Abstract

We have made an unexpected discovery of highly nonlinear optical behavior in certain praseodymium-doped lanthanum halides. When a LaCl₃ or LaBr₃ crystal containing Pr³⁺ ions is exposed to green laser radiation slightly in excess of a certain critical intensity, Pr fluorescence increases by orders of magnitude. Infrared output near 4.5-μm wavelength is associated with the large visible output, and pump power is strongly absorbed. We call this new phenomenon a photon avalanche. Critical pump power density is in the 1–15 W/cm² range. At higher pump rates, a population inversion and lasing action take place between the lowest infrared levels of the Pr ion. Experimental results are presented and compared to predictions of a photon avalanche model based on resonant energy transfer.

© 1985 Optical Society of America