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We report multistate optical switching among high-order bouncing-ball modes (“ripples”) and whispering-gallery modes (“petals”) of exciton-polariton condensates in a laser-generated annular trap. By tailoring the diameter and power of the annular trap, the polariton condensate can be switched among different trapped modes, accompanied by redistribution of spatial densities and superlinear increase in the emission intensities, implying that polariton condensates in this geometry could be exploited for an all-optical multistate switch. A model based on non-Hermitian modes of the generalized Gross-Pitaevskii equation reveals that this mode switching arises from competition between pump-induced gain and in-plane polariton loss. The parameters for reproducible switching among trapped modes have been measured experimentally, giving us a phase diagram for mode switching. Taken together, the experimental result and theoretical modeling advance our fundamental understanding of the spontaneous emergence of coherence and move us toward its practical exploitation., United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0001088)

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2017., Cataloged from PDF version of thesis., Includes bibliographical references (pages 249-265)., 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 wi, by Yongbao Sun., Ph. D.

The experimental realization of Bose-Einstein condensation (BEC) with atoms and quasiparticles has triggered wide exploration of macroscopic quantum effects. Microcavity polaritons are of particular interest because quantum phenomena such as BEC and superfluidity can be observed at elevated temperatures. However, polariton lifetimes are typically too short to permit thermal equilibration. This has led to debate about whether polariton condensation is intrinsically a nonequilibrium effect. Here we report the first unambiguous observation of BEC of optically trapped polaritons in thermal equilibrium in a high-Q microcavity, evidenced by equilibrium Bose-Einstein distributions over broad ranges of polariton densities and bath temperatures. With thermal equilibrium established, we verify that polariton condensation is a phase transition with a well-defined density-temperature phase diagram. The measured phase boundary agrees well with the predictions of basic quantum gas theory., United States. Department of Energy (DE-SC0001088), National Science Foundation (U.S.) (PHY-1205762), National Science Foundation (U.S.) (DMR-1104383), Gordon and Betty Moore Foundation (GBMF4420), National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (DMR-1420541)