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Advanced Solid-State Photonics (TOPS)

Anisotropy of nonlinear change of refractive index in Cr4+:YAG at continuous-wave resonant excitation

Alexander V. Kir’yanov, Juan Carlos Bermudez G.

Published Jan 1, 2004• 0 citations
Publisher

Abstract

The nonlinear change of refractive index in Cr4+:YAG under the powerful resonant continuous-wave excitation is modelled. The intensity- and thermo-induced anisotropy of refractive index nonlinear change are addressed for the first time to our knowledge.
DOI
10.1364/assp.2004.wb4

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A novel method is reported for calculating the nonlinear resonant change in refractive index of a doped saturable medium at the short-pulse excitation. The derived system of equations for energy density and phase of a short pulse propagating in the doped medium allows one to address a rather general case of the resonant index non-linearity caused by the dope's population perturbation (bleaching) effect. As an example, the energy-dependent changes in the refractive index and transmission coefficient of a Cr4+:YAG crystal under the ns-range (wavelength 1.06 μm) excitation are modeled.
The mechanism for refractive-index changes that accompany changes in population during pumping and amplification of an optical beam is studied in a Nd:YAG laser crystal by both resonant four-wave mixing and interferometric techniques. It is found that these refractive-index changes (connected with the difference in polarizability of excited and unexcited Nd3+ ions) have two components: a small noninertial component that accompanies an amplified optical pulse and the major changes that are intensified with a delay time of ∼3 µs. The latter effect is explained by population (or depopulation) of a higher-lying energy level 2(F2)5/2 of the 4f electron shell of Nd3+ ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the inverted Nd:YAG caused by the refractive-index changes is also discussed.
We studied both experimentally and theoretically the transmission and polarization characteristics of a Cr4+ : YAG saturable absorber crystal as a function of the intensity and polarization state of an incident laser beam. We found that a birefringent absorption behaviour is induced and we show that the experimental results are well described by a full numerical model which includes excited-state absorption as well as a ground-state cross-absorption term. By using pump-probe measurements we show that a finite cross-saturation of the absorption is achieved along the crystal axes orthogonal to the polarization of the optical radiation. No induced refractive index birefringence is observed.
A polarising interferometer was used to investigate changes in the refractive index of an Nd:YAG laser crystal associated with changes in the populations of the electronic energy levels characterised by different polarisabilities. The study was carried out on diode-pumped crystals as well as on crystals which were flashlamp-pumped under conditions of saturation of the gain experienced by the optical beam. The changes in the refractive index depended nonlinearly on the pump power. The ratio of the real and imaginary parts of the nonlinear susceptibility of an Nd:YAG laser amplifier was determined: this ratio rose from 0.2 to 1.2 when the gain increased from 0.2 to 0.45 cm -1 .
An investigation was made of changes in the profile and state of polarisation of a short (25 ns) resonant light pulse (λ ~ 1.06 μm) during its propagation in a YAG : Cr4+ crystal. The experimental results were in good agreement with a theory in which the absorption of light by the Cr4+ impurity centres in a YAG crystal is described phenomenologically as a consequence of the interaction of light with three orthogonal groups of resonant linear dipoles. The result demonstrated the fleasibility of controlling the profile of a light pulse propagating through polariser — YAG : Cr4+ crystal — polariser system. The application of a single-frequency pulse to this system generated output pulses with two or three intensity maxima. This was a direct result of a self-induced change in the state of the polarisation of the light in the YAG : Cr4+ crystal at the saturation absorption stage.
Two optical waves (at 1064 nm) were used to resonantly induce a population grating in a flash-lamp-pumped Nd:YAG rod, and a nonresonant wave (at 1054 or 694 nm) was used to read (with a diffraction efficiency of as much as 1% or 0.3%, respectively) the refractive-index grating caused by the difference in the polarizability of the excited and the unexcited Nd ions. Comparisons of experimental results and numerical calculations gave the dependence of the ratio of the real and the imaginary parts of the resonant susceptibility of Nd:YAG at 1064 nm on the pump power.