Doping nematic

The concept of local perturbations of the director around nanoparticles, often linked to homeotropic anchoring to the nanoparticle surface, is a concept often brought forward in discussions of thermal, optical and electro-optic properties of nanoparticle-doped nematic liquid crystals, which adds a slightly different perspective to the invisibility of smaller particles in aligned nematics. This appears to be of particular relevance for particles coated with either hydrocarbon chains or pro-mesogenic as well as mesogenic units. 

BaTiC>3 particles are another very attractive and intensively studied type of nanoparticles in nematic liquid crystals. Cook et al. reported on an asymmetric Freedericksz transition, where doping nematic TL205 with single domain ferroelectric BaTiC>3 nanoparticles (9 nm in diameter) reduced or increased the threshold voltage by 0.8 V depending on the polarity of the applied voltage [149]. 

The viscosity of a typical cholesteric made by doping a nematic with a modest amount of chiral nematic is much lower (around 1 P or so) than that of the typical pure eholesteric. Perhaps this is because the pitch of the doped nematic is higher than that of the typical pure cholesteric, or because the twist elastic constant of the doped nematic is much lower. 

Van Deun et al. were the first to observe near-infrared luminescence from lanthanide-doped liquid crystal mixtures They studied the spectroscopic properties of the lanthanide(III) / -diketonate complexes [L (dbm)3(phen)], where Ln = neodymium, erbium, ytterbium, and dbm is dibenzoylmethane, in the liquid crystal MBBA. By incorporation of an erbium(III)-doped nematic liquid crystal (ErCls dissolved in E7) in the pores of microporous silicon, narrowing of the erbium(III) emission band in the near-infrared was observedJ Luminescent optically active liquid crystals were obtained by doping [Eu(tta)3-3H20] into a mixture of cho-lesteryl nonanoate, cholesteryl tetradecanoate and the ternary liquid crystal mixture ZLI1083 from MerckJ ... 

The linear stability analysis of the stationary distorted state above the OFT induced by obliquely incident, linearly polarized light was repeated in [12], this time without the numerous approximations used in earlier works, using numerical methods. In addition, dye-doped nematics were considered, for which the threshold for the OFT is much lower and thus permits the experimental realization of a light beam that is much wider than the cell width. For this case, however, absorption also has to be taken into account, which means that the OFT does not happen at p = 1 for perpendicular incidence (see Fig. 16). Taking all of this into account, the exact reorientation profiles for the director were calculated numerically first, and its stability was investigated with respect to spatially periodic perturbations (proportional to exp[i qx + py)]). Then the... 

Figure 16. Bifurcation diagram of a dye-doped nematic excited by obliquely incident, linearly...

Chen AG, Brady DJ. 1993. Two wavelength reversible holograms in azo dye doped nematic liquid crystals. Appl Phys Lett 62(23) 2920 2922. 

Kreuzer M, Marrucd L, Paparo D. 2000. Light induced modification of kinetic molecuclar properties enhancement of optical Kerr effect in absorbing liquids, photoinduced torque and molecular motors in dye doped nematics. J Nonlin Optical Phys Mater 9 157 182. 

Another bistable device is the Binem display, which does not involve flexoelectric effects but is sufficiently similar for a brief mention. This device uses a chirally doped nematic in a cell with parallel alignment directions... 

Motoc, C. lacobescu, G. Birefringence of azo-dye doped nematic liqnid crystals. J. Optoelectron. Adv. Mater. 2006, 8, 295-298. 

Chen, A. Brady, D. J. (1993). Two-wavelength Reversible Holograms in Azo-dye Doped Nematic Liquid Crystals. Appl. Phys. Lett., 1993, 62,2920/1-3. 

T. T. Alkeskjold, J. Lsegsgaard, A. Bjarklev, et al., All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers. Opt. Express 12, 5857 (2004). 

M. Warenghem, J. F. Henninot, and G. Abbate, Nonlinearly induced self waveguiding structure in dye doped nematic liquid crystals confined in capillaries, Opt. Express 2, 483 (1998). 

FIGURE 4.27. Transmission characteristics of a twist-cell in parallel polarizers at oblique incidence [159], ie = 30°. (a) Nematic mixture without dye and (b) dye-doped nematic mixture. Curves 1, 2, 3 correspond to different voltages Ui[Pg.185]

The conduction regime as described by Figs. 2 to 4 occurs in suitably doped nematic liquid crystals of negative dielectric anisotropy. This can be seen qualitatively by studying the denominator of Eq. [32]. The first (viscous) term must be larger than the second (dielectric) term for the denominator to be positive and instability to occur. This restricts negative values of Ae to the range —2 to 0 for materials... 

Ilchishin, I. R Maslov, P. Yu. Tikhonov, E. A. Lipnitsky, S. O. Stepanov, A. A. Lasing in dye-doped nematic liquid crystals at a dynamic distributed feedback for two-scheme excitation. Mol. Cryst. Liq. Cryst. 2007, 467, 235-245. 

Deshmukh RR, Malik MK (2013b) Photo-polymerization kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer. Liq Cryst 40 1050-1059... 

Figure 1.16. Alignment of a dichroic dye-doped nematic liquid crystal (a) before application of switching electric field (b) switching field on.

Khoo, I. C., S. Slussarenko, B. D. Guenther, and W. V. Wood. 1998. Optically induced space charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals. Opt. Lett. 23 253-255. 

Khoo, I. C. 1996. Holographic grating formation in dye- and fiiUerene C60-doped nematic liquid crystal film. Opt. Lett. 20 2137. 

Khoo, I. C., J. Ding, Y. Zhang, K. Chen, andA. Diaz. 2003. Supra-nonlinear photoreffac-tive response of single-wall carbon nanotube- and C60-doped nematic liquid crystal. Appl. 

Khoo, I. C., Kan Chen, andY. Zhang Williams. 2006. Orientatioiral photorefiactive effect in undoped and CdSe nano-rods doped nematic liquid crystal Bulk and interface contri-butioirs. lEEEJ. Sel. Top. Quantum Electron. 12(3) 443 50. 

An interesting and important point is that, at low optical power, the orientational effect actually is the dominating one, even in such highly absorptive material. In other recent studies " of dye-doped nematic liquid crystals, observed nonlinearities have approached the so-called supranonlinear scale characterized by refractive index coefficients n2 1 cm /W. The observed nonlinearity is orders of magnitude larger than the so-called giant optical nonlinearity of pure liquid crystals. In particular, methyl-red dye-doped nematic liquid crystals were first observed to have a nonlinear index... 

Khoo, I. C., P. H. Chen, M. Y. Shih, A. Shishido, andS. Slussarenko. 2001. Supraoptical nonlinearities of methyl-red and azobenzene liquid crystal-doped nematic liquid crystals. Mol. Cryst. Liq. Cryst. 358 1-13 364 141-149. 

Khoo, 1. C., Kan Chen, and Y. Wilhams. 2006. Orientational photorefractive effect in undoped and CdSe nano-rods doped nematic liquid crystal Bulk and interface contributions. J. Sel. Top. Quantum Electron 12(3) 443 50 see also Khoo, I. C., Yana Zhang Wilhams, B. Lewis, and T. Mallouk. 2005. Photorefractive CdSe and gold nanowire-doped hquid crystals and polymer-dispersed-liquid-crystal photonic crystals. Mol. Cryst Liq. Cryst. 446 233-244. 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

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