Metasurface thermal emitters enabled chip scale mid infrared spectroscopic sensing: An 'instant-camera-like' miniature mid-infrared spectral sensing platform

Mid-infrared spectroscopy, with its unique molecular fingerprint recognition capability, serves as a core means for precise "optical decoding" of chemical substances, playing an irreplaceable role in environmental pollutant monitoring, biomedical diagnosis, and industrial chemical analysis. However, traditional mid-infrared spectrometers suffer from large size, complex systems, operational difficulties, and high cost, making them unable to meet the technical demands for portable applications in fields such as intelligent sensing and the Internet of Things. Improving the integration of optical components, detection units, and light source systems is key to overcoming this challenge.

In recent years, micro-spectrometer technologies based on micro-nano optics and advanced materials have developed rapidly. Compact spectral sensing and spectroscopy platforms can be achieved through integration of detector arrays with filter arrays, a single detector with a tunable filter, or direct electrical tuning of the spectral response of a photodetector. However, these technologies generally rely on external light sources, and due to the angular sensitivity of various micro-nano optical filtering technologies, they require extra collimation unit. Consequently, the size of the entire spectral detection system is limited by the light source and the collimation unit, while assembly and operational difficulties increase. Moreover, most current research focuses on the visible and near-infrared bands, leaving challenges in developing miniaturized, high-performance mid-infrared spectral sensing and analysis platforms.

In response to this situation, the team of Prof. Qin Chen from Jinan University published breakthrough research in PhotoniX, reporting a chip-scale mid-infrared spectral sensing technology based on metasurface thermal emitter array. Unlike previous on-chip integration strategies based on dispersive elements or photoelectric response regulation in micro-spectrometer technologies, this work adopts a light source-side regulation strategy for the first time, i.e., using a metasurface array as a wavelength-selective thermal emission light source. By encoding substance-specific wavelength-selective absorption information into the intensity distribution of the metasurface thermal emitter array at the imaging end, and processing the thermal image similar to a two-dimensional code, a one-to-one correspondence between substance information and images is established, thereby realizing the "instant camera" substance sensing and analysis function. Benefiting from the angle-insensitive metasurface design, the system's imaging angular tolerance exceeds ±40°, greatly improving the applicability of the "instant-camera-like" working mode. The research team conducted in-depth optimization of metasurface thermal emitters, comprehensively evaluating optical efficiency, resonance linewidth, angular sensitivity, and processability. Ultimately, a metal-dielectric-metal microstructure array device architecture was adopted, achieving high emissivity, narrow linewidth, and angle-insensitive thermal emission characteristics, which ensures high signal-to-noise ratio, resolution, and stability in spectral sensing applications. By adjusting the unit size of the metal microstructure array, the resonance wavelength can be tuned, enabling multi-channel spectral sampling across the 8-14 μm mid-infrared band required for material analysis. In experiments, the thermal emitters were fabricated on silicon wafers using standard microfabrication processes. The thermal emitter temperature was controlled using a semiconductor heating plate, and a low-cost thermal camera imaged the spatial radiation intensity distribution through the measured substance. Image data can be acquired on a mobile phone, with substance information decoded using algorithms such as quadratic discriminant analysis. Using this method, the team fabricated two types of metasurface thermal emitters (single-mode and dual-mode resonance) and conducted experiments including organic solvent classification, drug identification, and mixed organic solvent concentration measurement, all demonstrating exceptional accuracy. Particularly due to the angle-insensitive property, no optical collimation unit was needed, and high-precision substance analysis was maintained even at 40-degree imaging angles. The work also demonstrated scanning spectral imaging capability for polyethylene-covered steel rings. This operational scheme is simple, highly integrated, and low-cost, exhibiting excellent mid-infrared substance analysis capability with strong applicability for portable detection, providing a new approach to solving the miniaturization challenge of mid-infrared spectral detection platforms. 

This research demonstrates three primary innovations constituting significant technical breakthroughs. First, the team addressed spectral sensing integration challenges through a novel light source-side regulation strategy. By constructing a metasurface thermal emitter array as an integrated tunable-emission light source, it achieves for the first time monolithic integration of three traditional functions—light source, collimating optical path, and dispersive element—providing new solutions for mid-infrared spectral platform miniaturization. Second, the team established an innovative spectral sensing mechanism based on thermal imaging of metasurface emitters. By recording radiation distribution changes induced by substance wavelength-selective absorption, it realizes an "instant camera" spectral sensing mode, enabling portable spectral detection. Third, the developed metal-dielectric-metal metasurface thermal emitters exhibit high emissivity, narrow linewidth, and angle-insensitive characteristics. This enables high-precision qualitative/quantitative multi-type spectral sensing while maintaining detection accuracy within ±40° imaging angles, demonstrating exceptional system robustness and applicability. These innovations form a major advancement in portable mid-infrared spectral sensing technology, extending micro-spectral detection applications to longer wavelengths while achieving full functional integration in a miniaturized platform with excellent practicality.