Ultrasound modulation of coherent light in a multiple-scattering medium: experimental verification of nonzero average phase carried by light

We demonstrate the phase fluctuation introduced by oscillation of scattering centers in the focal volume of an ultrasound transducer in an optical tomography experiment has a nonzero mean. The conditions to be met for the above are: (i) the frequency of the ultrasound should be in the vicinity of the most dominant natural frequency of vibration of the ultrasound focal volume, (ii) the corresponding acoustic wavelength should be much larger than l(n)*, a modified transport mean-free-path applicable for phase decorrelation and (iii) the focal volume of the ultrasound transducer should not be larger than 4 - 5 times (l(n)*)(3). We demonstrate through simulations that as the ratio of the ultrasound focal volume to (l(n)*)(3) increases, the average of the phase fluctuation decreases and becomes zero when the focal volume becomes greater than around 4(l(n)*)(3); and through simulations and experiments that as the acoustic frequency increases from 100 Hz to 1 MHz, the average phase decreases to zero. Through experiments done in chicken breast we show that the average phase increases from around 110 degrees to 130 degrees when the background medium is changed from water to glycerol, indicating that the average of the phase fluctuation can be used to sense changes in refractive index deep within tissue.

Raman scattering, microstructural and dielectric studies on Ba1-xCaxBi4Ti4O15 ceramics

Polycrystalline powders of Ba1-xCaxBi4Ti4O15 (where x = 0, 0.25, 0.50, 0.75 and 1) were prepared via the conventional solid-state reaction route. X-ray diffraction (XRD) and Raman scattering techniques have been employed to probe into the structural changes on changing x. XRD analyses confirmed the formation of monophasic bismuth layered structure of all the above compositions with an increase in orthorhombic distortion with increase in x. Raman spectra revealed a redshift in A(1g) peak and an increase in the B-2g/B-3g splitting with increasing Ca content. The average grain size was found to increase with increasing x. The temperature of the maximum dielectric constant (T-m) increased linearly with increasing Ca-content whereas the diffuseness of the phase transition was found to decrease with the end member CaBi4Ti4O15 showing a frequency independent sharp phase transition around 1048 K. Ca doping resulted in a decrease in the remnant polarization and an increase in the coercive field. Ba0.75Ca0.25Bi4Ti4O15 ceramics showed an enhanced piezoelectric coefficient d(33) of 15 pC N-1 at room temperature. Low values of dielectric losses and tunability of temperature coefficient of dielectric constant (tau(epsilon)) in the present solid-solution suggest that these compounds can be of potential use in microwave dielectrics at high temperatures. (C) 2012 Elsevier B.V. All rights reserved.

RCS ENHANCEMENT DUE TO BRAGG SCATTERING

With the advances of techniques for RCS reduction, it has become practical to develop aircraft which are invisible to modern day radars. In order to detect such low visible targets it is necessary to explore other phenomenon that contributes to the scattering of incident electromagnetic wave. It is well known from the developments from the clear air scattering using RASS induced acoustic wave could be used to create dielectric constant fluctuation. The scattering from these fluctuations rather than from the aircraft have been observed to enhance the RCS of clear air, under the condition when the incident EM wave is half of the acoustic wave, the condition of Bragg scattering would be met and RCS would be enhanced. For detecting low visibility targets which are at significant distance away from the main radar, inducement of EM fluctuation from acoustic source collocated with the acoustic source is infeasible. However the flow past aircraft produces acoustic disturbances around the aircraft can be exploited to detect low visibility targets. In this paper numerical simulation for RCS enhancement due to acoustic disturbances is presented. In effect, this requires the solution of scattering from 3D inhomogeneous complex shaped bodies. In this volume surface integral equation (VSIE) is used to compute the RCS from fluctuation introduced through the acoustic disturbances. Though the technique developed can be used to study the scattering from radars of any shape and acoustic disturbances of any shape. For illustrative condition, enhancement due to the Bragg scattering are shown to improve the RCS by nearly 30dB, for air synthetic sinusoidal acoustic variation profile for a spherical scattering volume

Layer-dependent resonant Raman scattering of a few layer MoS2

We report resonant Raman scattering of MoS2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS2 in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single- and few-layer MoS2 samples which are absent in the bulk. The Raman mode at similar to 230 cm(-1) appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at similar to 179 cm(-1) shows asymmetric character for a few-layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the G-M direction. The most intense spectral region near 455 cm(-1) shows a layer-dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm(-1) is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS2 layers down to a few atoms thick. Copyright (C) 2012 John Wiley & Sons, Ltd.

Scattering of carriers by charged dislocations in semiconductors

The scattering of carriers by charged dislocations in semiconductors is studied within the framework of the linearized Boltzmann transport theory with an emphasis on examining consequences of the extreme anisotropy of the cylindrically symmetric scattering potential. A new closed-form approximate expression for the carrier mobility valid for all temperatures is proposed. The ratios of quantum and transport scattering times are evaluated after averaging over the anisotropy in the relaxation time. The value of the Hall scattering factor computed for charged dislocation scattering indicates that there may be a factor of two error in the experimental mobility estimates using the Hall data. An expression for the resistivity tensor when the dislocations are tilted with respect to the plane of transport is derived. Finally, an expression for the isotropic relaxation time is derived when the dislocations are located within the sample with a uniform angular distribution.

Interconnected, ultrafine osmium nanoclusters: preparation and surface enhanced Raman scattering activity

Interconnected Os nanochains consisting of ultrafine particles prepared using a simple procedure yield a coupled surface plasmon peak in the visible region and can be used as substrates for surface enhanced Raman scattering of various analytes.

Bioconvection in an anisotropic scattering suspension of phototactic algae

Phototaxis is a directed swimming response dependent upon the light intensity sensed by microorganisms. Positive phototaxis denotes motion directed towards the source of light and negative phototaxis is motion directed away from it. In this paper, we investigate the onset of bioconvection in a suspension of anisotropic scattering phototactic algae illuminated by collimated radiation at the top. The basic state of the system is defined by the zero fluid flow and the up and down swimming, caused by the positive and negative phototaxis, is balanced by the diffusion. A comprehensive numerical study of the linear stability is presented with particular emphasis on the forward scattering effect. The onset of bioconvection occurs either via a stationary mode or an oscillatory mode. The transition from a stationary mode to an oscillatory mode or vice versa has been observed as the anisotropic coefficient is varied for certain parameter values. (C) 2012 Elsevier Masson SAS. All rights reserved.

Lead-free piezoelectric system (Na0.5Bi0.5)TiO3-BaTiO3: Equilibrium structures and irreversible structural transformations driven by electric field and mechanical impact

The structure-property correlation in the lead-free piezoelectric (1 - x)(Na0.5Bi0.5)TiO3-(x)BaTiO3 has been systematically investigated in detail as a function of composition (0 < x <= 0.11), temperature, electric field, and mechanical impact by Raman scattering, ferroelectric, piezoelectric measurement, x-ray, and neutron powder diffraction methods. Although x-ray diffraction study revealed three distinct composition ranges characterizing different structural features in the equilibrium state at room temperature: (i) monoclinic (Cc) + rhombohedral (R3c) for the precritical compositions, 0 <= x <= 0.05, (ii) cubiclike for 0.06 <= x <= 0.0675, and (iii) morphotropic phase boundary (MPB) like for 0.07 <= x < 0.10, Raman and neutron powder diffraction studies revealed identical symmetry for the cubiclike and the MPB compositions. The cubiclike structure undergoes irreversible phase separation by electric poling as well as by pure mechanical impact. This cubiclike phase exhibits relaxor ferroelectricity in its equilibrium state. The short coherence length (similar to 50A degrees) of the out-of-phase octahedral tilts does not allow the normal ferroelectric state to develop below the dipolar freezing temperature, forcing the system to remain in a dipolar glass state at room temperature. Electric poling helps the dipolar glass state to transform to a normal ferroelectric state with a concomitant enhancement in the correlation length of the out-of-phase octahedral tilt.

Penetrative phototactic bioconvection in an isotropic scattering suspension

Phototaxis is a directed swimming response dependent upon the light intensity sensed by micro-organisms. Positive (negative) phototaxis denotes the motion directed towards (away from) the source of light. Using the phototaxis model of Ghorai, Panda, and Hill ''Bioconvection in a suspension of isotropically scattering phototactic algae,'' Phys. Fluids 22, 071901 (2010)], we investigate two-dimensional phototactic bioconvection in an absorbing and isotropic scattering suspension in the nonlinear regime. The suspension is confined by a rigid bottom boundary, and stress-free top and lateral boundaries. The governing equations for phototactic bioconvection consist of Navier-Stokes equations for an incompressible fluid coupled with a conservation equation for micro-organisms and the radiative transfer equation for light transport. The governing system is solved efficiently using a semi-implicit second-order accurate conservative finite-difference method. The radiative transfer equation is solved by the finite volume method using a suitable step scheme. The resulting bioconvective patterns differ qualitatively from those found by Ghorai and Hill ''Penetrative phototactic bioconvection,'' Phys. Fluids 17, 074101 (2005)] at a higher critical wavelength due to the effects of scattering. The solutions show transition from steady state to periodic oscillations as the governing parameters are varied. Also, we notice the accumulation of micro-organisms in two horizontal layers at two different depths via their mean swimming orientation profile for some governing parameters at a higher scattering albedo. (C) 2013 AIP Publishing LLC.

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.

Spontaneous assembly of iridium nanochain-like structures: surface enhanced Raman scattering activity using visible light

A facile, environmentally friendly approach to synthesize branched Ir nanochain-like structures under mild conditions, using polyfunctional capping molecules in an aqueous medium is reported; the nanostructures exhibit a surface plasmon resonance peak (SPR) in the visible region and serve as an active substrate for surface enhanced Raman scattering studies.

Electrical switching, SET-RESET, and Raman scattering studies on Ge15Te80-xIn5Agx glasses

Bulk Ge15Te85-xIn5Agx glasses are shown to exhibit electrical switching with switching/threshold voltages in the range of 70-120V for a sample thickness of 0.3 mm. Further, the samples exhibit threshold or memory behavior depending on the ON state current. The compositional studies confirm the presence of an intermediate phase in the range 8 <= x <= 16, revealed earlier by thermal studies. Further, SET-RESET studies have been performed by these glasses using a triangular pulse of 6 mA amplitude (for SET) and 21 mA amplitude (for RESET). Raman studies of the samples after the SET and RESET operations reveal that the SET state is a crystalline phase which is obtained by thermal annealing and the RESET state is the glassy state, similar to the as-quenched samples. It is interesting to note that the samples in the intermediate phase, especially compositions at x = 10, 12, and 14 withstand more set-reset cycles. This indicates compositions in the intermediate phase are better suited for phase change memory applications. (C) 2014 AIP Publishing LLC.

Revised density of magnetized nuclear matter at the neutron drip line

We study the onset of the neutron drip in high-density matter in the presence of a magnetic field. It has been found that, for systems having only protons and electrons, in the presence of a magnetic field greater than or similar to 10(15) G, neutronization occurs at a density that is at least an order of magnitude higher compared to that in a nonmagnetic system. In a system with heavier ions, the effect of the magnetic field, however, starts arising at a much higher field, greater than or similar to 10(17) G. These results may have important implications for high-magnetic-field neutron stars and white dwarfs and, in general, in nuclear astrophysics when the system is embedded within a strong magnetic field.

Synthesis of self-light-scattering wrinkle structured ZnO photoanode by sol-gel method for dye-sensitized solar cells

A special morphological zinc oxide (ZnO) photoanode for dye-sensitized solar cell was fabricated by simple sol-gel drop casting technique. This film shows a wrinkled structure resembling the roots of banyan tree, which acts as an effective self scattering layer for harvesting more visible light and offers an easy transport path for photo-injected electrons. These ZnO electrode of low thickness (similar to 5 mu m) gained an enhanced short-circuit current density of 6.15 mA/cm(2), open-circuit voltage of 0.67 V, fill factor of 0.47 and overall conversion efficiency of 1.97 % under 1 sun illumination. This shows a high conversion efficiency and a superior performance than that of ZnO nanoparticle-based photoanode (eta similar to 1.13 %) of high thickness (similar to 8 mu m).

Phonon anomalies, orbital-ordering and electronic raman scattering in iron-pnictide Ca(Fe0.97Co0.03)(2)As-2: temperature-dependent Raman study

We report inelastic light scattering studies on Ca(Fe0.97Co0.03)(2)As-2 in a wide spectral range of 120-5200 cm(-1) from 5 to 300 K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at T-sm similar to 160 K. The mode frequencies of two first-order Raman modes B-1g and E-g, both involving the displacement of Fe atoms, show a sharp increase below T-sm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below T-sm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400 and 1200 cm(-1) are attributed to electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and yz d-orbital levels is shown to be similar to 25 meV, which increases as temperature decreases below T-sm. A broad Raman band observed at similar to 3200 cm(-1) is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.