Homeotropic cell

In homeotropic cells, however, in-plane rotations of the director are reflected in a net azimuthal rotation of the optical axis (and the light polarization) across the cell which has allowed a detailed exploration of the characteristics of the NR-AR transition. Experiments have shown an excellent agreement with the predictions of generalized Ginzburg-Landau models [36],... 

The handedness inversion behaviors upon UV irradiation were observed in both wedge cells and homeotropic cells with the cholesteric (N )—nematic (N)— cholesteric (N ) phase transition sequence as confirmed by the Cano s lines and fingerprint texmres (Fig. 5.12). The mechanism of the helix inversion is proposed... 

Fig. 5.12 Handedness inversion of 0.27 mol % (S, R, S)-17 in 5CB observed in wedge cell (a-c) and homeotropic cell (d-f) from right-handed CLC (a, d) to left-handed CLC (c, f) through transient nematic phase (b, e). Reproduced with permission from [90]. Copyright 2013 John Wiley Sons...

Homeotropic cells offer another way to detect the flexoelectric response via observing the bend director distortions induced by an electric field parallel to the substrates (the Helfrich method ). Takezoe s group applied it to ClPbislOBB and found 63 20 pCm (which is the order of flexocoef-... 

We measure pyroelectric coefficient y = dP/dT, using heating the hybrid cell by short ( 10 ns) laser pulses, as shown in Fig. 10.13. The only difference from the surface polarization measurements is using a hybrid cell instead of uniform (planar or homeotropic) cells [28]. The laser pulse produces a temperature increment about AT 0.05 K and the flexoelectric polarization changes. To compensate this change, a charge passes through the external circuit and the current i = dqldt is measured by an oscilloscope. From the identity (A is cell area)... 

Due to the linear profile of 9(z) it is very easy to calculate the phase retardation of the initially homeotropic cell for the normal light incidence, kHz. Without electric field, the longest axis of the dielectric ellipsoid coincides with the director axis z. Therefore, refraction index for any polarization is o = With increasing field E, due to deflection of the director within plane xz, the y- and -components of the refraction index will correspond to the ordinary and extraordinary rays, Uy = no = n , rix(z) = rig(z). Integration provides us with the average extraordinary index ... 

H. Wang, T. X. Wu, X. Zhu, and S. T. Wu, Correlations between liquid crystal director reorientation and optical response time of a homeotropic cell, J. AppL Phys. 95, 5502 (2004). 

Let us first give a qualitative picture of the Fredericks transition. When a plane light wave falls at normal incidence on a homeotropic cell the electric field of the wave is exactly perpendicular to the director. At a positive value of it would be energetically favorable to orient the director in the direction of the field. However, this is prevented by the homeotropic orientation of the director by the curvette walls. Furthermore, in the first approximation in the light intensity the orientational effect of the field on the unperturbed director is absent, in other words, for E=e,E,j the function exact solution to equation (5). 

We also introduce the parameter/ which gives the incident power density relative to the LFT threshold for an ordinary homeotropic cell of the same width L ... 

An experimental investigation was made of aberration self-focusing due to orientational deformation of a nematic with a hybrid orientation subjected to the held of an incident ordinary optical wave. When the radiation was incident on a sample from the homeotropic substrate side, the reorientation occurred at intensities higher than the threshold for a photoinduced Freedericksz transition in a homeotropic cell of the same thickness, and it depended strongly on the angle of incidence of the ordinary wave on the sample. The effect was not observed when light was incident from the planar substrate side. 

Among the most interesting of these features are, in particular, a strong dependence of the threshold of the PFT induced in a homeotropic cell by an obliquely incident ordinary (o) wave on the angle of incidence of this wave and the oscillatory time dependence of the above-threshold distribution of the orientation in this geometry. These effects are... 

In view of this information, it seemed of interest to investigate the PFT in cells with an initially inhomogeneous orientation, particularly in a hybrid cell where the state of polarization of the exciting radiation is perturbed much more strongly than in a homeotropic cell of the same thickness. The PFT in a hybrid cell in the field of an ordinary wave has already been considered theoretically. Unfortunately, even linearized equations for the near-threshold perturbation of the orientation in a hybrid cell have no analytic solutions, so that this treatment is limited to purely qualitative estimates. 

Figure 2 shows the experimental dependences of the number of self-focusing rings on the laser radiation power W obtained for different angles of incidence Oq (the arrow on the abscissa corresponds to the specially measured threshold power for the PIT in a homeotropic cell with the same thickness and created in the same transverse cross section of the beam under normal incidence conditions). 

For normal incidence (ao = 0) the reorientation threshold was approximately three times higher than the PFT threshold in a homeotropic cell of the same thickness. It is clear from Fig. 2 that such reorientation (and the value of N) depended strongly on the angle of incidence, namely it increased on reduction in Uq. Moreover, in addition to the threshold reorientation, a weak subthreshold reorientation also took place. This could be explained qualitatively as follows the power density of the exciting wave in our experiments was - 3 kW/cm, so that even weak ( 1%) spontaneous scattering of the incident o wave into an e wave created an e wave of --30 W/cm power density, which was quite... 

The transmittance angular spectra T i) of the initially homeotropic cell in electric field is given in Fig. 4.35 (i-incident angle). According to the energy conservation law, in nonabsorbing media the reflectance (or reflectivity) spectra R are complementary to the transmittance spectra, as... 

Homeotropic orientation in a spatially nonuniform field was considered in [159] for large values of As/e , It was shown that near the edges of the pixel in a matrix display, based on the homeotropic cell, a reversed tilt occurred which reduced the contrast of the display [170]. 

Figure 9.8. Schlieren texture observed in a homeotropic cell of the SmCA phase of MHPOBC. Note the existence of two-brush defects.

To identify the antiferroelectric phase, texture observation of the homeotropic cells of racemic compounds is very effective. In the SmC phase, only the schlieren texture with four brushes is observable and that with two brushes is prohibited, because of the head-and-tail inequivalence of the C-director. In the SmCA phase, however, the schlieren texture with two brushes is sometimes seen, as shown in Figure 9.8 [18], [19]. The existence can be explained by taking into account a screw dislocation, as illustrated in Figure 9.9. The discontinuous change (7r-wall) of the C-director is compensated by the screw dislocation. This defect is a combined defect of a disclination and a dislocation, i.e., adispiration [18], [19]. 

For the bend transition in homeotropic cells this holds analogously, but and 33 change their roles. 

The contrast ratio is related to high voltages (V> V, ) and the use of polarized light. In planar cells the E vector is parallel to n. In homeotropic cells the E vector agrees with n after the dielectric reorientation. 

Consider the limiting case of director axis reorientation of a TN cell at high appUed voltage, or a homeotropic cell at F = 0. In that case, we have... 

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