Ultralow-threshold, continuous-wave upconverting lasing from subwavelength plasmons.

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb3+/Er3+-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W cm-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light

Expanding the I-II-V Phase Space: Soft Synthesis of Polytypic Ternary and Binary Zinc Antimonides

Soft chemistry methods offer the possibility of synthesizing metastable and kinetic products that are unobtainable through thermodynamically-controlled, high-temperature reactions. A recent solution-phase exploration of Li-Zn-Sb phase space revealed a previously unknown cubic half-Heusler MgAgAs-type LiZnSb polytype. Interestingly, this new cubic phase was calculated to be the most thermodynamically stable, despite prior literature reporting only two other ternary phases (the hexagonal half-Heusler LiGaGe-type LiZnSb, and the full-Heusler Li2ZnSb). This surprising discovery, coupled with the intriguing optoelectronic and transport properties of many antimony containing Zintl phases, required a thorough exploration of syn-thetic parameters. Here, we systematically study the effects that different precursor concentrations, injection order, nucleation and growth temperatures, and reaction time have on the solution-phase synthesis of these materials. By doing so, we identify conditions that selectively yield several unique ternary (c-LiZnSb vs. h*-LiZnSb), binary (ZnSb vs. Zn8Sb7), and metallic (Zn, Sb) products. Further, we find one of the ternary phases adopts a variant of the previously observed hexagonal LiZnSb struc-ture. Our results demonstrate the utility of low temperature solution phase—soft synthesis—methods in accessing and mining a rich phase space. We anticipate that this work will motivate further exploration of multinary I-II-V compounds, as well as encourage similarly thorough investigations of related Zintl systems by solution phase methods

A generalized approach to photon avalanche upconversion in luminescent nanocrystals

Photon avalanching nanoparticles (ANPs) exhibit extremely nonlinear upconverted emission valuable for sub-diffraction imaging, nanoscale sensing, and optical computing. Avalanching has been demonstrated with Tm3+, Nd3+ or Pr3+-doped nanocrystals, but their emission is limited to 600 and 800 nm, restricting applications. Here, we utilize Gd3+-assisted energy migration to tune the emission wavelengths of Tm3+-sensitized ANPs and generate highly nonlinear emission of Eu3+, Tb3+, Ho3+, and Er3+ ions. The upconversion intensities of these spectrally discrete ANPs scale with the nonlinearity factor s = 10-17 under 1064 nm excitation at power densities as low as 6 kW/cm2. This strategy for imprinting avalanche behavior on remote emitters can be extended to fluorophores adjacent to ANPs, as we demonstrate with CdS/CdSe/CdS core/shell/shell quantum dots. ANPs with rationally designed energy transfer networks provide the means to transform conventional linear emitters into a highly nonlinear ones, expanding the use of photon avalanching in biological, chemical, and photonic applications.Comment: 13 pages, 5 figure

Millisecond Kinetics of Nanocrystal Cation Exchange UsingMicrofluidic X-ray Absorption Spectroscopy

We describe the use of a flow-focusing microfluidic reactorto measure the kinetics of theCdSe-to-Ag2Se nanocrystal cation exchangereaction using micro-X-ray absorption spectroscopy (mu XAS). The smallmicroreactor dimensions facilitate the millisecond mixing of CdSenanocrystal and Ag+ reactant solutions, and the transposition of thereaction time onto spatial coordinates enables the in situ observation ofthe millisecond reaction with mu XAS. XAS spectra show the progression ofCdSe nanocrystals to Ag2Se over the course of 100 ms without the presenceof long-lived intermediates. These results, along with supporting stoppedflow absorption experiments, suggest that this nanocrystal cationexchange reaction is highly efficient and provide insight into how thereaction progresses in individual particles. This experiment illustratesthe value and potential of in situ microfluidic X-ray synchrotrontechniques for detailed studies of the millisecond structuraltransformations of nanoparticles and other solution-phase reactions inwhich diffusive mixing initiates changes in local bond structures oroxidation states

Giant nonlinear optical responses from photon avalanching nanoparticles

Avalanche phenomena leverage steeply nonlinear dynamics to generate disproportionately high responses from small perturbations and are found in a multitude of events and materials, enabling technologies including optical phase-conjugate imaging, infrared quantum counting, and efficient upconverted lasing. However, the photon avalanching (PA) mechanism underlying these optical innovations has been observed only in bulk materials and aggregates, and typically at cryogenic temperatures, limiting its utility and impact. Here, we report the realization of PA at room temperature in single nanostructures--small, Tm-doped upconverting nanocrystals--and demonstrate their use in superresolution imaging at near-infrared (NIR) wavelengths within spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave or pulsed lasers and exhibit all of the defining features of PA. These hallmarks include excitation power thresholds, long rise time at threshold, and a dominant excited-state absorption that is >13,000x larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26th power of pump intensity. This enables the realization of photon-avalanche single-beam superresolution imaging (PASSI), achieving sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational analysis. Pairing their steep nonlinearity with existing superresolution techniques and computational methods, ANPs allow for imaging with higher resolution and at ca. 100-fold lower excitation intensities than is possible with other probes. The low PA threshold and exceptional photostability of ANPs also suggest their utility in a diverse array of applications including sub-wavelength bioimaging, IR detection, temperature and pressure transduction, neuromorphic computing, and quantum optics.Comment: 14 pages, 4 figure

Precursor reaction kinetics control compositional grading and size of CdSe1-xSx nanocrystal heterostructures

We report a method to control the composition and microstructure of CdSe1-xSx nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 degrees C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k(E)) spanning 1.3 x 10(-5) s(-1) to 2.0 x 10(-1) s(-1). Depending on the relative reactivity (k(Se)/k(S)), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe1-xSx alloys (k(Se)/k(S) = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed