Widely Tunable Second Harmonic Generation in Low-Loss Thin-Film LiNbO₃ Membrane

The growing demand for broadband and tunable frequency converters in fields such as quantum computing, environmental sensing, and optical communications has driven significant advancements in nonlinear photonic platforms. However, existing solutions based on type-0 quasi-phase-matching face spectral limitations [1], offering tunability of≈ 10 nm or less [2]. In addition, LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (LN) nano-waveguide-based platforms encounter two primary challenges: (1) stringent fabrication tolerances, requiring sub-nanometer thickness precision down to 2.2 nm for a 5 mm interaction length [3], and (2) a trade-off between fabrication simplicity and low insertion losses, where additional processing steps are required to achieve coupling losses below 1 dB/facet [4]. These constraints significantly affect their applicability in demanding applications that require both broad spectral tunability and efficient frequency conversion such us broadband spectroscopy [5] or high brightness single photon sources [6].