Optical and microwave spectroscopy of Rydberg excitons in CuO

Abstract

An experiment was developed to create and study excitonic energy levels in cuprous oxide at 4 K. Excitons were excited using both one-photon and two-photon excitation schemes. A variety of methods were used to characterise the resulting emission, including time-resolved single photon counting and high-resolution spectroscopy. Additionally, microwave antennae were developed to apply microwave fields to the sample with the aim of driving electric dipole transitions between excitonic energy levels. The main result presented in this thesis are: (1) A comparative study of synthetic and natural material revealed the presence of copper vacancies in the synthetic material, which was responsible for limiting the Rydberg series. (2) Narrowband second harmonic generation spectroscopy was used to study Rydberg excitons up to n=12 and allowed the lineshape of the high n even-parity exciton states to be studied. (3) The addition of a microwave field significantly modified the exciton absorption lineshape. In the two-photon regime, applying a microwave field demonstrated coherent modulation of the second harmonic, with sidebands observed. The results were modelled based on microwave-driven electric dipole transitions between Rydberg states. With a simple microwave antenna it was possible to reach a regime where the microwave coupling (Rabi frequency) was comparable to the non-radiatively broadened linewidth of the Rydberg excitons. These result demonstrate the first coupling of Rydberg excitons and microwave fields, provide a new way to manipulate excitonic states, and open up the possibility of a cryogenic microwave to optical transducer based on Rydberg excitons.