Mid-Infrared Single-Photon Edge Enhanced Imaging Based on Nonlinear Vortex Filtering

Edge enhanced imaging via the spiral phase contrast enables to reveal the phase or amplitude gradients of a target, which has been proved useful in feature recognition, machine vision, and object identification. A long quest is to extend the operation wavelength into the mid-infrared (MIR) region, as highly demanded in various fields including infrared sensing, astronomic observation, and biomedical diagnosis. Here, ultra-sensitive MIR imaging at the single-photon level based on nonlinear frequency upconversion is demonstrated, where the spectrally converted replica of the MIR object image at 3070 nm is captured by a silicon electron multiplying charged coupled device. The imaging sensitivity is significantly improved by the coincidence pulsed pumping with a spectro-temporal optimization. Furthermore, the edge enhancement is realized by imprinting the spiral phase pattern of the pump onto the upconverted field at the Fourier plane within the nonlinear crystal. Such a nonlinear spatial filter not only provides an effective way to implement the required high-fidelity vortex screening in the edge enhanced detection, but also renders the MIR illumination into a visible image in an efficient and low-noise fashion. The presented system for MIR edge enhanced imaging might facilitate immediate applications in label-free histopathological diagnosis and non-destructive defect inspection.