Probing Orbital Angular Momentum States of Exciton-Polariton Condensates
Abstract
Condensates of microcavity exciton-polaritons have garnered much attention over the past decade for their potential to act as test beds for experiments with open dissipative bosonic condensates, and their capacity to host quantised vortices. A exciton-polariton is a bosonic quasiparticle consisting of an electron-hole bound pair (an exciton) strongly coupled to a photon typically confined by an optical microcavity. Polariton condensates can host vortices with quantised orbital angular momentum (OAM). The discrete, unbounded basis that these quantised OAM states comprise is attractive for storing or processing data with these systems, and thus methods for measuring and manipulating polariton vortex states are desirable. Fortunately, the state of a polariton condensate is easily accessible by measurement of its optical emission, unlocking the possibility to borrow OAM measurement techniques that are developed in the field of optics to be repurposed for use on polariton condensates. Currently, there is one method for performing these measurements that is universally used throughout the field. This is to produce an interferogram from the polariton emission, and infer the OAM and position of any quantised vortices present by an inspection of the fringe pattern. However, this method is limited in its ability to discern high-OAM states, and make quantitative measurements. In this thesis, I present an investigation into an alternative technique for measuring OAM that utilises a cylindrical lens. This technique is borrowed from applications in optics, and has not yet been applied to polariton emission. During this investigation, I found that the conventional technique that I employed to create stable polariton vortices was unexpectedly leading to an oscillatory switching between opposite OAM states. I propose a mechanism for this oscillation, and find that the predictions of this mechanism agree with the dynamics that were observed.