Abstract

Avalanche upconversion, characterized by the sudden appearance of increasing absorption and upconversion emission above a threshold intensity in heavily doped Tm:YALO and Tm:LiYF₄, has been reported previously.¹ A density matrix theory has been proposed to account tor this phenomenon, comprised of two sequential processes, namely near-neighbor cross relaxation followed by excitation migration between more distant impurity ions.¹ Here we present results to test the model. They show that avalanche threshold displays a minimum at about 110 K in Tm:LiYF₄, (at λ_ex.= 648.24 um) and that cross relaxation occurs with a rate which maximizes at about 30 K. Our analysis reveals that cross relaxation is initiated from the second Stark level of the ³Ή₄ state and that its temperature dependence can be explained on the basis of specific phonon-assisted transitions among Stark levels. This result is relevant both to models of the avalanche process and to operation of mid-infrared Tm^3,4 lasers which rely on cross relaxation.

© 1994 Optical Society of America