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

A track record: Upconverting nanoparticles (UCNPs) were tracked in living HeLa cells and their active transport by motor proteins was visualized in real time. The remarkable photostability of the UCNPs and the noninvasiveness of near‐infrared excitation allowed continuous observation of living cells for as long as 6 h.

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