Compensation of thermal refraction effect in high-$Q$ toroidal microresonator by polydimethylsiloxane coating

We experimentally and theoretically characterize the thermal refraction effect in a silica microtoroid and demonstrate that such effect can be reduced or even eliminated by applying a thin layer of polydimethylsiloxane (PDMS) to the surface of the silica resonator. By increasing the coating thickness, the whispering gallery modes (WGMs) experience a transition from redshift to blueshift induced by thermal absorption. Experiment results demonstrate that at the thickness of $0.52μm$⁠, the fundamental WGM with observed $Q$ factor of $1.5×106$ shows no shift with the input optical power since the thermal refraction of the silica for this mode is compensated completely by the PDMS layer, which has an opposite thermal refraction effect. This work shows that the PDMS layer could be used to reduce thermal noise in high-$Q$ silica microcavities for applications in sensing, lasing, and nonlinear optics.