Non-reciprocal frequency conversion in a multimode nonlinear cavity

Nonlinear optics is the workhorse for countless applications in classical and quantum optics, from optical bistability to single photon pair generation. However, the intrinsic weakness of optical nonlinearity has largely limited the efficiency of nonlinear frequency conversion. Here, motivated by recent advances in using non-Hermitian photonics to enable non-reciprocal light transport, we explore how the interplay between non-Hermiticity and optical nonlinearity leads to a fundamentally new regime of nonlinear frequency conversion. We describe how nonreciprocity in coupling between discrete frequency modes can be engineered to yield unidirectional energy flow towards the boundary mode of a frequency comb, closely resembling a non-Hermitian skin effect. Applying this mechanism to an infrared (IR) comb with cascaded second-order nonlinearity, we demonstrate high (>85%) nonlinear conversion efficiency into the “skin” mode and high-power THz generation through enhancement of THz-creating nonlinear processes. We also show how these effects are robust to defects and disorder in the comb and can be harnessed to generate stable limit cycles and comb generation at IR and THz frequencies.