Reorientational birefringence

Figure 22. Refractive index modulation as a function of the external angle of the probe beam with respect to the sample normal for the material given in [20], when the writing beams are arranged at a constant 55° and 75° to the sample normal. A) angular variation if reorientational birefringence is dominant B) angular variation if Pockels response were the primary source of index contrast.

A second type of behavior existing in the PLZT s is the linear (Pockels) effect which is generally found in high coercive field, tetragonal materials (composition 3), This effect is so named because of the linear relationship between An and electric field. The truly linear, nonhysteretic character of this effect has been found to be intrinsic to the material and not due to domain reorientation processes which occur in the quadratic and memory materials. The linear materials possess permanent remanent polarization however, in this case the material is switched to its saturation remanence, and it remains in that state. Optical information is extracted from the ceramic by the action of an electric field which causes linear changes in the birefringence, but in no case is there polarization reversal in the material. 

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]... 

Ellipsometry can also be applied to transmission measurements linear birefringence and dichroism of an anisotropic sample cause differences in amplitudes and phase shifts for waves of different polarization azimuths. Such experiments seem to be of considerable interest for partially ordered systems such as liquid crystals (cf. Sec. 4.6), here the degrees of polarization P and Ppf, reveal information related to order and reorientation processes (Korte et al., 1993 Reins et al., 1993). 

Birefringence of Strongly Dipolar Microsystems. In dealing with strongly dipolar but weakly polarizable microsystems, i.e. when X < X the term in Xi in the Langevin function (234) can be omitted. The reorientation function (233) now becomes ... 

Birefringence of Non-dipolar Microsystems. In the case of anisotropic non-dipolar or very weakly dipolar molecules, Xf < and equation (233) leads to the following reorientation function ... 

Birefringence of Dipolar Anisotropically Polarizable Microsystems. In the general case of microsystems which are dipolar and at the same time anisotropically polarizable in an external field E, the reorientation function for the Kerr effect is given by equation (233). Graphs of this reorientation function are shown in Figure 13 against the parameter Xi at parametrical values of X = X ln for = 1,4,9,16,25, 36,. 

As mentioned in Section 5.1, the reorientation of azobenzene photo-chromic moieties under illumination with polarized light and the birefringence that is induced from it are well known and covered in many publications. 

In polyurethane 33, the azobenzene moieties are separated from each other along the polymer backbone by isophorone units and have the free volume necessary for isomerization and molecular reorientation. This polymer s azobenzene moieties can be photoisomerized readily in mixed LBK films, as demonstrated by measuring the optically induced birefringence that originates from the photoreorientation of the chromophore upon polarized irradiation see Section 6.5.2). 

The photoselection in azobenzene polymers (Figure 11) is rather a general phenomenon.77-84 Systematic studies have been conducted by Natansohn and coworkers on the photoinduced molecular reorientation of azobenzene units to induce birefringence and dichroism of thin films of acrylate amorphous polymers with Dispose Red (DR) pendant residues, which have been attracting extensive... 

A comparison has been made between the reorientational behavior of a series of copolymers of DR with MMA (13, Figure 4) and that of series of blends of a DR homopolymer (12) with PMMA to elucidate the sequential effect of the DR monomer units on the orientability caused by intramolecular interactions between dye chromophore groups in polymer solids.79 The maximum birefringence induced by linearly polarized light for the blends80 increased linearly with an increase in the DR fraction. In contrast, the copolymers showed a nonlinear... 

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