Quadrupole Contribution and Field-Induced Symmetry Breaking

Quadrupole Contribution and Field-Induced Symmetry Breaking 

The first term on the right-hand side of Equation (10.53) vanishes in media with cen-trosymmetiy, such as nematic liquid crystals. It is, however, nonvanishing if the nematic director n is distorted (e.g., curvature deformation that produces flexoelec-tric effects ). The second term is the quadmpole moment contribution, which is, in general, nonzero. 

By considering the symmetry and coordinate transformation properties of a nematic liquid crystal under the action of an applied electric field, these authors were able to obtain explicit expressions for the susceptibility tensors Xyj. and and explain the experimental observations of second-harmonic generations by several groups.  

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