This investigation demonstrates a simple but accurate method for measuring the helical twisting power of chiral doped liquid crystals using axially symmetrical photo-alignment in azo dye-doped liquid crystal films. As reported in our previous paper, a reversed twist effect produces a disclination line in photo-aligned axially symmetrical liquid crystal films. The pitch and helical twisting power can be obtained by measuring the rotation angle of the disclination line in chrial doped liquid crystal. This method is independent of cell gap and provide an error below 0.5%.
© 2009 OSA
Chiral materials have been extensively used in liquid crystal (LC) devices to provide a twisting configuration. The twisted nematic liquid crystal display (TN-LCD) [1,2] and cholesteric liquid crystals (CLCs) [3,4] are the most well-know examples. In fabricating a TN-LCD, a small amounts of chiral material is doped in a nematic LC to prevent reversed twisting in the cells . Sufficient chiral concentration in chiral doped LC can produce color reflective one-dimen photonic crystal CLCs structure [5,6], which are utilized in many important fields, such as CLC lasing [7,8] and reflective LC displays . Therefore, the knowledge of helical twisting power (H.T.P.) of a chiral dopant is important for these applications.Currently used methods to determine the H.T.P. are indirect. In general, the pitch length of a CLC is determined first, and then converted the pitch length to an H.T.P. value. The common method for measuring the pitch is based on a wedge cell [10,11]. Yet the error can be large because accurately controlling the wedge structure is difficult.
This investigation demonstrates a simple, but accurate method for determining the H.T.P. of a chiral dopant. The method is based on the shift of the disclination line that is formed in a photo-aligned axially symmetric liquid crystal film with various chiral concentrations [12,13]. The experimental result is compared with the theoretical value to evaluate the accuracy. The error is less than 0.5%.
2. Device fabrication
The LC and azo dye that were adopted in this experiment were E7 (Merck) and Methyl Red (MR; Aldrich), respectively. The mixing ratio of MR to E7 was 1:99 wt%. The azo dye-doped liquid crystal (DDLC) mixture was mixed with a chiral dopant in the ratio (100- c):c wt%. Two chiral dopants, CB15 and S811, were used in the experiment. The homogeneously mixed compound was then injected into an empty cell with a 12 um gap to generate a DDLC sample. Notably, one of the two glass substrates was coated with an alignment film, polyvinyl alchohol (PVA), and rubbed, while the other was not treated. The LC molecules aligned with each other close to the rubbed surface and extended throughout the bulk of the sample to another surface while the film was not aligned. The homogeneous alignment of the cell was verified using a conoscope.
The photo-alignment approach described in our earlier works [14,15] was used to align the LC molecules on the un-treated surface in a particular direction. Briefly, photo-alignment was achieved using a linearly polarized DPSS (Diode-Pump Solid State) laser (λ = 532 nm), whose wavelength was close to the peak of the Azo dye’s absorption spectrum. Figure 1 presents the setup. The pump laser beam, propagating along the z-axis with an intensity of 0.541 W/cm2, was expanded into a collimated beam with a diameter of 21 mm. It then passed through a linear mask with a line-width of ~200 um, and was focused using a cylindrical lens onto the cell from the un-treated surface. The polarization of the pump beam was parallel to the x-axis. The sample was attached to a rotating motor with a rotational speed of ~140 rpm and the duration illumination was ~20 minutes. The excited dyes undergo trans–cis isomerization, molecular reorientation, diffusion and finally adsorption on the un-treated ITO surface with their long axes perpendicular to the polarization of the pump beam. Notably, a few factors, such as the concentration of the dyes doped in LCs, the intensity of the pumped beam, and the duration of illumination determined the properties of the adsorbed dye layer. In the experiment, the adsorbed dye layer was reliably photo-aligned by rotating the sample at a rate of ~60 to 800 rpm under illumination for 20 minutes.
3. Results and discussions
The adsorbed dyes then caused LC molecules to reorient perpendicular to the polarization and propagation of the light wave. Since the sample was rotated, the x-axis-polarized pumping beam yielded a radial alignment film on the un-treated surface. Finally, a hybrid LC sample that was homogeneously aligned on one of the substrates and radially aligned on the other was formed; this sample is referred to as a homogeneous-radial sample (Fig. 2(a) ).
If no chiral agent is doped into a nematic liquid crystal, then the twist angle of the formed homogeneous-radial sample is always less than π /2 by the minimization of elastic free energy. Therefore, a reversed twist effect yields a disclination line, which is perpendicular to the rubbing direction in a homogeneous-radial sample, as presented in Fig. 2(b). Notably, when a small amounts of chiral dopant is added to the sample, the disclination line is not perpendicular to the rubbing direction because of the intrinsic twisting of the chiral doped LC materials. As presented in Fig. 2(c), the disclination line is rotated by an angle ϕ to that without chiral doping. Measuring the angle of ϕ yields the intrinsic twisting angle and helical twisting power of a chiral material.
Figure 3 presents images of the homogeneous-radial LC film with various concentrations of chiral dopant CB15, obtained under a crossed-polarized optical microscope (POM). In the POM setup, the front polarizer axis is parallel to the director of the rubbed homogeneous LCs. Since the homogeneous-radial LC sample satisfies Mauguin’s condition, Δnd >>λ in this case (where Δn, d and λ are the birefringence of LC, the cell gap and the wavelength of the probe beam, respectively). Therefore, when a linearly polarized light with its polarization parallel to the rubbing direction is used to probe the formed homogeneous-radial LC sample, the emerging beam is radially polarized. Therefore, the region from which the emerging beam is polarized perpendicular (parallel) to the analyzer axis is dark (bright), and a brightness gradient exists between these two directions. The yellow rectangle in Fig. 3 marked disclination line in a chiral-doped LC film. The black dotted line is the disclination line in a cell without chiral doping. As presented in Fig. 3, the disclination line rotates counterclockwise as the concentration of chiral dopant increases. Figure 4 presents similar images of homogeneous-radial LC films with various concentrations of chiral dopant, S811. Since the twisting direction of the chiral agent of S811 is opposite that of CB15, the disclination lines are observed to be rotated clockwise.
Figure 5 plots the variation of the angle ϕ with the concentration of the chiral agent, CB15, from 0.101 wt% to 0.291 wt% and of S811 from 0.042wt% to 0.19wt%. The angle ϕ is directly proportional to the chiral concentration. This finding is expected, since p and c satisfy H.T.P. = 1/(p.c), where p and c are the pitch length of the chiral doped liquid crystal and the concentration of the chiral dopant, respectively. From Eqs. (1) and (2), the H.T.P. of a chiral agent can be calculated.
Figure 6 plots both the experimental and the theoretical H.T.P of chiral agents, CB15 and S811. The experimental results agree well with theoretical values. The mean errors of the H.T.P. values for samples doped with CB15 and S811 are less than 0.37% and 0.26%, respectively. It is noticed that the disclination line is yielded in the middle between reversed twist structures. So the weak anchoring strength caused by photo-alignment won’t affect the determination of H. T. P. value. This measurement approach only needs small amounts of chiral dopant and independent of cell gap. It offer high accuracy required for some applications.
In order to compare the accuracy with other methods, the Cano wedge cell and reflective spectrum were used to determine the H.T.P. of CB15. Table 1 shows the H.T.P. values obtained by different methods. In order to measure the reflection spectrum, the concentration of chiral dopant was added to near 40wt%. The high concentration of chiral dopant affects the average refractive index of liquid crystal that will cause the error in the determining of pitch length. Besides, the Cano wedge cell needs accurate cell gap to obtain the slope of wedge. As Table 1 shows, the H.T.P. measured by axially symmetrical LC film is more accurate than other methods. This measurement approach only needs small amounts of chiral dopant which won’t change the properties of liquid crystal and has large tolerance for cell gap uniformity.
In conclusion, this work demonstrated a novel approach to determine the H.T.P. of a chiral dopant. It is based on the disclination line that is formed in an axially symmetric homogeneous-radial LC film. By measuring the rotation angle of the disclination line in chrial doped liquid crystal film, the helical pitch and twisting power can be obtained with an error below 0.5%. This approach is simple but accurate and independent of cell gap. Therefore, this method has greater potential for practical application in the field of materials science.
This work was supported by the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education and the National Science Council (NSC 94-218-M-008-009) of Taiwan. The authors would like to thank the National Science Council of the Republic of China (Taiwan), (Contract No. NSC 95-2112-M-006-022-MY3 and NSC 96-2112-M-110-015-MY3)for financially supporting this research.
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