Upconversion edge imaging using single-beam second-harmonic generation

Near-infrared (NIR) imaging is essential for biological and materials sensing, yet high-performance NIR detectors are often cost-prohibitive and plagued by high dark current. Nonlinear frequency upconversion offers a compelling solution by shifting NIR signals into the visible spectrum, allowing the use of mature silicon-based detectors. However, single-beam nonlinear imaging is often hampered by self-convolution effects in the spatial frequency domain (Fourier plane), leading to the loss of fundamental structural information. To circumvent this image degradation, upconversion edge imaging has traditionally required complex dual-beam configurations. Here, we demonstrate a single-beam nonlinear upconversion edge imaging scheme utilizing multilayer GaSe. By sequentially combining linear spatial filtering at the Fourier plane with frequency upconversion in real space, we successfully transform the NIR illumination into visible edge images. This approach achieves high-fidelity and high-resolution edge detection without the need for reference beams or stringent spatiotemporal alignment. Given that two-dimensional crystals bypass traditional phase-matching constraints, this compact and stable architecture provides a promising pathway for broadband, real-time infrared image processing and miniaturized optical computing.