• • The engineered NaGdF4:10%Ho@NaYF4 core/shell PA NPs (~19.6 nm) exhibit a high nonlinearity (N) of 17–22 and a mild excitation threshold (P_th) of ~22 kW cm−2, enabling super-resolution imaging with reduced phototoxicity and improved signal-to-noise ratio.
• • The parallel PA mechanism leverages dual reservoir levels (5I7 and 5I6) of Ho3+, achieving simultaneous red-green-blue emissions with a large N > 20, which is critical for multicolor sub-diffraction imaging and multiplexed detection.
• • The extremely high sigma_ESA/sigma_GSA ratio of ~75000 (exceeding the PA criterion of 10000) ensures efficient excited-state absorption and low ground-state absorption, enabling low-threshold PA at the nanoscale.
• • The fast response time of 343–371 ms and tunable chromaticity via host lattice screening and co-doping (Ce3+, Tm3+) allow dynamic multicolor imaging and potential applications in optical data storage and super-resolution microscopy.