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European Quantum Electronics Conference (EQEC)

Spectroscopic discrimination of resonant and nonresonant contributions to the nonlinear refractive index in ion doped solids

S.M. Lima, T. Catiinda

Published Nov 15, 2004• 0 citations
Publisher

Abstract

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.

DOI
10.1109/eqec.2003.1314041

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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.
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.
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.
Through the use of modulation spectroscopy we have measured the absorption spectrum experienced by a weak probe beam in the presence of a saturating pump beam at two different wavelengths within the broad homogeneous absorption band of alexandrite. At 584 nm the absorption saturates at high pump intensities, and a hole (of width 612 Hz) appears at frequencies near that of the pump laser. However, at 457 nm the absorption demonstrates the inverse of normal saturation because of strong excited-state absorption, and an antihole appears at the pump frequency. These results are in good quantitative agreement with the predictions of a model that ascribes the origin of these spectral features to the temporal modulation of the ground-state population at the beat frequency between the pump and the probe beams. These population oscillations are treated properly within a rate-equation approximation in which the amplitude modulation of the total optical intensity caused by the beating of the pump and the probe beams is taken into account.
The ratio of the imaginary to the real part of the complex nonlinear refractive index has been measured by two-wave mixing in acridine orange doped fluorophosphate glass, fluorescein doped boric acid glass, and ruby at argon-ion wavelengths. A new and simple two-wave mixing measurement is described in which one of the beams contributing to a stationary interference pattern in the nonlinear absorber is subject to a phase transient. This results in a variation of the energy coupling between the beams from which the phase of the nonlinear index and also the sign of its real and imaginary parts can be determined. A simple theory based on homogeneously broadened absorption agrees with the phase of the nonlinear index in acridine, but not in fluorescein where excited state absorption is determined to be greater than absorption from the ground state. These results are compared with new nondegenerate two-wave mixing measurements in which the contribution from the phase and absorption gratings can be determined independently.1 Comparison of results from the two approaches enables the contribution of excited state absorption to the two-wave mixing to be qualified.
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.