High-efficiency SHG in etchless lithium niobate waveguides enabled by polymer-induced bound states in the continuum

Bound states in the continuum (BIC) demonstrate significant potential for achieving strong optical confinement, characterized by their inherent elimination of complex etching processes and low propagation loss. However, efficient second-harmonic generation (SHG) within BIC platforms remains challenging. In this work, we propose an etchless, periodically poled thin-film lithium niobate (TFLN) optical waveguide loaded with a polymer cladding. A quasi-BIC waveguide mode is engineered, exhibiting simulated propagation losses of 0.0090 dB/cm and 0.2142 dB/cm for the fundamental wave and second-harmonic wave, respectively. Leveraging this low-loss BIC waveguide, we achieve enhanced SHG. Experimentally, the fabricated BIC waveguide demonstrates a normalized SHG conversion efficiency of 844% W^-1cm^-2, alongside advantages of straightforward fabrication and large process tolerance. This work provides what we believe to be a novel approach for nonlinear optical applications utilizing BIC waveguides.