Enhanced negative-index collimation by overlapped second band of square lattice sonic crystals

Abstract Acoustic wave collimation and enhancement are systematically investigated in the overlapped second and third bands of a two-dimensional square lattice sonic crystals by analyzing the band structure, equifrequency contour and field distribution. We find the collimation stems from a small negative-index of the second band, and an additional concave entrance can improve the collimation performance in the far field region. But for the third band, negative, zero, positive refraction and wave splitting may occur at the same time due to the simultaneous excitation of multiple Bloch modes, which ultimately destroy the collimation beam of the second band in the near field region. This work provides an insight into the collimation properties of the overlapped bands of sonic crystals with potential application in acoustic directional emission and wave manipulation.