The Influence of Photorefractive Index Change on Raman Scattering Intensities in LiNbO₃

Abstract

The time dependent varying shape of the laser beam within the scattering volume in Raman scattering experiments on LiNbO3 influences the recorded spectra and can be studied spectroscopically by forward-, right angle-, and backward-scattering geometries. The recorded spectra allow a time resolved insight into the microscopic local changes of the refractive index. The decay constant of the scattered phonon intensity is a well defined function of the laser power. It furthermore showed to be characteristic for the individual sample. Raman spectroscopy thus seems to provide a method to determine the concentration of Fe-impurities. We have observed that ordinary photons influence the optically damaged regions of the sample whereas undamaged regions are left almost unchanged. Damped cut-in oscillations recorded when illuminating the sample alternatingly with ordinary and extraordinary laser light seem to originate from local fluctuations of an excited cloud of impurity electrons. The observation of undamped periodic break downs of the scattering intensity from polaritons and back scattered phonons is reported.

Application of Raman spectroscopy for measurement of photorefractive damage profile in LiNbO 3 crystals

“…Our experimental results show that the spatial inhomogeneity of the Raman scattered light along the propagation direction of the high-power laser beam path is observed even after photo/thermo-erasure procedure, but it is less significant than before this procedure in congruently melting LiNbO 3 crystals. It is well known, that such an inhomogeneity is a feature characterizing the optical damage induced in LiNbO 3 [1][2][3][4]6]. It should be noted, that the irreversible fraction of this inhomogeneity has been observed only after the high-intensity (≥ 25 kW/cm 2 ) exposure for a long time (t ≥ 50 min) and it has accumulative character with exposure.…”

“…It has been established that the Raman spectroscopy is efficient tool for the quantitative study of optical damage effect in nominally pure LiNbO 3 crystals [5][6][7]. As, a change of the laser-beam trajectory in LiNbO 3 crystals caused by photorefractivity results in a significant decrease of the detected Raman intensity.…”

“…As, a change of the laser-beam trajectory in LiNbO 3 crystals caused by photorefractivity results in a significant decrease of the detected Raman intensity. Using the 90 o scheme for Raman scattering measurements, we can detect those changes of the laser-beam trajectory that are due to the alteration of the projected form of the scattered volume on the entrance-slit monochromator plane [6]. One of the benefits of using Raman spectroscopy for the investigation of photorefractivity is that this technique enables to record signals from any local region within a crystal.…”

“…It means that the Raman intensity is not proportional to path-way of probe beam in the LiNbO 3 crystal with a strong optical damage. Note, that assumption of such proportionality together with a priori known linear relation between a Raman intensity and P in were used in all previous studies [6][7][8] directed on evaluation of Raman efficiency.…”

“…It is very important with practical point of view to determine if structural changes and microdomains formation occur in such small areas of a nominally pure lithium niobate crystal under the influence of inhomogeneous high-intensity illumination. One from the most efficient techniques of such a characterization is the Raman scattering spectroscopy, since previous studies have shown [5][6][7] that photorefractive (PR) effect induces marked perturbations of a Raman signal in iron-and titanium-doped LiNbO 3 bulk crystals and channel waveguides. Besides, the optical requirements on a material for quantitative Raman data acquisition are often much less stringent than those for direct measurements of the PR effect.…”