Super-resolution microscopy aims to visualize subcellular details beyond the photon diffraction limit in non-invasive and real-time ways. Rare-earth-doped upconverting nanoparticles, characterized by high photostability and large anti-Stokes shifts, have shown excellent performance in microscopy probing. So the developments in upconverting super-resolution microscopy based on recent progress in chemical synthesis and microscopy photophysics are summarized next. Jin et al. realized low-power stimulated emission depletion (STED) of highly-doped Tm³⁺ in a single upconverting nanoparticle and obtained an imaging resolution of 28 nm. Zhan et al. reported a multi-color upconverting super-resolution method by depleting the excitation state of upconverting sensitizer Nd³⁺ ions. Zhan et al. also improved the fluorescence emission difference (FED) method by using multi-core/shell upconverting nanoparticles with orthogonal excitation and emission bands. The one-scan microscopy image had a resolution of 54 nm. Schuck et al. demonstrated room-temperature photon-avalanching of a single upconverting nanoparticle pumped by a continuous wave (CW) laser for the first time. The 26th-power nonlinearity of photon-avalanching resulted in a sub-70-nm resolution under single-beam confocal scanning. Zhan et al. introduced an upconverting energy migration process into the photon-avalanching of lanthanide-doped nanoparticles, which improved avalanching nonlinearity and imaging resolution to 46th-power and 62 nm, respectively.

Microlasers are promising reinforcements for integrated optical circuits, quantum physics, and bio-sensing. The major challenge of microlasers comes from energy losses in miniaturized optical cavities, which induce increasing pumping powers to reach the lasing threshold. The progress made with whispering-gallery mode micro-resonators for upconverting microlasers is summarized thirdly. Schuck et al. coupled energy-looping upconversion of Tm³⁺-doped nanoparticles to whispering-gallery modes of polystyrene microspheres. The coupled microsphere resonator realized CW upconverted lasing under excitation power as low as 14 kW·cm^-2. Yan et al. further decreased the pumping threshold power of microsphere upconverting lasers to 4 W·cm^-2. They successfully coupled the same whispering-gallery mode microsphere cavity to Yb³⁺/Tm³⁺, Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺, and Yb³⁺/Tm³⁺ doped nanoparticles for multi-band lasing spanning the full visible spectrum.

Rare earth (RE) doped luminescent materials are essentially important for application and research purposes due to their diverse optical properties. Rare-earth-doped upconverting materials, in particular, have piqued the majority of research interest from physics, chemistry, material engineering, biomedicine, and related fields. Photon upconversion refers to a nonlinear optical mechanism that converts two or more low-energy photons into one high-energy photon. The ladder-like energy levels rising from 4f electron configurations of lanthanide ions could stabilize both intermediate and high excited states for photon upconversion. Thus, the upconverting luminescent efficiency of lanthanide compounds could be several orders of magnitude higher than that of two-photon absorption and second harmonic generation mechanisms. Distinct from common phosphors with Stokes-shift photoluminescence, rare-earth-doped upconverting materials exhibit intrinsic anti-Stokes shifts between emission and excitation bands. It means that upconverting photon signals and background noises have little overlap in luminescent spectra. Taking advantage of sophisticated nanochemistry methods, people have prepared various rare-earth-based upconverting nanoparticles and developed upconverting luminescent probes for biomedical imaging applications. Many systematic review articles have discussed upconverting nanomaterials in aspects of structural design, chemical preparation, surface modification, biomolecular labeling, and imaging settings. The general merits of upconverting nanoparticles and related bio-probes, such as low autofluorescence and large imaging depth, are well known to multidisciplinary researchers. However, pioneer applications of upconverting materials face some new challenges, including single nanoparticle detection, multi-dimensional signal coding, novel micro-device conception, and so on. To meet these challenges, researchers need to thoroughly understand and rationally engineer the photophysics properties of nano- and micro-sized upconverting materials. Herein, we attempt to archive recent upconversion photophysics studies on rare-earth-doped materials below the micron scale. More specifically, this review article focuses on luminescent polarization, super-resolution luminescent imaging, and microcavity coupling of rare-earth-doped upconverting materials. These photophysical achievements could help our readers expand the frontier of photon upconversion applications.

The intrinsic characteristics of photon upconversion include excitation and emission wavelengths, intensity ratios of multi-bands, excited state lifetimes, and polarization. However, luminescent polarization is rarely studied, because it necessitates the use of single particle spectroscopy or collective particles in uniform self-assembly. The research achievements on upconverting polarization properties are summarized firstly. In 2013, Qiu et al. reported the polarized emission behavior of a single NaYF4∶RE nanorod. They demonstrated that the emissive polarization characteristics were determined by crystal local symmetry rather than the aspect ratio of the nanorod. Later, Qiu et al. discovered that the polarized orientation of the photoexcitation beam had a key effect on the upconverting emission intensity of a single particle. These studies established the paradigm of single particle polarized spectroscopy for upconverting materials. Researchers used polarized spectroscopy to identify the spatial orientations of upconverting particles, control their orientations via dipolar interactions, and sense local environment motions. Thus, luminescent polarization should be recognized as an important signal channel to increase the informational dimensions of upconverting materials.

In this review, we discussed the photophysics studies on nano- and micro-sized upconversion materials. These photophysical discoveries in rare-earth-doped upconverting materials, such as crystal field-dependent polarization, photon-avalanching nonlinearity, and microcavity-coupled lasing, lay the groundwork for the development of new applications. However, the photon upconversion of lanthanide phosphors has not been fully unraveled at the nanoscale and beyond. We still lack tools and methods to investigate the upconversion polarization of a single nanocrystal and upconversion lasing in a nanocavity. We hope that more interdisciplinary researchers will join us to advance the related research.