Broadband Upconversion Fluorescence Imaging in Mixed-Halide Perovskite Nanoplates through Direct Femtosecond Laser Writing

Efficient single-photon upconversion is observed in halide perovskites without rare metal doping. Using advanced femtosecond direct laser writing (fs-DLW), an upconversion fluorescence image is obtained spanning a broadband (green-to-red) wavelength region in mixed-halide perovskite FAPb(Br_xI_1–x)₃ nanoplatelets. After fs-DLW fabrication, a I–Br mixture component is formed within the mixed-halide perovskite nanoplatelets. The corresponding diversified band gap culminates in a broadband photoluminescence emission. Confocal microscopy images show that the upconversion signal is spatially distributed on the nanoplates. In addition, it is confirmed that the upconversion signal has a low excitation threshold. Efficient single-photon upconversion is observed, which we hypothesize is due to effective upconversion through a “lattice energy reservoir”. These findings are highly beneficial for further highly sensitive near-infrared photodetection and solar energy harvesting.