Nonlinear Optical Absorption via Photon Avalanche

Abstract

We report an investigation of a nonlinear optical absorption phenomenon observed in LaCl3 crystals doped with Pr. When these crystals are illuminated with a laser beam of 529 nm wavelength and intensity in excess of a certain critical value, the light is strongly absorbed. The laser radiation is resonant with a Pr3+ transition from the first excited state (3H5) to a high optical state (3P1), and absorption is accompanied by strong fluorescence output from 3p1 and 3p0 to several lower-lying levels. Since the resonant transition does not include the ground state, it is not expected a priori that any absorption should occur (see Fig. 1). This is an example of the so-called photon avalanche phenomenon1.