  Whispering-gallery-mode microcavities are known to have high optical quality
factor, making them suitable for nonlinear optical interactions. Here,
third-harmonic generation is observed using a relatively small radius wedge
silicon oxide optical microcavity. The small radii wedge microdisks can be
dispersion-tailored to obtain either normal or anomalous group velocity
dispersion. In our case, we operate in the normal dispersion regime preventing
frequency comb generation by suppressing infrared cascading four-wave mixing.
This allows for a clean third harmonic generation at phase-matched visible
optical modes. Tunability of the third-harmonic emission is obtained due to a
combination of the thermo-optical and Kerr effects. An additional thermal
control of the phase matching condition allows for optimizing third-harmonic
generation, and agreement between this process and couple-mode theory is
demonstrated.
