Evaluation of materials for tunable vibronic lasers

A comparison is made of the use of three techniques for determining the effects of stimulated emission on the spectra of solid-statevibronic laser materials: peak single-pass gain, fluorescence band narrowing, and lifetime shortening. Each of these was used to obtain the pump energy density and population inversion at threshold, the stimulated emission cross section, and the gain coefficient for two potential vibronic laser materials. The difficulty and accuracy of each technique is discussed. Lifetime shortening is the easiest experimental measurement to make. These results were analyzed by using numerical techniques to solve the coupled set of rate equations describing the concentration of photons and population of excited states of ions in an optically pumped material under delta-function pulsed excitation. The solutions predict an effective shortening of the fluorescence lifetime at high excitation levels with a distinct threshold where stimulated emission processes become important. The observed band narrowing for the same type of pumping conditions is shown to be consistent with the theoretical predictions of the model developed by McCumber.