Simultaneous self-collimation of fundamental and second-harmonic in sonic crystals

Simultaneous self-collimation of sound beams with different frequencies, corresponding to fundamental wave and to its second harmonic, is proposed theoretically and demonstrated experimentally. The result is obtained when the isofrequency contours for each harmonics, corresponding to different propagation bands, develop flat segments along the same direction. The effect can be utilized for managing (enhancement and control) of nonlinear frequency mixing of narrow sound beams.