‘Photon avalanche’ and energy migration in nonlinear laser materials

An analysis is made of saturation in a spatially disordered ensemble of three-level active centres with nonradiative transfer of the electron excitation energy and with cross-relaxation. This ensemble is used to model a medium for luminescent ‘up-conversion’ of the pump radiation. The results confirm the occurrence of a ‘photon avalanche’ under the conditions of an initially present and growing kinetic inequivalence of the active centres caused by fluctuations in the number of neighbours in the environment. It is shown that such a fluctuation inhomogeneity makes the system sensitive to intercentre energy redistribution resulting from migration of excitations in the ensemble and that this inhomogeneity can alter significantly the saturation dynamics.