Plasmonic Population Redistribution Expands the Super‐Linear Response Range of Single Upconversion Nanoparticles

ABSTRACT

The super‐linear optical response of upconversion nanoparticles (UCNPs) offers unique opportunities for biophotonic imaging and sensing. However, the usable nonlinear excitation range is limited by the saturation threshold. A narrow nonlinear dynamic range causes the super‐linear response to collapse at relatively modest excitation powers, thereby limiting the accessible nonlinear operating regime and reducing tolerance to excitation‐intensity fluctuations. Since this threshold is primarily determined by dopant concentration, expanding the nonlinear range generally requires compositional redesign. Here, we demonstrate a plasmonic modulation strategy that shifts the saturation threshold of UCNPs to higher excitation powers, thereby extending their super‐linear response range. Silver nanohole arrays are engineered to support surface plasmon polariton (SPP) resonances that selectively couple to higher‐energy transitions, accelerating upper‐level relaxation and redistributing intermediate‐state populations to delay saturation of the 800 nm emission. The saturation threshold is increased from 0.24 to 1.30 MW cm ^{− 2} , corresponding to a more than fivefold expansion of the nonlinear dynamic range. This work establishes plasmonic population redistribution as an external, composition‐preserving approach for engineering saturation behavior in UCNPs, providing a flexible route toward tunable nonlinear sensing and integrated lanthanide photonic devices.