Avalanche phenomena leverage steeply nonlinear dynamics to generate disproportionately high responses from small perturbations and are found in a multitude of events and materials 1 , enabling technologies including optical phase-conjugate imaging, 2 infrared quantum counting, 3 and efficient upconverted lasing 4-6 . However, the photon avalanching (PA) mechanism underlying these optical innovations has been observed only in bulk materials and aggregates 6,7 , and typically at cryogenic temperatures 5-8 , limiting its utility and impact in many applications. Here, we report the realization of PA at room temperature in single nanostructures -small, Tm 3+ -doped upconverting nanocrystals -and demonstrate their use in superresolution imaging at wavelengths that fall within near-infrared (NIR) spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by either continuous-wave or pulsed lasers and exhibit all of the defining features of PA. These hallmarks include clear excitation power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is >13,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26 th power of pump intensity, resulting from induced positive optical feedback in each nanocrystal. This enables the experimental realization of photon-avalanche single-beam superresolution imaging (PASSI) 7 , 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 9-11 , 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 7 including subwavelength bioimaging 7,12,13 , IR detection, temperature [14][15][16] and pressure 17 transduction, neuromorphic computing 18 , and quantum optics 19,20 . Main

Reducing the size of lasers to microscale dimensions enables new technologies that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors to live brain tissue. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs) as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm-doped UCNPs to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm. Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.

Multiphoton imaging techniques that convert low-energy excitation to higher energy emission are widely used to improve signal over background, reduce scatter, and limit photodamage. Lanthanide-doped upconverting nanoparticles (UCNPs) are among the most efficient multiphoton probes, but even UCNPs with optimized lanthanide dopant levels require laser intensities that may be problematic. Here, we develop protein-sized, alloyed UCNPs (aUCNPs) that can be imaged individually at laser intensities >300-fold lower than needed for comparably sized doped UCNPs. Using single UCNP characterization and kinetic modeling, we find that addition of inert shells changes optimal lanthanide content from Yb3+, Er3+-doped NaYF4 nanocrystals to fully alloyed compositions. At high levels, emitter Er3+ ions can adopt a second role to enhance aUCNP absorption cross-section by desaturating sensitizer Yb3+ or by absorbing photons directly. Core/shell aUCNPs 12 nm in total diameter can be imaged through deep tissue in live mice using a laser intensity of 0.1 W cm−2.

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in-vivo cellular imaging, solidstate lighting, and fast 3D sensing in smartphones 1-3. Continuouswave (CW) lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions 4,5. Plasmonic nanocavities integrated with gain can generate coherent light at sub-wavelength scales 6-9 , beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires 10,11. However, insufficient gain 1 with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or lowtemperature operation 6-9,12-15. Here we show CW upconverting lasing at room temperature with record-low thresholds and high photostability from sub-wavelength plasmons. We achieve selective, single-mode lasing from Yb 3+ /Er 3+-co-doped upconverting nanoparticles (UCNPs) conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and < /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. Lanthanide-based UCNPs are photostable solid-state nonlinear emitters that are efficient at sequentially absorbing multiple near-infrared (NIR) photons and emitting at visible and shorter-NIR wavelengths 16-19. Recently, UCNPs have been used as gain media in small lasers, and their integration with dielectric microcavities and hyperbolic metamaterials has resulted in multi-wavelength upconverted lasing 20-22. UCNPs also exhibit long radiative lifetimes (typically 100s of µs) compared to other gain materials 18,23,24 , which