Birefringence of nematic liquid crystals

The stimulated thermal wave mixing effect discussed above can be used for application in a broad spectral range, owing simply to the broadband birefringence of nematic liquid crystals. The required absorption constant for optimizing the processes can be tailored with appropriate dye-doping. 

Another type of LCD utilises the birefringence of nematic liquid crystals to change the state of polarisation. In Section 11.3, the optical retardation introduced by a liquid crystal... 

An essential condition when measuring the birefringence of nematic liquid crystals is... 

The birefringence of the liquid crystals is controlled electronically. Either ferroelectric or nematic liquid crystals are used for the tunable retarders. By varying the input voltage to the liquid crystals the birefringence of that material can be changed to vary the retardation and change the interference pattern in such a way as to scan the spectral region of interest. 

Thus the birefringence of the liquid crystal is much more critical in the fluorescence polarization studies. Measurements of the order parameter of excited molecules are possible if the rotational relaxation time of the solute is small compared with the lifetime of the emitting state. The birefringence of the liquid crystal is also of critical importance if one wishes to determine the solvent order from the absorption polarization spectrum of the solvent in its nematic phase. The complications arise from the strong refractive index dispersion in the wavelength regions of the solvent absorption bands. Saupe and Maier [104] have treated this case. The same difficulties arise of course if the solute and the solvent absorption overlap. 

One of the most striking anisotropic properties of nematic liquid crystals is their optical anisotropy, which is manifested as birefringence. It was this property of liquid crystals that led to their discovery about 100 years ago, and early in the history of liquid crystals attempts were made to determine their birefringence. The first rough estimation was made by Lehmann in 1905 [1], who placed the nematic phase of 4,4 -bis(methoxy)azoxybenzene between a plane glass plate and a lens with a large radius of curvature. The birefringence... 

This partial fluorination has a number of interesting and even surprising effects [181]. First, it almost always totally suppresses the nematic phase and strongly enhances the smectic phases, in general both smectic A and smectic C. Moreover, it lowers the birefringence of the liquid crystal An may go from typically 0.20 to 0.07 or 0.10. It also makes the smectic C phase less hardtwisted the helical pitch may increase from about 2 pm to 5 -10 pm. 

A thin slab of nematic liquid crystal is filled between two glass parallel plates for use as a quarter-wave plate with a 488 nm laser. If the liquid crystal has a birefringence of 0.22, calculate the thickness of the liquid crystal slab. 

F. J. Kahn. "Electric Field-Induced Orientational Deformation of Nematic Liquid Crystals Tunable Birefringence. Appl. Phys. Lett., Vol. 20, p. 199 (1972). 

Deuling HJ, Helfrich W (1974) Hysteresis in the deformation of nematic liquid crystal layers with homeotropic orientation. Appl Phys Lett 25 129-130 Durbin SD, Arakelian SM, Shen YR (1981) Optical-field-induced birefringence and Freedericksz transition in a nematic liquid crystal. Phys Rev Lett 47(19) 1411-1414 Freedericksz V, Zolina V (1933) Forces causing the orientation of an anisotropic liquid. Trans Faraday Soc 29 919-930... 

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

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]... 

As discussed in section 7.1.6.4, semidilute solutions of rodlike polymers can be expected to follow the stress-optical rule as long as the concentration is sufficiently below the onset of the isotropic to nematic transition. Certainly, once such a system becomes nematic and anisotropic, the stress-optical rule cannot be expected to apply. This problem was studied in detail using an instrument capable of combined stress and birefringence measurements by Mead and Larson [109] on solutions of poly(y benzyl L-glutamate) in m-cresol. A pretransitional increase in the stress-optical coefficient was observed as the concentration approached the transition to a nematic state, in agreement of calculations based on the Doi model of polymer liquid crystals [63]. In addition to a dependence on concentration, the stress-optical coefficient was also seen to be dependent on shear rate, and on time for transient shear flows. 

The last polymer mentioned in Table 10.9 is poly(p-phenylene terephthalamide). This polymer is spun from its solution in pure sulphuric acid (100%), a dope that exhibits mesomorphic (=liquid crystalline) behaviour it is optically anisotropic and is nematic in character. A number of other polymers, containing rigid elements in the chain have melts of polymer liquid crystals, with a high birefringence and a non-linear optical behaviour in electric fields. 

Figure 2.8 The dependence of the refractive indices, n and ng, of the ordinary and extraordinary rays, respectively, on the temperature, T.for a typical nematic liquid crystal. Above the clearing point, Tg, there is no birefringence and only one refractive index,is observed. ...

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