The first reported SmAPF phase, formed from the bent core mesogen W586, exhibits interesting and useful electrooptics.(1) Specifically, in teflon-aligned cells, upon application of an electric field, modulation of the birefringence is observed without rotation of the apparent optic axis. In addition, optical latching is observed, providing the long sought but never before seen “infinitely multistable" ferroelectric LC electrooptic switch.
Based upon careful measurements of the cell behavior, the polar order parameter at temperatures just above the transition to a lower-lying smectic phase is close to unity. This suggested potential application of the phase as a host for controlled polar alignment of guest molecules.
One clear application of the system is for electronic electrooptic modulation of light at telecommunications wavelengths (Kerr effect), which can be obtained in materials with polar orientation of visible dyes.
Background and results of recent experiments in the host-guest chemistry of the W586 SmAPF phase, showing good orientation of designed NLO dyes, will be presented.
(1) a) Reddy, R. A.; Zhu, C.; Shao, R.; Korblova, E.; Gong, T.; Shen, Y.; Garcia, E.; Glaser, M. A.; Maclennan, J. E.; Walba, D. M.; Clark, N. A. "Spontaneous ferroelectric order in a bent-core smectic liquid crystal of fluid orthorhombic layers," Science 2011, 332, 72-77. b) Korblova, E.; Walba, D.; Gong, T.; Reddy, A.; Zhu, C.; Shao, R.; Maclennan, J.; Glaser, M.; Clark, N. "Design and synthesis of an achiral ferroelectric smectic liquid crystal," Proc. SPIE 2011, 8114, (1), 81140X-1 - 81140X-9.
(2) United States Patent: Walba US 9,187,500 B2, Nov. 17, 2015
Design and synthesis of a bent-core mesogen possessing the achiral ferroelectric SmAPF phase is reported. The design
approach is based upon the discovery of Sadashiva and Reddy, et al. that bent-core mesogens possessing only one tail
gave biaxial SmA phases shown to be antiferroelectric (SmAPA). The SmAPA phase shows antiferroelectric layer
stacking with synclinic layer interfaces, as expected based upon typical behavior seen in most bent-core mesogens. In
order to obtain the target ferroelectric phase, several tails known to allow the formation of anticlinic layer interfaces
were incorporated into the basic Sadashiva/Reddy structure. These tails are thought to suppress out of layer fluctuations,
thereby removing the strong entropically driven tendency for synclinic layer interfaces, and allowing formation of
anticlinic layer stacking driven by a more subtle, and unknown, factor in the free energy of the system. In the event, the
tricarbosilane-terminated alkoxy tail proved effective, providing the first known low molar mass SmAPF material.
Optical electric field sensors have been used for the measurement of high-voltages found in power substations. Typical
sensors are based on electro-optic crystals and hence require the coupling of light into and out of the crystals from optical
fibers. This coupling is difficult and costly. The objective of the work presented here is the design and implementation of
an optical electric field sensor that uses an entirely fiber-based sensor-head. The sensor-head is comprised of a D-shaped
optical fiber with its flat side coated with liquid crystals. D-fibers allow easy access to the evanescent optical field and
replacement of part of the cladding with an external medium allows for modulation of this optical field. We are
investigating the use of chiral Smectic A liquid crystals, which respond linearly to electric fields through the electroclinic
effect. The propagation characteristics of the D-fiber for various distances between the fiber core edge and flat and for
various refractive indices of the external medium are theoretically investigated and experimentally verified. Preliminary
experimental results for a prototype electric field sensor are presented. The sensor responds in a linear fashion to an
applied electric field.
Possible applications of the novel antiferroelectric/ferroelectric phases exhibited by achiral bent-core mesogens (termed banana phases, B phases, or bow- phases in the literature) are considered. Specifically, in a ferroelectric bow-phase, alignment such that the molecular bow plane of the bent mesogens is parallel to the substrates, anchoring the polarization parallel to the substrates, should afford an ideal geometry for analog `V- shaped switching' devices.
We have studied the three-component liquid crystal mixture reported by Inui et al. to exhibit `thresholdless antiferroelectricity', as well as some other materials with the same V-shaped (linear) electro-optic switching behavior. We find that thresholdless behavior is obtained in the absence of antiferroelectricity. The characteristic response of these materials is due to the field-induced switching of a twisted smectic C* structure stabilized by strong polar surface interactions. The twist is confined to thin surface regions by polarization charge effects leaving the bulk of the cell uniform, which gives almost complete extinction at zero field. Model calculations of the electro-optic response are in good agreement with the experimental data. In sufficiently thin cells, such thresholdless switching can in fact be followed down to much lower temperatures, where the bulk would be antiferroelectric but the cell is maintained in the ferroelectric state by hysteresis.