Photon Avalanching Nanoparticles

ConspectusAvalanches within nanoparticles seem like science fiction, but if they are avalanches of photons, they open up real-world innovations in imaging, sensing, optical computing, and other unexplored light-driven technologies. Avalanches are outsized events arising from the integration of many smaller inputs, and photon avalanching (PA) was first reported in bulk crystals in 1979 as an unexpectedly large jump in luminescence as excitation intensity was slowly increased. It would be 41 years before PA would be observed at the nanoscale in photon avalanching nanoparticles (ANPs), Tm3+-doped upconverting nanoparticles that show excited-to-ground state absorption inversion greater than 10,000:1 and emission that scales nonlinearly up to the 32nd power of the pump intensity. This extreme nonlinearity enables a real-time 5-fold improvement in the 150-year-old Abbe limit of spatial resolution, achieving 70 nm resolution using only simple scanning confocal microscopy. This extreme nonlinearity also gives rise to a series of highly unusual optical and sensing properties. Tm3+ ANPs show NIR-controlled bidirectional photoswitching, lasting over 1000 cycles in ambient or aqueous conditions with no measurable sign of photodegradation. This enables 2- and 3-dimensional optical nanoscale patterning with full erase and rewrite capabilities. Unlimited photoswitching also underlies the super-resolution technique INPALM, which is capable of sub-Ångstrom localization precision and resolving individual ANPs within tightly packed clusters. Nd3+-based ANPs show the peculiar property of intrinsic optical bistability (IOB), a form of memory in which emission depends on whether the ANPs have previously undergone PA. This stable, history-dependent contrast makes these ANPs analogous to optical transistors and promising materials for optical computing, neuromorphic circuitry, and related photonic technologies. The steep nonlinearity of PA also makes ANPs exceptional sensors of external perturbations, as tiny environmental changes may be amplified into large changes in optical output. As force sensors, Tm3+ ANPs are able to detect forces over a dynamic range of 4 orders of magnitude, from piconewtons to micronewtons, a range that will enable force sensing in complex systems across scales. Application of current ANP designs to imaging and devices, discovery of new PA-associated phenomena, and design of new ANPs with unique properties are all underway as the novelty of this technology cascades toward new fundamental discoveries and applications.