Backscattering Of Waves Research Articles

Imaging coherent transport in graphene (part II): probing weak localization

Graphene has opened new avenues of research in quantum transport, with potentialapplications for coherent electronics. Coherent transport depends sensitively on scatteringfrom microscopic disorder present in graphene samples: electron waves traveling alongdifferent paths interfere, changing the total conductance. Weak localization is produced bythe coherent backscattering of waves, while universal conductance fluctuations are createdby summing over all paths. In this work, we obtain conductance images of weaklocalization with a liquid-He-cooled scanning probe microscope, by using the tipto create a movable scatterer in a graphene device. This technique allows us toinvestigate coherent transport with a probe of size comparable to the electronwavelength. Images of magnetoconductance versus tip position map the effectsof disorder by moving a single scatterer, revealing how electron interference ismodified by the tip perturbation. The weak localization dip in conductivity atB = 0 isobtained by averaging magnetoconductance traces at different positions of the tip-created scatterer. Thewidth ΔBWL of the dip yields an estimate of the electron coherence lengthLϕ at fixed charge density. This ‘scanning scatterer’ method provides a new way ofinvestigating coherent transport in graphene by directly perturbing the disorderconfiguration that creates these interferometric effects.

Backscattering of electromagnetic waves by the vertex area of a thin conducting screen

The backscattering of the electromagnetic waves by the vertex area of a thin metal screen is investigated. A special case when the magnetic field of the incident wave is perpendicular to the diagonal of the vertex area is considered. The dependence of the vertex-scattered signal on the grazing angle is experimentally found. At a certain angle, an extremum in the radar scattering is discovered. At the extremum, good agreement between the results predicted by the asymptotic theory and the experimental results is observed.

Electromagnetic plasma emission during beam‐plasma interaction: Parametric decay versus induced scattering

Two‐dimensional electromagnetic particle‐in‐cell simulations are performed for examination of electromagnetic plasma emission at twice the electron plasma frequency. Electromagnetic “2fp” waves are considered to be excited by nonlinear three‐wave processes in beam‐plasma interactions. In this paper, nonlinear development of an electron‐beam‐plasma instability is studied as an initial value problem. The present simulation result confirmed that electromagnetic 2fp waves are strongly enhanced by the wave‐wave interaction between forward and backward Langmuir waves, which is in agreement with the previous studies. It is also demonstrated that large‐amplitude forward Langmuir waves decay into backward Langmuir waves and ion acoustic waves via a parametric decay and that electromagnetic 2fp waves are also enhanced by the decay of Langmuir waves. However, the growth rate of the electromagnetic 2fp waves due to the parametric decay of Langmuir waves is not as high. It should be noted that induced backscattering of Langmuir waves by enhanced thermal fluctuations of ions cannot be neglected in the excitation of backward Langmuir waves. Hence, low‐noise simulations are necessary to suppress the effect of enhanced thermal fluctuations in the particle‐in‐cell method.

Nonlinear ultrasound scattering by a cloud of microbubbles

In this study, we theoretically analyze the interaction of ultrasound waves in a liquid with suspended gas microbubbles confined in region of the liquid. In ultrasound harmonic imaging, gas microbubbles are used to contrast image by accentuating the generation of backscattered second-harmonic waves. However, these waves are attenuated twice as much as the fundamental. To delineate the harmonic amplification effect from excess attenuation, we study the nonlinear interaction of ultrasound with a region containing microbubbles. Considering an incident plane wave interacting with a spherical cloud of microbubbles, the solutions for the linear and the second-harmonic scattered waves are derived. The results are obtained for both monochromatic and period pulsed waves, including the effects of attenuation, and diffraction. These results provide the mathematical framework for estimating and compensating the effect of attenuation in the harmonic backscattered wave.

Applying time domain physical optics to acoustic wave backscattering problem

The interest in transient analysis of acoustic waves has been growing in recent years, due to the advance of wide-band sonars. In this paper, a transient analysis method for acoustic backscattering signals is proposed based on the time domain physical optics (TDPO). TDPO is formulated via a theoretical inverse Fourier transform of the conventional physical optics formula used in the frequency domain wave scattering analyses. A hidden surface removal algorithm using an adaptive triangular beam method and a virtual surface concept are adopted to explain shadow effects and multiple reflections among elements, respectively. Numerical analyses are carried out for two kinds of underwater targets: a submarine pressure hull and an idealized submarine, in order to validate the proposed method. The result of the submarine pressure hull shows good agreements between the proposed method and conventional physical optics based on inverse fast Fourier transform. Additionally, the result of the idealized submarine shows that the proposed method is efficient for finding highlights including their contribution to the whole backscattering signal.

Novel features of non-linear Raman instability in a laser plasma

The electron phase space evolution in a non-relativistic and homogeneous laser plasma generated by a nanosecond laser in a near infrared region in the presence of stimulated Raman scattering is investigated by a numerical simulation. The mechanism of electron acceleration in the potential wells of the plasma wave accompanying the Raman back-scattering is analyzed in a 1D Vlasov-Maxwell model. The dominant wave modes are both the backward and the forward propagating Raman waves, each accompanied by a daughter electrostatic wave. In addition to a strong interaction of plasma electrons with the primary electrostatic wave in the case of back-scattering, a cascading is observed consisting in a secondary scattering of the primary Raman back-scattered wave. This phenomenon reduces the Raman reflectivity and causes an acceleration of electrons against the direction of the heating laser beam. Moreover, the strong trapping in the primary electrostatic wave generated by the Raman back-scattering leads due to the trapped particle instability to a significant spectral broadening of the original plasma wave and a subsequent intermittent behaviour of the scattering process. The high phase velocity electrostatic daughter wave of the forward Raman scattering cannot trap the electrons directly, but there is an indication of non-resonant quasi-modes combined of this wave and of the simultaneously existing electrostatic daughter wave accompanying the Raman back-scattering. The transform method is used for a solution of the set of partial differential equations, which consists of the Vlasov equation and of the full set of Maxwell equations in a 1D approximation. A simplified Fokker-Planck collision term is added to overcome the numerical instabilities during the simulation. The model has relevance to a long scale plasma geometry, such as occurring in the indirect drive experiments near the light entrance holes of target hohlraum.

Evaluation of fracture damage of SUS306 structural steel using ultrasonic testing

High frequency ultrasonic nondestructive testing was conducted using a direct contact method for SUS306 stainless steel treated by high temperature and fracture tensile tests. Reflected ultrasonic echoes were analyzed. The relationships between the ultrasonic attenuation coefficient, strength of backscattering wave and the elongation at break of the samples were obtained. The damages were evaluated by using these results together with the analysis of microstructure and mechanics of the tested material.

Numerical implementation of local unified models for backscattering from random rough sea surfaces

In the context of electromagnetic wave backscattering from ocean-like surfaces, by using the lowest order of the SSA (SSA-1) model, Bourlier et al. proposed an original technique to reduce the number of numerical integrations to two for easier numerical implementation. To be consistent with microwave measurements, closed-form expressions of the Fourier coefficients with respect to the wind direction of the backscattering normalized radar cross-section (NRCS) are obtained. For Gaussian statistics, previous work is extended in this paper to kernels of unified models expanded up to second order, like full SSA and full LCA. Thus, with the help of Bessel functions and by analytical integrations over the azimuthal angles, the second-order backscattering (BNRCS) is expressed in terms of two-fold integrations and another independent integration instead of four-fold integrations, if no analytical integration is made. This approach allows us to obtain fast results (less than one second). Numerical results are then presented for different microwave frequencies and wind speeds.

A finite element model for acoustic propagation in shallow water waveguides.

A finite element model is developed using a commercial code to calculate the backscattering of acoustic waves in a two-dimensional shallow water waveguide with rough interfaces. Finite elements approach an exact solution to the Helmholtz equation as the discretization density increases. A time series for the backscattering is computed from time harmonic computations using Fourier synthesis. A parametric study of the roughness parameters for the sediment/water interface and the air/water interface will be presented in order to assess the range of validity of approximate methods. [Work sponsored by Internal Research and Development, Applied Research Laboratories.]

Using ADV backscatter strength for measuring suspended cohesive sediment concentration

Laboratory experiments were conducted at two institutes to reveal the relationship between acoustic backscatter strength and suspended sediment concentration (SSC). In total, three acoustic Doppler velocimeters (ADVs) with different frequencies (5, 10 and 16 MHz) were tested. Two different commercial clays and one natural sediment from Clay Bank site in the York River were checked for acoustic responses. The SSCs of selected sediments were artificially changed between a selected low and a high value in tap or de-ion water. Each ADV showed quite different backscatter responses depending on the sediment type and SSC. Not all devices had a good linear relationship between backscatter strength and SSC. Within a limited range of SSC, however, the backscatter strength can be well correlated with the SSC. Compared with optical backscattering sensor (OBS), the fluctuation of ADV backscatter signals was too noisy to be directly converted to the instantaneous changes of SSC due to high amplification ratio and small sampling volume. For the more accurate signal conversion for finding the fluctuation of SSC, the ensemble average should be applied to increase the signal-to-noise ratio. There are unexpected responses for the averaged backscatter wave strength: (1) high signals from small particles but low signals from large particles; and (2) two linear segments in calibration slope. These phenomena would be most likely caused by the different gain setting built in ADVs. The different acoustic responses to flocculation might also contribute somewhat if flocs are tightly packed. This study suggests that an ADV could be a useful instrument to estimate suspended cohesive sediment concentration and its fluctuation if the above concerns are clarified.

Nonlinear coherent backscattering

We theoretically examine diffusive transport and coherent backscattering of waves in dilute disordered media with weak nonlinearity. Depending on the type of nonlinearity, the coherent backscattering interference is either reduced or enhanced, as compared to the linear case.

Backscatter Compensation in IFOG Based Inertial Measurement Units With Polymer Phase Modulators

Precision guidance in navigation systems requires highly accurate, compact, and low cost inertial measurement units (IMUs). The key active guided-wave component of the IMU is the phase modulator. In our approach, electro-optic polymers have been utilized in fabricating low loss phase modulators with low half-wave drive voltage using advanced hybrid waveguide fabrication processes and novel optical integration techniques. However, the interference between the primary wave and the backscatter waves generated by the phase modulator and/or the interference between the two counter-propagating backscatter waves at the detector of the IMU has been a major source of error in this approach. A novel technique was introduced in assessing the error caused by backscatter and an offset waveguide design was developed to suppress the interference of backscatter light. The novel design not only preserved the miniaturization, but also improved the insertion loss with the use of a shorter waveguide. The gyro level tests performed with the backscatter compensated modulators showed about 5 times improvement of the average bias uncertainty over gyros integrated with a standard symmetric phase modulator.

Stimulated Brillouin backscattering and ion acoustic wave secondary instability

A study of the secondary instability of a finite-amplitude ion acoustic wave (IAW) affecting the saturation of stimulated Brillouin backscattering (SBS) of laser light in a plasma is presented. The secondary instability of the SBS IAW provides a nonlinear dissipation mechanism for the SBS IAW and can reduce the SBS reflectivity. To better understand the physics of the secondary instability and SBS, particle-in-cell kinetic simulations, analysis of dispersion relations, and integration of coupled mode equations have been undertaken and compared. Among the effects examined are the influences on the secondary instability of the frequency of the primary IAW, the second-harmonic IAW, and ion trapping in the primary SBS IAW which affects Landau damping and the IAW frequency.

A two-way paraxial system for simulation of wave backscattering by a random medium

This paper presents a probabilistic analysis of an iterative two-way paraxial scheme for the simulation of wave propagation in anisotropic random media. This scheme has the computational cost of the standard one-way paraxial wave equation but has the accuracy of the full wave equation in a regime beyond the classical paraxial regime. More precisely, it accurately predicts the statistics of the reflected wave field. The accuracy depends on two parameters: the order of the iterative scheme and the ratio of the random backscattering intensity over the random forward-scattering intensity.

The mechanism of backscattering of electromagnetic waves by a pine forest in the meter wave band

A synthetic-aperture multifrequency polarimetric radar complex is used for experiments on studying the features of reflection of electromagnetic waves from pine forests. It is found that the intensity of the reflected power is proportional to the biometric characteristics of the forests. The hydrology and spatial fluctuations of the dielectric characteristics of the upper layer of the forest soil are studied at different depths. It is shown that spatial fluctuations of the soil’s dielectric characteristics can be used to explain the features of reflection from the forest. Reflection from a forest in the meter wave band at horizontal (HH) and vertical (VV) polarizations is simulated. It is found that mechanisms of reflection from the forest may be different for the HH and VV polarizations. Experimental data confirming the presence of specific effects in the radar images obtained in the meter wave band at the HH and VV polarizations are presented.

Velocity and turbulence measurements by ultrasound pulse Doppler velocimetry

This paper describes a velocimeter, based on back-scattering of ultrasonic waves by particles, designed for velocity measurement of suspended sediments within the flow of sewer systems. The ultrasound pulse Doppler velocimeter (UPDV) needs no calibration and is therefore a potentially useful tool for measuring velocities in laboratory experiments or in sewer systems. The potential of the developed system to determine the velocity in turbulent pipe flow was investigated. Two different approaches were used to estimate velocity: the well-known temporal method, i.e., the pulse–pair technique and the spectral identification based method, developed by our research group. Measurements have demonstrated the ability of the apparatus, on the one hand, to measure unsteady turbulent velocities, and on the other hand, to investigate experimentally the statistical properties of homogeneous and isotropic turbulence. A good agreement between experimental results and theory was observed. The validation of these laboratory measurements permits to extrapolate them to sewer systems.

Isolation of backscattering resonances of a thin spherical shell using iterative time reversal

The backscattering spectrum of thin spherical shells show resonance peaks due to several different physical processes including high frequency Lamb wave excitation and low frequency modal ringing [Kaduchak et al., J. Acoust. Soc. Am. 97, 2699‐2708 (1995)]. These different processes can be isolated in both time and frequency by using simulated iterative time reversal. This is accomplished by windowing in the time domain and/or filtering in the frequency domain. Iterative time reversal techniques developed for buried target detection [Waters et al., J. Acoust. Soc. Am. 122, 3023 (2007)] are applied to the partial wave series solution for the backscattering of plane acoustic waves by a thin‐walled spherical shell. Tank experiments are performed to verify theoretical results. Targets loaded with both water and sediment are considered. Understanding the relationship between the time reversal window's size/position and the dominant target scattering mechanisms is key to the development of time reversal as a detection and identification technique. [Work supported by The Office of Naval Research.]

Backscattering and reflection of acoustic waves in the stable atmospheric boundary layer

Backscattering and reflection of acoustic waves in the stable atmospheric boundary layer

Backscattering and reflection of acoustic waves in the stable atmospheric boundary layer

Among different applications of sodars in studies of the atmospheric boundary layer (ABL), the remote sensing of the temperature structure parameter CT2 and the closely associated structure parameter of refraction index Cn2 occupies an important place. These parameters are calculated from sodar return signal with the help of the theory of Bragg volumetric scattering of sound by small-scale turbulent irregularities of temperature. According to the Kolmogorov-Oboukhov conception, the irregularities are assumed to be locally homogeneous and isotropic. The theory was confirmed by experiments and universally recognized as a classical one. However, detailed comparisons are still being made between CT2 values derived from sodar data and from aircraft and meteorological-tower measurements. The results of such comparisons performed by different researchers under different conditions contradict to each other. In this paper, the results of comparisons between remote and in situ measurements of CT2 in the ABL are briefly reviewed. Some examples of a systematic overestimation of CT2 calculated from sodar data obtained under stable stratification of the ABL are given. Technical and physical causes of these discrepancies are discussed. The following factors are considered as possible physical factors: contribution of a partial (Fresnel) reflection of sound from layers with a sharp temperature jump, anisotropy of small-scale turbulence, intermittency, and contribution of quasi-wave air-density inhomogeneities. It is shown that these factors can explain some short-lived increase of the sodar return signal, but not a systematic overestimation of CT2. The role of sound-beam refraction (which regularly occurs in the stable ABL due to strong air-temperature and wind-speed gradients) is discussed. Due to a refraction turn of sodar beam, the angle of scattering changes, and scattering by turbulent wind-speed irregularities could permanently contribute to return signal of a monostatic sodar.

Enhanced imaging from multiply scattered waves

Many imaging methods involve probing a material with a wave and observing the back-scattered wave. The back-scattered wave measurements are used to compute an image of the internal structure of the material. Many of the conventional methods make the assumption that the wave has scattered just once from the region to be imaged before returning to the sensor to be recorded. The purpose of this paper is to show how this restriction can be partially removed and also how its removal leads to an enhanced image, free of the artifacts often associated with the conventionally reconstructed image.