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.

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.

Optoelectronic excitations in monolayer MoS2 manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena.Investigating the fundamental interactions underpinning these phenomenacritical to both many-body physics exploration and device applications -presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. Here, optical detection of bound-and freecarrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies. The results explicitly disentangle the competing effects and highlight longstanding theoretical predictions of large carrier-induced band gap and exciton renormalization in 2D semiconductors. 2/20