Coherent interactions between phonons and exciton or exciton-polariton condensates

We analyze the interaction of exciton and exciton-polariton condensates in a semiconductor microcavity with a coherent acoustic wave. An analytical solution for the dispersion of excitations of the coupled condensate-phonon system is found in the approximation of k-independent interactions. Accounting for k dependence results in a stronger modification of the dispersion, and even in the appearance of a roton instability region.