Adiabatic Frequency Conversion in Non-Centrosymmetric High-Q Optical Microresonators

Microresonators are a versatile and efficient system platform for optical frequency conversion. Here, the non-linear response of material polarization is used for frequency synthesis. For example in WGRs made of non-centrosymmetric materials, tunable OPOs have been realized, and in WGRs made of centrosymmetric materials frequency combs were generated. Another way for frequency conversion is adiabatic tuning. Here, the optical length of the circumference of the resonator is changed during its ringdown time. This induces a frequency shift of the circulating light. So far different methods have been demonstrated. Free electrons generated by laser pulses, induce a change of the refractive index. By this method the wavelength can be tuned by some nanometers. The free electrons however lower the quality factor of the resonator. Moreover the repetition rate is limited by the time it takes for the free electrons to recombine. Another method is to change the frequency adiabatically via the AC-Kerr effect. Here, again second pump laser is needed. With this method the quality factor is not affected, but the achieved frequency shift is smaller compared to that for the free-electron method.