Refractive index, anisotropy

Formula (4.370) leads to a simple expression for the macroscopic optical anisotropy. If the Oz axis of the coordinate framework coincides with the applied held direction h, then the uniaxial symmetry condition yields (n2) = (n2) and vx = vy. Hence for the refractive index anisotropy (birefringence) we have... 

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

Perfluoroalloxy liquid crystals were prepared by the Williamson ether synthesis using perfluoro iodopropene. These materials can be mixed with nematic liquid crystal materials to provide liquid crystal compositions having low viscosity, low refractive index anisotropy, high dielectric anisotropy, and broad nematic phase ranges. 

Linearly polarized pumping light induces refractive index anisotropy in the sample, and linearly polarized probing light is changed to elliptically polarized light. 

Figure 15.20 shows schematically refractive index ellipsoids of polyimide prepared on isotropic substrates and imiaxially drawn polyimide. The films prepared on an isotropic substrate have no refractive index anisotropy in the... 

Results for a variation of An from 0.2-0.1 are given in Fig. 10. In this case the characteristic curves are steeper for smaller An but there is a residual transparency of about 10% in the off state. The curves are calculated for 633 nm laser-light. The optimized refractive index anisotropy for a 10 ym TN cell should therefore have a value between An=0.2 and An=0.1. 

Our calculations are summarized in Table 1. From a theoretical point of view the optical characteristics of a TN display can be optimized by using a material with a small K33/K2] in a cell with low tilt angle at the surfaces. In addition the cell thickness and the refractive index anisotropy must be adjusted. 

Fig. 4. Dependence of tilt determined from Eq. (1) on the refractive index anisotropy for several values of symmetry offset angle F and twist 90 .

Table 6. Dependence of figures of merit on the refractive index anisotropy.

For higher refractive index anisotropy n, the geometrical requirements on the nematic structure which need to be satisfied in order to achieve compensation of birefringence become more stringent. Thus, the transmission versus voltage curve for normal incidence shifts toward higher voltages and becomes less steep. 

Koyama H (2004) In-plane refractive-index anisotropy in porous silicon layers induced by polarized illumination during electrochemical etching. J Appl Phys 96(7) 3716-3720 Kiinzner N et al (2001) Giant birefringence in anisotropically nanostructured silicon. Opt Lett 26(16) 1265-1267... 

Krzyzanowska H, Kulik M, Zuk J (1999) Ellipsometric study of refractive index anisotropy in porous silicon. J Lumin 80 183-186... 

The role of push-pull molecules in PR systems is to provide refractive index modulation in response to an electric field. In polymers with a low glass transition temperature (Tg), where the molecules are relatively free to rotate due to a higher free volume, the chromophores can reorient in the applied/generated electric field. Because of the anisotropy in polarizability of the chromophores, such a reorientation will induce high refractive index anisotropy termed as orientational birefringence. The ability of a chromophore molecule to induce refractive index change is usually expressed as the figure of merit (FOM) defined by Equation 8.4 [44],... 

Here / is transmission for parallel polarizers, cell thickness, is the angle between the polarization din on and tbe director B , is the refractive index anisotropy, and is the vacuum optical wavelength. Maximum transmiaBion and contrast are ac vable for... 

The mainstream method at that time of producing a pressure-sensitive adhesive tape was to coat the adhesive directly onto the substrate. However, there was no technology to precisely coat the adhesive material onto the polarizers that had a low scratch threshold and low heat resistance. There were very difficult problems of contamination and air bubbles, limits to the drying process, and the strict demand for small refractive index anisotropy. Slight uneven thickness of the adhesive will lead to defects in the display, even for pressure-sensitive adhesives. AU this was completely different from the demands of customers of electronic adhesive tape so far. The aforementioned problems all led to visible optical distortions in the polarizer film. For this reason, the coating facility to apply the adhesive had to be installed in a clean room, the coating method of the pressure-sensitive adhesive transfer had to be studied, a stacked twist suppression technology had to be established, and the formation of microcrystals in the pressure-sensitive adhesive had to be suppressed, to satisfy customer requirements. 

We succeeded in fabricating polymer-stabilized ferroelectric liquid crystals exhibiting a transmittance as a function of applied voltage with a V-shaped switching with a 3 pm of cell gap as shown Fig. 8.24. The ceU thickness was 1.8 and 3 pm. As shown in Fig. 8.24, the 3 pm cell does not show any inferior uniaxial alignment compared with the 1.8 pm cell. This means that productirMi of PSV-FLCDs becomes possible for 3 pm cells. However, the maximum transmittance is lower for the 3 pm cell, because the retardation of the liquid crystal (xdAnIX) depends on An (refractive index anisotropy) and d (cell thickness). An improvement of the maximum transmittance in thicker cells is expected in the future because the development of ferroelectric liquid crystal with a low An is in progress. 

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