[PhD Thesis Presentation] ‐ Sho Kasumie - Whispering Gallery Resonators for Sensing and Nonlinear Optics

Presenter: Sho Kasumie

Title: Whispering Gallery Resonators for Sensing and Nonlinear Optics

Supervisor: Professor Sile Nic Chormaic

Unit: Light-Matter Interactions for Quantum Technologies Unit

ABSTRACT:

In this thesis work, a number of optical phenomena, such as nonlinear optics, optomechanics, sensing devices, and microlaser fabrication, have been studied using glass whispering gallery mode resonators (WGMRs). WGMRs are ultra-high Q optical resonators, with a Q-factor of at least 10^7 being easily achieved using a relatively simple fabrication platform. This ensures a strong interaction between light and the material of the resonator. Sensing application using WGMRs are based on detecting dispersion and dissipation changes to the cavity resonances.
We first show that dispersion and dissipation can be distinguished by measuring parameters associated with the signals obtained from the peak cavity ring-up spectroscopy (CRUS). As CRUS happens in a very short (nanosecond) timescale, sensing of transient phenomena is possible by applying the techniques we developed. Another form of light-cavity interaction can be seen via the evanescent field of cavity-coupled light. Two cavities, i.e. a microsphere and a microbubble, interact with each other through their evanescent fields to form a photonic molecule. Theory and experimental results give insight on the evanescent field coupling in the system.
Finally, the interaction between light and the cavity material is studied. Light coupled to a silica WGMR induces parametric oscillation. While the light-matter interaction is strong, the geometrical boundary condition of the cavity limits the parametric oscillation to occur only near the zero-dispersion wavelength (ZDW). This restriction is partially removed if the ZDW is adjustable. In this work, we use a microbubble resonator and we control the thickness of its wall during the fabrication process. This can change the geometrical dispersion of the cavity and enables adjustment of the ZDW. As a result, a four-wave-mixing frequency comb centred around 770 nm is generated. Stimulated Raman scattering is another form of interaction between light and the material. The theory developed as part of this thesis work shows that Raman scattering not only provides a source of light, but also has dynamical behaviour. In the simplest case, two Raman modes excited adjacent to each other are studied theoretically and experimentally, showing that a switching process between two modes is enabled by changing the pump lasser power. Finally, as a simple demonstration, we coat a microbubble with a thin solgel layer doped with erbium ions in order to create a tunable laser. Overall, the work contained in this thesis makes several contributions to the field of research using WGMRs and also impacts areas beyond this design of resonator through some of the phenomena studied and observed.