Dielectric relaxation process of a partially unwound helical structure in ferroelectric liquid crystals

The fluctuations of unwound helical structure have been observed in deformed helix ferroelectric liquid crystal (DHFLC) and conventional FLC sample cells. The helix is partially unwound by strong anchoring on the substrates. In such sample cells, the helical decarlization lines are not observed in the texture under crossed polarized microscope. The dielectric spectroscopy is employed to observe the behavior of dielectric relaxation processes in these sample cells. A dielectric relaxation process is observed at a lower frequency than the Goldstone mode processes in DHFLC and FLC, which we call partially unwound helical mode (p-UHM). However, the p-UHM process is not observed in the sample cell in which the helical lines appear. The application of various amplitudes of probing ac voltages on this mode has shown the higher frequency shift, i.e., the larger the amplitude of ac voltage, the higher is the relaxation frequency of p-UHM. At sufficient amplitude of applied probing ac voltage, the p-UHM merges with the Goldstone mode process and is difficult to detect. However, the Goldstone mode relaxation frequency is almost independent of the cell geometry and sample configuration. The electro-optical behavior of the p-UHM has also been confirmed by electro-optical technique. The dielectric relaxation of UHM at a frequency lower than the Goldstone mode is interpreted as the fluctuation of partially unwound helix.

Figure 1

(a) Real part of the complex dielectric permittivity versus frequency and (b) the tan δ versus frequency at various amplitudes of probing ac voltage in DHFLC material. Insets of (b) show the behavior of optical micrographs of DHFLC at different frequencies during dielectric spectroscopy.

Figure 2

(a) Real part of the complex dielectric permittivity versus frequency and (b) the tan δ versus frequency as a function of amplitude of probing ac voltage in conventional FLC (SCE-13) material. Insets of (b) show the behavior of optical micrographs FLC at different frequencies during dielectric spectroscopy.

Figure 3

Schematic of the helical unwinding at the interface of FLC/DHFLC and substrate. The depth (shaded region) of the unwinding could vary with the rubbing strength and amplitude of the applied electric field.

Figure 4

Dielectric strength of p-UHM process of DHFLC (LAHS-2) and FLC (SCE-13) as a function of amplitude of probing ac voltage.

Figure 5

Relaxation frequency of p-UHM versus amplitude of probing ac voltage of DHFLC (LAHS-2) and FLC (SCE-13).

Figure 6

(a) Real part of the complex dielectric permittivity versus frequency and (b) tan δ versus frequency at various bias electric voltages. The amplitude of probing ac voltage is kept at 100 mV in DHFLC (LAHS-2) material.

Figure 7

(a) Real part of the complex dielectric permittivity versus frequency and (b) tan δ versus frequency at various bias electric voltages. The amplitude of probing ac voltage is kept at 1 V in DHFLC (LAHS-2) material.

Figure 8

Optical micrographs of DHFLC (LAHS-2) under crossed polarized microscope at fixed 100-mV ac voltage at (a) frequency 100 Hz and (b) at 100 kHz.

Figure 9

Optical micrographs of DHFLC (LAHS-2) under crossed polarized microscope at fixed 1-V ac voltage at (a) frequency 100 Hz, (b) at 1 kHz, and (c) at 100 kHz.

Figure 10

Optical micrographs of FLC (SCE-13) at fixed amplitude of 1-V ac voltage (a) before measurement, (b) at 50 Hz, (c) at 221 Hz, and (d) at 100 kHz.

Figure 11

Optical micrographs of DHFLC (LAHS-2) at fixed frequency 100 Hz of ac signal and at (a) 500-mV, (b)1-V, (c) 2-V, (d) 4-V, (e) 6-V, and (f) 8-V ac voltage. Inset of (a) and (b) shows the optical micrographs of material before applying any field.

Figure 12

Optical micrographs of FLC (SCE-13) at fixed frequency 100 Hz of ac voltage and at amplitude of (a) 500-mV, (b) 1-V, (c) 2-V, (d) 4-V, (f) 6-V, and (e) 8-V ac voltage. Inset of (a) and (b) shows the micrograph of sample before applying any voltage.