Photon avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000‐fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano‐bio‐technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two‐color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super‐resolution imaging is realized for the first time in 3% Tm3+ doped LiYF4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials.

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Photon avalanche (PA) is a highly nonlinear luminescence phenomenon that occurs in lanthanide doped materials. PA exhibits a very steep power law relationship between luminescence intensity and the optical pump power. Due to the mechanism of PA emission, even weak perturbations to the energy looping and energy distribution within excited levels of lanthanide emitters are expected to significantly modify luminescent properties. Therefore, in this work, we experimentally study the impact of temperature (from – 175 to 175 °C, with 25 °C steps) on the sensitized PA emission in NaYF4 nanoparticles co-doped with 15% of Yb3+ and 0.5% of Pr3+ ions under 852 nm pumping wavelength. Significant variations of the PA nonlinearity ( S = 4.5–9), PA gain (from 50 up to 175), and PA threshold (from 100 up to 700 kW/cm2) were observed under temperature rise from – 175 to 175 °C, respectively. The relative temperature sensitivities based on luminescence intensity changes were larger than 1.5% °C–1 in the whole temperature range, reaching the maximal value of 7.5% °C–1 at 0 °C. Moreover, a new thermometric parameter was proposed, namely, the PA pump power threshold, which exhibited over 0.5% °C–1 relative sensitivities in the same wide temperature range. Owing to PA properties, the temperature sensitivity range and the corresponding relative sensitivities may be intentionally tuned by selecting the appropriate pump intensity in respect to the power dependence relationship. These studies not only provide a better understanding of fundamental processes and susceptibility of the sensitized photon avalanche emission to temperature variation, but also show the possibility of using PA materials as sensitive (nano)thermometers.