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. 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 power1,2,3. At the nanometre scale, lanthanide-doped upconversion nanoparticles (UCNPs) can now be made with precisely controlled phase, dimension and doping level4,5. When excited in the near-infrared, these UCNPs emit stable, bright visible luminescence at a variety of selectable wavelengths6,7,8,9, with single-nanoparticle sensitivity10,11,12,13, 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 high Tm3+ doping concentration leads to intense cross-relaxation, inducing a photon-avalanche-like effect that rapidly populates the metastable 3H4 level, resulting in population inversion relative to the 3H6 ground level within a single nanoparticle. As a result, illumination by a laser at 808 nanometres, matching the upconversion band of the 3H4 → 3H6 transition, can trigger amplified stimulated emission to discharge the 3H4 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.

Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy

Upconversion laser processes

Photon avalanche upconversion in rare earth laser materials

The design and characterization of new luminescent materials is an active area of research. Here we present several current topics in the area of upconversion by transition-metal and rare-earth-metal doped halide lattices. Following introduction to the necessary background material related to upconversion mechanisms and kinetics, a series of topics are discussed which illustrate some key areas of developing interest in the field. These include the use of unconventional experimental and theoretical techniques for gaining insight into upconversion processes in rare-earth-doped lattices (e.g., power-dependence measurements, two-color laser excitation schemes, and correlated crystal field calculations), as well as several specific examples of exciting and unusual upconversion behavior in both transition-metal and rareearth-metal systems. Finally, we discuss the variety of interesting effects host-lattice variation can have on the upconversion processes of a dopant ion, ranging from multiphonon relaxation properties to exchange interactions.

Upconversion Processes in Transition Metal and Rare Earth Metal Systems

Rare earth (RE) doped oxyfluoride glass ceramics possess interesting optical properties with applications in telecommunications and optoelectronics, such as solid state lasers, optical amplifiers, etc. These materials combine the transparency and mechanical and chemical resistance of aluminosilicate glasses with the low phonon energy and facile incorporation of RE ions in the fluoride crystals. The incorporation of RE ions in the crystalline phases enhances the laser emission intensity, a major property of these materials. Transparency is achieved when crystal size is in the nanometric scale, usually below 40 nm, which avoids light scattering. A strict control of the nucleation and crystal growth processes is therefore necessary which requires a deep knowledge of the crystallisation mechanisms. The great activity and publications in this field in the last decades merit a review providing a comparative study of the different nanoglass ceramic systems, their structural and optical characterisation and their main properties and applications. This is the objective of this review paper which includes 227 references. A general discussion on glass nucleation and crystallisation theories and more relevant crystallisation parameters and characterisation techniques are put forward in the first section of the review, focused on nanocrystallisation processes in oxyfluoride systems. In the second section, the principal RE doped glass ceramics are presented. After a general introduction about the luminescence processes, including up- and down-conversion, the behaviour of RE elements in glasses and crystals are discussed. Glass ceramic compositions have been divided as follows: glass ceramics with a glass composition following Wang and Ohwaki’s oxyfluoride glass ceramic, and glass ceramics with different matrix compositions, arranged by crystalline phases. Relevant properties, mainly optical and laser, are described in each system along with the most relevant applications of these materials.

/pdf/nanocrystallisation-in-oxyfluoride-systems-mechanisms-of-3vsuzuu2ij.pdf

Nanocrystallisation in oxyfluoride systems: mechanisms of crystallisation and photonic properties

Avalanche upconversion in LaF3:Tm3+

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Avalanche upconversion in LaF3:Tm3+