Backscattering Configuration Research Articles

Online Raman spectroscopy for quantitative detection and characterization of aerosolized soot.

Ex-situ Raman spectroscopy is a well-established method for the structural characterization of soot but necessitates a sampling step before analysis is made. This work studied the potential to perform Raman spectroscopic measurements of soot directly in the combustion exhaust gases, i.e., online Raman measurements. Two types of soot were produced from a Mini-CAST soot generator, one immature with high organic content and one mature with low organic content. Also, two different measurement configurations were used, a 90-degree and a backscattering configuration, as well as different polarization combinations of the laser radiation and the detected photons. Spectrally resolved Raman signals were successfully recorded and analyzed for the two soot types showing differences between the characteristic D and G peaks as well as the photoluminescence signal. Additionally, the Raman cross section was determined for both soot types and found to be higher for the immature soot. It was also found that a horizontally polarized laser excitation and a 90-degree collection angle had the best potential to reduce the interference from ambient gases. This study can be seen as a first step towards remote sensing of soot.

The respective and dependent effects of scattering and bone matrix absorption on ultrasound attenuation in cortical bone

Cortical bone is characterized by a dense solid matrix permeated by fluid-filled pores. Ultrasound scattering has potential for the non-invasive evaluation of changes in bone porosity. However, there is an incomplete understanding of the impact of ultrasonic absorption in the solid matrix on ultrasound scattering. In this study, maps were derived from scanning acoustic microscopy images of human femur cross-sections. Finite-difference time domain ultrasound scatter simulations were conducted on these maps. Pore density, diameter distribution of the pores, and nominal absorption values in the solid and fluid matrices were controlled. Ultrasound pulses with a central frequency of 8.2 MHz were propagated, both in through-transmission and backscattering configurations. From these data, the scattering, bone matrix absorption, and attenuation extinction lengths were calculated. The results demonstrated that as absorption in the solid matrix was varied, the scattering, absorption, and attenuation extinction lengths were significantly impacted. It was shown that for lower values of absorption in the solid matrix (less than 2 dB mm−1), attenuation due to scattering dominates, whereas at higher values of absorption (more than 2 dB mm−1), attenuation due to absorption dominates. This will impact how ultrasound attenuation and scattering parameters can be used to extract quantitative information on bone microstructure.

Competing effects of ultrasound absorption and scattering in porous bone

The mechanisms and effects of ultrasonic attenuation in porous cortical bone are poorly understood, and it is necessary to better understand them to evaluate bone porosity noninvasively using ultrasound. A finite-difference time domain numerical study was conducted in which ultrasound propagation was simulated in human femur cross-sections obtained via scanning acoustic microscopy. The effect of absorption on overall attenuation was studied by varying the nominal absorption level attributed to the solid matrix. Ultrasound pulses were emitted with a central frequency of 8 MHz in through-transmission and backscattering configurations. From these data, the respective extinctions lengths due to overall attenuation, scattering, and absorption were obtained. Two regimes seem to exist depending on the nominal absorption value. At low absorption values, scattering dominates overall attenuation, scattering and absorption appear to have a synergistic effect on overall attenuation, and the diffusion constant decreases with increasing average pore diameter. At high absorption values, absorption dominates overall attenuation, the extinction length for scattering increases with pore diameter, and the diffusion constant increases with increasing average pore diameter. These regimes affect how ultrasound parameters, such as the extinction lengths due to scattering, absorption, and overall attenuation, should be used to evaluate the porosity of cortical bone.

Mueller matrix model of polarized light propagation in layered human skin in backscattering configuration

We report a two-layer Mueller matrix model of polarized light propagation through layered human skin. Our model is based on single scattering approximation and the fact that the main scatterers in the top layer are various types of cells and the scatterers beneath it consist of fibers. By modeling the first layer mainly with spherical particles inducing the characteristics represented by Mueller matrices of depolarization and diattenuation, the second layer mainly with cylindrical scatterers inducing the retardance caused by scattering and birefringence, and considering the effects of the first layer to the second one, and the equal weight of contributions from both layers to a scatterer, it is possible to separate polarization properties of the medium induced by light propagation and scattering processes. One advantage of our model is that its extension to multi-layer tissues is straightforward. Polarization effects of human nail fold are identified.

Wide-field Mueller matrix polarimetry for spectral characterization of basic biological tissues: Muscle, fat, connective tissue, and skin.

This study investigates the polarimetric properties of skin, skeletal muscle, connective tissue, and fat using Mueller matrix imaging. It aims to compare the polarimetric characteristics of these tissues and explore how they evolve with wavelength. Additionally, the temporal evolution of certain tissues during meat aging is studied, providing insights into the dynamic behavior of polarimetric properties over time. The research employs back-scattering configuration and the differential decomposition analysis method of Mueller matrix images. Both in-vivo and ex-vivo experiments were conducted using a consistent instrument setup to ensure reliable analysis. The results reveal wavelength-dependent variations in tissue properties, including an increase in depolarization with wavelength. Significant differences in the polarimetric characteristics of meat tissues, particularly for skeletal muscle, are observed. Over a 24-h period, intensity, diattenuation, and retardation experience alterations, being the decreased retardation in skeletal muscle and the increased retardation in fat the most notable ones.

Evaluation of Anisotropic Biaxial Stress in Extremely-Thin Body (100) Silicon-Germanium-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect-Transistor by Oil-Immersion Raman Spectroscopy

We evaluated the anisotropic biaxial strain in the channel region of extremely-thin body silicon-germanium-on-insulator p-MOSFETs by oil-immersion Raman spectroscopy. Oil-immersion Raman spectroscopy can measure the transverse optical (TO) phonon mode which cannot be detected by conventional Raman spectroscopy under the backscattering configuration for (001) substrate. Therefore, we can calculate the anisotropic biaxial stress in the SiGe channel region and investigate the channel width dependence of stress. From the Raman spectra obtained in the polarization configurations of longitudinal optical (LO)-active and TO-active, we confirmed that compressive stress is induced in the SiGe channel region. In addition, we observed that the strain state of the SiGe channel region becomes quasi-uniaxial strain by narrowing the channel width.

Identifying anisotropic structural features beneath the human nail with differential polarization parameters

In this work, we report a method of identifying anisotropic structural features beneath the nail with differential polarization parameters. First the form of the differential Mueller matrix of medium in terms of the average and the fluctuation parameters of the differential polarization parameters in backscattering configuration is derived. The results are then employed to identify anisotropic structures beneath the human nail. For the first time, to our knowledge, it was found that collagen fibrils of the layers of epidermal cells of nail bed, the layers of actively dividing cells of the stratum germinativum of nail matrix, and the longitudinal striations in nail root can be related to the covariance of the linear birefringence along the x-y laboratory axes and the 45°axes. Although some tissue constituents can be observed in images of both macroscopic and differential parameters, the tissue components revealed by differential polarization parameters cannot be discerned in any image of the macroscopic parameters.

Probing hyperbolic and surface phonon-polaritons in 2D materials using Raman spectroscopy

The hyperbolic dispersion relation of phonon-polaritons (PhPols) in anisotropic van der Waals materials provides high-momentum states, directional propagation, subdiffractional confinement, large optical density of states, and enhanced light-matter interactions. In this work, we use Raman spectroscopy in the convenient backscattering configuration to probe PhPol in GaSe, a 2D material presenting two hyperbolic regions separated by a double reststrahlen band. By varying the incidence angle, dispersion relations are revealed for samples with thicknesses between 200 and 750 nm. Raman spectra simulations confirm the observation of one surface and two extraordinary guided polaritons and match the evolution of PhPol frequency as a function of vertical confinement. GaSe appears to provide relatively low propagation losses and supports confinement factors matching or exceeding those reported for other 2D materials. Resonant excitation close to the 1s exciton singularly exalts the scattering efficiency of PhPols, providing enhanced scattering signals and means to probe the coupling of PhPols to other solid-state excitations.

Polarized laser speckle images produced by calibrated polystyrene microspheres suspensions: comparison between backscattering and transmission experimental configurations

In this paper, we analyze polarized laser speckle images produced by calibrated polystyrene microspheres suspensions and discuss the results obtained while considering two different experimental configurations for light detection: speckle-transmission setup and speckle-backscattering one. The degree of light polarization (DOP) and speckle grain size (dx) were monitored in both cases. Results show that, when backscattered photons are detected, the degree of circular polarization allows a fine monitoring of the variation of scatterers concentration in a sample. A helicity flipping is detected as the scatterers concentration increases, showing that a transition from a simple scattering regime to a multiple scattering one is taking place. When transmitted scattered photons are detected dx is a better parameter than DOP for following changes in scattering regime: dx values initially increase slightly then decrease as the scattering coefficient increases. The observed maximum corresponds to the shift from a simple to a multiple scattering regime.

Ultraviolet Raman spectroscopy for remote detection of chlorine gas

As a primary material frequently used in industry, chlorine is relatively easy to obtain and available even in large quantities. Despite its high toxicity, molecular chlorine is readily available since it is an essential educt in the chemical industry. Over the past decades, numerous accidents involving injured and dead victims have occurred. Furthermore, it was already misused as a warfare agent at the beginning of the last century with still reported attacks. Early detection, localization, and monitoring of sources and cloud movements are essential for protecting stationary facilities, mobile operations, and the public. In contrast to most chemical hazardous materials, where it is possible to detect them by vibrational spectroscopic methods (e.g., passive hyper-spectral absorption technologies in the infrared), halogens are inactive to infrared absorption. Raman-based technologies rely on changes in the polarizability of the molecule and provide vibrational-spectroscopic access to such diatomic molecules and therefore close the gap in infrared detection capabilities. Here we present a straightforward approach for a standoff Raman detector in a backscattering configuration. This paper uses a simplified model to discuss optimum excitation wavelengths in achievable detection ranges. We validate the model by spontaneous (vibrational) Raman spectroscopic measurements between 20 and 60 m standoff distance. We also briefly discuss detection performances and technical and physical aspects as prospects of system design.

Development, characterization and photocatalytic study of biocomposites based on PTFE, TiO2 and LuffaCylindrica fibers

The present study focuses on the use of Luffa Cylindrica fibers as a reinforcing agent in a fluoroplastic (PTFE) polymer matrix and TiO2 nanoparticles as a photocatalytic agent with different mass fractions (%), for the purpose of the preparation and structural characterization of a new biocomposite material. The granulometry and zeta potential were measured using dynamic light scattering and laser Doppler velocimetry respectively. Structural and vibrational properties of the PTFE/TiO2/LC fibers composites were characterized by micro-Raman spectroscopy in oblique backscattering configuration and FTIR spectroscopy. Lastly, thermal properties were investigated using TGA and DSC. Further, the photocatalytic activity of samples was tested for the degradation of Methylene Blue dye (MB) in aqueous medium.

Chiral phonons: circularly polarized Raman spectroscopy and ab initio calculations in a chiral crystal tellurium.

Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudoangular momenta. In circularly polarized Raman spectroscopy, the peak split of the mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudoangular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe chiral phonons in a chiral unary crystal Te. The pseudoangular momentum of the phonon is obtained in Te by an ab initio calculation. From this calculation, we verified the conservation law of pseudoangular momentum in Raman scattering. From this conservation law, we determined the handedness of the chiral crystals. We also evaluated the true chirality of the phonons using a measure with symmetry similar to that of an electric toroidal monopole.

Raman tensor studies on defective non-van der Waals Bi2O2Se

The Raman tensors of the three modes at ∼55, ∼80, and ∼160 cm−1 for the non-van der Waals layered material Bi2O2Se, which were assigned to Eu, Eg, and A1g, respectively, were experimentally investigated. Two modes at ∼55 and ∼80 cm−1, which were not observable in perfect crystal Bi2O2Se in the backscattering configuration, owing to the Raman selection rule, were activated by defects. These two modes exhibit strong polarization dependence at line defects and the excitation energy; thus, their Raman polarizability tensors exhibit strong dependence on the defect morphology and geometric characteristics of Bi2O2Se. The results of this study confirm that the Raman tensors of nanocrystalline structures can be modulated by defects.

Weight of single and recurrent scattering in the reflection matrix of complex media.

In a heterogeneous medium, the wave field can be decomposed as an infinite series known as the Born expansion. Each term of the Born expansion corresponds to a scattering order, it is thus theoretically possible to discriminate single and multiple scattering contribution to the field. Experimentally, what is actually measured is the total field in which all scattering orders interfere. Conventional imaging methods usually rely on the assumption that the multiple scattering contribution can be disregarded. In a back-scattering configuration, this assumption is valid for small depths, and begins to fail for depths larger than the scattering mean-free path ℓ_{s}. It is therefore a key issue to estimate the relative amount of single and multiple scattering in experimental data. To this end, a single-scattering estimator ρ[over ̂] computed from the reflection matrix has been introduced in order to assess the weight of single scattering in the backscattered wave field. In this paper, the meaning of this estimator is investigated and a particular attention is given to recurrent scattering. In a diffraction-limited experiment, a multiple scattering sequence is said to be recurrent if the first and last scattering events occur in the same resolution cell. Recurrent scattering is shown to be responsible for correlations between single scattering and higher scattering orders of the Born expansion, inducing a bias to the estimator ρ[over ̂] that should rather be termed confocal scattering ratio. Interestingly, a more robust estimator is built by projecting the reflection matrix in a focused basis. The argument is sustained by numerical simulations as well as ultrasonic data obtained around 1.5MHz in a model medium made of nylon rods immersed in water. From a more general perspective, this work raises fundamental questions about the impact of recurrent scattering on wave imaging.

Adaptive design of dual excitation ultra‐low wavenumber Raman spectroscopy system

AbstractUltra‐low wavenumber or ultra‐low frequency (ULF) Raman spectroscopy using holographic optics is an emerging technique to obtain spectral signatures of chemicals in low energy corresponding to their phases, crystallinity, isomerism, and environmental conditions such as temperature. Commercial turn‐key systems employ mechanical turrets and other such optomechanical moving elements to accommodate multiple laser excitations, which are incident normally on the sample, and the Raman signal is collected in a 180° backscattering configuration. Here, we have developed a system consisting of two single‐longitudinal mode (SLM) lasers and a single‐stage spectrometer. Selected notch filters were based on the volume holographic grating technology providing output beam at a certain angle, which facilitates the possibility of different angles of incidence of the excitation laser on the sample. This adaptive ULF Raman system, therefore, is not only able to acquire high‐quality low‐wavenumber spectral fingerprints of various chemicals but can also be easily reconfigured for differential spectral studies.

The effects of nominal absorption in bone on the extinction lengths for scattering, absorption, and overall attenuation

In cortical bone, the effect of absorption in the solid matrix on ultrasound scattering and propagation is not well understood, and for absorption, it is not easily incorporated into simulations and is difficult to measure independently from scattering attenuation. In this study, finite-difference time-domain simulations were conducted in maps derived from scanning acoustic microscopy images of human femur cross-sections. These maps were modified by controlling pore density (fixed at 10 pores/mm2) and diameter distribution (ranging between 30 and 100 μm). Eight nominal absorption values, ranging from 0 to 100 dB/cm, were attributed to the solid matrix. Absorption in the pores was set to 0.01 dB/cm. Propagation of ultrasound pulses centered at 8 MHz was simulated in both backscattering and through-transmission configurations. The scattering, absorption, and overall attenuation extinction lengths were obtained. T-tests demonstrated that varying the absorption significantly impacts the extinction lengths for scattering, overall attenuation, and absorption. The differences in values for the scattering, attenuation, and absorption extinction lengths when absorption varied from 20 to 40 dB/cm are statistically significant (respective p-values: 9.1 × 10−14, 3.3 × 10−5, and 8.8 × 10−5). It was also demonstrated that attenuation due to scattering dominates overall attenuation at low-nominal absorption values, whereas the converse is true at higher absorption values.

TO-phonon anisotropies in a highly doped InP (001) grating structure

For zinc blende semiconductors, such as InP, the Raman selection rules for a backscattering configuration from the (001) surface forbid the transversal optical (TO) phonon mode, whereas the longitudinal optical mode is allowed. However, when InP is highly doped with Si atoms, InP-Si clusters with the reduced C3v symmetry allow TO modes in the Raman spectrum with the backscattering configuration. Here, we demonstrate that the amplitude of the TO modes can be modulated spatially by using a highly doped InP grating. By exciting the sample with a laser linearly polarized parallel and perpendicular to the grating grooves, we observe a change in amplitude of the phonon optical response for the TO mode.

Mueller matrix polarization parameter tomographic imaging method in the backscattering configuration

A Mueller matrix polarization parameter tomographic imaging (MMPPTI) in backscattering configuration for generating the depth-resolved cross sectional images of the polarization parameters of biological tissue is described. First, a theoretical model is proposed, which can be used to directly determine the polarization properties of anisotropic medium from the measured Mueller matrix. Several polarization parameters such as diattenuation of the medium can be expressed in terms of the elements of the measured Mueller matrix. The model proposed can be regarded as a physical interpretation of the symmetric decomposition of a Mueller matrix. To calibrate the new method for generating depth-resolved Mueller matrix images of the media with several polarization effects simultaneously, it is proposed in this work to use porcine liver as a mostly isotropic standard sample. The depth-resolved cross sectional images of 16 elements of the Mueller matrix, and the polarization properties of tissues in vivo and ex vivo are presented to demonstrate the capability of our method. Our MMPPTI method has the potential for being able to image all the polarization parameters of any depolarizing anisotropic medium. The model for MMPPTI in backscattering case can be modified to find the explicit expressions for the polarization parameters of media with some specific anisotropic structures or being imaged in transmitted mode.

Angle-Resolved Intensity of Polarized Micro-Raman Spectroscopy for 4H-SiC

Raman spectroscopy is an indispensable method for the nondestructive testing of semiconductor materials and their microstructures. This paper presents a study on the angle-resolved intensity of polarized micro-Raman spectroscopy for a 4H silicon carbide (4H-SiC) wafer. A generalized theoretical model of polarized Raman intensity was established by considering the birefringence effect. The distributions of angle-resolved Raman intensities were achieved under normal and oblique backscattering configurations. Experiments were performed on a self-built angle-resolved Raman system, which verified the validity of the proposed model and achieved the identification of crystal orientations of the 4H-SiC sample.

(Invited) Raman Spectroscopy of Confined Hyperbolic and Surface Phonon-Polaritons in 2D Materials

Phonon-polaritons are quasiparticles resulting from the strong coupling between polar phonons and the electromagnetic field. This hybrid matter-light quasiparticle exhibits remarkable characteristics enabling new or enhanced nanophotonics functionalities. Polaritons in 2D materials are particularly promising: the natural anisotropy leads to a hyperbolic frequency dispersion and enables for example sub-wavelength imaging, low-loss propagation, spontaneous emission enhancement, and extreme field confinement. In addition, 2D heterostructures allow unprecedented opportunities for polariton dispersion engineering.Due to the large momentum mismatch between polaritons and free space propagating fields, polariton measurements require either near-field coupling to its evanescent wave or a direct coupling to the underlying polar oscillation. So far, polaritons in 2D materials have been largely studied using the near-field coupling mediated by an AFM tip. This technique has been extensively applied to h-BN, where imaging of polaritons allows determining the dispersion relation, propagative behaviour, and confinement of polaritons. In contrast, Raman scattering probes polar phonons directly. However, polaritonic effects are manifest at very low wavenumber and can be observed only in a forward scattering configuration, which has made until now this important and ubiquitous characterization techniques ineffective for the study of polaritons in 2D materials.In this work, we demonstrate for the first time that Raman spectroscopy in a backscattering configuration is in fact an extraordinarily powerful and versatile tool for polariton studies in 2D materials. Usually forbidden, they can be readily observed due selection rule relaxation occurring in thin samples and the deep sub-wavelength confinement typical of hyperbolic materials. For the demonstration, we used gallium selenide owing to its strong polar resonances, its two hyperbolic regions and its nested reststrahlen structure.Polariton Raman scattering spectra are both calculated and experimentally measured, allowing an unambiguous identification of the polaritons involved. Measurements as function of incident angle directly reveal the dispersion of the upper extraordinary and surface polaritons. The dependence of polariton frequencies on sample thickness is reported and agrees very well with the calculated values, as long as the silicon oxide and silicon substrate on which the sample is deposited are included in the calculation. This demonstrates that polaritons are quite sensitive to their dielectric environment.Excitation near the 1s-exciton leads to a strong exaltation of the upper extraordinary polariton and the otherwise unobserved lower ordinary polariton. This resonant enhancement allows probing subtle effects related to propagation and confinement along lateral directions. The scattering efficiency, initially isotropic for large samples, becomes polarized along the weaker confinement direction. In addition, the polariton frequency becomes polarization dependent and is highest along the strongly confined direction. From these results, we find low propagation losses and a propagation figure of merit of γ-1 ≥ 39 is found. This value is among the highest reported and establishes GaSe as a promising 2D materials.More significantly, these results demonstrate that Raman spectroscopy, a simple, rapid, non-invasive and widely available technique, can effectively probe confined polariton states. Compatible with high spatial imaging and energy resolution, Raman spectroscopy will undoubtedly accelerate the development of a wider variety of polaritonic materials and complex 2D heterostructures for mid-infrared nanophotonics applications.