A polariton relaxation bottleneck is observed in angle-resolved measurements of photoluminescence emission from a semiconductor microcavity. For low power laser excitation, low k polariton states are found to have a very small population relative to those at high k. The bottleneck is found to be strongly suppressed at higher powers in the regime of superlinear emission of the lower polariton states. Evidence for the important role of carrier-carrier scattering in suppression of the bottleneck is presented. Semiconductor microcavities ͑MC's͒ with embedded quantum wells ͑QW's͒ have attracted great interest recently. In these structures, two-dimensional ͑2D͒ confinement of photon and exciton modes is realized. In high finesse MC's, as a result of exciton-photon coupling, mixed 2D polariton states occur 1 which exhibit a strong dependence of their properties on their relative exciton and photon contents. 2 In particular, due to the very small in-plane photon mass in MC's (M p Ϸ10 Ϫ4 m ex ), polaritons with a significant photon fraction have a very steep E -k dispersion curve, Of particular relevance to the present work, energy relaxation in the polariton system is expected to be strongly modified relative to that of uncoupled excitons. Acoustic phonon scattering rates from high k exciton states into polariton states with wave vectors corresponding to the strong coupling regime (kр5 -6ϫ10 4 cm Ϫ1 ) are expected to be small due to the large energy transfer required compared to typical acoustic phonon energies of 1 meV. Furthermore, the strongly coupled states have very short radiative lifetime ͑ϳ1 psec͒ due to their high photon fraction, thus leading to a nonthermal polariton population and the occurrence of a relaxation bottleneck. Such bottleneck effects have been much discussed theoretically for the cavity polariton system. 7 A manifestation of the bottleneck has been observed in time-resolved measurements of the polariton emission in II-VI based microcavities. In this paper, we present a definitive observation of the relaxation bottleneck for lower branch polaritons ͑LP's͒ from angle resolved photoluminescence ͑PL͒. Nonresonant cw laser excitation creates excitons at high energy which relax to form a reservoir of high k (Ͼ10 5 cm Ϫ1 ) excitons. These excitons scatter into the observable polariton states and give rise to the PL. For low density excitation, low k (Ͻ1.5 ϫ10 4 cm Ϫ1 ) occupancies nearly an order of magnitude smaller than those at higher k are found. Strong redistribution of the polariton population, and suppression of the bottleneck, is found in the regime where the LP emission exhibits superlinear behavior with increasing excitation intensity The sample studied is a high quality single MC structure grown by metal organic vapor phase epitaxy. The Bragg reflectors are composed of 17 ͑20͒ repeats of /4 Al 0.13 Ga 0.87 As/AlAs layers in the top ͑bottom͒ mirrors. The 3/2 GaAs cavity contains six 10-nm In 0.06 Ga 0.94 As/GaAs QW's. The vacuum Rabi splitting was ⍀Ϸ6 meV, with linewidths of Ͻ1 meV. Nonuniformities of the cavity width permit tuning of the photon mode energy over a wide range of negative and positive detunings (⌬ϭE cav ϪE ex ). Measurements were carried out with the sample immersed either in superfluid He or in He gas. A Ti-sapphire laser was used for nonresonant ͑1.56 eV͒ excitation. The PL signal was collected by a lens which imaged the excited sample region ͑collection angle Ͻ2°) on to a fiber. Both the lens and the fiber were mounted on a rotating rail permitting precise angle tuning. The signal was detected by a single monochromator/ CCD system. PL spectra for a detuning of ⌬ϭϪ4.3 meV and temperature Tϭ34 K over a wide range of collection angles (⌽, RAPID COMMUNICATION
