44 research outputs found
Dephasing of Local Vibrations in a Planar Lattice of Adsorbed Molecules
We investigate anharmonically coupled high- and low-frequency excitations in
a planar lattice of adsorbed molecules interacting with phonons of a crystal.
Dephasing of high-frequency local vibrations by low-frequency resonance modes
is described in terms of temperature Green's function. The equations obtained
are solved, first, with a small ratio of the anharmonic coupling coefficient
for high- and low-frequency modes to the resonance width, and second, in the
low-temperature limit. High-frequency spectral line positions and widths depend
on dispersion laws and resonance mode lifetimes. It is shown that lateral
interactions of low-frequency modes of adsorbed molecules can lead to a
significant narrowing of high-frequency spectral lines, which is consistent
with experimental data.Comment: REVTeX, 11 pages, no figure
Photoinduced ordering and anchoring properties of azo-dye films
We study both theoretically and experimentally anchoring properties of
photoaligning azo-dye films in contact with a nematic liquid crystal depending
on photoinduced ordering of azo-dye molecules. In the mean field approximation,
we found that the bare surface anchoring energy linearly depends on the azo-dye
order parameter and the azimuthal anchoring strength decays to zero in the
limit of vanishing photoinduced ordering. From the absorption dichroism spectra
measured in the azo-dye films that are prepared from the azo-dye derivative
with polymerizable terminal groups (SDA-2) we obtain dependence of the dichroic
ratio on the irradiation dose. We also measure the polar and azimuthal
anchoring strengths in nematic liquid crystal (NLC) cells aligned by the
azo-dye films and derive the anchoring strengths as functions of the dichroic
ratio. Though linear fitting of the experimental data for both anchoring
strengths gives reasonably well results, it, in contradiction with the theory,
predicts vanishing of the azimuthal anchoring strength at certain nonzero value
of the azo-dye order parameter. By using a simple phenomenological model we
show that this discrepancy can be attributed to the difference between the
surface and bulk order parameters in the films.Comment: revtex4, 25 pages, 9 figure
Kinetics of photoinduced ordering in azo-dye films: two-state and diffusion models
We study the kinetics of photoinduced ordering in the azo-dye SD1
photoaligning layers and present the results of modeling performed using two
different phenomenological approaches. A phenomenological two state model is
deduced from the master equation for an ensemble of two-level molecular
systems. Using an alternative approach, we formulate the two-dimensional (2D)
diffusion model as the free energy Fokker-Planck equation simplified for the
limiting regime of purely in-plane reorientation. The models are employed to
interpret the irradiation time dependence of the absorption order parameters
extracted from the available experimental data by using the exact solution to
the light transmission problem for a biaxially anisotropic absorbing layer. The
transient photoinduced structures are found to be biaxially anisotropic whereas
the photosteady and the initial states are uniaxial.Comment: revtex4, 34 pages, 9 figure
Photoinduced 3D orientational order in side chain liquid crystalline azopolymers
We apply experimental technique based on the combination of methods dealing
with principal refractive indices and absorption coefficients to study the
photoinduced 3D orientational order in the films of liquid crystalline (LC)
azopolymers. The technique is used to identify 3D orientational configurations
of trans azobenzene chromophores and to characterize the degree of ordering in
terms of order parameters. We study two types of LC azopolymers which form
structures with preferred in-plane and out-of-plane alignment of
azochromophores, correspondingly. Using irradiation with the polarized light of
two different wavelengths we find that the kinetics of photoinduced anisotropy
can be dominated by either photo-reorientation or photoselection mechanisms
depending on the wavelength. We formulate the phenomenological model describing
the kinetics of photoinduced anisotropy in terms of the isomer concentrations
and the order parameter tensor. We present the numerical results for absorption
coefficients that are found to be in good agreement with the experimental data.
The model is also used to interpret the effect of changing the mechanism with
the wavelength of the pumping light.Comment: uses revtex4 28 pages, 10 figure
Recommended from our members
Flexible electronics: The next ubiquitous platform
Thin-film electronics in its myriad forms has underpinned much of the technological innovation in the fields of displays, sensors, and energy conversion over the past four decades. This technology also forms the basis of flexible electronics. Here we review the current status of flexible electronics and attempt to predict the future promise of these pervading technologies in healthcare, environmental monitoring, displays and human-machine interactivity, energy conversion, management and storage, and communication and wireless networks
Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of nematic liquid-crystal easy axis
We theoretically study the kinetics of photoinduced reordering triggered by
linearly polarized (LP) reorienting light in thin azo-dye films that were
initially illuminated with LP ultraviolet (UV) pumping beam. The process of
reordering is treated as a rotational diffusion of molecules in the light
intensity-dependent mean-field potential. The two dimensional diffusion model
which is based on the free energy rotational Fokker-Planck equation and
describes the regime of in-plane reorientation is generalized to analyze the
dynamics of the azo-dye order parameter tensor at varying polarization azimuth
of the reorienting light. It is found that, in the photosteady state, the
intensity of LP reorienting light determines the scalar order parameter (the
largest eigenvalue of the order parameter tensor), whereas the steady state
orientation of the corresponding eigenvector (the in-plane principal axis)
depends solely on the polarization azimuth. We show that, under certain
conditions, reorientation takes place only if the reorienting light intensity
exceeds its critical value. Such threshold behavior is predicted to occur in
the bistability region provided that the initial principal axis lies in the
polarization plane of reorienting light. The model is used to interpret the
experimental data on the light-induced azimuthal gliding of liquid-crystal easy
axis on photoaligned azo-dye substrates.Comment: 27 pages, 11 fugure
Simple Topological Estimation of the First Electronic Hyperpolarizability of Polymethine Compounds
Quenching of CdSe quantum dot emission, a new approach for biosensing
The emission of CdSe quantum dots linked to the 5'-end of a DNA sequence is efficiently quenched by hybridisation with a complementary DNA strand with a gold nanoparticle attached at the 3'-end; contact of the quantum dot and gold nanoparticle occurs