Quasi-Phase-Matched Thin-Film Lithium Tantalate Waveguides for On-Chip Fourth-Harmonic Generation Toward the Ultraviolet

Thin-film lithium tantalate (LiTaO3) is a promising platform for integrated nonlinear photonics owing to its strong second-order nonlinearity, broad transparency window, and favorable photorefractive resistance. Here, we numerically investigate cascaded fourth-harmonic generation in quasi-phase-matched thin-film LiTaO3 waveguides, targeting ultraviolet generation at 387.5 nm from a 1550 nm pump. Three poling schemes, including square-wave periodic poling, generalized quasi-periodic superlattice (GQPS), and dual-period poling (DPP), are designed to simultaneously compensate the phase mismatch of second-harmonic generation and fourth-harmonic generation. Under a 10 mW average input power and 250 fs pulse duration, the simulated fourth-harmonic conversion efficiencies reach 42.7%, 35.7%, and 57.1%, respectively. The DPP structure provides the highest efficiency by supporting both nonlinear processes with relatively strong low-order reciprocal–vector components. The influence of waveguide geometry errors and temperature tuning is further analyzed, showing that the fourth-harmonic process is more temperature-sensitive than the second-harmonic process. In addition, the feasibility of extending this scheme toward 300 nm deep-ultraviolet generation is discussed. These results provide a design route for compact, efficient, and fabrication-compatible on-chip ultraviolet sources based on thin-film LiTaO3 nonlinear photonics.