Backscattering Configuration Research Articles

Concentration dependence in coherent Raman scattering

We investigate the concentration dependence of the femtosecond coherent anti-stokes Raman scattering (CARS) signals collected from several mixtures of molecules. We demonstrate a clear quadratic dependence of the transmitted CARS signal on the number of molecules of pyridine in water by looking at the Raman signal at 1030 cm−1. Using a backscattering configuration we study the quadratic concentration dependence in solid mixtures of dipicolinic acid, calcite, and gypsum in powder form.

Determining the stress tensor in strained semiconductor structures by using polarized micro-Raman spectroscopy in oblique backscattering configuration

We present a characterization technique for the determination of the stress tensor as well as of the crystallographic orientation of strained semiconductor structures. The technique is based on a polarized oblique incidence micro-Raman experiment in a backscattering configuration. A methodology relating the stress-induced frequency shifts and linewidths of the phonon peak to the stress tensor components within the adopted experimental configuration was developed. The method consists in monitoring the variations of the stress-sensitive peak frequencies and linewidths while rotating stepwise the sample about its normal. The practical application of the technique is illustrated on a Si∕SiGe sample microelectronic structure demonstrating a full plane stress tensor determination.

Escape time of an acoustic nanocavity mode

We present a study of the escape time of a 1 THz nanocavity phonon mode as a function of the Q-factor of the cavity. We compare results from picosecond acoustics and high resolution Raman scattering experiments, the latter obtained by means of a Fabry-Perot/triple-additive spectrometer tandem. A nanocavity consists of a GaAs spacer enclosed by two acoustic GaAs/AlAs Bragg mirrors. The number of periods of the inner mirror varies from sample to sample, spanning a range 64<Q<2470. This means that the cavity mode tunnels through the inner mirror to the substrate at theoretical time intervals that vary from 64 to 2470 ps for the different samples. An optic AlGaAs/AlAs Bragg mirror was grown between the substrate and the inner acoustic mirror in order to allow for the observation of the cavity mode in a backscattering configuration of the Raman experiments, otherwise forbidden by symmetry. At room temperature we observe escape times that vary from 65 to 278 ps. The theoretical values match the experimental results if a 3.0-GHz-wide lorentzian convolution is included to account for broadening effects. Possible explanations for this broadening will be discussed, as well as low temperature results.

Low-Threshold Stimulated Emission in ZnO Thin Films Grown by Atomic Layer Deposition

In this study, high-quality ZnO thin films were grown on sapphire substrates by atomic layer deposition (ALD), followed by high-temperature postdeposition annealing. A thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer was subsequently deposited by ALD on the ZnO surface to reduce detrimental surface states. Photoluminescence measurements conducted in a backscattering configuration at room temperature show that the ZnO film exhibited stimulated emission with a low threshold intensity of 35.1 kW/ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This may be attributed to the high-quality ZnO film and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> surface passivation layer grown by ALD, as well as the Al doping effect caused by the thermal diffusion of Al from the sapphire into the ZnO. Results show that ZnO films grown by the ALD technique are applicable to next-generation short-wavelength photonic devices.

Spectral discrimination of live prostate and bladder cancer cell lines using Raman optical tweezers

An investigation into the use of Raman optical tweezers to study urological cell lines is reported, with the ultimate aim of determining the presence of malignant CaP cells in urine and peripheral fluids. To this end, we trapped and analyzed live CaP cells (PC-3) and bladder cells (MGH-U1), because both prostate and bladder cells are likely to be present in urine. The laser excitation wavelength of 514.5 nm was used, with Raman light collected both in back- and forward-scattering geometric configurations. For the backscattering configuration the same laser was used for trapping and excitation, while for forward scattering a 1064 nm laser provided the trapping beam. Analysis of cell-diameter distributions for cells analyzed suggested normal distribution of cell sizes, indicating an unbiased cell-selection criterion. Principal components analysis afforded discrimination of MGH-U1 and PC-3 spectra collected in either configuration, demonstrating that it is possible to trap, analyze, and differentiate PC-3 from MGH-U1 cells using a 514.5 nm laser. By loading plot analysis, possible biomolecules responsible for discrimination in both configurations were determined. Finally, the effect of cell size on discrimination was investigated, with results indicating that separation is based predominantly on cell type rather than cell size.

Simulations of the bistatic scattering using two-scale model and the unified sea spectrum

This paper deals with the simulations of scattering by the sea surface using Two- Scale Model (TSM) and the realistic unified sea spectrum in various bistatic configurations. After a brief theoretical presentation, our numerical results (scattering matrix coefficients) are compared with measured data, with a semi-empirical model (CMOD5) and with other analytical models (Small Slope Approximation) in backscattering configuration. Then, we study the influence of sea parameters (wind speed and wind direction) upon the co- and cross-polarization signature in various bistatic configurations. Finally, we present the simulation of the signal received by an observer above the sea when a plane incident wave impinges on the ocean surface.

A simplified asymptotic theory for ocean surface electromagnetic wave scattering

The normalized radar cross-section (NRCS) expression of the Local Curvature Approximation (LCA-1) is derived to first order. The polarization sensitivity of this model is compared to the Kirchhoff Approximation (KA), Two-Scale Model (TSM), Small Slope Approximation (SSA-1) and Small Perturbation Method (SPM-1) to first order in the backscattering configuration. Analytical comparisons and numerical simulations show that LCA-1 and TSM could be rewritten with the same formulation and that their polarization sensitivities are comparable. Comparisons with experimental data acquired in C- and Ku-band reveal that the polarization sensitivities of these models are not adequate. However, the NRCS azimuth modulation predicted by LCA-1 is found to be dependent on polarization and sea surface roughness. This property of the LCA-1 model yields to an azimuth modulation for the polarization ratio. Based on the surface curvature correction concept, a simplified electromagnetic model is proposed. The curvature correction is restricted to the resonant wave-number of the sea roughness spectrum. This is found to reproduce the polarization ratio given by experimental data versus incidence angle and wind speed.

Temperature dependence of the Raman-active modes in the nonpolar a-plane GaN film

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the nonpolar a-plane (or [112¯0]-oriented) GaN thin film are analyzed in the range from 100to570K. The nonpolar a-plane GaN film is grown on an r-plane [or (11¯02)-oriented] sapphire substrate by metal organic chemical vapor deposition. The spectral features of the Raman shifts, intensities, and linewidths of the active phonons modes A1(TO), E1(TO), and E2(high) are significantly revealed, and corresponding temperature coefficients are well deduced by the empirical relationships. With increasing the measurement temperature the Raman frequencies are substantially redshifted and the linewidths gradually broaden. The compressive-strain-free temperature for the nonpolar a-plane GaN film is found to be at about 400K. Our studies will lead to a better understanding of the fundamental physical characteristics of the nonpolar GaN film.

Polarized Raman scattering studies of nonpolar a ‐plane GaN films grown on r ‐plane sapphire substrates by MOCVD

AbstractNonpolar (11$ \bar 2 $0) a ‐plane GaN thin films were grown on r ‐plane (1$ \bar 1 $02) sapphire substrates by low‐pressure metal organic chemical vapor deposition (MOCVD). The stress characteristics of the a ‐plane GaN films were investigated by means of polarized Raman scattering spectra in backscattering configurations. The experimental results show that there are strong anisotropic in‐plane stresses within the epitaxial a ‐plane GaN films by calculating the corresponding stress tensors. The temperature dependence of Raman scattering spectra was studied in the range from 100 K to 550 K. The measurements reveal that the Raman phonon frequencies decrease with increasing temperature. The temperature at which nonpolar a ‐plane GaN films are strain free is discussed. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Coherent backscattering in nonlinear atomic media: Quantum Langevin approach

In this theoretical paper, we investigate coherence properties of the near-resonant light scattered by two atoms exposed to a strong monochromatic field. To properly incorporate saturation effects, we use a quantum Langevin approach. In contrast to the standard optical Bloch equations, this method naturally provides the inelastic spectrum of the radiated light induced by the quantum electromagnetic vacuum fluctuations. However, to get the right spectral properties of the scattered light, it is essential to correctly describe the statistical properties of these vacuum fluctuations. Because of the presence of the two atoms, these statistical properties are not Gaussian : (i) the spatial two-points correlation function displays a speckle-like behavior and (ii) the three-points correlation function does not vanish. We also explain how to incorporate in a simple way propagation with a frequency-dependent scattering mean-free path, meaning that the two atoms are embedded in an average scattering dispersive medium. Finally we show that saturation-induced nonlinearities strongly modify the atomic scattering properties and, as a consequence, provide a source of decoherence in multiple scattering. This is exemplified by considering the coherent backscattering configuration where interference effects are blurred by this decoherence mechanism. This leads to a decrease of the so-called coherent backscattering enhancement factor.

Modification of the surface state of rough substrates by two different varnishes and influence on the reflected light

Modification of the visual appearance when a rough surface is covered by a varnish is mostly attributed to the levelling of the substrate surface, which depends on the molecular weight of the varnish. The topography of varnished surfaces, however, has never been measured directly. Surfaces of varnishes applied over glass substrates of varying roughness were studied, therefore, using mechanical profilometry. Two different varnishes made with a low and a high molecular weight resin were studied. Both varnishes lower the r.m.s. roughness of the substrates and filter the high spatial frequencies. These results are amplified for the varnish containing the low molecular weight resin. The light reflected by the varnished samples is modelled from these topographical data. Its angular distribution, calculated from the probability density of slopes is presented, taking into account separately the air/varnish and the varnish/substrate interfaces. These analyses are presented in a back-scattering configuration. They show that varnishing significantly reduces the angular width of the reflected light and that this effect is magnified for the low molecular weight resin. Modelling furthermore shows that the influence of the roughness of the varnish/substrate interface is negligible in the total reflected light.

Near-field reflection backscattering apertureless optical microscopy: Application to spectroscopy experiments on opaque samples, comparison between lock-in and digital photon counting detection techniques

An apertureless scanning near-field optical microscope (ASNOM) in reflection backscattering configuration is designed to conduct spectroscopic experiments on opaque samples constituted of latex beads. The ASNOM proposed takes advantage of the depth-discrimination properties of confocal microscopes to efficiently extract the near-field optical signal. Given their importance in a spectroscopic experiment, we systematically compare the lock-in and synchronous photon counting detection methods. Some results of Rayleigh's scattering in the near field of the test samples are used to illustrate the possibilities of this technique for reflection backscattering spectroscopy.

Light Scattering Studies on a Siloxane Polymer-Chiral Nematic Liquid Crystal Blend

ABSTRACT Dynamic light scattering experiments using circularly polarized light in the back-scattering configuration were used to study the effect of polymer stabilization on the amplitude of fluctuations in two of the five structural modes present in the isotropic phase of the chiral nematic liquid crystal (S)-(+)-4-(2-methyl-butyl) phenyl 4-decyloxybenzoate (CE6). From these data, the second-order transition temperatures of these modes, which grow further apart if chirality increase were determined. When the chiral liquid crystalline polymer was added, the fluctuations of the modes deviated from the normal temperature dependence and the separation of the second-order transition temperatures increased.

Structure–property relationships in porous GaN generated by Pt-assisted electroless etching studied by Raman spectroscopy

Porous GaN (PGaN) produced by Pt-assisted electroless etching has been studied by Raman spectroscopy. Three different unintentionally doped GaN films grown by hydride vapor phase epitaxy on sapphire were used for this study. Pt-assisted electroless etching takes place without external contact to a current source, simplifying the generation of porous semiconductors. Raman spectra were excited with both subband gap (441.6 nm) and above-band gap (325 nm) radiation, and collected in an unpolarized backscattering configuration. In both cases, a strong increase in scattering is observed for the Raman modes, depending on the depth of the porous layer. With visible excitation, symmetry-forbidden A1(TO) and E1(TO) are observed in PGaN, and spectral line shape analysis reveals the presence of the forbidden E1(LO) mode as well. Raman spectra were also analyzed using an effective medium theory, revealing extraordinary LO phonon scattering with visible, and Fröhlich scattering with UV, excitation. In addition a quasi-LO phonon is observed in the visible spectrum. All of the characteristic features of PGaN Raman scattering: symmetry-forbidden modes, quasi-LO scattering, and strong Fröhlich mode scattering, can be attributed to the dendritic porous morphology of PGaN.

Detection of PAHs in seawater using surface-enhanced Raman scattering (SERS)

The laboratory characterization of a field-operable surface-enhanced Raman scattering sensor (SERS optode) is presented for the detection of aromatic hydrocarbons in seawater. The sensor has been developed for deployment with a robust underwater spectrograph. To meet the demands of the harsh seawater application, sol–gel derived SERS substrates were used. The calibration curves of six PAHs were determined to be of Langmuir adsorption isotherm type with limits of detection ranging from the μg l −1 to ng l −1 level. The experimentally determined adsorption constants varied strongly with the molecular weight of the analytes and correlated with their solubility. A mixture of five PAHs dissolved in seawater was investigated to demonstrate the utility of this method for screening. Emphasis was put on the interference from suspended particulate matter (SPM). The Raman measurement with backscattering configuration was shown to be immune against turbidities up to 1000 NTU. The physico-chemical interference arising from adsorption by the sediment was measured on-line by adding sediment to a PAH-spiked solution. According to the calibration curve, the PAH concentration decrease corresponded to more than 98% of the analyte being scavenged by the sediment.

Development of an apertureless near-field optical microscope for fluorescence imaging and spectroscopy

We describe an apertureless-scanning near-field optical microscope (A-SNOM) in a backscattering configuration for fluorescence imaging. The feasibility of the setup was tested on 100 nm latex spheres doped with dye-molecules. Near-field Rayleigh scattering and fluorescence images of this sample were acquired. The near-field image demodulated at the frequency of the tip vibration displays a set of fringes which disappear at higher demodulated harmonics. A far-field spectrum of an area of this test sample proves that the corresponding near-field image acquired in the same conditions of illumination and detection contains near-field fluorescence information.

Color change due to surface state modification

AbstractThe dependence of color on the surface state of an object is calculated in the context of spectrophotometric measurements in a back‐scattering configuration. A modification of the surface roughness leads to a vertical shift of the reflectance spectrum. This translation is related, on a physical basis, to the relevant characteristic of the topography: the ratio h/l (ie, r.m.s. roughness/surface correlation length). Changes in L*a*b* colorimetric coordinates associated with this translation are computed. Finally, the color change ΔE is related to the surface state modification via the ratio h/l. Computations show that the color becomes lighter and less saturated when the surface becomes rougher. The color change is more important for dark or saturated initial objects, and also more important for a yellow surface than for a blue one. Finally, the minimum roughness modification that can induce a visible color change is determined. These results could be applied for industrial needs (quality control), or in the artistic field of conservation or restoration (to follow the color of paintings). © 2002 Wiley Periodicals, Inc. Col Res Appl, 28, 45–49, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.

Monostatic and bistatic scattering from a density contrast surface

Chu’s formalism for the impulse response of a point source to a density contrast isovelocity wedge [J. Acoust. Soc. Am. 86, 1883–1896 (1989)] is of significant practical importance, since it facilitates the extension of wedge diffraction boundary scattering models to rough penetrable surfaces. The method is used here to study scattering from an experimental surface described by Li et al. [J. Acoust. Soc. Am. 96, 3715–3720 (1994)]. The geometry contains both interior and exterior wedges, and allows the diffracted scattering component to be studied independently of the reflected component. For density contrast values which emulate rigid boundary conditions, the formalism correctly models the experimental results, and duplicates predictions of the Biot–Tolstoy solution for a rigid wedge [J. Acoust. Soc. Am. 29, 381–391 (1957)]. The formalism is then used to investigate an assumption commonly made in boundary scattering work, i.e., that scattering levels can be effectively estimated by scaling the predictions of a rigid surface scattering model by the reflectivity of the surface. The results suggest that the assymption is most valid for monostatic (backscattering) configurations, and less good for bistatic cases, with increasing differences occurring between interior and exterior wedge geometries. [Work supported by ONR.]

Pigment Identification by Fiber-Optics Diffuse Reflectance Spectroscopy

Fiber-optics reflectance spectroscopy is used to identify pigments in pictorial layers of works of art thanks to a spectra database of dry powdered mineral pigments. Measurements are noninvasive, without any contact, and can be implemented in situ, without moving the work of art under investigation from its conservation place. The experimental device, using the special back-scattering configuration, is briefly presented. The protocol leading to the constitution of the spectra database of dry mineral pigments is described. Unlike other studies, this protocol has been developed to emphasize multiple scattering of light by elementary pigments in comparison with specular reflection on the surface of the sample. In these conditions, the diffuse reflectance spectrum is the label of the mineral pigment. The numerical processing of pigment identification is detailed. Both the influences of the roughness of the studied surface and of a possible varnish layer are taken into account when numerical identification is implemented. Several applications on patrimonial works of art are reported.

Polarized Raman scattering of epitaxial Pb(Zr,Ti)O3 thin films in the tetragonal-phase field

In view of the difficulty in preparing single crystals, a c-axis oriented epitaxial lead zirconate titanate (PZT) film was prepared as a model system for the single-crystalline PZT. By employing an extraordinary backscattering configuration in which the relevant phonon wave vector, k, is parallel to the film/substrate interface, we have successfully isolated the E [transverse optical (TO)] symmetry phonons from A1(TO) phonons. Compared with polarized Raman spectra of the c-axis oriented PbTiO3 film, the polarized spectra of the epitaxial PZT film (with Zr/Ti=40/60) exhibited significant asymmetric line broadening with a noticeable downward shift of the mode frequencies. In addition to this, we observed a splitting of the “silent” mode into the B1-symmetry mode and the E(TO) mode using the parallel and cross polarization configurations, respectively.