Bose-Einstein Condensation of Microcavity Polaritons

The strong coupling of light and excitons in a two-dimensional semiconductor microcavity results in a new eigenstate of quasiparticles called polaritons. Microcavity polaritons have generated much interest due to the wealth of interesting optical phenomena recently observed in these systems such as nonlinear emission, macroscopic coherence, and bosonic stimulated scattering. The efficiency of amplification, parametric oscillation, and coherent emission of light makes it promising for applications in coherent control, microscopic optical switching, and other opto-electronic devices. Most of all, because of their light mass and bosonic character, these particles are predicted to undergo Bose-Einstein condensation (BEC) at much higher temperatures and lower densities than their atomic counterparts.Standard methods of growing semiconductor microcavities are quite inefficient in producing well-tuned samples with strong coupling of light and excitons. Wafers with continuously varying thicknesses are often produced, leaving only tiny regions with strong coupling. In our experiments, an inhomogeneous stress is applied to the microcavity in order to actively couple naturally detuned exciton and cavity modes at fixed $k_{||} = 0$, and at the same time, create an in-plane spatial trap, potentially making BEC of polaritons possible.Our recent experiments with exciton-polaritons in the stress trap have shown compelling evidence of BEC. At the bottom of the trap where the coupling is strongest, line narrowing and nonlinear increase of photoluminescence intensity are observed. Also a single, spatially narrow condensate of polariton gas is formed analogous to the case of atoms in a three-dimensional harmonic potential. Above a critical density, we observe a massive occupation of polaritons in the ground state, spontaneous build-up of linear polarization, and macroscopic coherence of the condensate all in agreement with predictions. The results are similar to what is observed in the naturally resonant unstressed case. Comparison with the stressed trap and the nonstressed case, however, revealed that stress traps play a significant contribution in forming a polariton condensate. Furthermore, the stress trap case has shown, where the unstressed case has not, two distinct thresholds, one from photon lasing and another from a BEC transition