Optical applications, chiral nematics

The application of an electric field above the threshold value results in a reorientation of the nematic liquid crystal mixture, if the nematic phase is of negative dielectric anisotropy. The optically active dopant then applies a torque to the nematic phase and causes a helical structure to be formed in the plane of the display. The guest dye molecules are also reoriented and, therefore, the display appears coloured in the activated pixels. Thus, a positive contrast display is produced of coloured information against a white background. The threshold voltage is dependent upon the elastic constants, the magnitude of the dielectric anisotropy, and the ratio of the cell gap to the chiral nematic pitch ... 

The pitch of the helix for compound 42 was found to be approximately 0.2-0.3 xm, thus the material selectively reflects visible light over a wide temperature range. Moreover, the pitch is relatively temperature insensitive thus the material can be used in large area non-absorbing polarizers, or in optical notch filters or reflectors. In addition, in the glassy state the helical macrostructure of the chiral nematic phases is retained, thus similar applications are possible. 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

Chiral nematic mesoporous films of Eu " doped Zr02 have been produced via a hard-templating approach using nanoctystalline cellulose-templated silica (Fig. 11). It was found that these chiral nematic nanostructures are capable of modulating the spontaneous emission of the Eu ions. The emission lines of the Eu " at 596 nm, 613 and 625 nm were significantly suppressed, and an increase in the luminescence lifetime is observed. It was suggested that these new chiral luminescent nanomaterials could find potential applications in sensing and new optical nanodevices. 

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

All interest in the unique optical properties of the chiral nematic phase stems from the two-fold optical activity of the phase (5, 6-9]. That is, the mesophase displays (1) molecular optical activity - the phase being composed of optically active molecules and (2) macromolecular optical activity - arising from the macroscopic helical twist induced by the chiral molecules in the phase (sometimes termed form chirality). These features are of immense importance technologically, as many of the applications of such materials depend on one if not both of these phenomena. 

As the temperature is decreased the chiral nematic structure transforms to a higher order phase. The phase may go through a first order phase transition and crystallize in which case the optical properties are of little interest herein. It may transform to a glass, in which case the optical properties, such as birefringence, pitch, etc., are frozen and may be used in static, or time and environment-independent devices or applications (as discussed in Sec. 2.5 of this Chapter), or it may go through a second order or second order plus a weak first order phase transition to a higher order liquid crystalline phase. Here, for simplicity, we are not considering the so-called re-entrant phases [ 14]... 

The application of chiral nematics to THG processes has long been recognized [198-200]. These are passive applications in which mirrorless optical bistability has been predicted [198] and the theory modi-... 

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