Optical and Nonlinear Optical Properties of Rb2ZnCl4 in the Incommensurate and Ferroelectric Phases

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Abstract
The incommensurate-commensurate phase transition in Rb2ZnCl4 has been investigated by means of refractive index measurements and second-harmonic generation experiments. The anomalies of the refractive indices n1, and n3 at the phase transition temperature T c can be explained by assuming Bloch type light waves propagating in the incommensurate structure, vanishing gradient terms in the Landau expansion of the free energy density and a discontinuity of the amplitude of the order parameter at T c. In the incommensurate phase the observed second-harmonic signals are generated by the symmetry-allowed coefficients of the ferroelectric phase and are explained to be due to the presence of a defect-induced unipolar domain structure. In the ferroelectric phase the nonlinear optical coefficients d32, d33, and d24 are shown to be proportional to the spontaneous polarization.

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