Other Electrooptical Effects

The three types of electrooptical eflFects considered above are the most interesting and useful for practical applications. However, in FLCs there were observed other electrooptical eflFects which will be discussed in this section. 

Another electrooptical mode is a transition of the scattering helical state, with random orientation of the helical axes into the transparent state where the helix is completely unwound [112-115]. According to (7.54), the threshold field for helix unwinding is E cx i.e., can be suflSciently small ( 0.5 Y/fiia) for high spontaneous polarization Pg. 

The scattering effects observed during the deformation of the ferroelectric helix have not yet been satisfactorily investigated [115]. For instance, one should explain the correlation between temperature dependence of the helix pitch and intensity of the scattered light [113], as well as the effect of FLC physical parameters on the response times and hysteresis behavior of transmission-voltage characteristics. Moreover, these effects have not been studied in commercial FLC mixtures operating at room temperature. Nevertheless, these electrooptical modes might be useful for applications in nonpolaroid FLC displays for realization of the optical memory, etc. 

An unusual effect was observed in an antiferroelectric liquid crystal (AFLC) [116-125]. The switching is associated with the appearance of the third state, in addition to the two bistable up and down states known for the Clark-Lagerwall effect. The corresponding hysteresis of electrooptical switching is shown in Fig. 7.23. We can see that the third state at the zero field is stable, and can be transformed either into up or down states if not applying a rather high switching field. 

FIGURE 7,23. Tristable electrooptical switching. Above hysteresis of the apparent tilt angle, which correlates with the average value of the polarization in the cell versus the applied voltage. Below the microscopic interpretation of the antiferroelectric state in the helical and imwound versions. 

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At present, the electroclinic effect is the fastest one among the other electrooptical effects in liquid crystals. References [20, 36, 105,108] demonstrate that the response times for the effect, measured at room temperature (T = 25 C), do not exceed =600 ns for the voltages of about 10-40 V and layer thicknesses of about 2 jim. The effect can be used in a wide spectral range, including the visible and near IR region. Placing two electroclinic cells one after another can ... 

Besides the ambitious efforu to arrive at well-aligned SSFLC structures, some work has been dedicated to other electrooptic effects as are well known with LMM FLCs, such u the electroclinic (SMFLC). the antifenoclectric (AFLC), and the deformed helix ferroelectric effect (DHF). 

Further research work has been devoted to creating new FLC polymers with multifunctional properties. Recently, Scherowsky reported on fluorescent FLC polymers [136]. The endeavor to produce FLC polymers with strong SHG activity will be dealt with in Section VI. So tar we have discussed variouB applications with an eleclrooptic effect aiudogous to the SSFLC Qark-Lagerwall effect observed on LMM FLCs. htoy of these applications are conceivable with other electrooptic effects such as the antifer-roelectric. electrodink. or deformed helix ferroelectric effect, in particular if gray scale is required. 

For a nematic LC, the preferred orientation is one in which the director is parallel everywhere. Other orientations have a free-energy distribution that depends on the elastic constants, K /. The orientational elastic constants K, K22 and K33 determine respectively splay, twist and bend deformations. Values of elastic constants in LCs are around 10 N so that free-energy difference between different orientations is of the order of 5 x 10 J m the same order of magnitude as surface energy. A thin layer of LC sandwiched between two aligned surfaces therefore adopts an orientation determined by the surfaces. This fact forms the basis of most electrooptical effects in LCs. Display devices based on LCs are discussed in Chapter 7. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

It was realized in the early 1970s that the unusual properties of thermotropic liquid crystals held great promise for use in flat-panel electronic displays and other optical control applications. The advantages particular to Uquid crystals of a very large (if not especially fast) electrooptic effect induced by CMOS-compatible voltages and of microwatts per square centimeter power consumption were identified at an early stage. With the discovery of chemically stable nematic liquid crystals, such as the... 

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). 

The theoretical and experimental results on physical properties of liquid crystals were reviewed by de Gennes [15], Chandrasekhar [16], de Jeu [17], Sonin [18], Belyakov and Sonin [19], Vertogen and de Jeu [20], and others. The electrooptical effects were discussed by Kapustin [21], Pikin [22], and Blinov [23]. Recent results on liquid crystalline materials and their application in devices can be found in [24-26, 29]. 

The first observation of natural optical anisotropy was made in 1669 by Bartolinius in calcite crystals, in which light travels at different velocities depending on the direction of propagation relative to the crystal structure. The electrooptic effect, electric-field-induced anisotropy, was first observed in glass in 1875 by J. Kerr. Kerr found a nonlinear dependence of refractive index on applied electric field. The term Kerr effect is used to describe the quadratic electrooptic effect observed in isotropic materials. The linear electrooptic effect was first observed in quartz crystals in 1883 by W. Rontgen and A. Kundt. Pockels broadened the analysis of this relationship in quartz and other crystals, which led to the term Pockels effect to describe linear behavior. In the 1960s several developments... 

A further interesting use of the focal-conic to homeotropic texture transition is in infrared modulation [272]. Here it was found possible to modulate infrared light at A=8-12 pm with a maximum transmission of 87%, a contrast of 93%, and turn on and off times of 1 ms and 125 ms, respectively. A further window examined was 3-5 pm, and this work suggests that other chiral nematic electrooptic effects could be exploited in the near infrared. In communications technology a 2x2 optical switch for fiber-optics has been developed [273] using a chiral nematic film and two switchable nematic waveplates. It has been demonstrated that this is suitable for LED or laser sources. The device worked at 1.318 pm and had switching times of 40 ms with -26 dB crosstalk between unselected fibers. There will clearly be further advances in this use of the unique optical properites of chiral nematics. 

The flexoelectrooptic effect is a field-sensitive electrooptic effect (it follows the sign of the field), which is fast (typically 10-100 p.s response time) with two outstanding characteristics. First, the induced tilt (p has an extremely large region of linearity, i.e., up to 30° for materials with dielectric anisotropy Ae=0. Second, the induced tilt is almost temperature-independent. This is illustrated in Fig. 39 for the Merck cholesteric mixture TI 827, which has a temperature-independent pitch but not designed or optimized for the flexoelectrooptic effect in other respects. 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2o) = co + co + 0) in a quasi-third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect (co = co -I- 0), or the quadratic electrooptic (Kerr) effect ( = -I- 0 -I- 0). EFISH is discussed in this article, however, for the important role that it has played in the characterization of nonlinear optical materials for other applications. 

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