High-efficiency second-harmonic generation in ultra-compact Z-cut lithium niobate waveguides via lateral selective domain engineering.

We propose and theoretically demonstrate a high-efficiency second-harmonic generation (SHG) scheme in ultra-compact Z-cut thin-film lithium niobate (TFLN) waveguides. By employing lateral selective domain engineering (LSDE) to achieve modal phase matching (MPM), we leverage a sub-wavelength 735-nm-wide architecture that exploits strong optical confinement to enhance nonlinear interactions while maintaining a small footprint. To overcome the intrinsic symmetry-induced overlap integral limitations typical of high-order modes, we introduce a localized domain inversion specifically in the central region of the waveguide. This strategy effectively rectifies the nonlinear polarization of the mode, converting spatial phase cancellation into constructive interference. Numerical results indicate that the proposed LSDE-MPM scheme achieves a normalized conversion efficiency of 6200% W-1cm-2, representing a 40.9% enhancement over conventional periodically poled lithium niobate (PPLN) waveguides of identical dimensions. Furthermore, we demonstrate that this approach eliminates the need for complex interdigital electrodes and offers superior fabrication tolerance, including a significantly reduced thickness sensitivity of 0.9 nm/nm and high immunity to stochastic poling jitter. This provides a robust, CMOS-compatible pathway for scalable nonlinear integrated photonic circuits.