Significant enhancement of the photon upconversion of a single fluorescent microsphere via annular near-field localization.

Fluorescent microspheres exhibit unique emissions at the microscale and have been widely used as probes for immunoassays and advanced micro-sensors. Herein, we report the localized enhancement of the photon upconversion of a single fluorescent microsphere via annular near-field confinement. The single fluorescent microspheres were composed of a 5 μm polystyrene (PS) microsphere coated with upconversion nanoparticles. Ten gold microspheres with a diameter of 5 μm were used for the construction of a "pagoda"-like microcavity. Interestingly, the emission of a single fluorescent microsphere on the "pagoda"-like microcavity was enhanced by tens of times (e.g., 540 nm and 650 nm emissions of Er³⁺ were increased by 46 times and 40 times, respectively). Theoretical calculations based on the FDTD method confirmed that the confined annular near field overlapped well with the upconversion nanoparticles distributed on the surface of the single fluorescent microsphere so that the emission of the single fluorescent microsphere was significantly enhanced. [ABSTRACT FROM AUTHOR]