Electro-optic geometry

Zenithal bistable devices have been used in several types of electro-optic geometry. In the VAN-HAN mode, Fig. 7.15, one ZBD surface is combined with a fiat homeotropic surface. In the planar ZBD state (Fig. 7.15 right), the nematic is hybrid aligned with the director plane rotated 45° with... 

More complex geometries have been developed [40] and the influence of the geometrical structure has been examined. Although straight-through microchannel emulsification has been developed [39,41], the production rates are still low compared to those obtained with standard emulsification methods. However, the very high monodispersity makes this emulsification process very suitable for some specific fechnological applicafions such as polymeric microsphere synfhesis [42,43], microencapsulation [44], sol-gel chemistry, and electro-optical materials. 

We have seen that a bulk transparent electro-optic crystal changes its index by a small amount, if an electric field is applied. Figure 4.9 shows a phase modulator for transverse geometry. Let us look at some numbers for LiNb03 d = 1 mm, L = 10 mm, r33 = 30.9 pm/V, ne = 2.2, U = 180 V, and A0 = 0.6 ptm. We find... 

Figure 4.9 Schematic of bulk electro-optic phase modulator in transverse geometry (a) standard electrode arrangement for static measurements, (b) travelling wave configuration...

After the switching off of the electric field, the electro-optical signal decays exponentially with time t, and this decay is simply related to the particle geometry [46] ... 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

Here, is the electro-optic (or Pockels) coefficient for a given geometry and n is the refractive index. 

In the past few decades the technological possibilities and interests have boosted research in systems in highly restricted geometries in almost every field of physics — recently down to lengthscales close to or even below the molecular level. In the field of liquid crystals, the importance of electro-optical applications which incorporate ordered liquid materials [1-3] has focused the research on LC systems with high surface-to-volume ratio [4]. In order to provide mechanically stable applications, liquid crystals are dispersed in polymers, stabilized by a polymer network, fill the cavities in porous materials, etc. [5,6]. The major technological interest concerns the scattering, reflective and bistable displays, optical switches, and others. 

Toliver P., R. J. Runser, I. Glesk, and P. R. Pmcnal (2000) "Comjjarison of three nonlinear optical switch geometries," in Proc. Conf. on Lasers and Electro-Optics (CLEO 2000), San Francisco, CA, May 7-12, p>p.254-255. 

Fig. 2. Oblique geometry typically used for characterizing bulk photorefracti ve polsmaers. The magnified portion illustrates the orientational enhancement effect described in the text due to the modulated poling of the electro-optic chromophore in the combined Eb + Esc fields. The beam and grating geometry applies for both four-wave mixing (beams 1-4) and two-beam coupling (beams 1 and 2).

There are also electro-optic effects using either a different geometry of surface stabilization or a completely different mechanism In the twisted ferroelectric smectic-C cell [54] the moleeules form in the zero field state a quarter helix which is removed when a dc field of either polarity is applied the optical effect is achieved in the same way as in a twisted nematic cell. Compounds with a short chiral smectic-C pitch in a thick cell are used for the distorted helix ferroelectric (DHF) device [55] this effect uses the optical difference between the zero-field state eharacterized by a fully developed short-pitch helix, and structures with a distorted or almost unwound helix in the presence of an applied field optically addressed spatial light modulators can take advantage of the DHF effect [56]. Further applications of ferroelectric liquid crystals are switchable diffraction gratings [57]. 

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