  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.
