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Nonlinear spectroscopy of the alexandrite R 1 line

T. Catunda, L. A. O. Munes

Published May 21, 1990• 0 citations
OpenAlex

Abstract

The nonlinear effect in Cr+3 doped crystals has been studied by populating the excited metastable level 2E through the 4T2 absorption band due to its higher absorption cross section compared with the R lines. The excited state 2E has a complex susceptibility different from that of the ground state so the material susceptibility changes in proportion to the excited state population.1 However, when the laser is sintonized near one of the R lines (the direct transitions between the ground and excited states), we must also consider the effect of this resonant interaction on the nonlinearity.

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The nonlinear effect in Cr+3 doped crystals has been studied by populating the excited metastable level 2E through the 4T2 absorption band due to its higher absorption cross section compared with the R lines. The excited state 2E has a complex susceptibility different from that of the ground state so the material susceptibility changes in proportion to the excited state population.1 However, when the laser is sintonized near one of the R lines (the direct transitions between the ground and excited states), we must also consider the effect of this resonant interaction on the nonlinearity.
This work intends to distinguish resonant and nonresonant contributions to the nonlinear refractive index in ion doped solids using a two-color nonlinear technique. In this experiment, the alexandrite metastable excited state (/sup 2/E) is populated via the /sup 4/T/sub 2/ absorption band using a 514 nm pump beam (Ar/sup +/ laser), while the refractive index change is probed by a tunable dye laser, in resonance with the R lines (from ground to metastable transition at /spl lambda/ /spl sim/ 680 nm). The absorption spectrum with the two characteristic R lines is also shown just for comparison of the spectra. Observations reveal that an absorption saturation signal follows the linear absorption spectrum.
The degenerate-multiwave-mixing spectroscopy of alexandrite at 77 K using a pulsed dye laser in resonance with the R1m{\mathit{R}}_{1\mathit{m}}R1m​ line was studied in this work. Unusual asymmetric line shapes that can be explained by an interference effect between resonant and nonresonant contributions of the media susceptibility were observed. The resonant effect is caused by the saturation of the two-level system susceptibility and the nonresonant contribution comes from the linear polarizability difference between excited and ground states. The present work shows an observation of this kind of interference phenomena in degenerate-wave-mixing experiments. Furthermore, we note that it is an observation of saturation line splitting in degenerate wave mixing in a narrow line in solids.
Using a method for nonlinear laser spectroscopy which eliminates the effects of laser jitter in nearly degenerate four-wave mixing, we present the first direct measurement of the ground-state relaxation rate of optically excited Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in Cr: YAlO3{\mathrm{O}}_{3}O3​. The Lorentzian linewidth of the emission resonance is 9.8 Hz (FWHM). The mechanism of ground-state relaxation is established to be fluorescent decay of the 2E^{2}E2E metastable level.
Nonlinear refraction spectroscopy measurements were performed in resonance with absorption lines in Nd3+ and Cr3+ doped crystals. The observed line shapes can be explained by the interference of resonant and nonresonant contributions to the nonlinear refractive index. These effects were fully discriminated using a pump and probe configuration, in which a dispersive line shape was observed on the top of a plateau that was attributed to the polarizability difference between excited and ground states (Deltaalpha). A theoretical model was used to obtain Deltaalpha from the experimental data, and the results are in agreement with several previous experiments. Slow and fast light propagation are also discussed.
We have performed nondegenerate four-wave mixing measurements on Cr(3+) ions in three crystals: BeAl(2)O(4), Gd(3)Sc(2)Ga(3)O(12), and LaMgAl(11)O(19). Two laser beams of the same frequency were used to resonantly pump one of the absorption bands of Cr(3+), and probe beams with three different wavelengths ranging from 442 to 1064 nm were used to generate a signal from the laser-induced population grating. The observed four-wave mixing signal strength was found to increase as the wavelength of the probe beam was decreased. A model was utilized to explain these results based on the change in polarizability of the Cr(3+) ions when they are in the metastable state versus the ground state, and it was found that the dispersion effects can be described by a single-oscillator model with the lowest-energy charge-transfer transition of Cr(3+) making the dominant contribution to the signal.
Degenerate four-wave mixing was observed in alexandrite crystals (BeAl2{\mathrm{Al}}_{2}Al2​O4{\mathrm{O}}_{4}O4​: Cr3+{\mathrm{Cr}}^{3+}Cr3+), and the signal beam efficiency and decay rate were measured as functions of pump beam-crossing angle, wavelength, and power. The results are consistent with scattering from excited-state population gratings related to the difference in dispersion of the Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in the ground and metastable states. These gratings can be selectively established with Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions in the inversion or mirror sites depending on the excitation wavelength. Strong scattering occurs only for pump beams polarized parallel to the bbb direction of the crystal.