Metallic Nanoparticles Arranged in a Helical Geometry: Route Towards Strong and Broadband Chiro-optical Response

Recent advances in nanotechnology have paved ways to various techniques for designing and fabricating novel nanostructures incorporating noble metal nanoparticles, for a wide range of applications. The interaction of light with metal nanoparticles (NPs) can generate strongly localized electromagnetic fields (Localized Surface Plasmon Resonance, LSPR) at certain wavelengths of the incident beam. In assemblies or structures where the nanoparticles are placed in close proximity, the plasmons of individual metallic NPs can be strongly coupled to each other via Coulomb interactions. By arranging the metallic NPs in a chiral (e.g. helical) geometry, it is possible to induce collective excitations, which lead to differential optical response of the structures to right-and left circularly polarized light (e.g. Circular Dichroism - CD). Earlier reports in this field include novel techniques of synthesizing metallic nanoparticles on biological helical templates made from DNA, proteins etc. In the present work, we have developed new ways of fabricating chiral complexes made of metallic NPs, which demonstrate a very strong chiro-optical response in the visible region of the electromagnetic spectrum. Using DDA (Discrete Dipole Approximation) simulations, we theoretically studied the conditions responsible for large and broadband chiro-optical response. This system may be used for various applications, for example those related to polarization control of visible light, sensing of proteins and other chiral bio-molecules, and many more.

Biomimetic molecular organization of naphthalene diimide in the solid state: tunable (chiro-) optical, viscoelastic and nanoscale properties

The interfacing of aromatic molecules with biomolecules to design functional molecular materials is a promising area of research. Intermolecular interactions determine the performance of these materials and therefore, precise control over the molecular organization is necessary to improve functional properties. Herein we describe the tunable biomimetic molecular engineering of a promising n-type organic semiconductor, naphthalene diimide (NDI), in the solid state by introducing minute structural mutations in the form of amino acids with variable Ca-functionality. For the first time we could achieve all four possible crystal packing modes, namely cofacial, brickwork, herringbone and slipped stacks of the NDI system. Furthermore, amino acid conjugated NDIs exhibit ultrasonication induced organogels with tunable visco-elastic and temperature responsive emission properties. The amino acid-NDI conjugates self-assemble into 0D nanospheres and 1D nanofibers in their gel state while the ethylamine-NDI conjugate forms 2D sheets from its solution. Photophysical studies indicated the remarkable influence of molecular ordering on the absorption and fluorescence properties of NDIs. Interestingly, the circular dichroism (CD) and X-ray diffraction (XRD) studies revealed the existence of helical ordering of NDIs in both solution and solid state. The chiral amino acids and their conformations with respect to the central NDI core are found to influence the nature of the helical organization of NDIs. Consequently, the origin of the preferential handedness in the helical organization is attributed to transcription of chiral information from the amino acid to the NDI core. On account of these unique properties, the materials derived from NDI-conjugates might find a wide range of future interdisciplinary applications from materials to biomedicine.

Optical, photo-acoustic and electrical switching studies of amorphous GeS2 thin films

This paper reports optical, photo-acoustic and electrical switching investigations of GeS2 amorphous thin films of different thicknesses, deposited on glass substrates in vacuum. The Tauc parameter (B (1/2)) and Urbach energy (E (U)) have been determined from the transmittance spectra, to understand the changes in structural disorder; it is found that B (1/2) increases whereas E (U) decreases as the thickness of the films increases. Based on the results, it is suggested that bond re-arrangement, i.e. transformation from homopolar bonds to heteropolar bonds, takes place with increase in thickness. The thermal diffusivity values of GeS2 thin films also show the presence of a chemically ordered network in the GeS2 thin films. Further, it is found that these films exhibit memory-type electrical switching. The observed variation in the switching voltages has been understood on the basis of increase in chemical order.

Ferroelectric and optical properties of Ba5Li2Ti2Nb8O30 ceramics potential for memory applications

Giant grained (42 mu m) translucent Ba5Li2Ti2Nb8O30 ceramic was fabricated by conventional sintering technique using the powders obtained via solid state reaction route. These samples were confirmed to possess tetragonal tungsten bronze structure (P4bm) at room temperature. The scanning electron microscopy established the average grain size to be close to 20 mu m. The photoluminescence studies carried out on these ceramics indicated sharp emission bands around 433 and 578 nm at an excitation wavelength of 350 nm which were attributed to band-edge emission as the band gap was 2.76 eV determined by Kubelka-Munk function. The dielectric properties of these ceramics were studied over wide frequency range (100-1 MHz) at room temperature. The decrease in dielectric constant with frequency could be explained on the basis of Koops theory. The dielectric constant and the loss were found to decrease with increasing frequency. The Curie temperature was confirmed to be similar to 370 A degrees C based on the dielectric anomaly observed when these measurements were carried out over a temperature range of 30-500 A degrees C. This shows a deviation from Curie-Weiss behaviour and hence an indicator of the occurrence of disordering in the system, the gamma = 1.23 which confirms the diffuse ferroelectric transition. These ceramics at room temperature exhibited P-E hysteresis loops, though not well saturated akin to that of their single crystalline counterparts. These are the suitable properties for ferroelectric random access memory applications.

Incoherence-based optimal selection of independent measurements in diffuse optical tomography

An optimal measurement selection strategy based on incoherence among rows (corresponding to measurements) of the sensitivity (or weight) matrix for the near infrared diffuse optical tomography is proposed. As incoherence among the measurements can be seen as providing maximum independent information into the estimation of optical properties, this provides high level of optimization required for knowing the independency of a particular measurement on its counterparts. The proposed method was compared with the recently established data-resolution matrix-based approach for optimal choice of independent measurements and shown, using simulated and experimental gelatin phantom data sets, to be superior as it does not require an optimal regularization parameter for providing the same information. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

Molecular Dynamics Study of Phonon Screening in Graphene

Phonon interaction with electrons or phonons or with structural defects result in a phonon mode conversion. The mode conversion is governed by the frequency wave-vector dispersion relation. The control over phonon mode or the screening of phonon in graphene is studied using the propagation of amplitude modulated phonon wave-packet. Control over phonon properties like frequency and velocity opens up several wave guiding, energy transport and thermo-electric applications of graphene. One way to achieve this control is with the introduction of nano-structured scattering in the phonon path. Atomistic model of thermal energy transport is developed which is applicable to devices consisting of source, channel and drain parts. Longitudinal acoustic phonon mode is excited from one end of the device. Molecular dynamics based time integration is adopted for the propagation of excited phonon to the other end of the device. The amount of energy transfer is estimated from the relative change of kinetic energy. Increase in the phonon frequency decreases the kinetic energy transmission linearly in the frequency band of interest. Further reduction in transmission is observed with the tuning of channel height of the device by increasing the boundary scattering. Phonon mode selective transmission control have potential application in thermal insulation or thermo-electric application or photo-thermal amplification.

Structural refinement, optical and electrical properties of Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 ceramics

Samarium doped barium zirconate titanate ceramics with general formula Ba1-x Sm-2x/3](Zr0.05Ti0.95)O-3 x = 0, 0.01, 0.02, and 0.03] were prepared by high energy ball milling method. X-ray diffraction patterns and micro-Raman spectroscopy confirmed that these ceramics have a single phase with a tetragonal structure. Rietveld refinement data were employed to model BaO12], SmO12], ZrO6], and TiO6] clusters in the lattice. Scanning electron microscopy shows a reduction in average grain size with the increase of Sm3+ ions into lattice. Temperature-dependent dielectric studies indicate a ferroelectric phase transition and the transition temperature decreases with an increase in Sm3+ ion content. The nature of the transition was investigated by the Curie-Weiss law and it is observed that the diffusivity increases with Sm3+ ion content. The ferroelectric hysteresis loop illustrates that the remnant polarization and coercive field increase with an increase in Sm3+ ions content. Optical properties of the ceramics were studied using ultraviolet-visible diffuse reflectance spectroscopy.

Correlation between Optical Properties and Nanomorphology of Fluoranthene-Based Conjugated Copolymer

Nanoparticles of conjugated polymers are receiving attention due to their interesting optical properties. Here we report nanoparticles of fluoranthene-based conjugated copolymer prepared by the Suzuki coupling reaction. The copolymer forms nanoparticles by the spontaneous self-assembly after evaporation of organic solvent. The mean diameter of the nanoparticles can be manipulated by varying solvent composition. We investigated the parameters that govern the nanostructured morphology of polymer by systematic variation of good and poor solvent. The UV vis and time-resolved fluorescence spectroscopy measurement reveal the use of poor solvent in the organization of nanostructures. Furthermore, transmission electron microscopy highlights the importance of rigidity of the polymer backbone in morphological development.

Nonlinear optical absorption in a graphene infrared photodetector

The photoresponse of the graphene photodetector elucidated strong dependence on several optical parameters, such as the angle of incidence and the incident power of infrared exposure at room temperature. The sinusoidal dependence of the photoresponse on incidence angle, which had not been realized before, has now been revealed. The combined effect of the photo excited charge carrier and the photon drag effect explain this nonlinear optical absorption in graphene at lower incident power. The nonlinear dependence of the charge carrier generation on the incident power revealed that this process contributed to the nonlinear photoresponse. However, a deviation is observed at a higher incident power due to the induction of thermal effects in the graphene lattice. This work demonstrates the tunability of the graphene photodetector under a systematic variation that involves both parameters.

Intense Acoustic Burst Ultrasound Modulated Optical Tomography for Elasticity Mapping of Soft Biological Tissue Mimicking Phantom: A Laser Speckle Contrast Analysis Study

This report addresses the assessment of variation in elastic property of soft biological tissues non-invasively using laser speckle contrast measurement. The experimental as well as the numerical (Monte-Carlo simulation) studies are carried out. In this an intense acoustic burst of ultrasound (an acoustic pulse with high power within standard safety limits), instead of continuous wave, is employed to induce large modulation of the tissue materials in the ultrasound insonified region of interest (ROI) and it results to enhance the strength of the ultrasound modulated optical signal in ultrasound modulated optical tomography (UMOT) system. The intensity fluctuation of speckle patterns formed by interference of light scattered (while traversing through tissue medium) is characterized by the motion of scattering sites. The displacement of scattering particles is inversely related to the elastic property of the tissue. We study the feasibility of laser speckle contrast analysis (LSCA) technique to reconstruct a map of the elastic property of a soft tissue-mimicking phantom. We employ source synchronized parallel speckle detection scheme to (experimentally) measure the speckle contrast from the light traversing through ultrasound (US) insonified tissue-mimicking phantom. The measured relative image contrast (the ratio of the difference of the maximum and the minimum values to the maximum value) for intense acoustic burst is 86.44 % in comparison to 67.28 % for continuous wave excitation of ultrasound. We also present 1-D and 2-D image of speckle contrast which is the representative of elastic property distribution.

Frustration in Optical Lattices of Interacting Bosons

The realization of optical lattices of cold atoms has opened up the possibility of engineering interacting lattice systems of bosons and fermions, stimulating a frenzy of research over the last decade. More recently, experimental techniques have been developed to apply synthetic gauge fields to these optical lattices. As a result, it has become possible to study quantum Hall physics and the effects of frustration in lattices of cold atoms. In this article we describe the combined effect of frustration and interactions on the superfluidity of bosons. By focussing on a frustrated ladder of interacting bosons, we show that the effect of frustration is for ``chiral'' order to develop, which manifests itself as an alternating pattern of circulating supercurrents. Remarkably, this order persists even when superfluidity is lost and the system enters a Mott phase giving rise to a novel chiral Mott insulator. We describe the combined physics of frustration and interactions by studying a fully frustrated one dimensional model of interacting bosons. The model is studied using mean-field theory, a direct quantum simulation and a higher dimensional classical theory in order to offer a full description of the different quantum phases contained in it and transitions between the different phases. In addition, we provide physical descriptions of the chiral Mott insulator as a vortex-anitvortex super solid and indirect excitonic condensate in addition to obtaining a variational wavefunction for it. We also briefly describe the chiral Mott states arising in other microscopic models.

Fienup Algorithm With Sparsity Constraints: Application to Frequency-Domain Optical-Coherence Tomography

We address the problem of reconstructing a sparse signal from its DFT magnitude. We refer to this problem as the sparse phase retrieval (SPR) problem, which finds applications in tomography, digital holography, electron microscopy, etc. We develop a Fienup-type iterative algorithm, referred to as the Max-K algorithm, to enforce sparsity and successively refine the estimate of phase. We show that the Max-K algorithm possesses Cauchy convergence properties under certain conditions, that is, the MSE of reconstruction does not increase with iterations. We also formulate the problem of SPR as a feasibility problem, where the goal is to find a signal that is sparse in a known basis and whose Fourier transform magnitude is consistent with the measurement. Subsequently, we interpret the Max-K algorithm as alternating projections onto the object-domain and measurement-domain constraint sets and generalize it to a parameterized relaxation, known as the relaxed averaged alternating reflections (RAAR) algorithm. On the application front, we work with measurements acquired using a frequency-domain optical-coherence tomography (FDOCT) experimental setup. Experimental results on measured data show that the proposed algorithms exhibit good reconstruction performance compared with the direct inversion technique, homomorphic technique, and the classical Fienup algorithm without sparsity constraint; specifically, the autocorrelation artifacts and background noise are suppressed to a significant extent. We also demonstrate that the RAAR algorithm offers a broader framework for FDOCT reconstruction, of which the direct inversion technique and the proposed Max-K algorithm become special instances corresponding to specific values of the relaxation parameter.

Rapid, microwave-assisted synthesis of Gd2O3 and Eu:Gd2O3 nanocrystals: characterization, magnetic, optical and biological studies

Ultra-small crystals of undoped and Eu-doped gadolinium oxide (Gd2O3) were synthesised by a simple, rapid microwave-assisted route, using benzyl alcohol as the reaction solvent. XRD, XPS and TEM analysis reveal that the as-prepared powder material consists of nearly monodisperse Gd2O3 nanocrystals with an average diameter of 5.2 nm. The nanocrystals show good magnetic behaviour and exhibit a larger reduction in relaxation time of water protons than the standard Gd-DTPA complex currently used in MRI imaging. Cytotoxicity studies (both concentration- and time-dependent) of the Gd2O3 nanocrystals show no adverse effect on cell viability, evidencing their high biological compatibility. Finally, Eu:Gd2O3 nanocrystals were prepared by a similar route and the red luminescence of Eu3+ activator ions was used to study the cell permeability of the nanocrystals. Red fluorescence from Eu3+ ions observed by fluorescence microscopy shows that the nanocrystals (Gd2O3 and Eu:Gd2O3) can permeate not only the cell membrane but can also enter the cell nucleus, rendering them candidate materials not only for MRI imaging but also for drug delivery when tagged or functionalized with specific drug molecules.

Tailoring the Microstructural, Optical, and Electrical Properties of Nanocrystalline WO3 Thin Films Using Al Doping

We have studied the influence of Al doping on the microstructural, optical, and electrical properties of spray-deposited WO3 thin films. XRD analyses confirm that all the films are of polycrystalline WO3 in nature, possessing monoclinic structure. EDX profiles of the Al-doped films show aluminum peaks implying incorporation of Al ions into WO3 lattice. On Al doping, the average crystallite size decreases due to increase in the density of nucleation centers at the time of film growth. The observed variation in the lattice parameter values on Al doping is attributed to the incorporation of Al ions into WO3 lattice. Enhancement in the direct optical band gap compared to the undoped film has been observed on Al doping due to decrease in the width of allowed energy states near the conduction band edge. The refractive indices of the films follow the Cauchy relation of normal dispersion. Electrical resistivity compared to the undoped film has been found to increase on Al doping.

Optical properties of an inhomogeneously broadened Lambda V-system with multiple excited states

We present a theoretical model using a density matrix approach to show the influence of multiple excited states on the optical properties of an inhomogeneously broadened Lambda V-system of the Rb-87 D2 line. These closely spaced multiple excited states cause asymmetry in absorption and dispersion profiles. We observe the reduced absorption profiles, due to dressed state interactions of the applied electromagnetic fields, which results the Mollow sideband-like transparency windows. In a room temperature vapor, we obtain a narrow enhanced absorption and steep positive dispersion at the line center when the strengths of control and pump fields are equal. Here, we show how the probe transmittance varies when it passes through the atomic medium. We also discuss the transient behavior of our system which agrees well with the corresponding absorption and dispersion profiles. This study has potential applications in controllability of group velocity, and for optical and quantum information processing.