Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cm À2 ) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. [1] Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core-shell structure, optical emission, and surface properties. [2][3][4][5] The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels. [6] Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb 3+ ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. [7] Excitation of conventional upconversion nanoparticles at other wavelengths has been proposed to minimize the effect of water absorption. [8] But the use of this technique is limited mainly by the largely sacrificed excitation efficiency. Efforts have also been devoted to tuning the NIR response of photon upconversion through integration of various sensitizers such as metal ions (e.g.; Nd 3+ , V 3+ or Cr 5+ ) and organic dyes. [9] The progress has resulted in visible emission by NIR excitation in the 700-900 nm range where the transparency of biological samples is maximal. [9e-h] However, upconversion emission across a broad range of spectra in these systems have not been demonstrated largely owing to the uncontrollable nonradiative processes. Herein, we describe a novel design, based on nanostructural engineering to separate unwanted electronic transitions for constructing a new class of materials displaying tunable upconversion emissions spanning from UV to the visible spectral region by single wavelength excitation at 808 nm. We also show that these nanoparticles can surpass the constraints associated with conventional upconversion nanoparticles for biological studies.The nanostructure design for management of energy transitions is depicted in Figure 1. A core-shell-shell nanoparticle platform is used to host light-harvesting, upconverting, and optical tuning processes at separate layers through doping of appropriate lanthanide ions. Interlayer energy exchange interactions are mediated by arrays of lanthanide migrator ions that can bridge efficient energy transfer across the core-shell interface while filtering unwanted crossrelaxations. As a result, incompatible optical processes can be rationally combined to achieve flexible and efficient photon energy conversions.As a proof-of-concept experiment, we employed a NaYbF 4 @Na(Yb,Gd)F 4 @NaGdF 4 core-shell-shell nanoparticle host....

Lasing action is realized in a ZnO/GaN heterojunction by employing a MgO interlayer. The MgO layer can confine electrons in the ZnO layer, while holes can pass through the MgO layer and enter into the n‐ZnO layer from the p‐GaN layer. The threshold of the lasing action is as low as 0.8 mA..

Lasing is observed from carbon nanodots (C-dots) dispersed into a layer of poly(ethylene glycol) coated on the surface of optical fibers under 266 nm optical excitation. This is due to the enhancement of photoluminescence intensity via the esterification of carboxylic groups of the C-dots, and the formation of high-Q cylindrical microcavities to support second-type whispering gallery modes.

Electrically pumped random lasers are realized in ZnO nanocrystallite films in a simple metal–oxide–semiconductor structure. By introducing an i‐ZnO layer, a threshold current of 6.5 mA is obtained. The reported results provide a simple route to electrically pumped random lasing (see figure) with relatively low threshold, a significant step towards the future applications of this kind of laser.

Lanthanide-doped nanocrystals (NCs), which found applications in bioimaging and labeling, have recently demonstrated significant improvement in up-conversion efficiency. Here, we report the first up-conversion multicolor microcavity lasers by using NaYF4:Yb/Er@NaYF4 core-shell NCs as the gain medium. It is shown that the optical gain of the NCs, which arises from the 2- and 3-photon up-conversion processes, can be maximized via sequential pulses pumping. Amplified spontaneous emission is observed from a Fabry-Perot cavity containing the NCs dispersed in cyclohexane solution. By coating a drop of silica resin containing the NCs onto an optical fiber, a microcavity with a bottle-like geometry is fabricated. It is demonstrated that the microcavity supports lasing emission through the formation of whispering gallery modes.

Blue emission at NIR excitation: A strategy, based on energy management in nanostructured materials, is reported for photon upconversion of near‐infrared light. Several optical processes can be integrated into a single nanoparticle (see picture). The effect offers upconversion emissions spanning from ultraviolet to the visible spectral region by excitation at 808 nm.

The luminescent capability of graphene quantum dots (GQDs) was investigated and compared with that of carbon nanodots (C-dots) obtained from the same functionalization process. It was found that the optical gain of GQDs is higher than that of C-dots due to their geometrical advantages such as larger surface area to volume ratio and smaller volume. Under optical excitation at 266 nm, lasing emission was observed from a Fabry-Perot cavity containing a mixture of GQDs and TiO(2) nanoparticles dispersed inside ethanol.

Electrically pumped random lasing has been realized in Au/MgO/ZnO structures. By incorporating Ag nanoparticles, whose extinction spectrum overlaps well with the emission spectrum of the structures, the threshold of the random lasing can be decreased from 63 mA to 21 mA. The decrease in the threshold has been attributed to the resonant coupling between the carriers in the active layer of the structures and the surface plasmon of the Ag nanoparticles.