Avalanche phenomena use steeply nonlinear dynamics to generate disproportionately large responses from small perturbations, and are found in a multitude of events and materials¹. Photon avalanching enables technologies such as optical phase-conjugate imaging², infrared quantum counting³ and efficient upconverted lasing^4,5,6. However, the photon-avalanching mechanism underlying these optical applications has been observed only in bulk materials and aggregates^6,7, limiting its utility and impact. Here we report the realization of photon avalanching at room temperature in single nanostructures—small, Tm³⁺-doped upconverting nanocrystals—and demonstrate their use in super-resolution imaging in near-infrared spectral windows of maximal biological transparency. Avalanching nanoparticles (ANPs) can be pumped by continuous-wave lasers, and exhibit all of the defining features of photon avalanching, including clear excitation-power thresholds, exceptionally long rise time at threshold, and a dominant excited-state absorption that is more than 10,000 times larger than ground-state absorption. Beyond the avalanching threshold, ANP emission scales nonlinearly with the 26th power of the pump intensity, owing to induced positive optical feedback in each nanocrystal. This enables the experimental realization of photon-avalanche single-beam super-resolution imaging⁷ with sub-70-nanometre spatial resolution, achieved by using only simple scanning confocal microscopy and without any computational analysis. Pairing their steep nonlinearity with existing super-resolution techniques and computational methods^8,9,10, ANPs enable imaging with higher resolution and at excitation intensities about 100 times lower than other probes. The low photon-avalanching threshold and excellent photostability of ANPs also suggest their utility in a diverse array of applications, including sub-wavelength imaging^7,11,12 and optical and environmental sensing^13,14,15.

雪崩现象使用陡峭的非线性动力学从小的扰动中产生不成比例的大响应，并在大量事件和材料中发现¹。光子雪崩使光学相位共轭成像²、红外量子计数³和高效上转换激光^4,5,6等技术成为可能。然而，这些光学应用背后的光子雪崩机制仅在散装材料和聚集体^6,7中被观察到，限制了它的效用和影响。在这里，我们报告了在室温下单纳米结构中光子雪崩的实现——小，Tm ³⁺-掺杂的上转换纳米晶体——并展示它们在具有最大生物透明度的近红外光谱窗口中的超分辨率成像中的用途。雪崩纳米粒子 (ANP) 可以被连续波激光器泵浦，并展现出光子雪崩的所有定义特征，包括清晰的激发功率阈值、异常长的阈值上升时间，以及超过比基态吸收大 10,000 倍。超过雪崩阈值，ANP 发射与泵浦强度的 26 次方成非线性比例，这是由于每个纳米晶体中诱导的正光反馈。这使得光子雪崩单光束超分辨率成像^{7的实验实现成为可能}具有亚 70 纳米的空间分辨率，仅使用简单的扫描共聚焦显微镜即可实现，无需任何计算分析。将其陡峭的非线性与现有的超分辨率技术和计算方法^8,9,10相结合，ANP 能够以比其他探头低约 100 倍的激发强度和更高分辨率进行成像。ANP 的低光子雪崩阈值和出色的光稳定性也表明它们在各种应用中的实用性，包括亚波长成像^7,11,12以及光学和环境传感^13,14,15。