Condensation of exciton–polaritons in complex potentials

Abstract

Bose–Einstein condensates (BEC) of exciton–polaritons represent a successful platform for studies of macroscopic quantum physics at elevated temperatures in a solid-state device. Despite the number of breakthroughs in both experiment and theory, there are still some gaps in our understanding of this part-light part-matter system. The difficulties in our interpretation of the system’s behavior arise from the inherent non-equilibrium nature of exciton–polaritons and their coupling with a reservoir of thermal excitons. This optically induced reservoir creates a repulsive potential and serves as a gain medium or source for exciton–polaritons, thus creating a complex-valued potential. In this Thesis, I summarize my PhD work on trapping, controlling, and manipulating exciton–polariton condensates using this complex potential. Chapter 1 of this Thesis introduces the topic of exciton-polariton condensation, the experimental and modeling techniques used in my work, as well as methods for potential landscape engineering for exciton–polaritons. Chapter 2 presents experiments on trapping the condensate in a one-dimensional array of photonic traps, and controlling the population of different energy states in the band-gap structure by applying a spatially structured pump. Chapter 3 demonstrates how the implementation of the finely controlled, fully optically-induced potentials allows us to finely tune the energy and linewidth of the condensate and elucidate its non-Hermitian nature through observation of non-Hermitian spectral degeneracies. Chapter 4 presents a detailed study of the condensation process in the presence of thermal reservoir, which is inherent in optically-induced trapping. Using an ultra-high-Q microcavity, we image single realizations of condensation with unprecedented detail, and observe filamentation of the condensate, which is a direct consequence of reservoir depletion. Chapter 5 presents further work performed in this “single-shot” regime, where we drive the condensate into the high-density regime, and, assisted by the reservoir depletion, observe a homogeneous profile characteristic of the Thomas–Fermi limit. Furthermore, the spectrum of the high-density condensate shows signatures of crossover from BEC to the Bardeen–Cooper–Schrieffer state, which represents a starting point for future studies beyond the scope of this Thesis. Show more