Cascaded second-order nonlinear process in a modal phase-matched lithium niobate waveguide for broadband wavelength conversion

We demonstrate broadband and efficient wavelength conversion via cascaded second-harmonic generation and difference-frequency generation process, utilizing modal phase matching in a dual-layer <i>x</i>-cut thin-film lithium niobate waveguide. By optimizing ferroelectric domain orientation, the nonlinear modal overlap between the interacting waves is significantly enhanced. The fabricated 6.3-mm-long waveguide exhibits a normalized conversion efficiency of 4508% W^-1cm^-2 at a pump wavelength of 1540.4 nm. The experimental results show that the device maintains high conversion efficiency over a 100 nm operational bandwidth (1505-1605 nm), while numerical simulations predict a theoretical 3 dB bandwidth of 173 nm.