The mechanisms behind the extreme susceptibility of photon avalanche emission to quenching.

The photon avalanche (PA) process that emerges in lanthanide-doped crystals yields a threshold and highly nonlinear (of the power law order >5) optical response to photoexcitation. PA emission is the outcome of the excited-state absorption combined with a cross-relaxation process, which creates positive and efficient energy looping. In consequence, this combination of processes should be highly susceptible to small perturbations in energy distribution and can thus be hindered by other competitive "parasitic" processes such as energy transfer (ET) to quenching sites. Although luminescence quenching is a well-known phenomenon, exact mechanisms of the susceptibility of PA to resonant energy transfer (RET) remain poorly understood, limiting its practical applications. A deeper understanding of these mechanisms may pave the way to new areas of PA exploitation. This study focuses on the investigation of the LiYF 4 :3%Tm 3+ PA system co-doped with Nd 3+ acceptor ions, which are found to impact both the looping and emitting levels. This effectively disrupts the PA emission, causing an increase in the PA threshold ( I th ) and a decrease in the PA nonlinearity ( S max ). Our complementary modelling results reveal that ET from the looping level increases I th and S max , whereas ET from the emitting level diminishes S max and the final emission intensity. Ultimately, significant PA emission quenching demonstrates a high relative sensitivity ( S R ) to infinitesimal amounts of Nd 3+ acceptors, highlighting the potential for PA to be utilized as an ultra-sensitive, fluorescence-based reporting mechanism that is suitable for the detection and quantification of physical and biological phenomena or reactions.