title: Bose-Einstein Condensate and phase transition of exciton polaritons in III/V microcavity samples.
Creators: name: Boozarjmehr, Maryam
Publisher: name: The Australian National University
Year: 2024

description: Exciton-polaritons, referred to as polaritons, present an ideal avenue for experimental investigations into condensation as one of the bosonic particles, given their comparatively lighter effective mass in contrast to excitons. Since the initial observation of particle condensation in 2002, numerous experiments have been undertaken to probe various aspects. These studies encompass examinations of macroscopic quantum or- der, quantum many-body effects, and trapping effects that influence the interaction and dispersion properties of polaritons. However, certain aspects, such as the impact of the reservoir, optical trapping, and energy differentials between exciton and photon energy on the quantum coherence properties of exciton-polaritons, remain open areas of exploration. Exciton-polaritons exhibit dual properties of both matter and light, contingent upon the fraction of exciton and photon components. Following the comprehension of the condensation theory of exciton-polaritons, experiments have delved into elucidating the fundamental characteristics of exciton-polariton coherence. These investigations specifically involve the measurement of first-order tempo- ral and spatial correlation functions, widely employed for characterizing the degree of coherence. These functions, by their nomenclature, reveal the coherence proper- ties of the particle wavefunction and elucidate how amplitude correlations manifest spatially and temporally. In this thesis, I have studied the spatial distribution of the excitonic reservoir below the condensation threshold to understand the behavior of the reservoir and to find out which parameters affect its spatial shaping, thus the transport length. Power- dependent analysis of the potential distribution shows a monotonic decrease in the effective transport length with increasing excitation power. We attribute it to the local heating of the sample at the pump spot and therefore the exciton bandgap renormalization and shrinking. We also observe the thermal-dependent energy redshift of the reservoir spectrum tens of microns away from the laser pump spot. The lo- cal potential fluctuations and imperfect thermalization and trapping of polaritons at lower temperatures compared to higher ones are responsible for the energy redshift. Then by optically trapping the exciton-polaritons condensation in the Thomas-Fermi regime and using the modified Michelson interferometer, I have measured the first- order correlation function to quantify the degree of the exciton-polariton coherence. I have used the Fourier transform and Kolmogorov-Smirnov (KS) test statistics meth- ods to analyze the data for matter-like or photon-like exciton-polariton properties. The results show that the trapping ring diameter size, reservoir presence, and energy fluctuations affect the coherence properties of exciton-polaritons with larger exciton or photon fractions differently when studying the spatial coherence properties of the trapped exciton-polariton condensation. The coherence for the photon-like polaritons decays faster than the matter-like polaritons. As the trap size gets larger, coherence length and time increase due to the smaller effects of the reservoir presence. Finally, the possible sources of decoherence for matter-like and photon-like polaritons are discussed.