Infrared Polarization Imaging via Parametric Upconversion

ABSTRACT Polarization of infrared light provides a non‐destructive probe of internal anisotropy and stress fields in optically opaque materials, offering information inaccessible to conventional intensity‐based imaging. However, measurement of complete infrared polarization information remains challenging, owing to the intrinsic limitations of infrared detectors. Here, we introduce a non‐phase‐matched parametric upconversion scheme that preserves and retrieves the full Stokes vector of infrared light. This is realized in a thin‐film lithium niobate, where polarization‐dependent sum‐frequency generation (SFG) with negligible longitudinal coherent buildup, thereby preserving faithful polarization transfer from the infrared signal to the upconverted field. By converting infrared signals into the visible band, this approach enables high‐fidelity, full‐Stokes polarization imaging with mature visible‐light detectors. We further demonstrate quantitative mapping of subsurface stress in opaque polymers and silicon wafers, revealing heterogeneities that evade conventional infrared imaging. Our framework offers a robust, broadly applicable route to polarization‐resolved infrared characterization, opening new opportunities for materials diagnostics and process monitoring.