The Research Team from the School of Information Engineering at Nanchang University has achieved important research progress in the field of nonlinear optical imaging. The team proposed a silicon nonlinear metasurface design based on quasi-bound states in the continuum (quasi-BICs), realizing high-efficiency infrared upconversion imaging. The related results were published online under the title "High-efficiency infrared upconversion imaging with nonlinear silicon metasurfaces empowered by quasi-bound states in the continuum" in Opto-Electronic Advances (Impact Factor: 22.4), a top-tier journal in the field of optics.

      Utilizing thermal signatures or the ability to penetrate specific atmospheric windows, infrared (IR) imaging plays a vital role in fields such as night vision, industrial inspection, biomedical diagnostics, and remote sensing. However, limited by narrow-bandgap semiconductor materials, conventional IR detectors face two major bottlenecks: first, they often require deep cryogenic cooling to suppress severe thermal noise, which significantly increases volume, power consumption, and cost; second, their sensitivity and response speed are generally inferior to mature silicon-based visible light detectors. To address this issue, nonlinear frequency upconversion technology provides an effective solution. Its core physical process involves converting incident infrared photons into the visible spectrum through nonlinear optical effects. In this way, infrared target information can be directly captured by highly sensitive, low-cost silicon-based CMOS or CCD cameras operating at room temperature. Early research primarily relied on traditional bulk nonlinear crystals, but their stringent phase-matching conditions limited the operating bandwidth and acceptance angle, while making the systems bulky and difficult to integrate. In recent years, with the development of nanofabrication technologies, nonlinear optical manipulation based on metasurfaces has become a research focus in this field. Composed of subwavelength nanostructure arrays, metasurfaces can achieve highly contrasted local electromagnetic field enhancement within an ultrathin physical thickness, breaking the phase-matching limitations inherent in traditional bulk materials. However, the nonlinear frequency conversion efficiency of metasurfaces has long struggled to meet the requirements of practical applications. How to further enhance light-matter interactions at the nanoscale and utilize high-quality-factor (Q-factor) resonances to boost nonlinear conversion efficiency remains a key scientific question for advancing IR imaging technology toward miniaturization and high performance.

Figure 1. Infrared upconversion imaging realized by quasi-BIC metasurfaces.

    To tackle this challenge, the research team designed a dimer unit cell consisting of a silicon circular nanodisk and an elliptical nanodisk (Figure 1(a)). Unlike conventional designs that introduce perturbations in multiple directions simultaneously, this work precisely controls the degree of in-plane symmetry breaking of the elliptical nanodisk along only one direction (the x-axis), transforming genuine BICs—which are normally decoupled from radiation modes in the continuum—into quasi-BIC resonances with a finite lifetime. This unidirectional perturbation strategy more effectively suppresses radiative loss. Experimentally, the metasurface exhibits high-Q resonant characteristics in the near-infrared band, with a measured Q-factor reaching up to 4000. To verify the application potential of this technology in IR imaging, the research team built a nonlinear upconversion imaging system and demonstrated it with various customized target patterns (Figures 1(b) and 1(c)). Using a Siemens star resolution target as the test object, the results show that the upconverted visible image can clearly resolve the fine stripes at the center of the target, achieving a spatial resolution of approximately 6 μm. Furthermore, the platform demonstrated excellent upconversion fidelity for complex character patterns (such as "NCU"), producing images with clear edges and a high signal-to-noise ratio.

     The nonlinear metasurfaces demonstrated in this research are fully CMOS-compatible and based on a mature semiconductor fabrication platform, possessing the potential for large-scale mass production and integration. Compared to traditional imaging schemes based on the sum-frequency generation (SFG) process, this approach requires only a single pump beam to achieve upconversion, significantly reducing system complexity. This achievement not only provides a new physical platform for studying light-matter interactions under strong-field conditions but also offers crucial technical support for the future development of miniaturized, high-performance infrared sensors and all-optical information processing devices operating at room temperature.

     Professor Tingting Liu is the first author, while Professor Qiegen Liu and Researcher Fellow Shuyuan Xiao are the co-corresponding authors. The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangxi Province, and the Young Elite Scientists Sponsorship Program by JXAST.

Paper Link: https://doi.org/10.29026/oea.2026.250257