High-efficiency infrared upconversion imaging with nonlinear silicon metasurfaces empowered by quasi-bound states in the continuum

Infrared imaging is indispensable for its ability to penetrate obscurants and visualize thermal signatures, yet its practical use is hindered by the intrinsic limitations of conventional detectors. Nonlinear upconversion, which converts infrared light into the visible band, offers a promising pathway to address these challenges. Here, we demonstrate high-efficiency infrared upconversion imaging using nonlinear silicon metasurfaces. By strategically breaking in-plane symmetry, the metasurface supports a high-$Q$ quasi-bound states in the continuum resonance, leading to strongly enhanced third-harmonic generation (THG) with a conversion efficiency of $3\times10^{-5}$ at a pump intensity of 10 GW/cm$^{2}$. Through this THG process, the metasurface enables high-fidelity upconversion of arbitrary infrared images into the visible range, achieving a spatial resolution of $\sim 6$ $\upmu$m as verified using a resolution target and various customized patterns. This work establishes a robust platform for efficient nonlinear conversion and imaging, highlighting the potential of CMOS-compatible silicon metasurfaces for high-performance infrared sensing applications with reduced system complexity.

💡 Research Summary

Infrared (IR) imaging is limited by the low sensitivity, high cost, and often cryogenic cooling required for conventional detectors. Nonlinear up‑conversion, which translates IR light into the visible band where silicon cameras are highly efficient, offers a promising solution. In this work, the authors demonstrate a high‑efficiency IR up‑conversion imaging platform based on a silicon metasurface that exploits a quasi‑bound state in the continuum (quasi‑BIC) resonance.

The metasurface unit cell consists of a pair of silicon nano‑disks (one circular, one elliptical) with a height of 220 nm, arranged on a glass substrate with periods Px = 1000 nm and Py = 500 nm. By varying only the minor axis r of the elliptical disk along the x‑direction, the structure introduces a controlled in‑plane asymmetry. This single‑direction symmetry breaking converts a symmetry‑protected BIC (which is completely decoupled from free‑space radiation) into a leaky quasi‑BIC with a high quality factor (Q). Numerical eigenmode analysis shows that when r = R = 180 nm the mode is a true BIC with even magnetic‑field symmetry; decreasing r opens a radiative channel, broadening the resonance and shifting its wavelength. Compared with designs that break symmetry in both directions, the one‑direction approach suppresses radiative loss more effectively, preserving higher Q values.

Fabrication was carried out on a silicon‑on‑insulator (SOI) wafer using electron‑beam lithography and inductively coupled plasma etching, yielding 800 × 800 µm² metasurfaces with r ranging from 120 nm to 180 nm in 10 nm steps. Scanning electron microscopy confirms uniformity and low roughness. Linear reflectance measurements under y‑polarized normal incidence reveal Fano‑shaped resonances; fitting extracts Q‑factors up to ~4000 for r = 170 nm and still ~500 for the most asymmetric case (r = 120 nm), in excellent agreement with simulations.

Third‑harmonic generation (THG) was measured in reflection using a femtosecond optical parametric amplifier (pump wavelength tunable from 1470 nm to 1550 nm, 200 fs pulses, 200 kHz repetition). The normalized conversion efficiency ζ = P_THG/(P_pump)³ reaches 3 × 10⁻⁵ at a pump intensity of 10 GW·cm⁻² for the r = 170 nm sample, corresponding to an absolute THG efficiency of the same order. This represents a >650‑fold enhancement over an unpatterned silicon film of equal thickness, confirming the strong field confinement provided by the quasi‑BIC. While ζ increases as r approaches R (higher Q), the growth slows because the resonance linewidth becomes narrower than the pump spectral bandwidth, limiting coupling efficiency—a practical constraint that highlights the need for laser bandwidth matching.

To demonstrate imaging, an IR mask (resolution target and custom patterns) was projected onto the metasurface at the quasi‑BIC resonance wavelength. The metasurface converted the spatially varying IR illumination into visible THG light, which was directly captured by a conventional camera. The up‑converted images exhibit a spatial resolution of ~6 µm, preserving fine details and contrast without any pixel‑wise phase or amplitude encoding on the metasurface. This pixel‑by‑pixel conversion simplifies system design, reduces fabrication tolerances, and enables parallel processing of the entire image.

In summary, the study provides a CMOS‑compatible silicon metasurface that leverages a single‑direction symmetry‑broken quasi‑BIC to achieve high‑Q resonances, strong third‑order nonlinearity, and efficient infrared‑to‑visible up‑conversion imaging. The demonstrated conversion efficiency (3 × 10⁻⁵) and micron‑scale resolution represent a significant advance over previous metasurface‑based up‑conversion schemes, which typically reported efficiencies ≤10⁻⁶. The work opens pathways toward compact, low‑cost IR sensing systems for night vision, thermal inspection, biomedical diagnostics, and remote sensing, and suggests future directions such as dynamic Q‑factor tuning, multi‑wavelength up‑conversion, and integration with real‑time image processing electronics.