Theory of $\chi^{(2)}$-microresonator-based frequency conversion

TL;DR

This paper develops a theoretical framework for $^{(2)}$-microresonator-based frequency conversion, showing that intentional phase mismatch can enhance efficiency and stability, challenging the traditional perfect matching requirement.

Contribution

It introduces universal conditions for maximizing efficiency and methods to compensate parasitic effects, expanding the design space for high-performance frequency converters.

Findings

Mismatches can be intentionally introduced to improve efficiency.

High-efficiency states are stable despite parasitic effects.

Universal conditions for optimal frequency conversion are identified.

Abstract

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 rigorously that these high-efficiency states are stable.