Xi'an Institute of Optics and Precision Mechanics Achieves Metasurface Mid-Infrared Upconversion Imaging

Recently, a team led by researcher Liu Hongjun from the Transient Optics Laboratory at the Xi'an Institute of Optics and Precision Mechanics (XIOPM), Chinese Academy of Sciences, made significant progress in metasurface-based nonlinear frequency upconversion imaging in the mid-infrared. The findings were published in Laser & Photonics Reviews. The first author is Zhang Congfu, a doctoral student at XIOPM, and the corresponding author is Liu Hongjun of XIOPM.

The mid-infrared band (3–5 μm) is an important atmospheric transmission window and plays a key role in fields such as biomedicine and environmental monitoring. Nonlinear frequency upconversion converts mid-infrared signal light, which is difficult to detect directly with high sensitivity, into the near-infrared or visible range, enabling high-performance detection with compact, high-quantum-efficiency silicon-based detectors. It is one of the core technology routes in mid-infrared detection today. Compared with conventional nonlinear optical devices, nonlinear metasurfaces can dramatically enhance light-matter interaction at subwavelength scales, overcoming the constraints of strict phase-matching conditions. In recent years, although high-quality-factor nonlocal metasurfaces have been used to enhance nonlinear effects, achieving efficient continuous-wave frequency upconversion under low-threshold, weak-light conditions while retaining flexible spectral control remains difficult, directly limiting the practical deployment of high-performance metasurface mid-infrared detection technology.

To address these issues, the team used thin-film lithium niobate as a nonlinear optical platform and proposed a nonlinear frequency upconversion method based on quasi-guided mode (QGM) control (Figure 1). The approach introduces quasi-guided-mode resonances through Brillouin zone folding, building stable high-quality-factor resonant modes over a wide wavevector range. It further exploits strong coupling between quasi-guided modes and guided-mode resonance (GMR), extending the mode-matching range of a single GMR system so that different resonant modes can all effectively enhance the nonlinear effect. Combined with resonance control over a wide wavevector range, the method significantly broadens the tuning range of frequency conversion, improving conversion efficiency and spectral tunability simultaneously.

Figure 1 Schematic of metasurface frequency upconversion (a) metasurface structure and QGM principle; (b) linear and nonlinear spectral characteristics of the metasurface

Based on this method, the team built a thin-film lithium niobate mid-infrared frequency upconversion imaging system. In experiments, they successfully demonstrated sum-frequency upconversion imaging of a low-intensity continuous-wave 4.33 μm mid-infrared signal (Figure 2). By tuning the near-infrared pump wavelength, the team verified pump-side resonance control and its influence on the nonlinear conversion process. Together with dispersion analysis, this further demonstrated the application potential of broadband tunable mid-infrared frequency conversion imaging.

Figure 2 Metasurface frequency upconversion imaging results

Zhang Congfu explained: "The nonlinear metasurface is only one-hundredth the thickness of a human hair, yet tens of thousands of nanostructures are integrated into an area the size of a sesame seed. Using the quasi-bound-state-in-the-resonance effect, we first let the structure receive only light of a specific wavelength, much like tuning a radio station, continuously collecting weak light energy; then, like a signal amplification tower, we concentrate that energy into an extremely thin film to amplify the effect of light. As a result, frequency conversion that once required high-power intense lasers can now be achieved with ordinary weak light. This solves a major challenge for miniaturized devices such as mid-infrared weak-light detection and non-invasive bioimaging."

This study extends the tunability of nonlocal metasurfaces to continuous-wave nonlinear frequency conversion. It not only offers a new technical route for miniaturized, low-cost, highly sensitive mid-infrared detection technology, but also, thanks to the technology's potential for on-chip integration, shows broad application prospects in gas detection, environmental monitoring, biomedical imaging, and integrated photonic sensing.

In recent years, Researcher Liu Hongjun's team has conducted systematic studies on nonlinear metasurface light-field control and enhancement mechanisms, with related results published in journals such as Laser & Photonics Reviews and Nanoscale Horizons.

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