Dimensional Crossover in a Photonic Quantum Gas

Abstract

For many-body quantum systems, dimensionality is known to exert a profound influence on the physical behaviour, allowing for the shaping of distinct phases of matter. In general, for lower dimensions, enhanced fluctuations serve to suppress long range order. For bosonic gases, for example, Bose-Einstein condensation in one dimension requires confinement that is stronger than linear in order to occur, in contrast to two-dimensional systems where harmonic confinement can suffice. <br/> In this thesis, the dimensional crossover between one and two dimensions in a harmonically trapped photon gas has been experimentally investigated. The photons were confined within a dye filled optical microcavity, where polymer nanostructures produced by direct laser writing defined the trapping potential. By systematically adjusting the aspect ratio of the harmonic trap, the confinement was varied from an isotropic two dimensional regime to a highly anisotropic one dimensional regime. <br/> The caloric properties of the photon gas were characterised across this transition, revealing that the sharp phase transition observed in two dimensions evolves into a smooth crossover in one dimension. This work enhances understanding of thermodynamic behaviour in photon Bose gases under confinement and demonstrates that here dimensionality can be used to tailor phase transition phenomena. <br/> Furthermore, polymer cavities fabricated via direct laser writing afforded precise control over the confinement geometry, enabling exploration of variable potential landscapes. Proof of concept studies of advanced geometries, previously inaccessible, are presented, opening new directions for research in driven dissipative Bose gases and photonic quantum simulation platforms.