The Mid-Infrared Challenge

The mid-infrared (MIR) spectral region (2.5–25 μm), often referred to as the “molecular fingerprint region”, reveals unique vibrational features of molecules and is thus invaluable for applications in medicine, food safety, environmental sensing, and materials analysis. However, conventional MIR cameras based on narrow-bandgap materials such as HgCdTe and InSb typically suffer from high noise at room temperature and require cryogenic cooling. Silicon detectors, in contrast, perform excellently at room temperature but cannot directly detect MIR signals.

To address this challenge, researchers have exploited the principle of non-degenerate two-photon absorption (ND-2PA): an MIR photon and a near-infrared (NIR) pump photon jointly excite carriers across the silicon bandgap, enabling indirect MIR detection. This process operates without stringent phase-matching and offers broadband response at room temperature. Nonetheless, it has remained difficult to combine wide field of view, high sensitivity, and fast acquisition—key requirements for practical MIR imaging.

Two-Photon Computational Imaging

The research team led by Prof. Heping Zeng and Prof. Kun Huang in East China Normal University (ECNU) have introduced ND-2PA into a single-pixel computational imaging framework, creating an integrated encoding–detection system (Fig. 1). In this scheme, spatially structured NIR pump light acts as an all-optical mask for the MIR signal. When the pump and signal photons jointly interact within a silicon detector, their energies exceed the bandgap, producing a measurable photocurrent (Fig. 2). Each structured pump pattern leaves a distinct “projection” in the detector response, which is later used for computational reconstruction.

By leveraging mature NIR modulation techniques, the system bypasses the diffraction limits of long-wavelength modulation and achieves ~7 μm spatial encoding precision, corresponding to 11 μm imaging resolution. These results experimentally confirm the feasibility and performance advantages of the approach.

Chemical Selectivity and Applications

Beyond principle validation, the team demonstrated the potential of this approach for multispectral and chemical-selective imaging. In experiments with thin films of polystyrene (PS) and polyvinyl chloride (PVC), the system successfully distinguished different absorption fingerprints across the 2.5–3.8 μm band (Fig. 3), underscoring its promise for chemical recognition and molecular sensing.

Looking forward, the technique could be combined with high-numerical-aperture optics and large-area detectors to achieve megapixel-level resolution for high-definition imaging. Optimizing pump wavelengths and detector materials may further extend the operational band, enhancing molecular fingerprint identification. Meanwhile, the synchronous pulsed gating could provide depth resolution, paving the way for high-resolution MIR 3D imaging.

For more details, please read our recent publication: Huijie Ma, Kun Huang*, Jianan Fang, Ziyu He, Yan Liang, and Heping Zeng*, “Mid-infrared single-pixel imaging via two-photon optical encoding”, Photonix 6, 34 (2025).