Dynamic response of a dual-frequency chiral hybrid aligned nematic liquid-crystal cell

The dynamic response of a chiral dual-frequency hybrid aligned nematic liquid-crystal cell to a multiple frequency pulse has been characterized using a time-resolved fully leaky guided-mode optical characterization technique. On application of a low-frequency voltage the cell is found to switch to homeotropic alignment, effectively destroying the inherent twist in the cell. When this voltage is immediately followed by a high-frequency voltage the structure is driven into a homogeneously aligned twisted structure. Analysis of the response of the director to this change in effective dielectric anisotropy of the material reveals a form of backflow. This arises due to the combination of the coupling between the rotation and flow of the director, the constraining effect of the pitch on the twist in the cell, and the driving of the director into homogeneous alignment. The measured director profiles have been compared to model profiles generated using the Leslie-Eriksen-Parodi nematodynamics theory, and the viscosity coefficients for the material have been determined.

Figure 1

Schematic diagram of the chiral HAN dual-frequency liquid crystal cell viewed in the $x$-$z$ and $x$-$y$ planes with (a) low-frequency high-voltage applied, (b) $0\mathrm{V}$ applied, and (c) high-frequency high-voltage applied.

Figure 2

Schematic diagram of the time-resolved fully leaky guided-mode experiment.

Figure 3

Comparison of measured (symbols) and modeled (lines) of the time evolution of $n_x$, $n_y$, and $n_z$ at points (a) $1\mathrm{\mu }\mathrm{m}$, (b) $2\mathrm{\mu }\mathrm{m}$, (c) $3\mathrm{\mu }\mathrm{m}$, and (d) $4\mathrm{\mu }\mathrm{m}$ through the cell. A low-frequency field $\left(2\mathrm{kHz}\right)$ is applied between $t=0$ and $t=20\mathrm{ms}$ and a high-frequency field $\left(64\mathrm{kHz}\right)$ is applied between $t=20\mathrm{ms}$ and $t=100\mathrm{ms}$. The values of the physical constants used are stated in the text. The dotted lines show the modeled natural relaxation of the cell when the voltage is removed at $t=20\mathrm{ms}$.

Figure 4

Schematic representation of the motion of the director, projected onto the $x$-$y$ plane for switching a dual-frequency CHAN cell into homeotropic alignment (0 to $20\mathrm{ms}$) and then (a) driving it into the twisted nematic state using a high-frequency pulse or (b) allowing it to relax into the $0\mathrm{V}$ state on removal of the voltage.

Figure 5

Modeled evolution of the twist and tilt profiles (deg) of the director with time during the low-frequency switching to the homeotropic state [(a) and (b)] and the high-frequency switching to the twisted homogeneous alignment [(c) and (d)].