Photon-Avalanching Holmium Nanoparticles for Multicolor Super-Resolution Imaging
Photon avalanche (PA) is a nonlinear optical phenomenon characterized by steep upconversion emission growth with excitation power. Since the first demonstration of PA on nanoscale and at room temperature in 2021, PA luminescence of lanthanides has attracted considerable attention in nano- and bio-photonics. However, PA nanoparticles (NPs) remain restricted to a limited range of material systems and lanthanide ions, facing challenges including inadequate nonlinearity (N), high excitation threshold (P_th), and deficient chromaticity. In a recent publication in Nature Photonics, Dong and co-workers reported a new class of PA nanosystem based on Ho3+-doped fluoride NPs, which afforded tunable PA chromaticity for multicolor sub-diffraction imaging. Unlike conventional PA systems reliant on a single reservoir level, this study introduced a novel 'parallel PA' (PPA) mechanism leveraging the dual long-lived intermediate reservoir levels (5I7 and 5I6) of Ho3+, facilitating simultaneous operation of two PA loops to generate multicolor emissions. Under 965 nm continuous-wave excitation, the PPA of Ho3+ produced simultaneous red-green-blue emissions with large N (>20). Differential rate equation modelling identified all PA signatures, including clear P_th, S-shaped curves, extremely high sigma_ESA/sigma_GSA ratio of ~75000, and volcano-shaped rise time. The engineered NaGdF4:10%Ho@NaYF4 core/shell PA NPs (~19.6 nm) exhibited remarkably high N of 17–22 with mild P_th of ~22 kW cm−2 and fast response time of 343–371 ms. By screening host lattices and introducing co-dopants, PA chromaticity was precisely tailored, demonstrating unparalleled emission tunability for multicolor super-resolution imaging.