Microwave Induced Optical Nonlinearities in Cuprous Oxide

Abstract

This thesis presents an experimental study of the transmission spectrum of cuprous oxide in the presence of a microwave field. Using one photon spectroscopy an excitonic population is created which can then be modified by the application of a microwave field.

This was achieved using a combination of microwave antennae and high-resolution spectroscopy. The main results presented in this thesis are: (1) Using photoluminescence and transmission spectroscopy the quality of natural and synthetic samples was measured, finding natural samples to be far superior. This was attributed to the presence of copper vacancies in the synthetic material. These vacancies were removed through thermal annealing, but this did not improve the quality of synthetic samples.
(2) Determination of temperature as the limiting factor to observe higher nP states. This was carried out through spectroscopy of cuprous oxide over a range of temperatures.
(3) Observation of large optical nonlinearities in the presence of a microwave field. These manifest as changes in absorption which is stronger for higher states, where for sufficiently strong fields it is no longer possible to resolve them. An additional impact of the microwave field is an increase in the absorption between states. These observations were modelled by treating excitons as spinless hydrogen enabling an extraction of the nonlinear susceptibility using an atomic physics approach. The change in absorption is well described by this model. Alongside this, a large cross-Kerr nonlinearity is estimated which is several orders of magnitude larger than that seen in conventional media. These results cement cuprous oxide as a viable platform for Rydberg physics wherein strong coupling to external fields can be detected. This also highlights the need to develop high-quality synthetic material to remove the reliance on naturally occurring samples.