Nonreciprocal sound propagation in space-time modulated media

Realization of nonreciprocal devices, such as isolators and circulators, is of fundamental importance in microwave and photonic communication systems. This can be achieved by breaking time-reversal symmetry in the system or exploiting nonlinearity and topological effects. However, exploration of nonreciprocal devices remains scarce in acoustic systems. In this work, sound propagation in a space-time modulated medium is theoretically studied. Finite-difference time-domain simulations are carried out to verify the results. Functionalities such as mode conversion, parametric amplification, and phase conjugation are demonstrated.