Nematic liquid crystals birefringence

A nematic liquid crystal cell, based on Merck Licrilite E202, was used in these experiments. The rod like liquid crystal molecules preferentially aligned themselves with each other and to an alignment surface in the liquid crystal device. Any birefringence. An, was given as the difference between the two orthogonal refractive indices. As a consequence, any resulting... 

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). 

Such polymers adopt, when affected by a mechanical field, an optically uniaxial homeotropic structure polymers B.1.2, B.1.7, B.1.8 (Table 8) have positive birefringence polymers B.1.1, B.1.8. (Table 9) have negative birefringence, which has not been reported to our knowledge, for low-molecular nematic liquid crystals. Although the authors do not comment on the cause for the observed phenomenon, the fact in itself is sufficiently uncommon. 

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]... 

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. 

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. ...

Gooch and Tarry plot of transmission versus the coefficient, u, and the corresponding birefringence values, An, for a Twisted Nematic Liquid Crystal Display TN-LCD of thickness 6.55 pm operating between parallel polari-... 

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)... 

Schematic diagrams of die cell and apparatus for measuring die temperature dependence of die optical rotation of an aligned nematic liquid crystal cell. The birefringence cell is formed from microscope slides and die lines in the cell denote the rubbing direction for the glass plates and thus the direction for the nematic director n.

Liquid crystals are optically anisotropic media with the exception of the cubic phases, such as the D-phase and blue phase. The refractive index or dielectric constant of liquid crystals varies according to the orientation of the molecular axes (or the optical axes). For example, the nematic liquid crystals are optically uniaxial and exhibit remarkable birefringence, that... 

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. 

I. C. Khoo, Y. Liang and H. Li, "Polarization Switching by Stimulated Orientational Scattering in Birefringent Nonlinear Nematic Liquid Crystal Film," lEEE-Lasers and Electro-Optics Society Technical Meeting, San Jose, CA (November 1993). 

---