Propagation of excitonic polaritons in a microcavity

By exciting resonantly a microcavity with a pulsed laser beam, we have obse rved the in-plane propagation of excitonic cavity polaritons on distances m uch greater than the wavelength. The initial in-plane wave vector is conser ved during several tens of picoseconds showing it is a good quantum number for cavity polaritons. The polariton group velocities were measured both as a function of the in-plane wave vector and detuning. Propagation is larger at negative detunings where polaritons have a light effective mass. The ob servations are in good agreement with a simple two-oscillator model.