Nonlinear virtual lens for programmable and multispectral infrared upconversion imaging

Conventional infrared (IR) imaging techniques depend on IR cameras based on narrow-bandgap semiconductors, which offer limited spectral bandwidth. Recently, advances in nonlinear flat optics have opened a pathway for converting IR signals into the visible through nonlinear generations, enabling the direct visualization of IR images using standard visible cameras. However, the narrow spectral bandwidth remains a key challenge. We address both issues via an adaptive and multifunctional IR-to-visible imaging platform offering tunable bandwidth and focusing simultaneously. We utilize sum-frequency generation (SFG) to convert IR light into the visible by introducing a pump beam modulated by a spatial light modulator (SLM) to construct a virtual metalens enabling precisely controlled focusing of the generated nonlinear optical field. As a result, we demonstrate both theoretically and experimentally an optical focusing mechanism with a tunable focal length, achieved by varying the pump and signal wavelengths and modulating the phase distribution. Furthermore, because the focal length depends on the input signal wavelength, the imaging plane position varies accordingly, indicating a promising potential for the multispectral IR imaging applications. Our upconversion platform delivers SLM-controlled, programmable multispectral focusing for next-generation IR imaging, opening new avenues in the fields of computational and multispectral imaging techniques.