Photon Avalanches in Nanoparticles

While the original applications of nanoscience to bioimaging used luminescent nanocrystals whose properties largely mirrored those of traditional probes, newer nanocrystals have been engineered with optical properties unlike anything found in organics or proteins. These include perfect photostability, multiphoton emission a billion-fold more efficient than standard multiphoton techniques, and most recently, photon avalanches hosted within nanostructures. In two recent studies,^1,2 we describe the engineering and imaging of avalanching nanoparticles (ANPs), which are Tm³⁺-doped NaYF₄upconverting nanoparticles that efficiently convert near infrared excitation to higher energy emission. Avalanches are steeply nonlinear events in which outsized responses arise from a series of minute inputs and, with light, photon avalanching had been observed only in bulk materials, often at cryogenic temperatures. The extreme nonlinearity of ANP emission enables sub-70 nm spatial resolution using only simple scanning confocal microscopy and before any computational analysis. Two-way NIR photoswitching of ANPs enables full optical control of photodarkening and photobrightening, and we find indefinite photoswitching of individual nanoparticles in ambient or aqueous conditions without measurable photodegradation. This enables unlimited photon collection for calculation of sub-Ångstrom localization accuracies, and we can distinguish individual ANPs within tightly packed clusters.<br/> <br/>1. Lee, C. <i>et al.</i> Giant nonlinear optical responses from photon-avalanching nanoparticles. <i>Nature</i> <b>589</b>, 230–235 (2021).<br/>2. Lee, C. <i>et al.</i> Indefinite and bidirectional near-infrared nanocrystal photoswitching. <i>Nature</i> <b>618</b>, 951–958 (2023).