204 research outputs found
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: III. Interference Caused by a Double Pulse
In order to study consequences of the differences between the
ionic-to-neutral and neutral-to-ionic transitions in the one-dimensional
extended Peierls-Hubbard model with alternating potentials for the TTF-CA
complex, we introduce a double pulse of oscillating electric field in the
time-dependent Schr\"odinger equation and vary the interval between the two
pulses as well as their strengths. When the dimerized ionic phase is
photoexcited, the interference effect is clearly observed owing to the
coherence of charge density and lattice displacements. Namely, the two pulses
constructively interfere with each other if the interval is a multiple of the
period of the optical lattice vibration, while they destructively interfere if
the interval is a half-odd integer times the period, in the processes toward
the neutral phase. The interference is strong especially when the pulse is
strong and short because the coherence is also strong. Meanwhile, when the
neutral phase is photoexcited, the interference effect is almost invisible or
weakly observed when the pulse is weak. The photoinduced lattice oscillations
are incoherent due to random phases. The strength of the interference caused by
a double pulse is a key quantity to distinguish the two transitions and to
evaluate the coherence of charge density and lattice displacements.Comment: 16 pages, 8 figure
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: II. Linear Behavior in Neutral-to-Ionic Transition
Dynamics of charge density and lattice displacements after the neutral phase
is photoexcited is studied by solving the time-dependent Schr\"odinger equation
for a one-dimensional extended Peierls-Hubbard model with alternating
potentials. In contrast to the ionic-to-neutral transition studied previously,
the neutral-to-ionic transition proceeds in an uncooperative manner as far as
the one-dimensional system is concerned. The final ionicity is a linear
function of the increment of the total energy. After the electric field is
turned off, the electronic state does not significantly change, roughly keeping
the ionicity, even if the transition is not completed, because the ionic
domains never proliferate. As a consequence, an electric field with frequency
just at the linear absorption peak causes the neutral-to-ionic transition the
most efficiently. These findings are consistent with the recent experiments on
the mixed-stack organic charge-transfer complex, TTF-CA. We artificially modify
or remove the electron-lattice coupling to discuss the origin of such
differences between the two transitions.Comment: 17 pages, 9 figure
Evidence of environmental strains on charge injection in silole based organic light emitting diodes
Using d. functional theory (DFT) computations, the authors demonstrated a
substantial skeletal relaxation when the structure of
2,5-bis-[4-anthracene-9-yl-phenyl]-1,1-dimethyl-3,4-diphenyl-silole (BAS) is
optimized in the gas-phase comparing with the mol. structure detd. from
monocrystal x-ray diffraction. The origin of such a relaxation is explained by
a strong environmental strains induced by the presence of anthracene entities.
Also, the estn. of the frontier orbital levels showed that this structural
relaxation affects mainly the LUMO that is lowered of 190 meV in the gas phase.
To check if these theor. findings would be confirmed for thin films of BAS, the
authors turned to UV photoemission spectroscopy and/or inverse photoemission
spectroscopy and electrooptical measurements. The study of the c.d. or voltage
and luminance or voltage characteristics of an ITO/PEDOT/BAS/Au device clearly
demonstrated a very unusual temp.-dependent behavior. Using a thermally
assisted tunnel transfer model, this behavior likely originated from the
variation of the electronic affinity of the silole deriv. with the temp. The
thermal agitation relaxes the mol. strains in thin films as it is shown when
passing from the cryst. to the gas phase. The relaxation of the intramol. thus
induces an increase of the electronic affinity and, as a consequence, the more
efficient electron injection in org. light-emitting diodes
Réalisation de micro-résonateurs organiques par procédés hybrides : Nouvelles perspectives en photonique intégrée
L’article consiste en une étude de micro-résonateurs (MRs) sphériques fabriqués à partir d’un matériau organique : le polymère NOA (Northland Optical Adhesive). Les auteurs présentent en un premier lieu le circuit micro fluidique spécialement adapté à la création de gouttes sphériques MRs.L’excitation des modes de galerie permet de déterminer les rayons des sphères. Dans une intégration photonique et pour un meilleur confinement, les sphères sont excitées en utilisant un guide d’onde réalisé par technique de « spin coating » à l’aide de polymère SU8 (bisphénol A d’éther glycide). Le couplage est réalisé à l’aide d’ondes évanescentes circulant du guide vers la sphère. La photographie des sphères excitées (pour deux configurations différentes all-pass ou add-drop) met en évidence les modes optiques situés à la surface des sphères synonymes de modes de galerie. L’analyse spectrale du signal à la sortie du guide met en évidence la présence de pics confirmant l’excitation de cette catégorie de modes au sein de la structure. Ces mesures, à l’aide de l’intervalle spectrale libre permettent de remonter aux diamètres des sphères. Les valeurs déterminées sont en bon accord avec celles mesurées à l’aide de la microscopie optique
Infrared study of spin crossover Fe-picolylamine complex
Infrared (IR) absorption spectroscopy has been used to probe the evolution of
microscopic vibrational states upon the temperature- and photo-induced spin
crossovers in [Fe(2-picolylamine)3]Cl2EtOH (Fe-pic). To overcome the small
sizes and the strong IR absorption of the crystal samples used, an IR
synchrotron radiation source and an IR microscope have been used. The obtained
IR spectra of Fe-pic show large changes between high-spin and low-spin states
for both the temperature- and the photo- induced spin crossovers. Although the
spectra in the temperature- and photo-induced high-spin states are relatively
similar to each other, they show distinct differences below 750 cm-1. This
demonstrates that the photo-induced high-spin state involves microscopically
different characters from those of the temperature-induced high-spin state. The
results are discussed in terms of local pressure and structural deformations
within the picolylamine ligands, and in terms of their possible relevance to
the development of macroscopic photo-induced phase in Fe-pic.Comment: 6 pages (text) and 6 figures,submitted to J. Phys. Soc. Jp
The Subaru Coronagraphic Extreme Adaptive Optics system: enabling high-contrast imaging on solar-system scales
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a
multipurpose high-contrast imaging platform designed for the discovery and
detailed characterization of exoplanetary systems and serves as a testbed for
high-contrast imaging technologies for ELTs. It is a multi-band instrument
which makes use of light from 600 to 2500nm allowing for coronagraphic direct
exoplanet imaging of the inner 3 lambda/D from the stellar host. Wavefront
sensing and control are key to the operation of SCExAO. A partial correction of
low-order modes is provided by Subaru's facility adaptive optics system with
the final correction, including high-order modes, implemented downstream by a
combination of a visible pyramid wavefront sensor and a 2000-element deformable
mirror. The well corrected NIR (y-K bands) wavefronts can then be injected into
any of the available coronagraphs, including but not limited to the phase
induced amplitude apodization and the vector vortex coronagraphs, both of which
offer an inner working angle as low as 1 lambda/D. Non-common path, low-order
aberrations are sensed with a coronagraphic low-order wavefront sensor in the
infrared (IR). Low noise, high frame rate, NIR detectors allow for active
speckle nulling and coherent differential imaging, while the HAWAII 2RG
detector in the HiCIAO imager and/or the CHARIS integral field spectrograph
(from mid 2016) can take deeper exposures and/or perform angular, spectral and
polarimetric differential imaging. Science in the visible is provided by two
interferometric modules: VAMPIRES and FIRST, which enable sub-diffraction
limited imaging in the visible region with polarimetric and spectroscopic
capabilities respectively. We describe the instrument in detail and present
preliminary results both on-sky and in the laboratory.Comment: Accepted for publication, 20 pages, 10 figure
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
FIRST, a pupil-remapping fiber interferometer at the Subaru Telescope: on-sky results
FIRST, the Fibered Imager foR a Single Telescope, is a spectro-imager using single-mode fibers for pupil remap- ping, allowing measurements beyond the telescope diffraction limit. Integrated on the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru Telescope, it benefits from a very stable visible light wave- front allowing to acquire long exposure and operate on significantly fainter sources than previously possible. On-sky results demonstrated the ability of the instrument to detect stellar companions separated 43mas in the case of the Capella binary system. A similar approach on an extremely large telescope would offer unique scientific opportunities for companion detection and characterization at very high angular resolution
FIRST, a pupil-remapping fiber interferometer at the Subaru Telescope: on-sky results
FIRST, the Fibered Imager foR a Single Telescope, is a spectro-imager using single-mode fibers for pupil remap- ping, allowing measurements beyond the telescope diffraction limit. Integrated on the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru Telescope, it benefits from a very stable visible light wave- front allowing to acquire long exposure and operate on significantly fainter sources than previously possible. On-sky results demonstrated the ability of the instrument to detect stellar companions separated 43mas in the case of the Capella binary system. A similar approach on an extremely large telescope would offer unique scientific opportunities for companion detection and characterization at very high angular resolution
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