Exciton-polariton lasing and amplification based on exciton-exciton scattering in CdTe microcavity quantum wells

We describe experiments demonstrating an exciton-polariton laser and amplifier based on an incoherent exciton-polariton reservoir in CdTe microcavity quantum wells. The gain mechanism is real excited exciton-exciton scattering, in which excitons created at large in-plane wave vectors are thermalized and accumulate at the bottleneck lower polariton states at smaller in-plane wave vectors. Because the exciton-exciton scattering rate for CdTe at the saturation density is higher than that for GaAs, the threshold for spontaneous polariton lasing is more easily reached in the case of CdTe with respect to GaAs. We demonstrate a high-gain amplification of bottleneck lower polaritons close to the lasing threshold. By performing a pulsed pump and probe experiment, we observe unambiguous evidence of real excited exciton-exciton scattering gain in the form of $\exp \left(\mathrm{const}{N}_{\mathrm{exc}}^2\right),$ where $N_{\mathrm{exc}}$ is the exciton-polariton reservoir population. This result is in sharp contrast to the recently demonstrated parametric polariton amplifier based on virtual coherent four wave mixing, in which gain is proportional to $\exp \left(\mathrm{const}{N}_{\mathrm{exc}}\right).\mathrm{}$ [P.G. Savvidis et al., Phys. Rev. Lett. 84, 1547 (2000)].