Modal phase matching in z-cut dual-layered lithium niobate rib waveguide for second harmonic generation

We report a comprehensive investigation of modal phase matching in z-cut dual-layered thin-film lithium niobate rib waveguides, emphasizing geometrical dispersion engineering for efficient second harmonic generation. Through systematic analysis, we examine the influence of waveguide cross-sectional dimensional variations on phase-matching conditions and conversion efficiency. Our findings reveal that the phase-matching wavelength exhibits substantially greater sensitivity to thickness fluctuations compared to variations in waveguide width and etch depth. By exploiting modal phase matching in z-cut thin-film lithium niobate, we demonstrate second harmonic generation with conversion efficiencies reaching the order of 10³% W^-1cm^-2. Our findings provide new insights into the optimization of optical frequency conversion processes and present significant potential for advancing on-chip nonlinear photonic devices based on thin-film lithium niobate platforms.