Abstract
Transmission Raman spectroscopy experiments were performed on iron doped congruent lithium niobate within two –in principle equivalent- configurations, namely Y(ZX)Y and Y(XZ)Y. While the former respects the Raman selection rules, the other configuration gives a time dependent spectrum that, after a transient time of several minutes, finally results in a mixture of expected and forbidden modes. This breaking of Raman selection rules is caused by the spontaneous conversion of a part of the ordinarily polarized pump beam into an extraordinarily polarized beam by photorefractive anisotropic self-scattering. A numerical modelling of the phenomenon is developed and fairly reproduces the time dependence of conversion energy.
Highlights
Lithium niobate (LN) has various advantageous properties such as large nonlinear optical coefficients, high transparency in the visible and near infrared range, technology for the manufacturing of waveguides[1,2] and domain structures[3,4]
Just a few papers are concerned by the relationship between Raman spectroscopy and non linear optical or photorefractive properties[22,23,24]
We reported a noticeable shift of Raman line with time, which was interpreted as caused by the strain associated with the space charge field Esc
Summary
The Fe:LN crystal sample studied here was grown by the Czochralski method from a congruent melt with concentration of 0.03 wt% Fe. One can notice the beam pattern evolution, showing that after some time a new light field extraordinarily polarized is produced along two lobes symmetrically placed with respect to the pump spot This explains the activation of A1[TO]Raman lines, the intensity of which increases with time: this part of the Raman spectrum is produced by the newly generated extraordinary beams which corresponds to aY(ZZ)Y scattering geometry. For sake of simplicity, we will consider that the extraordinary beams are emitted only in one specific direction, i.e. in the plane xy at an angle theta with the primary beam corresponding to the direction along which the scattered intensity is maximized The overall relative error in performing our simplified treatment is expected to be in the range of 10–20% depending on the parameters
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