Visible‐Readout Upconversion Nanotransducers for Mid‐Infrared Photodetection

Mid‐infrared (MIR) photodetection under ambient conditions is crucial for applications ranging from molecular fingerprinting to imaging and free‐space communications, yet conventional MIR photodetectors based on narrow‐bandgap semiconductors typically require cooling and relatively complex system integration. Lanthanide nanotransducers provide an attractive alternative by converting MIR perturbations into visible/near‐infrared readouts that are compatible with silicon photodetectors. However, many reported systems rely on near‐infrared readout and relatively high pump power. Here, we report Nd 3+ /Yb 3+ /Er 3+ triply doped NaYF 4 upconversion nanoparticles (UCNPs) with a precisely controlled Nd 40 Yb 15 @Yb 30 Er 2 @Y core–shell architecture and oleic‐acid surface ligands. Under 808 nm excitation, incident 8 µm radiation induces a photothermal response that modulates interfacial energy‐transfer dynamics, producing a ratiometric change in visible emission at pump powers down to 5 mW. Integrated with silicon photodetectors, the system achieves a specific detectivity of 3 × 10 8 Jones at room temperature. This design presents a materials‐based strategy for MIR sensing, imaging, and spectroscopy without cryogenic operation.