Sum-frequency generation in bound-state-in-continuum microring resonators.

Bound states in the continuum (BICs) can facilitate strong optical mode confinement for nonlinear optical applications. Recently, BICs were used for low-loss photonic integrated circuits, enabling waveguides to be fabricated without etching on material platforms which do not have mature low-loss etching processes. Here, we show experimentally that the losses in BIC microring resonators are sufficiently low for efficient sum-frequency generation (SFG) on an etchless lithium niobate platform. The normalized conversion efficiency is measured to be 6.45 × 10^-6mW^-1. These results establish the feasibility of this approach for exploring the use of what we believe to be novel functional materials for nonlinear integrated photonics.