High pump depletion second-harmonic generation using domain engineered thin-film lithium niobate waveguides.

Thin-film lithium niobate (TFLN) has emerged as a powerful platform for integrated nonlinear optics owing to its large χ (2) nonlinearity, tight confinement and flexible tunability. To fully excavate such superior nonlinear optical properties, domain engineering is commonly adopted to fulfill the phase matching condition of χ (2) processes. During the past decade, various domain engineered TFLN nonlinear optical devices have been demonstrated, showing extremely high length-normalized nonlinear optical conversion efficiencies. However, application-driven scenarios demand absolute energy conversion in nonlinear frequency conversion rather than length-normalized efficiencies, but the progress has been limited by imperfect fabrication processes. In this work, we realize effective on-chip nonlinear energy conversion by developing low-loss and high-quality domain engineered TFLN waveguides with long interaction length. Ion beam trimming (IBT) technique and an etching-prior-poling workflow are adopted for such fabrication. Optical characterization yields an overall second-harmonic generation (SHG) efficiency of 2,590 %/W. A high pump depletion of 85.7 % is demonstrated under continuous-wave operation, which directly reflects strong nonlinear energy conversion. These results may lead to breakthroughs in applications like classical optical frequency conversion, quantum frequency conversion, and quantum light generation.