Polariton condensates in optical traps and strong magnetic fields

Abstract

Semiconductor Microcavities in the strong coupling regime are an ideal test bed for studying light matter interactions at the micro-scale. The eigenstates of these
systems, exciton-polaritons, are bosonic hybrid light matter quasi-particles that have been demonstrated to undergo Bose condensation. Owing to their photonic
component polaritons are lighter than atoms so their Bose-Einstein condensation (BEC) is attainable at higher temperatures than traditional BEC phase transitions
in atomic systems, while the reduced dimensionality of the system has the implication that the BEC phase transition spontaneously occurs only in the presence of a confining potential. In this thesis, the underlying mechanisms of polariton condensation in optically imprinted trapping potential landscapes is examined. Condensation in the ground state of an optical trap, de-localised from the excitation light is demonstrated and investigated and the confined condensate is shown to exhibit well defined quantum mechanical properties.

A comparative study of the observed spectral features with a condensate formed with typical non-resonant excitation methods is conducted revealing a significant reduction of the excitation density threshold due to efficient trapping and relaxation of polaritons inside the trap. Decoupling of the optically induced excitonic reservoir results in increased temporal coherence in this system by suppression of the strong interactions with un-condensed particles. Modification of the geometrical properties of the trap results in single excited-state condensation. Contrary to defect and stress induced trapping schemes the condensation process is here driven by polaritons injected into the potential-trap from the trap barriers. This leads to more efficient pumping of the energetically higher modes extending into the trap boundary. We demonstrate how this feature can be exploited to manifest transitions between energetically neighbouring coherent quantum states in the steady state dynamic equilibrium regime and in the transient domain were the intensity tuning of coherent tunnelling modes is also examined. A by-product of the localisation of the condensate inside the photonic trap and the decoupling from the reservoir is the strong susceptibility of this system to small imbalances in the optical pumping of spin states.

The population of the two spin states of the condensate can be controlled by small imbalances of the circular components of the excitation. The high density regime in this configuration is then investigated where a linearisation of the polariton dispersion is observed under pulsed excitation. However, a vigorous examination of the transient dynamics in this regime demonstrates the artificial nature of this effect due to transient relaxation and momentum narrowing in the transition from photon lasing to a confined polariton condensate. The confined condensate is an ideal subject for studying strong magnetic field effects on the spin properties of polariton condensates as it emits in a single energy mode for a wide range of excitation powers above threshold compared to the "untrapped" case and doesn’t suffer from de-coherence effects induced from the reservoir that causes line broadening and inhibits the spectral resolution of the spin components. We have performed initial reference strong magnetic field experiments with "unconfined" polariton condensates and present the predicted density dependent collapse of the Zeeman splitting and the modulation of the previously observed paramagnetic screening by the polarisation of the exciting beam.

In the final chapter of the thesis we investigate the modulation of the condensation threshold for the non-optically confined case by the application of a magnetic field in the Faraday geometry. The experimental observations are explained by a model based on the suppression of diffusion in the reservoir and the shrinking of the Bohr radius by the application of the magnetic field.

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Identifiers

ORCID for Pavlos Lagoudakis: orcid.org/0000-0002-3557-5299

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