Special optical performance from single upconverting micro/nanoparticles

Rare-earth ion-doped upconversion (UC) micro/nanoparticles (UCM/NPs) could enable the conversion of near-infrared photons to high-energy emissions in the visible and ultraviolet regions. This unique property makes UCM/NPs highly attractive for applications in diverse fields, including materials science, photophysics, and biomedicine. However, ensemble spectroscopy ignores the heterogeneity in terms of size, shape, defects, surface groups, and charges of single UCM/NPs. Recently, the rapid development of single-particle spectroscopic techniques results in a gradual shift in the investigation of UC materials to heterogeneous nanoscale structures, unique photophysical phenomena, and advanced applications of single UCM/NPs, and the optical performance of a single UCM/NP has become a central topic in the community of UC materials. In this Perspective paper, we outline the characterization methods of a single UCM/NP and provide an overview of recent and on-going progress in investigations on single UCM/NPs, with a focus on their spectroscopic properties and applications in the polarization, waveguide, micro/nano-laser, super-resolution nanoscopy, and nanobarcode. Finally, current challenges and perspectives in this field are highlighted for future research.