Luminescence Characteristics of Rare-Earth-Doped Microsphere Cavities

Rare-earth-doped microsphere cavities have attracted significant interest for applications in miniaturized photonic devices due to their unique optical properties. In this work, Yb³⁺/Er³⁺ co-doped microsphere cavities were fabricated via a melting method, which enables uniform interior doping at high and tunable rare-earth concentrations through a simpler and more cost-effective process compared with existing coating and fiber-etching approaches. Whispering gallery modes (WGMs) enhanced upconversion luminescence, which was observed using tapered fiber coupling, producing a vivid green fluorescence ring near the equatorial region of the microsphere. The luminescence characteristics of the microsphere cavity were investigated by measuring the fluorescence spectra under varying excitation powers. The results indicated that the fluorescence emission follows a two-photon absorption process, consistent with the upconversion emission mechanism of Er³⁺. A finite difference time domain (FDTD) model was employed to simulate the optical field distribution within the microsphere cavity. At a microsphere diameter of 90 μm and a coupling gap of 0 μm, both the 980 nm pump light and the emitted light were effectively confined near the equatorial region of the microsphere, forming WGM confinement patterns. These findings are expected to advance the application of rare-earth-doped microsphere cavities in fields such as biosensing, bioimaging, optical communications, and upconversion microlasers.