Upconversion (UC) laser has the characteristics of anti-Stokes displacement, monochromatism, and high stability. Hence, rare earth (RE) ion-doped upconversion micro/nano laser has shown potential applications in various fields, such as biomedicine, holographic projection, visible light communication, data storage, and new-generation display technologies, resulting in extensive attention in recent years. The optical feedback from photon scattering of the porous upconversion nanoparticles clusters has been reported to produce upconversion random lasers. Light bouncing back and forth between two reflective surfaces or internal surfaces has been utilized to achieve modulated upconversion lasing emission. In addition, plasmonic cavities with enhanced electromagnetic fields can amplify the upconversion process within the sub-diffraction-limiated volumes and produce highly efficient upconversion lasers. In this review, the recent advances in RE ions-doped upconversion materials for random, Fabry?Perot (F?P)/whispering gallery mode (WGM) cavity-, and plasmonic cavity-modulated upconversion micro/nano lasers are overviewed. The main factors affecting the output of upconversion micro/nano lasers are summarized. Current challenges and future directions of the upconversion micro/nano lasers are also discussed.

First, upconversion nanoparticles (UCNPs) can be employed as cavities for feedback and resonance based on their own scattering effect to produce random lasing emission. For example, a type of high-temperature-operated compact self-cooling laser has been demonstrated using Ba₂LaF₇∶Yb³⁺, Er³⁺ nanocrystals (NCs)-embedded glass-ceramics. Additionally, by using core-shell UCNPs as the gain medium, a highly efficient single-segment white random laser and tunable random lasing emission from 309 to 363 nm have also been achieved.

Unlike random lasers induced by the scattering effect, F?P cavities and whispering gallery modes (WGMs) are commonly employed as micro-resonator geometries for upconversion lasing emission owing to their high quality factors. F?P microcavities, constructed with two or more parallel mirrors, allow light to bounce back and forth between the reflective surfaces. Resonance occurs when the optical path length equals an integer multiple of the light wavelength. Based on this principle, upconversion lasing emission has been achieved by designing an F?P cavity—consisting of a quartz tube sandwiched between a distributed Bragg reflector and an Al mirror—with a NaYF₄∶Yb³⁺, Er³⁺@NaYF? core-shell NC solution as the gain medium. However, the relatively long cavity length makes it challenging to obtain a finely structured lasing spectrum.

WGM microcavities can confine light in a narrow ring along the equatorial surface of the cavity via total internal reflection. WGM cavities, which have various shapes such as bottles, spheres, toroids, and rings, have been further explored and demonstrated as one of the optimal cavities for microlasers with low thresholds and narrow linewidths, owing to their high quality factors and small mode volumes. A bottle-like WGM microcavity enables upconverted blue, green, red, and deep ultraviolet lasing emission by coating a drop of silica resin containing UCNPs onto an optical fiber. To reduce the size of upconversion microlasers, a single hexagonal NaYF₄∶Yb³⁺, Tm³⁺, Er³⁺ microrod was utilized to achieve multicolor upconversion lasing emission, supported by total internal reflection between its six flat surfaces. After depositing efficient energy-looping NCs onto the surface of a polystyrene microsphere, low-threshold upconversion lasing emission pumped by continuous wave was achieved. Furthermore, WGM microcavities fabricated from RE ion-doped glass ceramics exhibited upconversion lasing emission with extremely low thresholds, reaching the microwatt level.

One strategy to lower the threshold of lanthanide upconverting lasers is to enhance the upconversion efficiency of the gain medium. Plasmonic structures with locally enhanced electromagnetic fields can amplify the upconversion process within sub-diffraction-limited volumes. As typical plasmonic materials, silver and gold nanoparticles with tailorable resonance modes matching upconversion excitation or emission wavelengths have been shown to enhance upconversion efficiency. Accordingly, plasmonic arrays consisting of silver nanopillars were fabricated to provide a high-quality single-mode lattice plasmon with a sharp resonance peak. After depositing NaYF₄∶20%Yb³⁺, 20%Er³⁺@NaYF₄ NPs onto the surface of the plasmonic nanoarray to form a microcavity, continuous-wave upconversion lasing with an ultralow threshold of 70 W/cm² was achieved.

a gain medium, a pumping source, and an optical cavity. These are also the main factors affecting the output of upconversion lasers. To achieve lasing emission, the gain medium must satisfy the requirement of population inversion, meaning that the population in the excited states should exceed that in the ground state. Furthermore, to realize net gain after each feedback cycle, the number of emitted photons in the gain medium must exceed the losses induced by scattering or re-absorption. Increasing the pumping power leads to successive amplified spontaneous emission and stimulated emission (lasing action). For the optical cavity, it must have a high quality factor, i.e., the optical loss should be low.

This review summarizes recent progress in RE ion-doped upconversion micro/nano lasers. The excellent frequency conversion properties of upconversion nanoparticles provide numerous opportunities for upconversion lasing, such as the upconversion random lasing, WGM/F?P cavity-modulated upconversion lasing, and plasmonic cavity-based upconversion lasing. Moreover, the emission wavelength of upconversion microlasers has been expanded from the deep ultraviolet to the near-infrared region. Low-threshold continuous-wave-pumped upconversion lasing has been achieved through the design of microcavities with WGM and lattice plasmon modes, showing potential applications in solid-state holographic display, underwater monitoring, high-speed information transmission, bioimaging and tracking, and water purification. However, challenges remain. For instance: (1) Further exploration is needed to improve upconversion efficiency, thereby reducing the lasing threshold; (2) It is necessary to enhance the thermal stability of the gain medium to avoid the influence of thermal effects on laser performance under high pumping power; (3) Smart cavities with high-Q-factor WGMs, F?P cavities, and plasmonic enhancement should be further developed for various applications.