Efficiency electrical birefringence

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

As shown previously, nematic liquid crystals reorient easily in weak electric fields and their high birefringence provides an efficient electro-optic mechanism that makes them excellent candidates for photorefractive materials. However, charge transport relies on the generation of mobile anions or cations. These mobile charges obey the current density (/) equations given by [82,83]... 

We discovered this phenomenon by chance when attempting to compare the efficiency of the photoinduced birefringence process with the efficiency of the refractive index modulation under electric field in photorefractive polymers. 

One very important advantage of FLCPs over amorphous poled polymers is the possibility of phase matching, which is a precondition for efficient SHG. The mismatch of the refractive indices of the fundamental wave and the second harmonic due to the dispersion may be overcome using the birefringence of the FLC material in the geometry shown in Fig. 37 [15]. Due to the C2 symmetry of the unwound smectic C phase (achieved by surface stabilization or by application of a DC electric field along the y direction), the tensor has only four independent elements, which may be denoted in the contracted d tensor notation as... 

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