Abstract

We have observed the sudden appearance of increasing absorption, upconversion emission, and phase conjugation above a threshold intensity in heavily doped Tm:YALO and Tm:YLF at room temperature. These effects occurred when incident light was tuned to exciled-state transitions of Tm ions rather than to ground-state transitions (649.5 nm for Tm: YALO, 628.3 nm and 648.4 nm for Tm:YLF). Our results indicate that the nonlinear coefficient n₂ measured by four-wave-mixing techniques at those unusual resonances agrees with theoretical analysis based on a density-matrix treatment of delocalized, cooperative dynamics of an avalanche process consisting of near-neighbor cross relaxation followed by excitation migration between more distant impurity ions.¹

© 1993 Optical Society of America