16 research outputs found
Conversion efficiency in a Josephson effect mixer
The RCSJ (Resistor-Capacitor Shunted Junction) model provides us an
analytical formula for the conversion efficiency for a resonant Josephson
effect mixer. An external rf signal is applied to a long Josephson junction
operating in the fluxon oscillating regime. The sine-Gordon equation is used to
investigate the dynamics of the phase-locking between the fluxon oscillations
and the external signal. We obtain that it is possible to have conversion
efficiency greater than unity.Comment: 6 pages, LaTeX article, packages: cite epsfig, 3 figures, 11
reference
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
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
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
Sensitive methods for estimating the anchoring strength of nematic liquid crystals on Langmuir-Blodgett monolayers of fatty acids
The anchoring of the nematic liquid crystal
N-(p-methoxybenzylidene)-p-butylaniline (MBBA) on Langmuir-Blodgett monolayers
of fatty acids (COOHCH) was studied as a function of the length
of the fatty acid alkyl chains, (). The monolayers were
deposited onto ITO-coated glass plates which were used to assemble sandwich
cells of various thickness that were filled with MBBA in the nematic phase. The
mechanism of relaxation from the flow-induced quasi-planar to the
surface-induced homeotropic alignment was studied for the four decreases
linearly with increasing the length of the alkyl chains which suggests that
the Langmuir-Blodgett film plays a role in the phenomenon. This fact was
confirmed by a sensitive estimation of the anchoring strength of MBBA on the
fatty acid monolayers after anchoring breaking which takes place at the
transition between two electric-field--induced turbulent states, denoted as
DSM1 and DSM2. It was found that the threshold electric field for the anchoring
breaking, which can be considered as a measure of the anchoring strength, also
decreases linearly as increases. Both methods thus possess a high
sensitivity in resolving small differences in anchoring strength. In cells
coated with mixed Langmuir-Blodgett monolayers of two fatty acids ( and
) a maximum of the relaxation speed was observed when the two acids were
present in equal amount. This observation homeotropic cells by changing the
ratio between the components of the surfactant film.Comment: LaTeX article, 20 pages, 15 figures, 17 EPS files. 1 figure added,
references moved. Submitted to Phys. Rev.