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Lanthanide-doped glasses and crystals are attractive for laser applications because the metastable energy levels of the trivalent lanthanide ions facilitate the establishment of population inversion and amplified stimulated emission at relatively low pump power^,,. At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be made with precisely controlled phase, dimension and doping level^,. When excited in the near-infrared, these UCNPs emit stable, bright visible luminescence at a variety of selectable wavelengths^,,,, with single-nanoparticle sensitivity^,,,, which makes them suitable for advanced luminescence microscopy applications. Here we show that UCNPs doped with high concentrations of thulium ions (Tm³⁺), excited at a wavelength of 980 nanometres, can readily establish a population inversion on their intermediate metastable ³H₄ level: the reduced inter-emitter distance at …