Optical whispering gallery mode resonators: analysing thermo-optic tuning in a silicon sphere

In this work, we discuss and experimentally investigate the whispering gallery modes (WGMs) in a 500 μm radius silicon sphere. We begin by reviewing the basics of WGM resonators, followed by simulations and experimental results obtained with the aforementioned silicon spherical WGM resonator. The recorded WGM signatures in the transmission and scattering spectra excited, in the near-infrared (near-IR) region, agree well with the simulations. Thermo-optic tuning of these WGMs is achieved by introducing a pump laser in the violet-blue region. Red-shifts in wavelength are observed, which increase with the increase in the pump power. A shift of 0.63 nm is observed at a pump power of 6.32 mW. We also study switching by analysing the transient response of the WGM spectra. The measured WGMs exhibit high quality factors ( $Q\sim {10}^5$ ), which along with the aforesaid tunability, demonstrates the potential of spherical silicon resonator as a platform for photonic applications, e.g. sensing and communication. In summary, this work contributes towards the understanding of the fundamental physics of WGMs and provides insights into silicon WGM resonators.