. The solution of this challenge requires to solve a complex problem: reducing the geometric dimensions of the structure while increasing the density of states at low frequencies and fulfilling the impedance conditions for the desired control. A new type of sub-wavelength metamaterials based on the concept of slow sound propagation will be presented. This type of metamaterials makes efficient use of strong dispersion for generating slow-sound conditions inside the material and, therefore, drastically decreasing the frequencies of the absorption peaks. Hence, the structure thickness becomes deeply sub-wavelength. These open systems are characterized by both the leakage rate of energy (i.e., the coupling of the resonant elements with the propagating medium) and the intrinsic losses of the resonator at the resonant frequency. We will discuss in this talk, the different possibilities offered by the balance between the leakage and the losses to activate the critical coupling condition for trapping the energy around the resonant elements and generating a maximum of energy absorption or to generate deep subwavelength diffusers that we call metadiffusers.