Acoustic anomalous reflectors based on diffraction grating engineering

We present an efficient method for the design of anomalous reflectors for acoustic waves. The approach is based on the fact that the anomalous reflector is actually a diffraction grating in which the amplitude of all the modes is negligible except for the one traveling towards the desired direction. A supercell of drilled cavities in an acoustically rigid surface is proposed as the basic unit cell, and analytical expressions for an inverse diffraction problem are derived. It is found that the the number of cavities required for the realization of an anomalous reflector is equal to the number of diffracted modes to cancel, and this number depends on the relationship between the incident and reflected angles. Then, the ``retroreflection'' effect is obtained by just one cavity per unit cell; also, with only two cavities it is possible to change the reflection angle of a normally incident wave, and five cavities are enough to design a general retroreflector changing the incident and reflected angles at oblique incidence. Finally, the concept of Snell's law violation is extended not only to the incident and reflected angles, but also to the plane in which it happens, and a device based on a single cavity in a square lattice is designed in such a way that the reflection plane is rotated /4 with respect to the plane of incidence. Numerical simulations are performed to support the predictions of the analytical expressions, and an excellent agreement is found.