Wavelength-selective nonlinear wavefront control in resonant thin-film lithium niobate metasurfaces

TL;DR

This work tackles the challenge of achieving wavelength-selective nonlinear wavefront control in compact devices. By engineering a two-region thin-film lithium niobate metasurface with region-specific Mie-type resonances, the authors imprint a resonant phase difference that shapes the SHG wavefront while preserving the pump. They demonstrate conversion of a pump near $1100~\text{nm}$ to SHG at $550~\text{nm}$ with the SHG output adopting an HG01-like profile near the region boundary, achieving a phase difference of $\Delta \approx 0.85\pi$ at the SHG wavelength. The approach opens a route to ultracompact, tunable nonlinear optical components for nonlinear holography and related applications, with potential extensions to other nonlinear processes and electro-optic tunability in TFLN.

Abstract

Nonlinear metasurfaces offer compact control over frequency conversion and wavefront shaping. However, existing approaches, often based on geometric phase, lack wavelength selectivity, resulting in static nonlinear responses. Here, we demonstrate a thin-film lithium niobate metasurface that enables spectrally selective shaping of second-harmonic generation through resonance-engineered phase control. The structure consists of two regions with distinct phase responses, realized via spectral tuning of Mie-type resonances. This design enables simultaneous frequency conversion and spatial mode shaping, transforming a Gaussian pump near 1100 nm into a first-order Hermite-Gaussian mode at 550 nm, while maintaining the pump profile. The demonstrated approach offers a pathway toward ultracompact and tunable components for nonlinear holography and related applications.

Figures (4)

• Figure 1: (a) Schematics of the metasurface, consisting of two regions (A and B) of truncated TFLN nanopyramids with a height of $h= 135~\text{nm}$, lattice constant $p=340$ nm an etching angle of $\alpha= 75^{\circ}$, and a varying side length $L$. The inset shows an oblique view of the unit cell. (b, c) Oblique-view SEM images of regions A ($L_1 = 195~\text{nm}$) and B ($L_2 = 252~\text{nm}$), respectively. The measured periodicity extracted from SEM data is 330 nm. (d, e) Larger-area SEM images from the interface area between regions A and B from a representative metasurface ($L_1=170$ nm, $L_2=220$ nm). SEM images were taken at a $30^\circ$ tilt and prior to the removal of the silicon nitride layer on top.
• Figure 2: (a,b) Measured extinction spectra ($1-T$) from regions A and B of the sample, corresponding to side lengths $L_1=170$ nm and $L_2=260$ nm, respectively. The plots show the experimental data (black dots), Lorentzian fits (solid dark lines), and the extracted phase responses (dotted red lines). (c) Phase difference between regions A and B in the visible spectral range.
• Figure 3: (a) Schematic top view of the sample, indicating the pump beam positions during the scan. The pump starts in region A and is translated along the x-axis in $20~\mu \text{m}$ steps until it reaches region B. (b) Corresponding measured SHG patterns at each pump position, showing the spatial evolution of the SHG mode from a Gaussian profile in region A, gradually transforming into an HG01-like mode near the center between the two regions, and reverting back to a Gaussian profile in Region B. (c) Mode-overlap between the measured SHG field and an ideal HG01 mode, exhibiting a maximum near the center between the two regions. The dashed portion indicates values artificially elevated due to camera saturation.
• Figure 4: Left: wavelength-dependent SHG measurements. Right: the overlap with the ideal HG01 mode as a function of the wavelength. The best mode quality is observed for pump wavelengths between 1100–-1200 nm, consistent with the expected phase difference across the metasurface.