Showing papers on "Light scattering published in 2006"

Gold and Silver Nanoparticles in Sensing and Imaging: Sensitivity of Plasmon Response to Size, Shape, and Metal Composition

Abstract: Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.

Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust

Abstract: [ 1] The possibility of using shape mixtures of randomly oriented spheroids for modeling desert dust aerosol light scattering is discussed. For reducing calculation time, look-up tables were simulated for quadrature coefficients employed in the numerical integration of spheroid optical properties over size and shape. The calculations were done for 25 bins of the spheroid axis ratio ranging from similar to 0.3 ( flattened spheroids) to similar to 3.0 ( elongated spheroids) and for 41 narrow size bins covering the size parameter range from similar to 0.012 to similar to 625. The look-up tables were arranged into a software package, which allows fast, accurate, and flexible modeling of scattering by randomly oriented spheroids with different size and shape distributions. In order to evaluate spheroid model and explore the possibility of aerosol shape identification, the software tool has been integrated into inversion algorithms for retrieving detailed aerosol properties from laboratory or remote sensing polarimetric measurements of light scattering. The application of this retrieval technique to laboratory measurements by Volten et al. ( 2001) has shown that spheroids can closely reproduce mineral dust light scattering matrices. The spheroid model was utilized for retrievals of aerosol properties from atmospheric radiation measured by AERONET ground-based Sun/sky-radiometers. It is shown that mixtures of spheroids allow rather accurate fitting of measured spectral and angular dependencies of observed intensity and polarization. Moreover, it is shown that for aerosol mixtures with a significant fraction of coarse-mode particles ( radii >= similar to 1 mu m), the nonsphericity of aerosol particles can be detected as part of AERONET retrievals. The retrieval results indicate that nonspherical particles with aspect ratios similar to 1.5 and higher dominate in desert dust plumes, while in the case of background maritime aerosol spherical particles are dominant. Finally, the potential of using AERONET derived spheroid mixtures for modeling the effects of aerosol particle nonsphericity in other remote sensing techniques is discussed. For example, the variability of lidar measurements ( extinction to backscattering ratio and signal depolarization ratio) is illustrated and analyzed. Also, some potentially important differences in the sensitivity of angular light scattering to parameters of nonspherical versus spherical aerosols are revealed and discussed.

Optical properties of deep glacial ice at the South Pole

Abstract: We have remotely mapped optical scattering and absorption in glacial ice at the South Pole for wavelengths between 313 and 560 nm and depths between 1100 and 2350 m. We used pulsed and continuous light sources embedded with the AMANDA neutrino telescope, an array of more than six hundred photomultiplier tubes buried deep in the ice. At depths greater than 1300 m, both the scattering coefficient and absorptivity follow vertical variations in concentration of dust impurities, which are seen in ice cores from other Antarctic sites and which track climatological changes. The scattering coefficient varies by a factor of seven, and absorptivity (for wavelengths less than ∼450 nm) varies by a factor of three in the depth range between 1300 and 2300 m, where four dust peaks due to stadials in the late Pleistocene have been identified. In our absorption data, we also identify a broad peak due to the Last Glacial Maximum around 1300 m. In the scattering data, this peak is partially masked by scattering on residual air bubbles, whose contribution dominates the scattering coefficient in shallower ice but vanishes at ∼1350 m where all bubbles have converted to nonscattering air hydrates. The wavelength dependence of scattering by dust is described by a power law with exponent -0.90 ± 0.03, independent of depth. The wavelength dependence of absorptivity in the studied wavelength range is described by the sum of two components: a power law due to absorption by dust, with exponent -1.08 ± 0.01 and a normalization proportional to dust concentration that varies with depth; and a rising exponential due to intrinsic ice absorption which dominates at wavelengths greater than ∼500 nm. Copyright 2006 by the American Geophysical Union.

Multiple Scattering of Light by Particles: Radiative Transfer and Coherent Backscattering

Abstract: 1. Introduction 2. Maxwell's equations, electromagnetic waves, and Stokes parameters 3. Basic theory of electromagnetic scattering 4. Scattering by a fixed multi-particle group 5. Statistical averaging 6. Scattering by a single random particle 7. Single scattering by a small random particle group 8. Radiative transfer equation 9. Calculations and measurements of single-particle characteristics 10. Radiative transfer in plane-parallel scattering media 11. Macroscopically isotropic and mirror-symmetric scattering media 12. Radiative transfer in plane-parallel, microscopically isotropic and mirror-symmetric scattering media 13. Illustrative applications of radiative transfer theory 14. Coherent backscattering.

Self-Assembly of β-Cyclodextrin in Water. Part 1: Cryo-TEM and Dynamic and Static Light Scattering

Abstract: In this article, we report evidence of β-cyclodextrin (β-CD) self-aggregation in water. A critical aggregation concentration (cac) between 2 and 3 mM was determined by using dynamic (DLS) and static (SLS) light scattering to investigate the presence of β-cyclodextrin aggregates. Transmission electron microscopy at cryogenic temperature (Cryo-TEM) was used to detect the structural features of cyclodextrin self-aggregates. The results show the occurrence of polymorphism depending on the β-CD concentration: polydisperse nearly spherical objects with diameters of about 100 nm are present at lower concentrations, whereas micrometer planar aggregates are predominant at higher concentrations.

Anomalous light scattering by small particles

Abstract: Light scattering by a small spherical particle with a low dissipation rate is discussed based upon the Mie theory. It is shown that if close to the plasmon (polariton) resonance frequencies the radiative damping prevails over dissipative losses, sharp giant resonances with very unusual properties may be observed. In particular, the resonance extinction cross section increases with an increase in the order of the resonance (dipole, quadrupole, etc.); the characteristic values of electric and magnetic near fields for the scattered light are singular in the particle size, while energy circulation in the near field is rather complicated, so that the Poynting vector field includes singular points whose number, types, and positions are very sensitive to fine changes in the incident light frequency. The results may provide new opportunities for a giant, controlled, highly frequency-sensitive enhancement and variation of electromagnetic field at nanoscales.

Resonances of individual metal nanowires in the infrared

Abstract: With infrared spectroscopic microscopy using synchrotron light, the authors studied resonant light scattering from single metal nanowires with diameters in the 100nm range and with lengths of a few microns. The Au and Cu nanowires were electrochemically grown in polycarbonate etched ion-track membranes and transferred on infrared-transparent substrates. Significant antennalike plasmon resonances were observed in good agreement with exact light-scattering calculations. The resonances depend not only on length and diameter but also on the dielectric surrounding of the nanowire. The observed maximum extinction at resonance corresponds to an electromagnetic far-field enhancement by a factor of about 5.

Illumination device for simulating neon or similar lighting in various colors

Abstract: An illumination device has a light source, a waveguide, and a light-transmitting medium The light source emits light of a first color The waveguide has both optical waveguide and light scattering properties The light-transmitting medium is composed of a matrix of substantially translucent material doped with a pigment, and is positioned between the light source and the waveguide such that a portion of the light emitted by the light source passes around the light-transmitting medium and reaches the waveguide directly, and a portion of the emitted light is received by the light-transmitting medium The pigment changes a portion of the received light to a light of a second color The waveguide receives and mixes the light of the first color and the light of the second color, and emits light of a combined color

The optical properties of mineral suspended particles: A review and synthesis

Abstract: Small mineral particles suspended in the sea are excellent at reflecting light and show up well in visible band satellite images. In order to make quantitative estimates of the particle concentration, and its effect on the penetration of sunlight into the sea, it is necessary to know how the absorption, scattering and backscattering coefficients of these inorganic particles change with concentration, the nature of the particles, and with wavelength. In this paper, observations from the literature are supplemented with a data set from the Irish Sea. The concentration-specific absorption coefficient of mineral particles am∗ is generally found to decrease exponentially with wavelength towards (in our data) a constant non-zero value in the red. Specific scattering coefficients show a tendency to decrease from the open ocean into energetic shelf seas and estuaries, but then to increase again within shelf seas as turbulent energy increases. The variation of specific scattering with turbulent energy in the Irish Sea is consistent with particle size scaling with the Kolmogorov microscale. Colour ratios (the ratio of two reflection coefficients) are less sensitive to variations in scattering, and we suggest that a combination of satellite measurements of brightness and colour in water with high mineral suspended sediment content will produce (1) a better estimate of concentration and (2) information on the variation of specific scattering.

Light scattering properties of natural and artificially demineralized dental enamel at 1310 nm

Abstract: A fundamental understanding of how near-IR light propagates through sound and carious dental hard tissues is essential for the development of clinically useful optical diagnostic systems, since image contrast is based on changes in the optical properties of these tissues on demineralization During the caries (decay) process, micropores are formed in the lesion due to partial dissolution of the individual mineral crystals Such small pores behave as scattering centers, strongly scattering visible and near-IR light The optical properties of enamel can be quantitatively described by the absorption and scattering coefficients, and the scattering phase function Our aim is to measure the optical scattering behavior of natural and artificial enamel caries Near-IR attenuation measurements and angular-resolved goniometer measurements coupled with Monte Carlo simulations are used to determine changes in the scattering coefficient and the scattering anisotropy on demineralization at 1310 nm An ultra-high resolution digital microradiography system is used to quantify the lesion severity by measurement of the relative mineral loss for comparison with optical scattering measurements The scattering coefficient increases exponentially with increasing mineral loss Natural and artificial demineralization increases the scattering coefficient more than two orders of magnitude at 1310 nm, and the scattering is highly forward directed

Theory of optical scattering by achiral carbon nanotubes and their potential as optical nanoantennas

Abstract: The Leontovich-Levin equation for optical scattering by an achiral carbon nanotube (CNT) of finite length is formulated, based on a quantum-mechanical microscopic model of the conductivity. Both approximate analytical and numerical solutions of the Leontovich-Levin equation yield a comparable surface current density distribution and scattering pattern. Applications over a wide frequency range from the terahertz to the ultraviolet are possible. The CNT polarizability in the low-frequency range and the scattering pattern in the range of optical interband transitions as well as in the vicinity of plasmon resonance are calculated. Geometric resonances of strongly retarded surface waves emerge and can be used for the qualitative interpretation of experimentally observed features in the optical response characteristics of CNT-based composite mediums. The potential of isolated CNTs as optical nanoantennas of both the receiving and transmitting types is established.

Random lasing in weakly scattering systems

Abstract: We present detailed experimental and numerical studies of random lasing in weakly scattering systems. The interference of scattered light, which is weak in the passive systems, is greatly enhanced in the presence of high gain, providing coherent and resonant feedback for lasing. The lasing modes are confined in the vicinity of the pumped volume due to absorption of emitted light outside it. In the ballistic regime where the size of the gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of the pumped region. It results in nearly constant frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume.

Large-scale supramolecular structure in solutions of low molar mass compounds and mixtures of liquids: I. Light scattering characterization.

Abstract: Static and dynamic laser light scattering were used to bring evidence of large-scale supramolecular structure in solutions of low molar mass electrolytes, nonelectrolytes, and mixtures of liquids. It was shown that solutes are distributed inhomogeneously on large length scales. Regions of higher and lower solute concentration exist in solution and give sufficient scattering contrast for experimental observation. A detailed light scattering study showed that these regions can be characterized as close-to-spherical discrete domains of higher solute density in a less dense rest of solution. These domains do contain solvent inside and can be therefore characterized as loose associates (giant clusters, aggregates). Their size distributions are significantly broad, ranging up to several hundreds of nanometers. Characteristic sizes of these inhomogenities thus exceed angstrom dimensions of individual molecules by several orders of magnitude. The number of solute molecules per domain varies approximately in the range 10(3)-10(8). Phenomena described were observed in a very broad range of solutes and solvents. Among others, selected data on most common substances of great chemical and biological importance such as sodium chloride, citric acid, glucose, urea, acetic acid, and ethanol are presented.

Direct measurement of the evanescent field profile produced by objective-based total internal reflection fluorescence.

Abstract: Total internal reflection fluorescence (TIRF) microscopy produces a thin excitation field (the evanescent field) that nominally decays exponentially. This field is ideal for selective excitation of fluorophores near the coverslip/sample interface. We present an experimental method, where the depth and axial profile of the evanescent field can be measured directly by microscopic observation of low refractive index fluorescently labeled spherical beads in an index-matched solution. To demonstrate the technique, through-the-objective TIRF is set up with laser excitation. In this configuration, the axial profile of the evanescent field created by either a 1.45-numerical aperture (NA) or a 1.65-NA objective fits well to a double exponential. At the coverslip/sample interface, about 90% of the evanescent field is represented by an exponential with a decay rate consistent with that expected for a theoretical evanescent field; the remaining 10% of the field is represented by an exponential with a much longer decay constant and is identified as scattering. The approach presented here is particularly useful for investigating the quality and axial profile of the evanescent field in both laser-based and mercury arc-based through-the-objective TIRF systems where a significant amount of light scattering can occur in the illumination optics.

Optical scattering resonances of single plasmonic nanoantennas

Abstract: We investigate the far-field optical resonances of individual dimer nanoantennas using confocal scattering spectroscopy. Experiments on a single-antenna array with varying arm lengths and interparticle gap sizes show large spectral shifts of the plasmon modes due to a combination of geometrical resonances and plasmon hybridization. All resonances are considerably broadened compared to those of small nanorods in the quasistatic limit, which we attribute to a greatly enhanced radiative damping of the antenna modes. The scattering spectra are compared with rigorous model calculations that demonstrate both the near-field and far-field characteristics of a half-wave antenna.

Planar lighting system

Abstract: PROBLEM TO BE SOLVED: To provide a planar lighting system having a low-profile shape and high light utilizing efficiency for emitting light with less brightness irregularity while actualizing middle-high or bell-shaped brightness distribution SOLUTION: The planar lighting system comprises: a light guide plate having two symmetrical inclined faces gradually distant from a light projection plane as tending from two light incident planes to the center of the light projection plane and a curved portion for joining the two inclined faces, and including scattering particles for scattering light conveyed inside, wherein a length between the two light incident planes, the thickness of each of the light incident planes, the center thickness of the curved portion, the curvature radius of the curved portion, and the taper of the inclined face each satisfy a predetermined range, and the particle sizes of the scattering particles, the concentration thereof, the light utilizing efficiency, and the middle-high degree of the brightness distribution of the light projection plane each satisfy a predetermined range; light sources; a casing; a fixing means for fixing the light source with distances between the light sources and the light incident planes of the light guide plate kept to be constant, and a sliding mechanism for sliding the fixing means relative to the casing COPYRIGHT: (C)2009,JPO&INPIT

Gold nanorods: Synthesis and optical properties

Abstract: The main results of studying the synthesis, growth mechanisms, and optical properties of gold nanorods published in the last 5–8 years are briefly reviewed. Hydrosols of gold nanorods with variable axial ratios are synthesized in the micellar solution of ionic surfactants by sead-mediated growth procedure using the stage of particle separation in the glycerol concentration gradient. Results of synthesis in systems containing one surfactant, albeit with different Ag/Au molar ratios and different amounts of gold seeding particles, agree with the published data. It is shown that, in the case of the mixture of two surfactants, the Ag/Au ratio is an efficient controlling parameter of the synthesis of nanorods with large axial ratios. The extinction and differential light scattering, spectra dynamic light scattering, and the depolarization of laser light scattering at 90° are used for the optical control of synthesis. Three fractions are observed in separated samples. One of these fractions is characterized by the only short wavelength plasmon resonance at 570 nm corresponding, in agreement with the published data, to cubic particles. Measurements of the extinction spectra of nanorods in water-glycerol mixtures reveal higher sensitivity of the longitudinal plasmon resonance to the dielectric environment relative to the transverse resonance. It is shown theoretically and experimentally that the relative shift of plasmon resonance is proportional to the relative increment of refractive index of the surrounding medium. To calculate optical properties of nanorods, we employed a model of cylinders with semispherical ends (s-cylinders) corresponding to the shape of real particles and admitting the exact solution by the T-matrix method with a computational burden that is an order of magnitude lower than that used in the discrete dipole method. The set of dependences of the longitudinal resonance wavelength on the axial ratio of different-thickness particles complies with our data and published measurements. Theoretical and experimental values of depolarization ratio I VH/I VV for nanorods and nanospheres with different sizes prepared with both citrate (15–46 nm) and original thiocyanate (90 nm) reduction of HAuCl4 are compared. It is shown that the depolarization parameter of light scattered by a nanorod suspension can exceed the theoretical limit (1/3) for common dielectric particles. The measured 10%-depolarization ratio for 90-nm spheres was far beyond the set of “size-depolarization” measurements for 15–46-nm-dia particles prepared by the citrate method and is indicative of the improved spherical morphology of 90-nm particles. This assumption was confirmed by TEM data, which also revealed both the presence of a noticeable amount of nanorods with a large axial ratio and “nanowires” of about the same thickness. A new analytic calibration for determining the diameter of spherical particles (5–100 nm) by the spectral position of the sol extinction maximum is proposed.

Tissue self-affinity and polarized light scattering in the born approximation: a new model for precancer detection.

Abstract: Light scattered from biological tissues can exhibit an inverse power law spectral component. We develop a model based on the Born approximation and von Karman (self-affine) spatial correlation of submicron tissue refractive index to account for this. The model is applied to light scattering spectra obtained from excised esophagi of normal and carcinogen-treated rats. Power law exponents used to fit dysplastic tissue site spectra are significantly smaller than those from normal sites, indicating that changes in tissue self-affinity can serve as a potential biomarker for precancer.

Ultrafast resonant optical scattering from single gold nanorods: Large nonlinearities and plasmon saturation

Abstract: We measure nonlinear optical scattering from individual Au nanorods excited by ultrafast laser pulses on resonance with their longitudinal plasmon mode Isolating single rods removes inhomogeneous broadening and allows the measurement of a large nonlinearity, much greater than that of nanorod ensembles Surprisingly, the ultrafast nonlinearity can be attributed entirely to heating of conduction electrons and does not exhibit any response associated with coherent plasmon oscillation This indicates an unanticipated damping of strongly driven plasmons

Theoretical studies of plasmon resonances in one-dimensional nanoparticle chains: Narrow lineshapes with tunable widths

Abstract: In this paper we describe a new configuration for producing narrow extinction lineshapes for light scattering from one-dimensional arrays of silver nanoparticles. In this configuration, which is specifically concerned with an array with a finite number of relatively large (radius greater than around 30 nm) nanoparticles, the wavevector of the light is chosen to be parallel to the array axis, while the polarization direction is perpendicular to the array axis. This leads to narrow plasmon/photonic lineshapes when the particle spacing is half the incident wavelength. This effect stands in contrast to the narrow lines previously found for wavevector and polarization vector perpendicular to the array axis, where the optimum spacing is close to the wavelength. The results are rationalized using a semi-analytical evaluation of the coupled dipole interaction, and it is demonstrated that the parallel and perpendicular chains have very different dependence on the number of particles in the chain. Results as a function of chain orientation relative to the wavevector are also considered, as is the possibility of sensing using an array configuration that combines the parallel and perpendicular chain directions.

Tunable and augmented plasmon resonances of Au∕SiO2∕Au nanodisks

Abstract: The plasmon resonance of Au∕SiO2 multilayered nanodisks was studied using light scattering spectroscopy and numerical calculations. Compared to single layered Au nanodisks, multilayered nanodisks exhibit several distinctive properties including significantly enhanced plasmon resonances and tunable resonance wavelengths which can be tailored to desired values by simply varying dielectric layer thickness while the particle diameter is kept constant. Numerical calculations show that slicing one metal layer into metal multilayers leads to higher scattering intensity and more “hot spots,” or regions of strong field enhancement. This tunable and augmented plasmon resonance holds a great potential in the applications of surface-enhanced Raman scattering (SERS).

Light scattering by complex ice-analogue crystals

Abstract: Angle-dependent light-scattering measurements on single ice analogues crystals are described. Phase functions and degree of linear polarization are measured for electrodynamically levitated crystals. A procedure for randomizing particle orientation during levitation is demonstrated. The dependence of scattering on the shape, complexity and surface roughness of the crystals is examined. The phase functions from complex crystals with smooth surfaces show little dependence on shape. There is close agreement between the measured functions and the analytic phase function for ice clouds. However, rosettes with rough surfaces have qualitatively different phase functions, with raised side and back scattering. The asymmetry parameter is typically about 0.8±0.04 and 0.63±0.05 for smooth and rough crystals, respectively. The 22° halo peak is present for smooth rosettes and aggregates but absent for rough rosettes. Two-dimensional scattering patterns from several crystals in fixed orientations are also shown. The results suggest that it may be possible to use such patterns to discriminate not only between crystals of different shape but also to obtain some information on surface properties.

Scattering of light from quasi-homogeneous sources by quasi-homogeneous media.

Abstract: The field generated by scattering of light from a quasi-homogeneous source on a quasi-homogeneous, random medium is investigated. It is found that, within the accuracy of the first-order Born approximation, the far field satisfies two reciprocity relations (sometimes called uncertainty relations). One of them implies that the spectral density (or spectral intensity) is proportional to the convolution of the spectral density of the source and the spatial Fourier transform of the correlation coefficient of the scattering potential. The other implies that the spectral degree of coherence of the far field is proportional to the convolution of the correlation coefficient of the source and the spatial Fourier transform of the strength of the scattering potential. While the case we consider might seem restrictive, it is actually quite general. For instance, the quasi-homogeneous source model can be used to describe the generation of beams with different coherence properties and different angular spreads. In addition, the quasi-homogeneous scattering model adequately describes a wide class of turbulent media, including a stratified, turbulent atmosphere and confined plasmas.

Approximate model for surface-plasmon generation at slit apertures

Abstract: We present a semianalytical model that quantitatively predicts the scattering of light by a single subwavelength slit in a thick metal screen. In contrast to previous theoretical works related to the transmission properties of the slit, the analysis emphasizes the generation of surface plasmons at the slit apertures. The model relies on a two-stage scattering mechanism, a purely geometric diffraction problem in the immediate vicinity of the slit aperture followed by the launching of a bounded surface-plasmon wave on the flat interfaces surrounding the aperture. By comparison with a full electromagnetic treatment, the model is shown to provide accurate formulas for the plasmonic generation strength coefficients, even for metals with a low conductivity. Limitations are outlined for large slit widths (>λ) or oblique incidence (>30°) when the slit is illuminated by a plane wave.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

Abstract: Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core.

Raman scattering from phonons and magnons in RFe3(BO3)(4)

Abstract: Inelastic light scattering spectra of several members of the RFe3(BO3)(4) family reveal a cascade of phase transitions as a function of temperature, starting with a structural, weakly first-order, phase transition followed by two magnetic phase transitions. Those consist of the ordering of the Fe-spin sublattice, revealed by all the compounds, and a subsequent spin-reorientational transition for GdFe3(BO3)(4). The Raman data evidence a strong coupling between the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice ordering leads to a strong suppression of the low-energy magnetic scattering, and a multiple peaked two-magnon scattering continuum is observed. Evidence for short-range correlations is found in the "paramagnetic" phase by the observation of a broad magnetic continuum in the Raman data, which persists up to surprisingly high temperatures.

Extracting chemical information from spectral data with multiplicative light scattering effects by optical path-length estimation and correction.

Abstract: When analyzing complex mixtures that exhibit sample-to-sample variability using spectroscopic instrumentation, the variation in the optical path length, resulting from the physical variations inherent within the individual samples, will result in significant multiplicative light scattering perturbations. Although a number of algorithms have been proposed to address the effect of multiplicative light scattering, each has associated with it a number of underlying assumptions, which necessitates additional information relating to the spectra being attained. This information is difficult to obtain in practice and frequently is not available. Thus, with a view to removing the need for the attainment of additional information, a new algorithm, optical path-length estimation and correction (OPLEC), is proposed. The methodology is applied to two near-infrared transmittance spectral data sets (powder mixture data and wheat kernel data), and the results are compared with the extended multiplicative signal correction (EMSC) and extended inverted signal correction (EISC) algorithms. Within the study, it is concluded that the EMSC algorithm cannot be applied to the wheat kernel data set due to core information for the implementation of the algorithm not being available, while the analysis of the powder mixture data using EISC resulted in incorrect conclusions being drawn and hence a calibration model whose performance was unacceptable. In contrast, OPLEC was observed to effectively mitigate the detrimental effects of physical light scattering and significantly improve the prediction accuracy of the calibration models for the two spectral data sets investigated without any additional information pertaining to the calibration samples being required.