  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 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 thus
can be hindered by other competitive 'parasite' processes such as energy
transfer (ET) to quenching sites. Although luminescence quenching is a
well-known phenomenon, the exact mechanisms of susceptibility of PA to resonant
energy transfer (RET) remain poorly understood limiting 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
was found to impact both the looping and emitting levels and thus to
effectively disrupt PA emission, causing an increase in the PA threshold
($I_{th}$) and a decrease in PA nonlinearity ($S_{max}$). Our complementary
modelling results revealed that the ET from the looping level increased
$I_{th}$ and $S_{max}$, whereas the ET from the emitting level diminished
$S_{max}$ 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.
