Backward Wave Second-Harmonic Generation in X-Cut Thin Film Lithium Niobate

Thin-film lithium niobate (TFLN) is of particular interest for nonlinear wave mixing processes thanks to its high refractive index, high second-order nonlinearity and ferroelectricity enabling efficient quasi-phase-matching (QPM). Nonlinear wave mixing in x-cut TFLN also offers cointegration with other nonlinear components such as high-speed electrooptic modulators. Although there are many demonstrations of forward second-harmonic generation (FSHG) in x-cut TFLN, the SH-peak position shifts severely with respect to the intrinsic fluctuations in waveguide dimensions [1], [2]. Alternatively, QPM in backward-wave second-harmonic generation (BWSHG) is intrinsically more tolerant to such fluctuations as the QPM wavelength is only dependent on the effective refractive index of the second harmonic (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\lambda_{\text{QPM}}=2 n_{2 \omega} \Lambda$</tex>) [3]. It also enables novel nonlinear effects such as mirrorless optical parametric oscillation [4] Here, we demonstrate for the first time BWSHG in x-cut TFLN.