Tunable parametric oscillation in radially poled whispering gallery resonators

Continuous-wave optical parametric oscillators (OPOs) combine single-frequency output with a huge tuning range. These properties have let them become working horses for spectroscopy in the near and mid infrared over the last decades. However, singly resonant OPOs require high pump powers. Multiply resonant systems operate at much smaller pump powers, but tunability is hindered by high demands to the dielectric coatings of resonator mirrors. Whispering gallery resonators (WGRs) are an elegant solution since resonance in these monolithic spheroids arises from total internal reflection: the OPO process can be triply resonant across broad wavelength ranges, with quality factors limited by material absorption, yielding μW pump thresholds.