Theory of χ(2)-microresonator-based frequency conversion

Microresonator-based platforms with chi((2)) nonlinearities have the potential to perform frequency conversion at high efficiencies and ultralow powers with small footprints. The standard doctrine for achieving high conversion efficiency in cavity-based devices requires perfect matching, that is, zero phase mismatch while all relevant frequencies are precisely at a cavity resonance, which is difficult to achieve in integrated platforms due to fabrication errors and limited tunabilities. In this Letter, we show that the violation of perfect matching does not necessitate a reduction in conversion efficiency. On the contrary, in many cases, mismatches should be intentionally introduced to improve the efficiency or tunability of conversion. We identify the universal conditions for maximizing the efficiency of cavity-based frequency conversion and show a straightforward approach to fully compensate for parasitic processes such as thermorefractive and photorefractive effects that, typically, can limit the conversion efficiency. We also show the design criteria that make these high-efficiency states stable against nonlinearity-induced instabilities. (C) 2021 Optical Society of America

Automated summary

Microresonator-based platforms with chi((2)) nonlinearities have the potential to achieve high-efficiency frequency conversion at ultralow powers. Perfect matching is not necessary for high conversion efficiency, and mismatches can even be intentionally introduced to improve efficiency or tunability. Design criteria must be met to ensure stability against nonlinearity-induced instabilities in these high-efficiency states.