Exciton-polaritons in thermal equilibrium : from Bose-Einstein condensation to exciton-polaritonics

Abstract

Coherent control has been at the heart of the study of physical chemistry. Great advancement has been achieved in the past few decades in coherent control of classical systems by using spatially and temporally shaped electromagnetic waves. In this dissertation, we extend the concept of coherent control to a purely quantum mechanical collective system, namely, microcavity exciton-polariton Bose-Einstein condensates. Microcavity exciton-polaritons, hereafter simply polaritons, are bosonic quasiparticles formed in a resonant semiconductor microcavity by coupling the excitonic polarizabilities in quantum wells to the transverse mode of the confined optical field in the cavity. The light-matter dual nature allows direct control of polaritons through either their excitonic or photonic components. By utilizing the fact that polariton-exciton and polariton-polariton interactions are repulsive, all-optical control of polaritons was realized. By shaping the intensity fronts of the optical beam incident on a microcavity, the potential landscape felt by polaritons can be easily tailored. This is the key ingredient of this dissertation work. The light-matter dual nature endows polaritons a very small effective mass that is one hundred million times less than that of a hydrogen atom, leading to the observation of quantum phenomena such as condensation, superfluidity and quantized vortices at temperatures ranging from tens of Kelvin up to room temperatures. However, debates persist over whether the observed phenomena can be related to Bose- Einstein condensation because polaritons are not in thermal equilibrium. By applying all-optical trapping to a high-quality microcavity structure, polaritons at both spatial and thermal equilibrium were achieved across a broad range of densities and bath temperatures, as evidenced by the observed equilibrium Bose-Einstein distributions. A phase diagram for Bose-Einstein condensation of polaritons was produced for the first time, which agrees with the predictions of basic quantum gas theory. The thermalization behavior depends crucially on the interactions among polaritons. By changing the underlying excitonic/photonic fractions in polaritons, the interaction strength of polaritons can be varied, leading to control between nonequilibrium and equilibrium behavior of the polariton gas. The interactions also play a crucial role in polaritonic device operations. However, an accurate measurement of the polariton-polariton interaction strength has been not possible because of the difficulty in separating polaritons and excitons that are created by the same optical excitation. After propagating to the center of a sufficiently large optically induced annular trap, polaritons were separated from the incoherent populations of free carriers and hot excitons. The polariton interaction strength was then extracted from energies measured as a function of the polariton density. The measured interaction strength was about two orders of magnitude larger than previous theoretical estimates, putting polaritons squarely into the strongly interacting regime. Optical control can also be utilized to directly manipulate polariton condensates. By tailoring the size and pumping intensity of the optical trap, polariton condensates can be switched among different high-order modes and the homogeneous condensate mode. The redistribution of spatial densities is accompanied by a superlinear increase in the emission intensity as a function of excitation power, implying that polariton condensates in this geometry could be exploited as a multistate switch. The parameters for reproducible switching between the high-order states in the optical trap have been measured experimentally, giving us a phase diagram for the mode switching. It will serve well to calibrate the implementation of an exciton-polaritonic multistate switch.