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

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.

💡 Research Summary

This work presents a large‑area, etch‑free lithium niobate (LN) metasurface that achieves highly efficient, narrowband second‑harmonic generation (SHG) with tunable wavelength and polarization control. The device consists of a slanted titanium‑dioxide (TiO₂) nanograting fabricated on a thin‑film LN (TFLN) substrate using scalable nano‑imprint lithography. The nanograting parameters (period = 910 nm, width = 530 nm, TiO₂ thickness = 510 nm, LN thickness = 610 nm) and a modest slant angle of 5° break the mirror symmetry of the unit cell, converting symmetry‑protected bound states in the continuum (BICs) into quasi‑BICs (q‑BICs). Simulations predict Q‑factors of ≈10⁵ for the TE₁₀ and TM₁₀ q‑BIC modes, while the leaky modes (TE₂₀, TM₂₀) exhibit lower, angle‑independent Q values.

Linear transmission measurements under both TE and TM illumination confirm the presence of four resonances and the strong angle dependence of the q‑BICs, matching the simulated dispersion. The experimentally observed Q‑factors are reduced relative to theory due to the finite spectral width of the supercontinuum source and fabrication imperfections, yet remain sufficiently high to enable strong field confinement within the LN layer.

For nonlinear experiments, a narrow‑band picosecond optical parametric oscillator (Δλ < 1 nm, Q ≈ 1400–2200) is used as the pump. By scanning the pump wavelength (≈1700–1800 nm) and the sample rotation angle (θ = 0°–4°), SHG maps are recorded for both polarizations. Under TE excitation (electric field parallel to the LN optic axis), the dominant d₃₃ = 29.1 pm/V coefficient is accessed, and the TE₁₀ q‑BIC yields a characteristic “V‑shaped” dispersion with a pronounced SHG peak. The TE₂₀ leaky mode shows an inverted “Λ‑shaped” response with lower conversion. Under TM excitation, the weaker d₃₁ and d₂₂ coefficients dominate, resulting in an order‑of‑magnitude lower SHG intensity for the TM₁₀ q‑BIC, but the same “Λ‑shaped” dispersion is observed. Optimal SHG occurs when the pump Q matches the q‑BIC Q, which experimentally is achieved at rotation angles of approximately ±0.2° to ±1°. At these angles the normalized SHG efficiency reaches 0.15 %·cm²/GW, one of the highest reported for LN metasurfaces.

A key advantage of the platform is the low pump peak intensity required (3.64 kW/cm²). Such modest intensities enable continuous‑wave (CW) operation, opening pathways to compact, low‑power nonlinear devices. By varying the incidence angle, the SHG wavelength can be tuned continuously from 870 nm to 920 nm while maintaining stable output power, and the polarization of the generated second harmonic can be switched between TE and TM by simply rotating the pump polarization. This dual tunability arises from the opposite angular dispersion of the q‑BIC and leaky modes.

The fabrication approach—nano‑imprint lithography of TiO₂ on an unetched LN film—produces a 1 mm × 1 mm metasurface in a single step, circumventing the damage and defect formation associated with direct LN etching. This scalable, low‑cost process is compatible with wafer‑scale manufacturing and can be readily integrated with existing photonic platforms.

Overall, the study demonstrates that (i) high‑Q quasi‑BIC resonances can be realized in an etch‑free LN metasurface, (ii) these resonances provide strong field enhancement leading to record‑high SHG efficiency at low pump powers, (iii) wavelength and polarization of the SHG output are readily controllable via off‑normal incidence, and (iv) the device can be fabricated over large areas using a commercially viable process. These attributes make the platform highly attractive for applications such as on‑chip frequency conversion, narrowband optical filtering, THz generation and detection, and ultrafast electro‑optic modulation in integrated photonic systems.