Bose-Einstein Condensate and phase transition of exciton polaritons in III/V microcavity samples.
Date
2024
Authors
Boozarjmehr, Maryam
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Abstract
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.
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