Electro-optic behaviour

Fig. 26a and b. Influence of the electric field frequency on the electro-optical behaviour of the nematic polymer XII and scheme of mesogenic groups orientation before (A) and after (B) the application of electric field (a) optical transmission as a function of time at different frequencies (U = 30 V T = 75 °C) (b) optical transmission as a function of time upon application of an electric field at U = 85 V (f = 50 Hz) (1) relaxation upon switching the electric field off (2), upon application of an electric field (U = 80 V) of different frequency f = I (3) 5 (4) 7 (5) and 20 kHz (6) during the relaxation process... 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

Cations that have a higher valency than those they replace, when present at levels exceeding about 0.5 cation percent, e.g. La3+ in place of Ba2+ or Nb5+ in place of Ti4+, generally inhibit crystal growth. This has the effect of raising the permittivity level below the Curie point as shown in Fig. 2.48. Crystal size is also controlled by sintering conditions. It has important effects on the electro-optical behaviour. 

The high anisotropy of Qd can influence the electro-optical behaviour of nematics even at low frequencies, especially for those materials, which have a low frequency inversion of dielectric anisotropy e. An example is shown in Fig. 7.24 related to a material with e inversion at/= 700 kHz. The electric conductivity begins to deviate from the ionic, low frequency plateau at 200 Hz and 10 kHz for the longitudinal and transverse components, respectively. At/ = 10... 

Such a great growth of conductivity anisotropy dramatically influences the electro-optical behaviour of nematics in this frequency interval. 

Clark, N. A. D. Coleman, and J. E. Maclennan. 2000. Electrostatics and the electro-optic behaviour of chiral smectics C Block polarization screening of applied voltage and V-shaped switching. Liq. Cryst. 27(7) 985-990. 

See, for example, Marshall, K. L., E. Kimball, S. Mcnamara, T. Z. Kosc, A. Trajkovska-Petkoska, and S. D. Jacobs. 2004. Electro-optical behaviour of polymer cholesteric liquid crystal flake/fluid suspensions in a microencapsulated matrix, in Liquid Crystals VIII, edited by 1. C. Khoo, SPIE Proceedings Vol. 5518 (SPIE Bellingham, WA 2004) pp. 170-181 and references therein see also, Marshall, K. L., T. Z. Kosc, S. D. Jacobs, S. M. Paris, and L. Li. 2003. Electrically switchable polymer liquid crystal and polymer birefringent flake/fluid host systems and optical devices utilizing same, U.S. Patent No. 6,665,042 B1 (Dec 2003). 

C.V. Brown, P.E. Dunn and J.C. Jones, The effect of the elastic constants on the alignment and electro-optic behaviour of smectic C liquid crystals, Euro. Jnl. of Applied Mathematics, 8, 281-291 (1997). 

Molecular flexibility can be estimated, but only to a limited extent. Because of the lack of suitable theories for the electro-optic behaviour of semi-flexible molecules, dynamic relaxation properties are generally studied. The field-free relaxatirm time T can be analysed in terms of the persistence length concept using suitable equations. In addition, the scattering parameter/ may be used. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

For an organic material to show electro-optical activity it must exhibit high second order non-linear behaviour. This is one of the terms in the equation that defines the polarisation (u) of an organic molecule as shown below, where E is the perturbing electric field. 

The simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

The properties of thin films are primarily determined by the type of chemical element or compound they comprise and by the film thickness. Their optical, electro-optical, electrical and mechanical behaviour is also determined by structure, microstructure, surface and interface morphology, chemical composition, purity and homogeneity. These are strongly influenced by the film preparation method, the chosen parameters, and by post-deposition treatments. 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

Evidently, these characteristic mesophase structures are responsible for the occurrence of anisotropic physical properties, manifested for example in the magnetic, electrical and optical behaviour [16]. Various successful applications, particularly in the field of electro-optical displays, take advantage of these prconinent features, explaining the common interest in LMLCs [12, 14]. 

Furthermore, they examined the performance of different density functionals, including a local-density approximation and a generalized-gradient approximation as well as the functional of van Leeuwen and Baerends that has been constructed to have the correct asymptotic behaviour. Moreover, they considered different frequency-dependent processes, including third-harmonic generation [THG, corresponding to y( 3electric-field-induced second harmonic generation (EFISH, y( 2electro-optic Kerr effect [EOKO, y(—ft> optical rectification [OR, /S(0 [Pg.161]

In electro-optical measurements, one determines essentially an electrical dipole moment which is related to the number and the spatial distribution of mobile ions in the vicinity of a colloid surface. This technique - light scattering in the presence of an electrical field - was abundantly us in the investigation of the electrical properties of colloids dispersed in water. Extensive monographs on colloid electro-optics were published (7-2). In recent years, the effect of water soluble polymers on the behaviour of hydrosols (especially colloids of mineral or metallic origin) began to raise interest. Polymers partly because of their large... 

The optical microscopy or TOA of the smectic side-chain polymers shows the important effect that viscosity has on the behaviour of these systems, i.e. the existence of T and the fluid region, the rate of growth of texture on cooling and the immobility of the texture below r. Since smectic materials will be more viscous than the nematic phases, this suggests again that the materials should be useful for the storage of electro-optic information if this information can be written on an acceptable timescale. 

Ling, H., Holland, W., and Gordon, M., DC electrical behaviour of polymers used in electro-optic devices, J. Appl. Phys., 70, 6669-6673 (1991). 

In this chapter, I will conduct a review on some of the fundamental material properties of relaxor ferroelectric PLZT ceramics, which include the dielectric, ferroelectric, electromechanical, electro-optical and thermo-optical behaviours. Further details on each section can be found in the references (Levesque and Sabat 2011 Sabat, Rochon, and Mukherjee 2008 Sabat and Rochon 2009b Sabat and Rochon 2009c Sabat and Rochon 2009a). 

A. General, physical and inorganic diemistry Nuclear physics and nuclear diemistry Ai Optical behaviour of matter At Electricity. Magnetism. Electro-chemistry At, Thermodynamics. Thermochemistry At CoUoid Chemistry. Boundary layer research At Structural research A, Equilibra. Kinetics... 

Fume H, Takahashi T, Kobayashi S, Yokoyama H (2002) Models of molecular alignment stmcture in polymer-stabilized ferroelectric liquid crystals. Jpn J Appl Phys 40(l) 7230-7233 Giepehnann F, Zugenmaier P (1995) Mean-field coefficients and the electroclinic effect of a ferroelectric hquid crystal. Phys Rev E 52(2) 1762-1772 Giepehnann F, Hermann A, ZugeranaiCT P (1997) Experimental determination of Landau-expansion coefficients in ferroelectric liquid crystals. Ferroelectrics 200 237-256 Guymon CA, Hoggan EN, Walba DM, Qark NA, Bowman CN (1995) Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal. Liq Cryst 19 719-727... 

---