Monitoring phase-matching of third harmonic generations in microbubble whispering gallery mode resonators by a...

High Q whispering gallery mode (WGM) optical resonators can achieve a high nonlinear frequency conversion efficiency with quite low input laser intensity. Third harmonic generation (THG) in a WGM resonator requires phase-matching which is conventionally accomplished by careful engineering of the dispersion of the cavity modes. Input laser power-induced temperature-dependent dispersion is often used to additionally compensate the residual phase mismatch. Tracing back the original resonant mode is difficult, especially when a dense resonant spectrum exists such as in a microbubble resonator (MBR). In this work, by using a novel dynamic opto-thermal disruption method to interrupt the thermal-drift of the fundamental resonance, we monitored the temperature-dependent THG phase-matching condition in MBRs step by step. We precisely determine the modal phase-matching wavelength that is 22.6 pm away from the cold cavity resonance, THG conversion bandwidth is 670 MHz at 517.7 nm. A peak THG conversion efficiency of 2.6×10^-9 with a pump power of 1.5 mW is achieved in a high Q (= 2.9×10⁷) MBR. The bandwidth can be substantially expanded combining with other dispersion control methods.