Mid‐ to Long‐Wave‐Infrared Nanoscopy With Lanthanide Transducers

ABSTRACT Mid‐ to long‐wave infrared (MIR–LWIR) microscopy provides a non‐invasive and label‐free tool to acquire rich spectroscopic and structural information about chemical materials and biomedical samples. However, the lateral resolution is typically limited by severe optical diffraction at long infrared wavelengths, which hinders imaging systems from observing intricate details beyond the diffraction limit. Here, we report a MIR–LWIR to near infrared (NIR) transducer based on a rare‐earth‐doped crystal, which enables room‐temperature MIR–LWIR imaging within a broad spectral coverage of 7–10.6 . The underlying mechanism relies on monitoring fluorescence intensity changes under infrared illumination, thus favoring far‐field upconversion operation without suffering from the stringent requirements of polarization control, phase matching, or nanocavity design, commonly encountered in previous upconversion imaging platforms. Moreover, the presented lanthanide‐based transducer is compatible with close positioning to nano‐/micro‐structures, facilitating near‐field MIR–LWIR imaging with an improved spatial resolution from 50 to sub‐. Notably, hidden objects can be accurately identified with high axial precision owing to the confocal excitation configuration, which enables high‐resolution MIR–LWIR depth imaging. In addition, experimental validation using 2D materials such as hexagonal boron nitride reveals distinct MIR–LWIR response characteristics, demonstrating the system's capability for high‐resolution imaging and spectroscopic characterization across extended infrared wavelengths.