Efficient and tunable narrowband second-harmonic generation by a large-area etchless lithium niobate metasurface

Yaping Hou; Yigong Luan; Yu Fan; Alfonso Nardi; Attilio Zilli; Bobo Du; Jinyou Shao; Marco Finazzi; Chunhui Wang; Lei Zhang; Michele Celebrano

arXiv · 2602.00830

Efficient and tunable narrowband second-harmonic generation by a large-area etchless lithium niobate metasurface

Yaping Hou·Yigong Luan·Yu Fan·Alfonso Nardi·Attilio Zilli·Bobo Du·Jinyou Shao·Marco Finazzi·Chunhui Wang·Lei Zhang·Michele Celebrano

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Abstract

Optical resonances in nanostructures enable strong enhancement of nonlinear processes at the nanoscale, such as second-harmonic generation (SHG), with high-$Q$ modes providing intensified light--matter interactions and sharp spectral selectivity for applications in filtering, sensing, and nonlinear spectroscopy. Thanks to the recent advances in thin-film lithium niobate (TFLN) technology, these key features can be now translated to lithium niobate for realizing novel nanoscale nonlinear optical platforms. Here, we demonstrate a large-area metasurface, realized by scalable nanoimprint lithography, comprising a slanted titanium dioxide (TiO$_2$) nanograting on etchless TFLN for efficient narrowband SHG. This is enabled by the optimal coupling of quasi-bound state in the continuum (q-BIC) modes with a narrowband pulsed laser pump. The demonstrated normalized SHG efficiency is $0.15\%\,\mathrm{cm}^2/\mathrm{GW}$, which is among the largest reported for LN metasurfaces. The low pump peak intensity ($3.64~\mathrm{kW}/\mathrm{cm}^2$) employed, which enables SHG even by continuous-wave pumping, allows envisioning integrated and portable photonic applications. SHG wavelength tuning from $870$ to $920~\mathrm{nm}$ with stable output power as well as polarization control is also achieved by off-normal pump illumination. This versatile platform opens new opportunities for sensing, THz generation and detection, and ultrafast electro-optic modulation of nonlinear optical signals.

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Yaping Hou, Yigong Luan, Yu Fan, Alfonso Nardi, Attilio Zilli, Bobo Du, Jinyou Shao, Marco Finazzi, Chunhui Wang, Lei Zhang, Michele Celebrano. 2026-01-31. Efficient and tunable narrowband second-harmonic generation by a large-area etchless lithium niobate metasurface. https://arxiv.org/abs/2602.00830

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