Phase Transitions of Exciton-Polaritons in Grating-Based Microcavities

Abstract

Exciton-polaritons in semiconductor microcavities have been a platform to study macroscopic quantum phenomena. They exhibit phases like Bose–Einstein condensate (BEC) and conventional photon lasing. Due to their fermionic constituents, a Bardeen–Cooper–Schrieffer (BCS)-like phase has also been predicted at density levels in between that of a BEC and photon lasing. The pumping and decay of polaritons not only modify these phases but may lead to a range of non-equilibrium phenomena that are more accessible in the polariton system than in closed systems. Studies of the BCS-like phase and other phenomena in the polariton system require the capability to control the polariton properties. In this dissertation, experimental methods are developed to control the polariton system by engineering its photonic component using grating-based microcavities. The tuning of energy, decay rate, polarization, and mode profile of the photonic mode in compact devices is demonstrated and utilized to identify and characterize new types of phases of exciton-polaritons.

A high-contrast grating that has polarization-selective high reflectivity is used as a reflector for a GaAs/AlGaAs microcavity with GaAs quantum wells. In the linear polarization with high-reflectivity, spontaneous transitions of exciton-polaritons from the normal phase to phases with coherent light emission under non-resonant pumping are observed, while the response of the underlying excitons can be probed using the orthogonal linear polarization. The features of the polariton emission spectra resemble a polariton BEC or a photon laser as detuning changes, while the reflection spectra of the excitons reveal that the coherent emission is due to stimulated emission under population inversion in both cases, ruling out the possibility of a polariton BEC. The comparison with a theoretical model suggests the possibility of the polariton BCS phase that has been postulated in the past but still lacks direct experimental confirmation. These observations contribute to the understanding of the phases of exciton-polaritons at high densities.

By altering the grating geometry, it is possible to independently control the resonant frequency and the decay rate of the cavity mode and therefore of the resulting polariton mode. The tuning of polariton energy and linewidth with about 1meV step size is demonstrated with fabricated devices. Combined with the capability of designing localized polariton modes, realizing these different parameters in a single device may facilitate the study of phase transitions arising from non-Hermitian physics.

A type of microcavity that utilizes the interference effect between the guided-mode resonance of the grating reflector and the background transmission continuum to achieve a high Q-factor is designed and tested. Reflection spectra measurements with a tunable cavity length setup show signatures of the cavity resonance. The Q-factor of the resonance has a strong frequency dependence that may allow leveraging the nonlinearity of polaritons for new devices.