Anisotropy, optical

Abstract Macromolecular coils are deformed in flow, while optically anisotropic parts (and segments) of the macromolecules are oriented by flow, so that polymers and their solutions become optically anisotropic. This is true for a macromolecule whether it is in a viscous liquid or is surrounded by other chains. The optical anisotropy of a system appears to be directly connected with the mean orientation of segments and, thus, it provides the most direct observation of the relaxation of the segments, both in dilute and in concentrated solutions of polymers. The results of the theory for dilute solutions provide an instrument for the investigation of the structure and properties of a macromolecule. In application to very concentrated solutions, the optical anisotropy provides the important means for the investigation of slow relaxation processes. The evidence can be decisive for understanding the mechanism of the relaxation. 

The bulk crystal structure of zinc-blende-type III-V semiconductors is cubic, that is, of point group symmetry (Section 13.2). Such crystals are optically isotropic in first-order approximation, that is, if symmetry reducing effects, such as finite k-vector electric fields and other effects are neglected [138]. Then the RAS signal should originate from the surface that is anisotropic due to the termination of crystal periodicity and due to the surface reconstraction [127-129]. Apart from the structural anisotropy and related effects such as strain, the surface electric field accompanied by the band bending will modify electronic eigenstates and induce an optical anisotropy as well. 

For noncubic compounds, the bulk crystal may be optically anisotropic. This is the case not only for wurtzite or chalcopyrite crystal structures but also for ternary and quaternary III-V and II-VI compounds showing ordering effects on the sublattices. In aU these cases, the compound semiconductor is an optically anisotropic material. The RAS signal is then a superposition of two contributions, one originating from the bulk and the other from the surface. In order to analyze the surface, the two contributions have to be separated, a task for optical modeling. 

In contrast, for studying materials with an isotropic bulk structure, for example, zinc blende, RAS is a very versatile optical method for surface analysis. In this case, the optical anisotropy is surface specific, regardless of the large penetration depth of the light, which usually far exceeds the surface region (a few atomic layers). Therefore, the RAS represents a quite usefiil optical method for surface investigations of zinc blende materials. 

Bulk- and surface-derived transitions are indicated as En and Sn, respectively. The origin 

FIGURE 2.10. Measuring liquid crystal electrical conductivity. 1. Electrometer. 2. Generator. 3. Measuring sample with the cell (4) filled with liquid crystal (5). 

If we consider the behavior of uniaxial liquid crystals at optical frequencies (cd o d 10 s ), the orientation polarization component will not enter into the discussion. A contribution to the electric polarizability of liquid crystal molecules, 7,f and 7, at optical frequencies is made only by the electronic and atomic parts. Consequently, both parts of the complex refractive index (i.e., the refractive index n and the absorption coeflScient k) become anisotropic and each has two principal components (n , nj and /c,  

Typical temperature dependences of the principal refractive indices for three nematic liquid crystals are given in Fig. 2.11 (information on the refractive indices of smectic crystals is given in [25]). 

The average value of the refractive indices in the nematic phase is given by the relationship 

According to (2.12) the optical anisotropy As = njf — n is completely defined by the anisotropy of polarizability, measured parallel and perpendicular to the long molecular axis A7. The values of optical anisotropy increase [14,28] with  

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Aspnes D E 1985 Above-bandgap optical anisotropies in cubic semiconductors a visible-near ultraviolet probe of surfaces J. Vao. Sc/. Teohnoi. B 3 1498-506... 

Wetzel, R., Zirwer, D., Becker, M. Optical anisotropy of oriented deoxyribonucleic acid films of diffi-rcrit water content. Biopolymers 8 (1969) 391-401... 

In the single-domain state, many ferroelectric crystals also exhibit high optical nonlinearity and this, coupled with the large standing optical anisotropies (birefringences) that are often available, makes the ferroelectrics interesting candidates for phase-matched optical second harmonic generation (SHG). 

Perhaps the most astounding property of this material is the high degree of transparency. This arises first because both molecules and crystals show little optical anisotropy and secondly because crystalline and amorphous zones have similar densities. They also have similar refractive indices and there is little scatter of light at the interfaces between amorphous and crystalline zones. 

The quantitative assessment of the overall orientation of PET fibers is generally made on the basis of fiber optical anisotropy measurements, i.e., measurements of the optical birefringence of the fiber. The determination of the value of optical birefringence makes it possible to determine the value of Hermans function of orientation based on the equation ... 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

Kashiwagi et al.10) determined the second moment anisotropy for the one-way drawn polyethylene terephthalate sheets discussed above. The three lattice sums S00, S2q and S4o were calculated from the crystal structure determination of Daubeny et al., the proton positions being calculated on the basis of known bond angles and lengths. The isotropic lattice sum S00 was adjusted to a value consistent with the measured isotropic second moment of 10.3G2. The values for P200, P220 etc. were then used to predict the optical anisotropy. The predicted refractive indices for the sheets of draw ratio 2 1 and 2.5 1 are shown in Fig. 10, together with the experimental... 

Fig. 2 Raman spectra of a single-crystal of orthorhombic Sg at three different polarizations in which the off-diagonal elements of the Raman scattering tensor are non-zero big, b2g, b g), after [105]. However, Raman intensities of other polarizations like flg components ( 54 cm ) penetrate in the spectra due to optical anisotropy in the crystal...

Hashimoto T., Shibayama M., Kawai H., and Meier D. J. Confined chain statistics of block polymers and estimation of optical anisotropy and domain size. Macromolecules, 18, 1855, 1985. 

Inset Figure 8 shows that the sizes of most of the particles remain similar. However, it can be observed that, in part of the TEM pattern, some particles form chains, which are either close together or coalesced. The presence of the two plasmon modes (Fig. 8B) indicates an optical anisotropy due to the nonspherical shape of the particles, as shown on the TEM grid (inset Fig. 8). 

Bone, R. A. and G. Draper (2007). Optical anisotropy of the human cornea determined with a polarizing microscope. Allied Optics 46 8351-8357. 

Zdzislaw, S. Gordon, T., Optical anisotropy in lipid bilayer membranes Coupled plasmon waveguide resonance measurements of molecular orientation, polarizability, and shape, Biophys. J. 2001, 80, 1557 1567... 

Figure 1.11. Schematic view of a zeolite L crystal loaded with type 1 (Fig. 1.8) dyes with electronic transition moments aligned along the axis of the channels. Left Side view of the morphology, size, and optical anisotropy of the material. Right Front view of a few individual dye-filled channels. The polarization of absorbed and emitted light is indicated.

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

K. Kinosita, Jr., S. Kawato, and A. Ikegami, Dynamic structure of biological and model membranes Analysis by optical anisotropy decay measurement, Adv. Biophys. 17, 147-203 (1984). 

Figure 4.1. Time scales for rotational motions of long DNAs that contribute to the relaxation of the optical anisotropy r(t). Experimental methods used to study these motions in different time ranges are also indicated along with the authors and dates of some early work in each case. FPA, Fluorescence polarization anisotropy (Refs. 15, 18-20, and 87) TPD, transient photodichroism (Refs. 28 and 62) TEB, transient electric birefringence (Refs. 26 and 27) DDLS, depolarized dynamic light scattering (Ref. 116) TED, transient electric dichroism (Refs. 25, 115, and 130) Microscopy, time-resolved fluorescent microscopy (Ref. 176).

Though derived for DNA, and nucleosome core particles, these twisting correlation functions are potentially useful for the analysis of other filamentous biopolymers. Such analyses of optical anisotropy data for actin filaments... 

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