Frequency Tuning of Whispering Gallery Mode Resonators with Planar Dielectric Substrates

Abstract

In this work we investigate the proposed relationship between planar dielectric substrates of increasing refractive index, and their effect on the frequency of a resonant mode confined in a whispering gallery mode resonator. We did this using a heavily automated setup to facilitate the collection of large quantities of data. Not all of this data is useful due to recurring instabilities within the experimental setup.

Despite the instabilities we were able to collect and analyse a sample of data for both TE and TM polarised modes when they interacted with a substrate. We used seven substrates which gave us a refractive index range of 1.43 to 4.0. We built plots that highlight the effect that these substrates have on both the frequency of the mode and the broadening of the mode for the substrates where mode-broadening was shown to occur.

We measured the effect of the substrates on the frequency of a whispering gallery mode resonance. These results are then compared to the theoretical predictions that inspired this research. We found that the figures we built showed qualitative agreement with the theory especially for the frequency tuning of TE polarised modes. The TM polarised modes also showed agreement with the theory. We also saw some agreement between TE mode broadening results and the theory but the TM mode broadening results are less conclusive when comparing to the theory.