869 research outputs found
Alignment of nematic liquid crystals on mixed Langmuir-Blodgett mono-layers
Mono-layers of stearic and behenic acids and mixtures of them in different
proportions, deposited with the Langmuir-Blodgett technique, were used to study
the alignment and the alignment dynamics in nematic liquid crystal cells. A
relaxation process from a splay-bend flow induced metastable orientation to the
homeotropic one occurs. The lifetime of the metastable state was found to
depend on the mono-layer composition. The transition between the homeotropic
and the conical anchoring was found to be irreversible in the case of the mixed
aligning mono-layers: on cooling from the isotropic phase a quasi-planar
nematic state (schlieren texture) appears. It is stable in a range of a few
degrees below the clearing point and, on decreasing the temperature, relaxes to
the homeotropic state in form of expanding domains.Comment: 9 pages, LaTeX2e article, 8 figures, 11 EPS files, submitted to Thin
Solid Film
Influence of the flow on the anchoring of nematic liquid crystals on a Langmuir-Blodgett monolayer studied by optical second-harmonic generation
The influence of capillary flow on the alignment of the nematic liquid
crystal MBBA on fatty acid Langmuir-Blodgett monolayers was studied by optical
second-harmonic generation. The surface dipole sensitivity of the technique
allows probing the orientation of the first liquid crystal monolayer in the
presence of the liquid crystal bulk. It was found that capillary flow causes
the first monolayer of liquid crystal molecules in contact with the fatty acid
monolayer to be oriented in the flow direction with a large pretilt (78
degrees), resulting in a quasi-planar alignment with splay-bend deformation of
the nematic director in the bulk. itself is affected by the flow. The
quasi-planar flow-induced alignment was found to be metastable. Once the flow
ceases, circular domains of homeotropic orientation nucleate in the sample and
expand until the whole sample becomes homeotropic. This relaxation process from
flow-induced quasi-planar to surface-induced homeotropic alignment was also
monitored by SHG. It was found that in the homeotropic state the first nematic
layer presents a pretilt of 38 degrees almost isotropically distributed in the
plane of the cell, with a slight preference for the direction of the previous
flow.Comment: LaTeX2e article, 11 figures, 17 EPS files, submitte
Telephone-cord instabilities in thin smectic capillaries
Telephone-cord patterns have been recently observed in smectic liquid crystal
capillaries. In this paper we analyse the effects that may induce them. As long
as the capillary keeps its linear shape, we show that a nonzero chiral
cholesteric pitch favors the SmA*-SmC* transition. However, neither the
cholesteric pitch nor the presence of an intrinsic bending stress are able to
give rise to a curved capillary shape.
The key ingredient for the telephone-cord instability is spontaneous
polarization. The free energy minimizer of a spontaneously polarized SmA* is
attained on a planar capillary, characterized by a nonzero curvature. More
interestingly, in the SmC* phase the combined effect of the molecular tilt and
the spontaneous polarization pushes towards a helicoidal capillary shape, with
nonzero curvature and torsion.Comment: Submitte
Phase-matched second-harmonic generation in a ferroelectric liquid crystal waveguide
True phase-matched second-harmonic generation in a waveguide of crosslinkable
ferroelectric liquid crystals is demonstrated. These materials allow the
formation of macroscopically polar structures whose order can be frozen by
photopolymerization. Homeotropic alignment was chosen which offers decisive
advantages compared to other geometries. All parameters contributing to the
conversion efficiency are maximized by deliberately controlling the
supramolecular arrangement. The system has the potential to achieve practical
level of performances as a frequency doubler for low power laser diodes.Comment: 4 pages, LaTeX2e article, 3 figures, 4 EPS files, submitted to
Physical Review Letter
High-contrast imaging of 180{\deg} ferroelectric domains by optical microscopy using ferroelectric liquid crystals
Ferroelectric liquid crystals (FLCs) couple the direction of their
spontaneous electric polarization to the direction of tilt of their optic axis.
Consequently, reversal of the electric polarization by an electric field gives
rise to an immediate and lasting optical response when an appropriately aligned
FLC is observed between crossed polarizers, with one field direction yielding a
dark image, and the opposite direction yielding a bright image. Here this
peculiar electro-optic response is used to image, with high optical contrast,
180{\deg} ferroelectric domains in a crystalline substrate of magnesium-doped
lithium niobate. The lithium niobate substrate contains a few domains with
upwards electric polarization surrounded by regions with downward electric
polarization. In contrast to a reference non-chiral liquid crystal that is
unable to show ferroelectric behavior due to its high symmetry, the FLC, which
is used as a thin film confined between the lithium niobate substrate and an
inert aligning substrate, reveals ferroelectric domains as well as their
boundaries, with strong black and white contrast. The results show that FLCs
can be used for non-destructive read-out of domains in underlying
ferroelectrics, with potential applications in e.g. photonic devices and
non-volatile ferroelectric memories.Comment: 12 pages, 3 figure
Nonlinear optical properties of a channel waveguide produced with crosslinkable ferroelectric liquid crystals
A binary mixture of ferroelectric liquid crystals (FLCs) was used for the
design of a channel waveguide. The FLCs possess two important functionalities:
a chromophore with a high hyperpolarizability and photoreactive groups.
The smectic liquid crystal is aligned in layers parallel to the glass plates in
a sandwich geometry. This alignment offers several advantages, such as that
moderate electric fields are sufficient to achieve a high degree of polar
order. The arrangement was then permanently fixed by photopolymerization which
yielded a polar network possessing a high thermal and mechanical stability
which did not show any sign of degradation within the monitored period of
several months. The linear and nonlinear optical properties have been measured
and all four independent components of the nonlinear susceptibility tensor
have been determined. The off-resonant -coefficients are remarkably
high and comparable to those of the best known inorganic materials. The
alignment led to an inherent channel waveguide for p-polarized light without
additional preparation steps. The photopolymerization did not induce scattering
sites in the waveguide and the normalized losses were less than 2 dB/cm. The
material offers a great potential for the design of nonlinear optical devices
such as frequency doublers of low power laser diodes.Comment: LaTeX2e article, 15 pages, 10 figures, 11 EPS files, submitted to
Physical Review
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