Flexoelectro-optic effect

At the same time experimental facts indicate that the difference between the flexocoefficients is non-zero and even rather large for a number of nematic materials, and it strongly depends on the absolute value and the orientation of the permanent dipole within the molecular structure.Moreover, the difference between the flexocoefficients determines the flexoelectro-optic effect, which has been extensively studied experimentally. There exist also some other experimental data which, in principle, allowed us to distinguish between dipolar and quadrupolar flexoelectricity. This can be considered as an argument in favour of the dipolar interpretation of the flexoelectric effect. On the other hand, the actual ratio of the dipolar and quadrupolar contributions to the flexocoefficients of particular nematic materials remains unknown. It is only possible to speculate... 

S. Ponti, P. Ziherl, C. Ferrero and S. Zumer, Flexoelectro-optic effect in a hybrid nematic liquid crystal cell, Liq. Cryst. 26(8), 1171-1177, (1999). doi 10.1080/026782999204192... 

The flexoelectro-optic effect in cholesterics (which is described in more detail in Chapter 7 by Rudquist and Lagerwall ) offers another way to measure 61 — 63. A tight-pitch helical structure can easily be induced in nematics by adding a few per cent of chiral dopant. Recently this technique has been adopted to test the flexoelectric response of another BC nematic, 4-cyano-l,3-phenylene bis [4-[4 -(hexyl) benzoyloxy] benzoate] (C6ban). ... 

Many bent-core molecules do not have nematic phases, but may be added to calamitic nematics to tune the flexoelectric behaviour. Several groups have reported measurement results in such guest-host systems ° using either HAN cells or the flexoelectro-optic effect mentioned above. It was found that some BC compounds used as dopants in small (< 10%) concentration may effectively increase ei —es of the host. ° Other, mainly asymmetric, BC compounds had hardly any influence on ei - - es of the host nematic. ... 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

The flexoelectro-optic effect should also be very interesting in a phase modulator, e.g. a spatial light modulator. The continuous switching of a retarder with constant birefringence gives a continuous phase-only modulation with circularly polarized light. 

An interesting feature of Eq. (7.6) is that for a cholesteric with a temperature-independent pitch, (j> is temperature independent, because both e and K are proportional to 5, where S is the nematic order parameter containing the temperature dependence. In Eq. (7.14), 7 and K only partly compensate each other in this respect. As it is possible to make a cholesteric with constant k independent of temperature, we can see that the flexoelectro-optic effect will have an induced tilt 4> that is temperature independent and a dynamics that is weakly temperature dependent and, in addition, independent of the applied field. The latter is true as long as Ae 0. 

Fig. 7.8. Even homeotropic boundary conditions allow for a standing helix structure. Thus, homeotropic anchoring conditions will not ensure a long-term stable lying helix structure for the flexoelectro-optic effect. ...

E70A ei - es 3.5 0.2 pC/m twisted cholesteric flexoelectro-optic effect 61... 

BEsllCB 6i — 63 9 1 pC/m ( = ) homeotropic cholesteric flexoelectro-optic effect 81... 

FFOSOCB 6i — 63 20.4 pC/m (0.84Tjvi) homeotropic cholesteric flexoelectro-optic effect 83... 

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