Showing papers on "Light scattering published in 2010"

Radio Tomographic Imaging with Wireless Networks

Abstract: Radio Tomographic Imaging (RTI) is an emerging technology for imaging the attenuation caused by physical objects in wireless networks. This paper presents a linear model for using received signal strength (RSS) measurements to obtain images of moving objects. Noise models are investigated based on real measurements of a deployed RTI system. Mean-squared error (MSE) bounds on image accuracy are derived, which are used to calculate the accuracy of an RTI system for a given node geometry. The ill-posedness of RTI is discussed, and Tikhonov regularization is used to derive an image estimator. Experimental results of an RTI experiment with 28 nodes deployed around a 441 square foot area are presented.

Optical response features of Si-nanoparticle arrays

Abstract: Periodic structures of spherical silicon particles are analyzed using the coupled-dipole equations for studying optical response features and local electromagnetic fields. The model takes into account the electric and magnetic dipole moments of the particles embedded in a homogeneous dielectric medium. Particles with radius of 65 nm and larger are considered. It is shown that, due to the large permittivity of silicon, the first two Mie resonances are located in the region of visible light, where the absorption is small and the extinction is basically determined by scattering. The main contribution is given by the induced magnetic and electric dipoles of the particles. Thus, in contrast to metal particle arrays, here is a possibility to combine separately either the electric or magnetic dipole resonances of individual particles with the structural features. As a result, extinction spectra can have additional narrow resonant peaks connected with multiple light scattering by the magnetic dipoles and displaying a Fano-type resonant profile. Reflection and transmission properties of the Si particle arrays are investigated and the conditions of low light reflection and transmission by the particle arrays are discussed, as well as the applicability of the dipole approach. It is shown that the light transmission of finite-size arrays of Si particles can be significantly suppressed at the conditions of the particle magnetic dipole resonance. It is demonstrated that, using resonant conditions, one can separately control the enhancements of local electric and magnetic fields in the structures.

Exploiting disorder for perfect focusing

Abstract: Optical microscopy and manipulation methods rely on the ability to focus light to a small volume. However, in inhomogeneous media such as biological tissue, light is scattered out of the focusing beam. Disordered scattering is thought to fundamentally limit the resolution and penetration depth of optical methods1,2,3. Here we demonstrate, in an optical experiment, that scattering can be used to improve, rather than deteriorate, the sharpness of the focus. The resulting focus is even sharper than that in a transparent medium. By using scattering in the medium behind a lens, light was focused to a spot ten times smaller than the diffraction limit of that lens. Our method is the optical equivalent of highly successful methods for improving the resolution and communication bandwidth of ultrasound, radio waves and microwaves4,5,6. Our results, obtained using spatial wavefront shaping, apply to all coherent methods for focusing through scattering matter, including phase conjugation7 and time-reversal4. Light is scattered out of a focusing beam when an inhomogeneous medium is placed between the lens and the focal plane. Now, scientists experimentally demonstrate that scattering can be exploited to improve, rather than deteriorate, the focusing resolution of a lens by using wavefront shaping to compensate for scattering.

Dual‐Function Scattering Layer of Submicrometer‐Sized Mesoporous TiO2 Beads for High‐Efficiency Dye‐Sensitized Solar Cells

Abstract: Submicrometer-sized (830 ± 40 nm) mesoporous TiO 2 beads are used to form a scattering layer on top of a transparent, 6-μm-thick, nanocrystalline TiO 2 film. According to the Mie theory, the large beads scatter light in the region of 600-800 nm. In addition, the mesoporous structure offers a high surface area, 89.1 m 2 g -1 , which allows high dye loading. The dual functions of light scattering and electrode participation make the mesoporous TiO 2 beads superior candidates for the scattering layer in dye-sensitized solar cells. A high efficiency of 8.84% was achieved with the mesoporous beads as a scattering layer, compared with an efficiency of 7.87% for the electrode with the scattering layer of 400-nm TiO 2 of similar thickness.

Plasmonic nanobubbles as transient vapor nanobubbles generated around plasmonic nanoparticles.

Abstract: We have used short laser pulses to generate transient vapor nanobubbles around plasmonic nanoparticles. The photothermal, mechanical, and optical properties of such bubbles were found to be different from those of plasmonic nanoparticle and vapor bubbles, as well. This phenomenon was considered as a new complex nanosystem—plasmonic nanobubble (PNB). Mechanical and optical scattering properties of PNB depended upon the nanoparticle surface and heat capacity, clusterization state, and the optical pulse length. The generation of the PNB required much higher laser pulse fluence thresholds than the explosive boiling level and was characterized by the relatively high lower threshold of the minimal size (lifetime) of PNB. Optical scattering by PNB and its diameter (measured as the lifetime) has been varied with the fluence of laser pulse, and this has demonstrated the tunable nature of PNB.

Effect of high- κ gate dielectrics on charge transport in graphene-based field effect transistors

Abstract: The effect of various dielectrics on charge mobility in single-layer graphene is investigated. By calculating the remote optical phonon scattering arising from the polar substrates, and combining it with their effect on Coulombic impurity scattering, a comprehensive picture of the effect of dielectrics on charge transport in graphene emerges. It is found that though high-$\ensuremath{\kappa}$ dielectrics can strongly reduce Coulombic scattering by dielectric screening, scattering from surface phonon modes arising from them wash out this advantage. Calculation shows that within the available choice of dielectrics, there is not much room for improving carrier mobility in actual devices at room temperatures.

On-chip stimulated Brillouin scattering

Abstract: We report the first demonstration of on-chip stimulated Brillouin scattering (SBS) with low average power. The measured Brillouin shift and line width are ∼7.7 GHz and ∼6 MHz in a 7 cm long chalcogenide waveguide.

Protein−Nanoparticle Interaction: Identification of the Ubiquitin−Gold Nanoparticle Interaction Site

Abstract: We demonstrate that it is possible to identify the protein−nanoparticle interaction site at amino acid scale in solution. Using NMR, chemical shift perturbation analysis, and dynamic light scattering we have identified a specific domain of human ubiquitin that interacts with gold nanoparticles. This method allows a detailed structural analysis of proteins absorbed onto surfaces of nanoparticles in physiological conditions and it will provide much needed experimental data for better modeling and prediction of protein−nanoparticle interactions.

How Noniridescent Colors Are Generated by Quasi‐ordered Structures of Bird Feathers

Abstract: We investigate the mechanism of structural coloration by quasi-ordered nanostructures in bird feather barbs. Small-angle X-ray scattering (SAXS) data reveal the structures are isotropic and have short-range order on length scales comparable to optical wavelengths. We perform angle-resolved reflection and scattering spectrometry to fully characterize the colors under directional and omni-directional illumination of white light. Under directional lighting, the colors change with the angle between the directions of illumination and observation. The angular dispersion of the primary peaks in the scattering/reflection spectra can be well explained by constructive interference of light that is scattered only once in the quasi-ordered structures. Using the Fourier power spectra of structure from the SAXS data we calculate optical scattering spectra and explain why the light scattering peak is the highest in the backscattering direction. Under omni-directional lighting, colors from the quasi-ordered structures are invariant with the viewing angle. The non-iridescent coloration results from the isotropic nature of structures instead of strong backscattering.

Strategies for three-dimensional particle tracking with holographic video microscopy

Abstract: The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track individual colloidal particles’ three-dimensional motions with nanometer resolution. In this work, we compare the performance of two complementary analysis techniques, one based on fitting to the exact Lorenz-Mie theory and the other based on phenomenological interpretation of the scattered light field reconstructed with Rayleigh-Sommerfeld back-propagation. Although Lorenz-Mie tracking provides more information and is inherently more precise, Rayleigh-Sommerfeld reconstruction is faster and more general. The two techniques agree quantitatively on colloidal spheres’ in-plane positions. Their systematic differences in axial tracking can be explained in terms of the illuminated objects’ light scattering properties.

Estimation of lipid and water concentrations in scattering media with diffuse optical spectroscopy from 900 to 1600 nm

Abstract: We demonstrate a method to estimate the concentrations of water and lipid in scattering media such as biological tissues with diffuse optical spectra acquired over the range of 900 to 1600 nm. Estimations were performed by fitting the spectra to a model of light propagation in scattering media derived from diffusion theory. To validate the method, spectra were acquired from tissue phantoms consisting of lipid and water emulsions and swine tissues ex vivo with a two-fiber probe.

Brillouin light scattering studies of planar metallic magnonic crystals

Abstract: The application of Brillouin light scattering to the study of the spin-wave spectrum of one- and two-dimensional planar magnonic crystals consisting of arrays of interacting stripes, dots and antidots is reviewed. It is shown that the discrete set of allowed frequencies of an isolated nanoelement becomes a finite-width frequency band for an array of identical interacting elements. It is possible to tune the permitted and forbidden frequency bands, modifying the geometrical or the material magnetic parameters, as well as the external magnetic field. From a technological point of view, the accurate fabrication of planar magnonic crystals and a proper understanding of their magnetic excitation spectrum in the gigahertz range is oriented to the design of filters and waveguides for microwave communication systems.

Direct evidence of dust growth in L183 from mid-infrared light scattering

Abstract: Context. Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission). Aims. Direct evidence of larger particles can be derived from scattered mid-infrared (MIR) radiation from a molecular cloud observed in a spectral range where little or no emission from polycyclic aromatic hydrocarbons (PAHs) is expected. Methods. We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree with the AV map and compare them to the Spitzer data. Results. The Spitzer data of L183 show emission in the 3.6 and 4.5 μm bands, while the 5.8 μm band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 μm, which is not seen. Hence, we interpret this emission to be MIR scattered light from grains located further inside the core, and call it ”coreshine”. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 μm. Conclusions. There is direct evidence of dust grain growth in the inner part of L183 from the scattered light MIR images seen by Spitzer.

Modeling of light scattering from micro- and nanotextured surfaces

Abstract: We present a calculation routine for the angular and spectral dependence of scattered light after transmission through textured interfaces. Based on a modified Rayleigh–Sommerfeld integral, the treatment requires only measured surface profiles, and the refractive indices of the two materials adjacent to the textured interface but no fitting parameter. For typical surface morphologies used in solar cell fabrication, the calculations correctly reproduce the angle resolved scattering at 543 nm and the total scattered light intensity in the spectral range from 400 to 2000 nm. The model is then applied to predict the behavior of the interface between ZnO and silicon in a thin film solar cell which is not experimentally accessible.

Accurate and versatile modeling of electromagnetic scattering on periodic nanostructures with a surface integral approach.

Abstract: A surface integral formulation for light scattering on periodic structures is presented. Electric and magnetic field equations are derived on the scatterers’ surfaces in the unit cell with periodic boundary conditions. The solution is calculated with the method of moments and relies on the evaluation of the periodic Green’s function performed with Ewald’s method. The accuracy of this approach is assessed in detail. With this versatile boundary element formulation, a very large variety of geometries can be simulated, including doubly periodic structures on substrates and in multilayered media. The surface discretization shows a high flexibility, allowing the investigation of irregular shapes including fabrication accuracy. Deep insights into the extreme near-field of the scatterers as well as in the corresponding far-field are revealed. This method will find numerous applications for the design of realistic photonic nanostructures, in which light propagation is tailored to produce novel optical effects.

Modulated surface textures for enhanced light trapping in thin-film silicon solar cells

Abstract: Substrates with a modulated surface texture were prepared by combining different interface morphologies. The spatial frequency surface representation method is used to evaluate the surface modulation. When combining morphologies with appropriate geometrical features, substrates exhibit an increased scattering level in a broad wavelength region. We demonstrate that the improved scattering properties result from a superposition of different light scattering mechanisms caused by the different geometrical features integrated in a modulated surface texture.

Anisotropic Propagation and Damping of Spin Waves in a Nanopatterned Antidot Lattice

Abstract: All-electrical spin-wave spectroscopy, Brillouin light scattering, as well as the magneto-optical Kerr effect are combined to study spin-wave propagation through a magnetic antidot lattice nanopatterned into a Ni80Fe20 thin film. The propagation velocities and, in particular, the relaxation are found to depend characteristically on the applied in-plane magnetic field. We explain the observed anisotropies by magnetic field-controlled spin-wave guiding in a network of interconnected nanowires which takes place over distances of up to 20 μm.

Temporal Coupled-Mode Theory for Fano Resonance in Light Scattering by a Single Obstacle †

Abstract: We present a theory for Fano interference in light scattering by individual obstacle, based on a temporal coupled-mode formalism. This theory is applicable for obstacles that are much smaller than the incident wavelength, or for systems with two-dimensional cylindrical or three-dimensional spherical symmetry. We show that for each angle momentum channel, the Fano interference effect can be modeled by a simple temporal coupled-mode equation, which provides a line shape formula for scattering and absorption cross-section. We validate the analysis with numerical simulations. As an application of the theory, we design a structure that exhibits strong absorption and weak scattering properties at the same frequency.

Effects of relative humidity on aerosol light scattering in the Arctic

Abstract: . Aerosol particles experience hygroscopic growth in the ambient atmosphere. Their optical properties – especially the aerosol light scattering – are therefore strongly dependent on the ambient relative humidity (RH). In-situ light scattering measurements of long-term observations are usually performed under dry conditions (RH>30–40%). The knowledge of this RH effect is of eminent importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. This study combines measurements and model calculations to describe the RH effect on aerosol light scattering for the first time for aerosol particles present in summer and fall in the high Arctic. For this purpose, a field campaign was carried out from July to October 2008 at the Zeppelin station in Ny-Alesund, Svalbard. The aerosol light scattering coefficient σsp(λ) was measured at three distinct wavelengths (λ=450, 550, and 700 nm) at dry and at various, predefined RH conditions between 20% and 95% with a recently developed humidified nephelometer (WetNeph) and with a second nephelometer measuring at dry conditions with an average RH

How does zero forward-scattering in magnetodielectric nanoparticles comply with the optical theorem?

Abstract: A few decades ago, Kerker et al. [J. Opt. Soc. Am. 73, 765-767 (1983)] theoretically pointed out the interesting possibility of conceiving small magnetodielectric spheres that may provide zero scattering in the forward direction, despite significantly larger scattering in any other direction. Recent experimental and theoretical papers on the topic have further discussed this possibility in more realistic scenarios. Inspecting some of their analyses, it seems indeed possible to conceive nanoparticles characterized by a scattering pattern with a sharp minimum, although not zero, in the forward direction. From a theoretical standpoint, however, it is well known that the total scattered power from any object has to be proportional to a portion of the scattered field in the forward direction, implying that very small or zero forward scattering should be synonymous to even smaller or zero total scattering, regardless of the nature of the object and of its design. Using analytical theory and an accurate scattering formulation, we clarify the nature of this apparent paradox and the limitations of this anomalous phenomenon in terms of particle size. In this way, we shed some new light on theoretical and experimental papers on the topic, identifying relevant missteps in some of their physical interpretation, and considering the general possibility of verifying these effects. This discussion may also be relevant to some cloaking applications using exotic artificial materials.

Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells

Abstract: We demonstrate enhanced external quantum efficiency and current-voltage characteristics due to scattering by 100 nm silver nanoparticles in a single 2.5 nm thick InGaN quantum well photovoltaic device. Nanoparticle arrays were fabricated on the surface of the device using an anodic alumina template masking process. The Ag nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency by up to 54%. Additionally, the short-circuit current in cells with 200 nm p-GaN emitter regions is increased by 6% under AM 1.5 illumination. AFORS-Het simulation software results were used to predict cell performance and optimize emitter layer thickness.

Static and dynamic light scattering of healthy and malaria-parasite invaded red blood cells.

Abstract: We present the light scattering of individual Plasmodium falciparum-parasitized human red blood cells Pf-RBCs, and demonstrate progressive alterations to the scattering signal arising from the development of malaria-inducing parasites. By selectively imaging the electric fields using quantitative phase microscopy and a Fourier transform light scattering technique, we calculate the light scattering maps of individual Pf-RBCs. We show that the onset and progression of pathological states of the Pf-RBCs can be clearly identified by the static scattering maps. Progressive changes to the biophysical properties of the Pf-RBC membrane are captured from dynamic light scattering. © 2010 Society of Photo-Optical Instrumentation Engineers.

Decoherence due to Elastic Rayleigh Scattering

Abstract: We present theoretical and experimental studies of the decoherence of hyperfine ground-state superpositions due to elastic Rayleigh scattering of light off resonant with higher lying excited states. We demonstrate that under appropriate conditions, elastic Rayleigh scattering can be the dominant source of decoherence, contrary to previous discussions in the literature. We show that the elastic-scattering decoherence rate of a two-level system is given by the square of the difference between the elasticscattering amplitudes for the two levels, and that for certain detunings of the light, the amplitudes can interfere constructively even when the elastic-scattering rates from the two levels are equal. We confirm this prediction through calculations and measurements of the total decoherence rate for a superposition of the valence electron spin levels in the ground state of 9 Be þ in a 4.5 T magnetic field.

Using laser diffraction data to obtain accurate particle size distributions: the role of particle composition

Abstract: Suspended particle composition has a large influence on light scattering by small ( 250 µm) produced little effect; however, particles smaller than the lower limit (<1.25 µm) influenced concentrations over the entire LISST size range and produced counterintuitive effects, such as increased concentrations in the largest size bins.

Levitated spinning graphene flakes in an electric quadrupole ion trap

Abstract: A method is described for levitating micron-sized few-layer graphene flakes in an electric quadrupole ion trap. Starting from a liquid suspension containing graphene, charged flakes are injected into the trap using the electrospray ionization technique and are probed optically. At micro-torr pressures, torques from circularly polarized light cause the levitated particles to rotate at frequencies $g1\text{ }\text{MHz}$, which can be inferred from modulation of light scattering off the rotating flake when an electric field resonant with the rotation rate is applied. Possible applications of these techniques will be presented, both to fundamental measurements of the mechanical and electronic properties of graphene and to new approaches to graphene crystal growth, modification, and manipulation.

Effects of spatial coherence on the angular distribution of radiant intensity generated by scattering on a sphere.

Abstract: In the analysis of light scattering on a sphere it is implicitly assumed that the incident field is spatially fully coherent. However, under usual circumstances the field is partially coherent. We generalize the partial waves expansion method to this situation and examine the influence of the degree of coherence of the incident field on the radiant intensity of the scattered field in the far zone. We show that when the coherence length of the incident field is comparable to, or is smaller than, the radius of the sphere, the angular distribution of the radiant intensity depends strongly on the degree of coherence. The results have implications, for example, for scattering in the atmosphere and colloidal suspensions.

Hydrodynamic radius of polyethylene glycol in solution obtained by dynamic light scattering

Abstract: In order to compare the size characterizations in poly(ethylene glycol) (PEG) obtained by dynamic light scattering (DLS) and small angle neutron scattering (SANS), DLS experiments were performed in various PEG solutions to ascertain the hydrodynamic radius. Data from the experiments were analyzed by using a method to eliminate effects of PEG aggregation on dynamic correlation functions. The results of the analysis were then compared to the radii of gyration reported from SANS experiments. The relation between the hydrodynamic radius, obtained by DLS, and the radius of gyration, obtained by SANS, in PEG in solution was found to be in agreement with a previously obtained relation for PEG, where the radius of gyration was found by static light scattering.

Structure and gelation mechanism of silk hydrogels

Abstract: Silk fibroin was regenerated from cocoons produced by the silkworm Bombyx Mori. Light scattering showed that an aqueous solution of the regenerated silk fibroin (RSF) was made of individual proteins with a weight average molar mass of about 4 × 105 g mol−1 and a hydrodynamic radius of about 10 nm. Gel formation of RSF in acidic solutions was investigated as a function of the pH (2–4), concentration (0.5–10 g L−1) and temperature (5–70 °C). The structure of the gels was studied using light scattering and confocal laser scanning microscopy. The structure was found to be self-similar from length scales of less than 15 nm up to length scales of about 1 μm, and characterized by a correlation length of a few microns. Gel formation was tracked using turbidity, rheology, light scattering and circular dichroism. Gelation involves the formation of self-similar aggregates with a growth rate that increases exponentially. The protein aggregation is correlated to, and perhaps caused by, the formation of β-sheets, the fraction of which also increases exponentially with time.