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 (Tm), excited at a wavelength of 980 nanometres, can readily establish a population inversion on their intermediate metastable H level: the reduced inter-emitter distance at high Tm doping concentration leads to intense cross-relaxation, inducing a photon-avalanche-like effect that rapidly populates the metastable H level, resulting in population inversion relative to the H ground level within a single nanoparticle. As a result, illumination by a laser at 808 nanometres, matching the upconversion band of the H → H transition, can trigger amplified stimulated emission to discharge the H 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 emission depletion (STED) microscopy and achieve nanometre-scale optical resolution (nanoscopy), imaging single UCNPs; the resolution is 28 nanometres, that is, 1/36th of the wavelength. These engineered nanocrystals offer saturation intensity two orders of magnitude lower than those of fluorescent probes currently employed in stimulated emission depletion microscopy, suggesting a new way of alleviating the square-root law that typically limits the resolution that can be practically achieved by such techniques.

Each single upconversion nanocrystal (UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near-infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers' concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb ions has been used to avoid the so-called "concentration quenching" effect. Here we report that the concentration quenching effect does not limit the sensitizer concentration and NaYbF is the most bright host matrix. Surface quenching and the large size of NaYbF nanocrystals are the only factors limiting this optimal concentration. Therefore, we further designed sandwich nanostructures of NaYbF between a small template core to allow an epitaxial growth of the size-tunable NaYbF shell enclosed by an inert shell to minimize surface quenching. As a result, the suspension containing 25.2 nm sandwich structure UCNCs is 1.85 times brighter than the homogeneously doped ones, and the brightness of each single 25.2 nm heterogeneous UCNC is enhanced by nearly 3 times compared to the NaYF: 20% Yb, 4% Tm UCNCs in similar sizes. Particularly, the blue emission intensities of the UCNCs with the sandwich structure in the size of 13.6 and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes. Maximizing the sensitizer concentration will accelerate the development of brighter and smaller UCNCs as more efficient biomolecule probes or photon energy converters.

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-For commercial one-sun solar modules, up to 80% of the incoming sunlight may be dissipated as heat, potentially raising the temperature 20°C-30°C higher than the ambient. In the long term, extreme self-heating erodes efficiency and shortens lifetime, thereby dramatically reducing the total energy output. Therefore, it is critically important to develop effective and practical (and preferably passive) cooling methods to reduce operating temperature of PV modules. In this paper, we explore two fundamental (but often overlooked) origins of PV self-heating, namely, sub-bandgap absorption and imperfect thermal radiation. The analysis suggests that we redesign the optical properties of the solar module to eliminate parasitic absorption (selective-spectral cooling) and enhance thermal emission (radiative cooling). Comprehensive opto-electro-thermal simulation shows that the proposed techniques would cool one-sun terrestrial solar modules up to 10°C. This self-cooling would substantially extend the lifetime for solar modules, with corresponding increase in energy yields and reduced levelised cost of electricity (LCOE).