Correlation between alignment geometries and memory effect in a surface-stabilized ferroelectric liquid crystal

Memory effect in weakly aligned surface stabilized ferroelectric liquid crystal (SSFLC) material has been investigated by electro-optical and dielectric spectroscopy in three configurations of alignment: antiparallel, $90^{\circ }$ twisted, and unaligned planar samples. It has been observed that two types of molecular dynamics exist in antiparallel rubbed cell in which memory effect is observed for longer duration than in other samples. One dielectric relaxation process is near the surface of the electrode and a second is in the bulk of the SSFLC. Both the molecular dynamics contribute in the switching process and affect the memory phenomenon in surface stabilized geometries. However, a single dielectric process is observed in twisted geometry in which the sample is showing shorter memory effect than in antiparallel and this is compared with unaligned samples also having cell thickness less than the pitch value of FLC. In an unaligned sample, a single dielectric process is observed and the smaple does not show memory effect at all. The investigation is significant to understand the anomalies occurring in memory observations in various geometries.

Figure 1

Optical micrographs at 10 V DC voltage for surface stabilized geometry of antiparallel rubbed cell having rubbing direction at (a) $-45^{\circ }$, (b) $0^{\circ }$, and (c) $45^{\circ }$ with respect to polarizer. In twisted structure SSFLC cell having rubbing direction (d) in scattered state at $-45^{\circ }$, (e) $0^{\circ }$, and (f) $45^{\circ }$. In no rubbing SSFLC cell (g) in scattered state at $-45^{\circ }$, (h) $0^{\circ }$, and (i) $45^{\circ }$ angles with respect to polarizer. These measurements have been performed at 30°C in SmC* phase. The crossed double headed arrows indicates the positions of polarizer (P) and analyzer (A).

Figure 2

Optical micrographs of surface stabilized geometry cell of antiparallel rubbed cell (a) with 10 V and (b) 2 min after removal of bias. Twisted SSFLC cell (c) with 10 V and (d) 2 min after removal of bias. Unaligned cell (e) with 10 V, and (f) 2 min after removal of bias. These measurements have been performed at 30°C in SmC* phase. The crossed double headed arrows indicates the positions of polarizer (P) and analyzer (A).

Figure 3

(a) Comparative data storage response (memory effect) for all the three samples with applied DC field in SmC* phase. Dielectric loss (ε″, imaginary part of complex dielectric constant) as a function of frequency without and with DC bias for (b) antiparallel, (c) twisted cell, and (d) unaligned cells. Insets of all the figures show tanδ as a function of frequency of corresponding cell.

Figure 4

Dielectric loss (ε″) as a function of frequency of FLC in SmC* phase with different probing voltages at room temperature in (a) antiparallel, (b) twisted, and (c) unaligned sample of surface stabilized geometries. Insets of all the figures show tanδ as a function of frequency with different probing AC voltages of corresponding cell.

Figure 5

Relaxation frequency of GM in SmC* phase at different amplitudes of probing AC voltage at room temperature in (a) antiparallel, (b) twisted, and (c) unaligned cells.

Figure 6

Relaxation frequency of GM and $p\text{−}\mathrm{UHM}$ processes in SmC* phase as a function of probing voltage at room temperature in antiparallel rubbed cell.

Figure 7

Dielectric permittivity as a function of frequency in SmC* phase at different probing voltages at room temperature in (a) antiparallel, (b) twisted, and (c) unaligned cells.

Figure 8

Dielectric loss (ε″) response of cell as a function of frequency in SmC* phase at 800 mV probing AC voltage at different temperatures in (a) antiparallel, (b) twisted, and (c) unaligned cells in SmC* and SmA* phases.