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Nonlinear Optical Properties of Materials

Nonlinear Optical Absorption via Photon Avalanche

A. W. Kueny, William E. Case, Mark E. Koch

Published Jan 1, 1988• 0 citations
Publisher

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.

DOI
10.1364/nlopm.1988.wb5

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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.
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.
Laser output at 4.9 μm has been obtained from a Pr doped LaCl3 crystal. Pump laser energy at 529 nm is coupled into the crystal via the photon avalanche process. This process involves metastable state ion absorption of a pump photon and subsequent ion‐ion energy transfer. The latter step generates two metastable state ions (self‐induced absorption). This highly nonlinear process is characterized by a sharp onset with pump intensity and, at higher intensities, depopulation of the ionic ground state. Experimental results will be compared with predictions from an existing rate equation model.
The phenomenon of photon avalanche, which involves both an excited state absorption process from a metastable level and a cross relaxation energy transfer process responsible for the large increases in population of this metastable level, has been investigated in LaCl3:Pr3+.{\mathrm{LaCl}}_{3}:{\mathrm{Pr}}^{3+}.LaCl3​:Pr3+. In this paper, we report on the intensity dependence of both the steady-state fluorescence and the temporal evolution of the avalanche signal as a function of temperature (80 K to room temperature). Comparisons are made between experimental results and mathematical models based on a six-level system described by a series of coupled rate equations combined with the propagation of a Gaussian beam. We develop a model with only one adjustable parameter, the excited state absorption cross section, which provides a good description of all of our experimental data. We have estimated values of the excited state absorption cross section for the various ${}^{3}{\stackrel{\ensuremath{\rightarrow}}{{\mathrm{H}}_{5}}}^{3}{\mathrm{P}}_{0}$ pumping transitions at different temperatures.