Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy

Super-resolution optical microscopy based on stimulated emission depletion effects can now be performed at much lower light intensities than before by using bright upconversion emission from thulium-doped nanoparticles. Breaking the law Improvements in super-resolution optical microscopy based on stimulated emission depletion (STED) effects have a problem: they are typically limited by a 'square-root law' regarding the number of photons required to achieve a gain in resolution. Yujia Liu and colleagues have found a way to bypass this troublesome law. As others have done before them, they adopt lanthanide-doped upconversion nanoparticles as the emitting species used to achieve high-resolution imaging. The difference this time is that the laser-like absorption and emission properties of these nanoparticles are engineered to facilitate STED-like microscopy at much lower light intensities. 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 1 , 2 , 3 . At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be made with precisely controlled phase, dimension and doping level 4 , 5 . When excited in the near-infrared, these UCNPs emit stable, bright visible luminescence at a variety of selectable wavelengths 6 , 7 , 8 , 9 , with single-nanoparticle sensitivity 10 , 11 , 12 , 13 , which makes them suitable for advanced luminescence microscopy applications. Here we show that UCNPs doped with high concentrations of thulium ions (Tm 3+ ), excited at a wavelength of 980 nanometres, can readily establish a population inversion on their intermediate metastable 3 H 4 level: the reduced inter-emitter distance at high Tm 3+ doping concentration leads to intense cross-relaxation, inducing a photon-avalanche-like effect that rapidly populates the metastable 3 H 4 level, resulting in population inversion relative to the 3 H 6 ground level within a single nanoparticle. As a result, illumination by a laser at 808 nanometres, matching the upconversion band of the 3 H 4 → 3 H 6 transition, can trigger amplified stimulated emission to discharge the 3 H 4 intermediate level, so that the upconversion pathway to generate blue luminescence can be optically inhibited. We harness these properties to realize low-power super-resolution stimulated emiss