Photon avalanche nanomaterials: from spark to surge

Abstract Photon avalanche (PA) upconversion, driven by a positive feedback loop that couples nonresonant ground-state absorption (GSA), resonant excited-state absorption (ESA), and highly efficient cross-relaxation (CR), gives rise to a threshold-triggered ultrahigh optical nonlinearity accompanied by uniquely prolonged rise-time dynamics. From spark to surge, this phenomenon can deliver tens to even hundreds of nonlinear orders at the nanoscale, redefining opportunities in imaging, sensing, and optical computing while opening a new paradigm for interrogating light–matter interactions.