Polarizer cholesteric liquid crystal

Other equally remarkable optical properties are associated with the selective reflection. At normal incidence, the reflected light is circularly polarized one circular component is totally reflected, while the other passes through unchanged. Also, quite contrary to what is found in normal substances, the reflected wave has the same sense of circular polarization as that of the incident wave. This is an important difference between the nature of the optical rotation of normal substances and of cholesteric liquid crystals. While the more familiar cases of optical rotation have their origin in the selective absorption of one circularly polarized component of the light, the form optical rotation of the twisted structure in cholesteric liquid crystals originates in the selective reflection of one circularly polarized component of the light. 

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

Thus, side-chain systems can exhibit many properties in between, well-oriented and solid materials. Many applications for cholesteric, nematic, and smectic cyclic siloxanes have been proposed. Most of them use cholesterics. Cholesteric liquid crystals (n ) or tilted smectic phases reflect the incident light in a specific wavelength range and with circular polarization. The... 

Photocyclization using circular polarized light yielded dihydro[5]helicene in a small enantiomeric excess (ee = 3 %). Attempts were also made to enantioselectively synthesize helicenes using chiral solvents as well as cholesteric liquid crystals Excellent enantiomeric excesses (up to 98%) were obtained through temporary introduction of optically active residues like mandelic acid, lactic acid derivatives and (—)-menthyl esters... 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

The Cano wedge can be used to measure the cholesteric pitch. Two substrates are wedged by a small angle 2y. The cholesteric liquid crystal between the substrates is homogeneously aligned. Under polarized microscopy, the equally spaced disclinations appear at the middle of the wedge. The separation of the lines 2d is associated with the pitch P... 

Cholesteric liquids can rotate polarized light to a large degree, such as some thousand degrees per 1 mm layer thickness for visible light [258]. Because of different absorption of the two polarized components in the cholesteric liquid, the material shows dichroism. Most of the effects are applied practically. The colour change in reflection with temperature of a cholesteric liquid can be used for very sensitive temperature measurements of 0.001°C It is even possible to construct infra-red/visible image converters with cholesteric liquid crystals. 

Cholesteric liquid crystals, e.g., those of cholesteroylnonaoate (see Sec. 3.2), produce a Bragg-type scattering, which depends on temperature and angles of incidence and observation. Either total reflection or total transmission of circular polarized light is observed, which effect provides the basis of the dark-bright liquid crystal display in the Schadt-Helfrich cell (Fig. 3.5.3) as well as color reflection. 

Fig. 4.1.15. First and second order reflexion spectra of a cholesteric liquid crystal film (0.45 0.55 mole fraction mixture of 4 -bis(2-methylbutoxy)-azoxybenzene and 4,4 -di-n-hexoxyazoxybenzene) 15 pitch lengths or 11.47 on thick. Angle of incidence 45°. Polarizer and analyser are parallel to the plane of reflexion for and normal to it for measurements. The small oscillations are interference fringes from the two cholesteric-glass interfaces. (After Berreman and Scheffer. )...

An example of this type of thermomechanical coupling appears to have been observed by Lehmann in cholesteric liquid crystals very soon after their discovery. He found that droplets of the material when heated from below seemed to be rotating violently, but from optical studies he concluded that it was not the drops themselves but the structure that was rotating. Fig. 4.4.1 shows a few of the many sketches that he made depicting his observations. Leslie s equations offer a simple explanation of the phenomenon because of the absence of mirror symmetry, an applied field, which is a polar vector, can result in a torque, which is an axial vector. 

Thus, all monomers of the ChMAA-n series fonn a monotropic liquid crystalline phase of the cholesteric type, whose temperature interval of existence depends on the rate of cooling. The liquid crystalline phase is unstable and is transformed to crystal phase so soon that X-ray examination of the mesophase structure becomes difficult. Nevertheless, polarization-optical studies have made it possible to draw certain conclusions as to the nature of the liquid crystalline phase of monomers. Cooling of isotropic melts of monomers results in a confocal texture which turns to a planar one when a mechanical field is superimposed on the sample, for example, by shifting a cover glass in the cell of the polarizing microscope (Figure 4). The observed planar texture exhibits the property of selective light reflection, which is typical of low-molecular cholesteric liquid crystals. 

Similar through-space asymmetric polymerization from achiral mono-, di-, or tri-thiophenes and pyrrole monomers was also achieved by the use of cholesteric liquid crystals as an asymmetric reaction solvent [19]. As no reaction occurred between the molecules of the liquid crystal and the monomers, the chiral morphology of the polymers (which have no chiral substituent) is considered to derive from the asymmetry produced by the chiral liquid crystal medium during polymerization. Heat treatment of the polymer causes disaggregation and a loss of chirality, and polymers prepared in this way exhibit an exiton splitting signal in the circular dichroism spectra in the absorbance region of the polymeric backbone they also display a circular polarized luminescence. A representative example is shown in Scheme 8.2 [19]. 

Fig. 4.30 A fingerprint texture of a cholesteric liquid crystal seen in a polarization microscope (the distance between stripes equals a half-pitch)...

Fig. 12.5 Comparison of the non-polarized light transmission by a stack of dielectric layers and a cholesteric liquid crystal (CLC). The two materials have the same Bragg reflection frequency (numerical calculations, for parameters see the text), (a) Transmission spectra on the frequency scale showing the absence of high harmonics in the case of CLC (b) blown transmission spectra at the wavelength scale showing the flat form of the CLC Bragg band and oscillations of transmission at the edges of the band...

We showed in last section that in a uniform anisotropic medium, for each propagation direction, there are two eigenmodes which are linearly polarized. The polarization state of the eigenmodes is invariant in space. In this section, we discuss the propagation of light in a special case of a non-uniform anisotropic medium a cholesteric liquid crystal which locally is optically uniaxial, but the optic axis twists uniformly in space [6,7]. Choose the z axis of the lab frame to be parallel to the helical axis of the cholesteric liquid crystal. The pitch P of the liquid crystal is the distance over which the liquid crystal director twists In. The components of the liquid crystal director of a right-handed cholesteric liquid crystal q > 0) are given by... 

Cell thickness-dependence of the reflection of a cholesteric liquid crystal in the planar state. The pitch of the liquid crystal is P = 350 nm. The refractive indices of the liquid crystal are tig = 1 -7 and = 1.5. The liquid crystal is sandwiched between two glass plates with the refractive index = 1.6. The incident light is circularly polarized with the same helical handedness as the liquid crystal. Neglect the reflection from the glass-air interface. Use two methods to calculate the reflection spectrum of the liquid crystal with the following cell thicknesses P, 2P, 5P and lOP. The first method is the Berreman 4x4 method and the second method is using Equation (2.186). Compare the results from the two methods. 

Use the Berreman 4x4 method to calculate the reflection spectra of the cholesteric film under the polarization conditions specified in Figure 3.10. The parameters of the cholesteric liquid crystal are also given in Figure 3.10. 

Cholesteric liquid crystals consist of chiral molecules and therefore do not have reflection symmetry. The symmetry group of cholesteric hquid crystals is >2 [1,3]- A cholesteric liquid crystal is invariant for the two-fold (180°) rotation around n, which rules out the possibility of spontaneous polarization perpendicular to n. It is also invariant for the two-fold rotation around an axis that is perpendicular to the n - (the hehcal axis) plane, which mles out the possibility of spontaneous polarization parallel to n. Therefore there is no ferroelectricity in the cholesteric phase. 

S. V. Shiyanovskii, 1.1. Smalyukh, and O. D. Lavrentovich, Computer simulations and fluorescence con-focal polarizing microscopy of structures in cholesteric liquid crystals, p. 229, in Defects in liquid crystals computer simulations, theory and experiments (Kluwer Academic Publishers, Netherland, 2001). 

This is the typical optical rotatory power of cholesteric liquid crystals with pitch shorter than the hght wavelength. In the blue phases, the double-twist cylinders orient along many different directions, and therefore the optical rotatory power is smaller than that of cholesteric phase. The thickness of blue phases displays are typically a few microns, and over this distance the hehcal structure in the blue phases does not change much the polarization state of light. 

Cholesteric liquid crystals (CLCs) can also be used to make reflective polarizers. CLCs reflect circularly polarized hght with the same handedness as the helical stmcture of the liquid crystal. An unpolarized incident hght can be decomposed into a left-handed circular polarized hght and a right-handed circular polarized hght. One component is reflected and the other component is transmitted. The reflected hght is reflected toward the CLC polarizer by a back mirror and its handedness is converted to the opposite handedness and thus it passes the CLC polarizer. The transmitted circular polarized hght is converted into hnear polarization by a quarter waveplate. 

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