Supersonic exciton gratings: coherent inter-polariton scattering in semiconductor microcavities

Summary form only given. We report on a coherent nonlinear phenomenon in a semiconductor microcavity (SMC), which has no parallel for QW excitons. When two different polariton modes of the SMC are impulsively excited they undergo normal mode oscillations (NMOs) with coherent energy exchange between the exciton and the cavity mode. In our experiment the two polaritons are excited with slightly different angles resulting in a travelling wave exciton grating. When a test polariton mode is excited it will scatter in the travelling grating producing amplitude modulation sidebands. This phenomenon produces a transient four-wave mixing (TFWM) signal, which is shifted in frequency from that of the test beam by the NMO frequency, in our case, in the THz range corresponding to a grating velocity = 1 /spl times/ 10/sup 7/ m/s, which is four orders of magnitude larger than the sound velocity. The sample under investigation is a GaAs/AlGaAs /spl lambda/ cavity with a single 25 nm GaAs QW at the center.