Predicting the impact of temperature dependent multi-phonon relaxation processes on the photon avalanche behavior in Tm3+: NaYF4 nanoparticles

Photon avalanche (PA) is an anti-Stokes process in which lanthanide-doped materials can emit upconverted luminescence in a highly non-linear manner. PA was recently demonstrated in thulium (III)-doped NaYF4 colloidal nanoparticles at room temperature, with reported nonlinearities (power factors) exceeding 30. Importantly, good rate-equations model agreement with experimental data was achieved, which enabled the derivation of the rates of energy transfer processes, providing a foundation for further developments and optimization of similar materials and their new applications. In order to apply PA to, for example, luminescent (nano)thermometry, we need to have a better understanding of the susceptibility of photon avalanche phenomenon to temperature variation. For this purpose, we study how the temperature and energy-gap-dependent multiphonon relaxation and inter-ionic energy transfer processes affect the photon avalanche behavior. As a result of these simulations, high susceptibility to temperature changes with relative sensitivity over 40 %⋅K−1 above 400 K (up to 15 %⋅K−1at room temperature) was noted. Moreover, by knocking-out the temperature dependences of the individual processes involved in the PA, we found the temperature dependence of the multiphonon relaxation of the intermediate F34 state to the ground state to be the primary one responsible for the simulated temperature dependence of PA emission. This process is observed to be more significant than temperature dependencies of energy looping and phonon assisted ground state non-resonant absorption. Although there are challenges with a single band technique, these results still show the great potential, especially when used ratiometrically with different excitation wavelengths.