Observation of Rydberg excitons in nanofabricated Cu₂O crystals and cavities

Abstract

Photons, the fundamental particles of light, hold potential for quantum applications. While creating, manipulating, and detecting photons is possible, achieving strong interactions at the single-photon level is challenging. Although photons in a vacuum do not interact, nonlinear interactions with quantum matter can mediate photon-photon interactions, known as single-photon nonlinearity regime. Exciton-polaritons, which are hybrid light-matter quasiparticles, offer a promising path for these interactions. Conventional materials like GaAs struggle to achieve the desired levels of single-photon nonlinearity. Cu₂O Rydberg excitons are promising contenders, due to their giant optical nonlinearities from higher-order Rydberg states that couple strongly with cavity photons. This thesis explores spatial confinement in Cu₂O nanostructured cavities as a viable method to achieve single-photon nonlinearity.

Major results of this work are: (a) Optically tuned Rydberg excitons in Cu₂O pillars fabricated using a focused ion beam, while preserving excitonic properties. We show a size-dependent redshift in excitonic energies which can be harnessed for generating strongly correlated lattices of Rydberg polaritons. (b) Encapsulating Cu₂O nanostructures in microcavities, where a strong coupling of Rydberg excitons (up to n = 8) to cavity photons is first observed by absorption spectroscopy. A thin layer of ITO deposition on the bulk cavity helps in better etching. Photoluminescence of Rydberg excitons (up to n = 5) is observed in the nanostructured and bulk cavities, with an enhancement in the scaling of the nanostructures with reducing dimensions. Understanding the underlying phenomena of the enhancement is key to exploiting the coupling of Cu₂O Rydberg excitons to cavity photons and generating the predicted nonlinearity. The findings pave the way to establish Cu₂O Rydberg polaritons as a scalable platform for photonic quantum simulations.

Date of Award	30 Jun 2025
Original language	English
Awarding Institution	University of St Andrews Macquarie University
Supervisor	Hamid Ohadi (Supervisor) & Thomas Volz (Supervisor)