75 research outputs found
Dielectric spectroscopy of ferroelectric nematic liquid crystals: Measuring the capacitance of insulating interfacial layers
Numerous measurements of the dielectric constant of the recently
discovered ferroelectric nematic () liquid crystal (LC) phase report
extraordinarily large values of (up to ~30,000). We show that
what is in fact being measured in such experiments is the high capacitance of
the non-ferroelectric, interfacial, insulating layers of nanoscale thickness
that bound the material in typical cells.
We analyze a parallel-plate cell filled with material of
high-polarization , oriented parallel to the plates at zero applied
voltage. Minimization of the dominant electrostatic energy renders
spatially uniform and orients it to make the electric field in the as
small as possible, a condition under which the voltage applied to the cell
appears almost entirely across the high-capacity interfacial layers. This
coupling of orientation and charge creates a combined polarization-external
capacitance (PCG) Goldstone reorientation mode requiring applied voltages
orders of magnitude smaller than that of the layer alone to effectively
transport charge across the layer. The layer acts as a low-value
resistor and the interfacial capacitors as reversible energy storage
reservoirs, lowering the restoring force (mass) of the PCG mode and producing
strong reactive dielectric behavior. Analysis of data from several experiments
on ferroelectric liquid crystals (chiral smectics C, bent-core smectics, and
the phase supports the PCG model, showing deriving that deriving
dielectric constants from electrical impedance measurements of
high-polarization ferroelectric LCs, without properly accounting for the
self-screening effects of polarization charge and the capacitive contributions
of interfacial layers, can result in overestimation of the
values of the LC by many orders of magnitude.Comment: 26 pages of text, 10 figures, 49 references (40 pages total
Topological defect coarsening in quenched smectic-C films analyzed using artificial neural networks
Mechanically quenching a thin film of smectic-C liquid crystal results in the
formation of a dense array of thousands of topological defects in the director
field. The subsequent rapid coarsening of the film texture by the mutual
annihilation of defects of opposite sign has been captured using high-speed,
polarized light video microscopy. The temporal evolution of the texture has
been characterized using an object-detection convolutional neural network to
determine the defect locations, and a binary classification network customized
to evaluate the brush orientation dynamics around the defects in order to
determine their topological signs. At early times following the quench,
inherent limits on the spatial resolution result in undercounting of the
defects and deviations from expected behavior. At intermediate to late times,
the observed annihilation dynamics scale in agreement with theoretical
predictions and simulations of the D XY model.Comment: 11 pages, 10 figure
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Scanned conical illumination as a probe of electro-optic retro-reflection
We describe a prototype element for use in probing electro-optic retro-reflection in sensor applications, illuminating a planar-aligned nematic liquid crystal electro-optic cell with convergent light having a single, tunable angle of incidence (tunable conical illumination). This illumination is generated using a 100X, high numerical aperture, long working-distance microscope objective under conditions of extreme spherical aberration. The electro-optic effect observed is multiple-beam optical interference between polarized reflections from the two bounding plates of the cell, rendered tunable with voltage-controlled refractive index changes induced by molecular reorientation of the liquid crystal. Characterization of the reflectivity vs. angle of incidence and applied voltage enables identification of conditions of high-contrast, low power, electro-optic reflectivity control applicable to fiber optics.</p
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