Strong coupling enables high-efficiency sum-frequency generation in a DBR-based lithium niobate waveguide

Sum-frequency generation (SFG) is of particular importance in coherent light sources, single photon detection, hyperspectral imaging, etc. However, high-efficiency SFG remains a challenge. Here, based on the strong coupling of two high-quality (Q) guided mode resonances (GMRs), we propose an effective means to realize high-efficiency SFG in a distributed Bragg reflector (DBR)-based etchless lithium niobate (LN) waveguide structure. Two prominent resonances in the near-infrared region are generated due to the spectrum splitting induced by strong coupling of GMRs respectively supported by the LN waveguide (GMRLN) and DBR (GMRDBR), which create a remarkable condition for high-efficiency SFG. A high SFG conversion efficiency of 2.13 × 10−2 is achieved even though the intensity of input beams is low at 0.01 MW/cm2. Furthermore, high-efficiency SFG exhibits wide-angle insensitivity to the incident light and perfect unidirectional emission in the backward direction due to nearly unchanged high-Q factors of GMRs and perfect blocking of forward emission by the DBR, respectively. Our results provide an excellent paradigm for enhancing light–matter interactions and boosting nonlinear conversion efficiency without breaking the in-plane structural symmetry, which may find possible applications in quantum nanophotonics, optical communication, and nonlinear light sources.