Doped Liquid Crystals

From the standpoint of optical properties, the doping of liquid crystals by appropriately dissolved concentrations and types of dyes clearly deserves special attention. The most important effect of dye molecules on liquid crystals is the modification of their well-known linear, and more recently observed nonlinear, optical properties (see Chapters 8 and 12). 

An obvious effect of dissolved dye is to increase the absorption of a particular liquid crystal at some specified wavelength region. If the dye molecules undergo some physical or orientational changes following photon absorption, they could also affect the orientation of the host liquid crystal, giving rise to nonlinear or storage-type optical effects (see Chapter 8). 

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Electro-Optical Properties of Bimetallic Nanoparticle-Doped Liquid Crystal Displays... 

Electro-optic effects induced by doping liquid crystals with one-dimensional metal nanoparticles were not only investigated in standard electro-optic test cells, but also in costume-made cells consisting of a thin layer of liquid crystal either deposited onto a thin film of alumina with embedded GNRs [443], or using rubbed polyimide alignment layers modified with solution-cast GNR [444]. In both cases, surface plasmon resonance frequencies of the GNR integrated into these liquid crystal cells could be electrically controlled. 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

One can certainly expect more work in the area of organoclay-doped liquid crystals in the near future. The hope is that these studies will not be limited to polar 5CB as a host material only, which is not necessarily an ideal host, but explore other nematic systems and other liquid crystal phases. 

The field of liquid crystal nanoscience has experienced tremendous growth over the past decade. Research on nanoparticle-doped liquid crystals makes use of a wide variety of nanoparticles differing in size, shape, core material, and coating (Fig. 23). 

Figure 5 Superior photoconductivity, normalized to an absorption of 1 cm" perylene and NI doped liquid crystal versus R6G doped sample.

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 ... 

Legge CH, Mitchell GR. 1992. Photo induced phase transitions in azobenzene doped liquid crystals. J Phys D Appl Phys 25 492 494. 

M. Yaegashi, A. Shishido, T. Shiono and T. Dceda, Effect of ester moieties in dye structures on photoinduced reorientation of dye-doped liquid crystals, Client. Mater., 17, 4304-4309 (2005). 

Kato, J. Yamaguchi, I. Defect enhancement of periodic stmcture using dye-doped liquid-crystal cells. Proc. SPIE-Int. Soc. Opt. Eng. 1996, 2778, 557-558. 

Kato, J. Yamaguchi, I. Tanaka, H. Nonlinear spatial filtering with a dye-doped liquid-crystal cell. Opt. Lett. 1996, 21, 767-769. 

Chen, A. G. S. Brady, D. J. Surface-stabihzed holography in an azo-dye-doped liquid crystal. Opt. Lett. 1992,17, 1231-1233. 

M. Warenghem, J. F. Henninot, F. Derrin, and G. Abbate, Thermal and orientational spatial optical solitons in dye-doped liquid crystals. Mol. Cryst. Liq. Cryst. 373, 213 (2002). 

X. Li, C. P. Chen, H. Y. Gao, et al., Video-Rate Holographic Display Using Azo-Dye-Doped Liquid Crystal, J. Disp. Technol. 10, 6 (2014). 

If (5.20) is not valid other types of electrooptical effects take place and the modulated FVederiks transition cannot be observed in experiment. Figure 5.4 shows how doping liquid crystals with conducting and dielectric impurities can violate the inequality (5.20) and, consequently, the electro-hydrodynamic instabilities 1 and 5 (Table 5.1) are observed within the whole frequency range (curve B). Considerable change in the threshold voltage and inversion frequency also takes place for different values of the low-frequency dielectric anisotropy (curves A and C). 

Photoinduced conformational changes in azo-dye-doped liquid crystals have been used for real-time holography [65]. The orientation of azo dyes by an optical field, the Weigert effect [66], has been used to induce orientation of liquid crystals both in the bulk and in alignment layers [67, 68]. 

A twist angle 0>9O° can be achieved by doping the nematic liquid crystal mixture with a cholesteric liquid crystal. For a cell with zero pretilt the twist angle

cell-thickness d to the pitch p of the doped liquid-crystal mixture ... 

Todorov T, Nikolova L, Stoyanova K, Tomova N (1985) Polarization holograph. 3 some applications of polarizatirai holographic recording. Appl Opt 24 785-788 Wu WY, Mo TS, Fuh AYG (2006) Polarization charactmstics of diffracted beams from twisted nematic gratings fabricated by the photoalignment effect in dye-doped liquid crystal films. J Opt Soc Am B 23 1737-1742... 

Lucchetti, L., M. Di Fabrizio, O. Francescangeh, and F. Simoni. 2004. Colossal optical nolinearity in dye-doped liquid crystals. Opt. Commun. 233 417. 

Khoo, I. C., Y. Z. Williams, B. Lewis, and T. Mallouk. 2005. Photorefractive CdSe and gold nanowire-doped liquid crystals and polymer-dispersed-liquid-crystal photonic crystals. Mol. Cryst Liq. Cryst. 446 233-244. 

Figure 8.11. Schematic diagram of the optical fields and their propagation direction in a planar aligned dye-doped liquid crystal. " The two fields Ei and E2 are coherent beams derived from splitting a pump laser to induce dynamic grating in the liquid crystal sample. E3 is the probe beam.

Gibbons, W. M., P. J. Shannon, S.-T. Sun, and B. J. Swetlin. 1991. Surface-mediated alignment of nematic liquid crystals with polarized laser light. Nature (London). 351 49-50 see also Chen, A. G.-S., and D. J. Brady. 1992. Surface-stabilized holography in an azo-dye-doped liquid crystal. Opt. Lett. 17 1231-1233. 

Khoo, I. C., H. Li, and Yu Liang. 1993. Optically induced extraordinarily large negative orientational nonlinearity in dye-doped liquid crystal. lEEEJ. Quantum Electron. QE-29 1444. 

In the experiment, the liquid crystal used is methyl-red-doped (1% by weight) 5CB or E7. The cell is made by sandwiching the hghtly doped liquid crystal between two rubbed polyvinyl alcohol (PVA) coated glasses. The director axis of the nomadic hquid crystal hes on the plane of the cell windows, which are placed such that the director axis of the nomadic film rotates by 90° from the incident to the exit windows (see Fig. 12.3). 

Table 12.3. Comparison of the Performance Characteristics of Commercial Liquid Crystal Spatial Light Modulator (LCSLM) with What Could he Expected of Dye-Doped Liquid Crystal (DDLC) Film...

Khoo, I. C., M. V. Wood, M. Y. Shih, and P. H. Chen. 1999. Extremely nonlinear photosensitive liquid cry stals for image sensing and sensor protectioa Opt Express. 4(11 ) 431 142 Shih, M. Y., I. C. Khoo, A. Shishido, M. V. Wood, and P. H. Chen. 2000. All-optical image processing with a supra nonlinear dye-doped liquid crystal film. Opt Lett. 25 978-980. 

In [26], we report a detailed study on TRPS and steady-state fluorescence of HK271 in different liquids as a function of the excitation-light wavelength, A. These experiments have been motivated by the observed A-dependence of the merit figure n in certain dye-doped liquid crystals [27,28]. In particular. 

Marrucci,L.,Paparo, D.,Vetrano,M.R.,Colicchio, M., Santamato,E.,and Viscardi, G. (2000). Role of dye structure in photoinduced reorientation of dye-doped liquid crystals./. Chem. Phys. 113 10361-10366. 

Kreuzer, M., Hanisch, E, Eidenschink, R., Paparo, D., and Marrucci, L. (2002). Large deuterium isotope effect in the optical nonlinearity of dye-doped liquid crystals. Phys. Rev. Lett. 88 013902(1 ). 

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