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A theoretical model for the investigation of the dynamics of microcavity polaritons in the strong-coupling regime is proposed. The resulting photoluminescence dynamics at small angles is studied as a function of the angle of observation, the cavity detuning, the lattice, and the free-carrier temperature. For small detunings, the strong dispersion of the microcavity polaritons at small angles results in a bottleneck relaxation dynamics, similar to the bulk one. However, important differences, related to the reduced dimensionality of the system, are found. In particular, a larger emission from the upper polariton with respect to the lower polariton is found for any temperature. Moreover, a two-lobe angular emission from the lower branch is also expected. In the case of large pump excess energies, when hot carriers are injected into the system, longitudinal-optical-phonon emission has also been considered as a possible polariton formation mechanism, and shown to reduce only partially the effects summarized above. ͓S0163-1829͑97͒06036-0͔

The spectral and dispersive emission properties are analytically determined for the twodimensional system of exciton-polaritons in microcavities excited by a resonant and coherent optical pump. New collective excitations result from the anomalous coupling between one generic polariton state and its idler, created by the scattering of two pumped polaritons. The corresponding parametric correlation is stimulated by the emitter and idler populations and drives very efficiently the luminescence. The intrinsic properties of the collective excitations determine a peculiar emission pattern.In their pioneering experiments, Weisbuch et al.[1] discovered a new kind of two-dimensional quasi-particles, resulting from the strong coupling between quantum well excitons and confined photons in a semiconductor microcavity. These peculiar particles, called microcavity polaritons, have a very sharp dispersion due to the very light mass of the cavity photon. Remarkably, this represents a condensed matter system of small mass quasiparticles which can be manipulated through laser beams both in frequency and momentum space. In principle, the low polariton density of states could allow large occupation numbers at relatively small densities of particles , well below the critical saturation value due to the fermionic nonlinearities. In other words, the polariton system could exhibit bosonic properties [2]. Furthermore, unlike unbound electron-hole pairs in ordinary semiconductor lasers, microcavity polaritons have a relatively short time of recombination into external photons. All these ingredients are indeed very promising for applications in the domain of ultrafast all-optical switching and amplification.Recently, Dang et al.[3] measured photoluminescence spectra from a II-VI microcavity excited with a nonresonant pump. A threshold was observed in the dependence of the polariton luminescence intensity as a function of the input power. Similar results were reported by Senellart and Bloch in a III-V microcavity [4]. These experiments have been interpreted in terms of enhanced scattering of reservoir excitons into the emitting polariton modes. The origin of the enhancement has been attributed to bosonic stimulation due to final state occupation.More recently, great insight into the subject has been given by angle-resolved experiments under resonant excitation [5][6][7][8]. In this kind of experiments, polaritons are optically excited at a desired energy and momentum, allowing a direct control of the polariton dynamics. In particular, Savvidis et al. [5] have uncovered a new kind of polariton parametric amplifier through pump-probe experiments. The angular selection rules for the parametric conversion of pumped polaritons into the probe and idler modes are unambiguously given by the energy and momentum conservation for the polariton scattering. In the case of an applied probe, the nonlinearity can be explained in terms of phase-matched wave-mixing of polariton matter beams [9]. Remarkably, giant nonlinearities occur also when the probe bea...