An Investigation into topological defects in polariton condensates

Abstract

Polaritons are quasiparticles arising from the state of strong coupling between a an exciton - another quasiparticle, made from an electron hole pair - with a photon trapped in a microcavity. The high binding energy of Frenkel excitons allows for polariton condensation to occur at room temperature, leading to the possibility for polaritons to be used in optoelectronic devices. The weaker-bound Wannier-Mott excitons in inorganic semiconductors require cryogenic temperatures to be excited, but, due to years of investigation, are being made into low-disorder cavities, allowing for the possible study of more exotic phenomena, such as vorticity, bistability, condensate coupling and lattices of condensates. Both regimes have their clear advantages and disadvantages, but ultimately, it is the study of both that will lead to a deeper understanding of the field.
This thesis looks at the coherence in between coupled condensates in a polariton dyad, extracting the coherence length for the system. Then, the creation of vortices varying topological charge is demonstrated by injecting an odd number of polariton condensates at the vertices of a regular polygon and imposing frustration into the system by controlling the interactions across vertices. Stable spatially localised circular energy flows can be observed circulating around a central vortex. The creation of so-called giant vortices, formed at the centre of a polygon on a non-zero density background, present as platform for the study of vorticity in superfluids.
Polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye BODIPY-Br is shown. The experimental fingerprints of polariton condensation under non-resonant optical excitation are presented , including the non-linear dependence of the emission intensity, linewidth narrowing and wavelength blueshift with increasing excitation density, obtained from single pulse dispersion imaging which allows us to visualise the collapse of the energy distribution upon reaching threshold. Finally, single shot interferometry is used to observe unpinned spontaneous vortices in organic polariton condensates.

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Identifiers

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

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