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Synthesis, electrochemical and optical studies of new cyanopyridine based conjugated polymers as potential fluorescent materials
No full textRestricted AccessIn this communication we describe the design and synthesis of four new conjugated polymers (P1eP4)
carrying cyanopyridine in their backbone via Wittig condensation technique. Their structures were well
established by FTIR, 1H NMR, elemental analysis and gel permeation chromatographic techniques. They
exhibited good thermal stability with the onset decomposition temperature around 300e350 C in
nitrogen atmosphere. Further, their electrochemical, linear and third-order non-linear optical properties
have been investigated. The polymers showed low electrochemical band gaps in the range of 1.77
e1.99 eV and displayed very good fluorescence emission property in various polar solvents. The fluorescence
quantum yields of polymers P1eP4 in tetrahydrofuran (THF) were found to be 35, 42, 38 and
34%, respectively. The effective two-photon absorption coefficients (b) for the polymers were determined
by Z-Scan technique and were found to be 1.34 x 10, 1.24 x 10, 3.04 x 10 and 1.85 x 1011 m/W
for polymers P1eP4, respectively
Optical nonlinearity and power limiting in organic molecules and nanocomposites
No full textRestricted Access.Nonlinear optical transmission in materials has several applications including laser mode-locking, pulse shaping, optical
bistability, optical switching, and optical power limiting. Organic molecules suitable for functionalization have been
extensively investigated for their third order nonlinearities. We have measured the optical nonlinearity of different novel
organic and composite systems including nanocomposite polymer films of Au, Ag and Pt, Organic ionic crystals
(pyridinium and quinolinium salts), Au-alkanethiol clusters, thiophene based polymers, and Schiff base complexes, using
the z-scan and degenerate four wave mixing techniques, employing laser pulses of nanosecond and femtosecond
durations respectively. Most of these materials are found to be efficient optical power limiters under our excitation
conditions, and their nonlinear extinction coefficients have been calculated. Enhancement in the optical nonlinearity was
sometimes obtained even by mixing of two organic media. From degenerate four wave mixing experiments, the third
order nonlinear susceptibility χ(3) and figure of merit χ(3)/α values also have been determined for some of these
materials. The above experiments conducted in a large number of organic and composite materials unravel the potential
of these media for diverse photonic applications
CdSe quantum dots in a columnar matrix
No full textOpen accessCadmium selenide quantum dots (QDs) have been dispersed in a
discotic liquid crystal columnar matrix. Thermophysical properties
of these liquid crystal nanocomposites were investigated by UV-Vis
spectroscopy, photoluminescence spectroscopy, differential scanning
calorimetry, polarizing optical microscopy, DC conductivity and
small angle X-ray diffraction
Integrin adhesion drives the emergent polarization of active cytoskeletal stresses to pattern cell delamination
No full textRestricted Access.Tissue patterning relies on cellular reorganization through the
interplay between signaling pathways and mechanical stresses.
Their integration and spatiotemporal coordination remain poorly
understood. Here we investigate the mechanisms driving the
dynamics of cell delamination, diversely deployed to extrude dead
cells or specify distinct cell fates. We show that a local mechanical
stimulus (subcellular laser perturbation) releases cellular prestress
and triggers cell delamination in the amnioserosa during Drosophila
dorsal closure, which, like spontaneous delamination, results in
the rearrangement of nearest neighbors around the delaminating
cell into a rosette. We demonstrate that a sequence of “emergent
cytoskeletal polarities” in the nearest neighbors (directed myosin
flows, lamellipodial growth, polarized actomyosin collars, microtubule
asters), triggered by the mechanical stimulus and dependent
on integrin adhesion, generate active stresses that drive
delamination. We interpret these patterns in the language of active
gels as asters formed by active force dipoles involving surface
and body stresses generated by each cell and liken delamination
to mechanical yielding that ensues when these stresses exceed
a threshold. We suggest that differential contributions of adhesion,
cytoskeletal, and external stresses must underlie differences
in spatial pattern
A high speed networked signal processing platform for multi-element radio telescopes
No full textRestricted Access. An open-access version is available at one of the alternative locations.A new architecture is presented for a Networked Signal Processing
System (NSPS) suitable for handling the real-time signal processing of multielement
radio telescopes. In this system, a multi-element radio telescope is
viewed as an application of a multi-sensor, data fusion problem which can
be decomposed into a general set of computing and network components for
which a practical and scalable architecture is enabled by current technology.
The need for such a system arose in the context of an ongoing program for reconfiguring
the Ooty Radio Telescope (ORT) as a programmable 264-element
array, which will enable several new observing capabilities for large scale surveys
on this mature telescope. For this application, it is necessary to manage,
route and combine large volumes of data whose real-time collation requires
large I/O bandwidths to be sustained. Since these are general requirements of
many multi-sensor fusion applications, we first describe the basic architecture
of the NSPS in terms of a Fusion Tree before elaborating on its application
for the ORT. The paper addresses issues relating to high speed distributed
data acquisition, Field Programmable Gate Array (FPGA) based peer-topeer
networks supporting significant on-the fly processing while routing, and
providing a last mile interface to a typical commodity network like Gigabit
Ethernet. The system is fundamentally a pair of two co-operative networks,
among which one is part of a commodity high performance computer cluster and the other is based on Commercial-Off The-Shelf (COTS) technology with
support from software/firmware components in the public domain
Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films
No full textOpen Access.We report a dramatic change in effective three-photon absorption coefficient of amorphous
Ge16As29Se55 thin films, when its optical band gap decreases by 10 meV with 532 nm light
illumination. This large change provides valuable information on the higher excited states, which are
otherwise inaccessible via normal optical absorption. The results also indicate that photodarkening
in chalcogenide glasses can serve as an effective tool to tune the multiphoton absorption in a rather
simple way
Record statistics of continuous time random walk
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The statistics of records for a time series generated by a continuous time random walk is studied, and found to be independent of the details of the jump length distribution, as long as the latter is continuous and symmetric. However, the statistics depend crucially on the nature of the waiting-time distribution. The probability of finding M records within a given time duration t, for large t, has a scaling form, and the exact scaling function is obtained in terms of the one-sided Levy stable distribution. The mean of the ages of the records, defined as < t/M >, differs from t/< M >. The asymptotic behaviour of the shortest and the longest ages of the records are also studied
Quantum walk of light in frequency space and its controlled dephasing
No full textOpen AccessWe implement, using a coherent source, a coined quantum walk of light in frequency space in a tabletop experiment utilizing a series of modified Michelson interferometers that incorporate polarization optics and acousto-optic modulators. Manipulating the phase of the radio frequency that governs the acousto-optic interaction, we achieve symmetric or asymmetric quantum walks. Introducing rapid random phase shifts electronically, while regulating the amplitude, we cause controlled dephasing and thus simulate a gradual transition from the quantum walk to the classical random walk
18 GHz SZ measurements of the bullet cluster
No full textOpen AccessWe present 18 GHz observations of the Bullet cluster using the Austalia Telescope Compact Array (ATCA), which show structure in the Sunyaev-Zeldovich effect; in particular, a deep, compact feature which does not correspond to any bright feature in X-ray, optical or lensing maps. In general, the relatively deeper SZE features appear to avoid the regions with the most intense X-ray emission. SZE displaced from X-ray centres implies that modeling cluster dynamics is non-trivial. The SZE distribution in the western parts of the cluster are co-spatial with the radio halo indicative of a common origin for the hot and relativistic electrons in the turbulent wake of the Bullet
Double relics in the outskirts of A3376: Accretion flows meet merger shocks?
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts. We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al. 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large-scale accretion flows on the outgoing merger shocks as reported in the recent simulations