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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Focus: Phase-resolved nonlinear terahertz spectroscopy—From charge dynamics in solids to molecular excitations in liquids

Intense terahertz (THz) electric field transients with amplitudes up to several megavolts/centimeter and novel multidimensional techniques are the key ingredients of nonlinear THz spectroscopy, a new area of basic research. Both nonlinear light-matter interactions including the non-perturbative regime and THz driven charge transport give new insight into the character and dynamics of low-energy excitations of condensed matter and into quantum kinetic phenomena. This article provides an overview of recent progress in this field, combining an account of technological developments with selected prototype results for liquids and solids. The potential of nonlinear THz methods for future studies of low-frequency excitations of condensed-phase molecular systems is discussed as well.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Analysis of linear and nonlinear conductivity of plasma-like systems on the basis of the Fokker-Planck equation

The problems of high linear conductivity in an electric field, as well as nonlinear conductivity, are considered for plasma-like systems. First, we recall several observations of nonlinear fast charge transport in dusty plasma, molecular chains, lattices, conducting polymers, and semiconductor layers. Exploring the role of noise we introduce the generalized Fokker-Planck equation. Second, one-dimensional models are considered on the basis of the Fokker-Planck equation with active and passive velocity-dependent friction including an external electrical field. On this basis, it is possible to find the linear and nonlinear conductivities for electrons and other charged particles in a homogeneous external field. It is shown that the velocity dependence of the friction coefficient can lead to an essential increase of the electron average velocity and the corresponding conductivity in comparison with the usual model of constant friction, which is described by the Drude-type conductivity. Applications including novel forms of controlled charge transfer and non-Ohmic conductance are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

A nonlinear analysis of the terahertz serpentine waveguide traveling-wave amplifier

A nonlinear model for the numerical simulation of terahertz serpentine waveguide traveling-wave tube (SW-TWT) is described. In this model, the electromagnetic wave transmission in the SW is represented as an infinite set of space harmonics to interact with an electron beam. Analytical expressions for axial electric fields in axisymmetric interaction gaps of SW-TWTs are derived and compared with the results from CST simulation. The continuous beam is treated as discrete macro-particles with different initial phases. The beam-tunnel field equations, space-charge field equations, and motion equations are combined to solve the beam-wave interaction. The influence of backward wave and relativistic effect is also considered in the series of equations. The nonlinear model is used to design a 340 GHz SW-TWT. Several favorable comparisons of model predictions with results from a 3-D Particle-in-cell simulation code CHIPIC are presented, in which the output power versus beam voltage and interaction periods are illustrated. The relative error of the predicted output power is less than 15% in the 3 dB bandwidth and the relative error of the saturated length is less than 8%.The results show that the 1-D nonlinear analysis model is appropriate to solve the terahertz SW-TWT operation characteristics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonlinear absorption coefficient of pulsed laser deposited MgZnO thin film

We report the imaginary part of 3{sup rd} order nonlinear susceptibility and the nonlinear absorption coefficient of Mg doped ZnO thin film using standard Z-scan technique. The origin of nonlinear absorption is attributed to the two photon absorption followed by the free carrier absorption because of the presence of oxygen vacancy defects. We have also confirmed the experimental results with the theoretical results obtained by considering the steady state response of a two level atom with the monochromatic field models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Nonlinear regime of electrostatic waves propagation in presence of electron-electron collisions

The effects are presented of including electron-electron collisions in self-consistent Eulerian simulations of electrostatic wave propagation in nonlinear regime. The electron-electron collisions are approximately modeled through the full three-dimensional Dougherty collisional operator [J. P. Dougherty, Phys. Fluids 7, 1788 (1964)]; this allows the elimination of unphysical byproducts due to reduced dimensionality in velocity space. The effects of non-zero collisionality are discussed in the nonlinear regime of the symmetric bump-on-tail instability and in the propagation of the so-called kinetic electrostatic electron nonlinear (KEEN) waves [T. W. Johnston et al., Phys. Plasmas 16, 042105 (2009)]. For both cases, it is shown how collisions work to destroy the phase-space structures created by particle trapping effects and to damp the wave amplitude, as the system returns to the thermal equilibrium. In particular, for the case of the KEEN waves, once collisions have smoothed out the trapped particle population which sustains the KEEN fluctuations, additional oscillations at the Langmuir frequency are observed on the fundamental electric field spectral component, whose amplitude decays in time at the usual collisionless linear Landau damping rate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Ion scale nonlinear interaction triggered by disparate scale electron temperature gradient mode

We have observed that the disparate scale nonlinear interactions between the high-frequency (∼0.4 MHz) electron temperature gradient (ETG) mode and the ion-scale low-frequency fluctuations (∼kHz) were enhanced when the amplitude of the ETG mode exceeded a certain threshold. The dynamics of nonlinear coupling between the ETG mode and the drift wave (DW) mode has already been reported [C. Moon, T. Kaneko, and R. Hatakeyama, Phys. Rev. Lett. (2013)]. Here, we have newly observed that another low-frequency fluctuation with f ≃ 3.6 kHz, i.e., the flute mode, was enhanced, corresponding to the saturation of the DW mode growth. Specifically, the bicoherence between the flute mode and the DW mode reaches a significant level when the ∇T{sub e}/T{sub e} strength exceeded 0.54 cm{sup −1}. Thus, it is shown that the ETG mode energy was transferred to the DW mode, and then the energy was ultimately transferred to the flute mode, which was triggered by the disparate scale nonlinear interactions between the ETG and ion-scale low-frequency modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonlinear photoacoustic spectroscopy of hemoglobin

As light intensity increases in photoacoustic imaging, the saturation of optical absorption and the temperature dependence of the thermal expansion coefficient result in a measurable nonlinear dependence of the photoacoustic (PA) signal on the excitation pulse fluence. Here, under controlled conditions, we investigate the intensity-dependent photoacoustic signals from oxygenated and deoxygenated hemoglobin at varied optical wavelengths and molecular concentrations. The wavelength and concentration dependencies of the nonlinear PA spectrum are found to be significantly greater in oxygenated hemoglobin than in deoxygenated hemoglobin. These effects are further influenced by the hemoglobin concentration. These nonlinear phenomena provide insights into applications of photoacoustics, such as measurements of average inter-molecular distances on a nm scale or with a tuned selection of wavelengths, a more accurate quantitative PA tomography.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Nonlinear evolution of stimulated Raman scattering near the quarter-critical density

Nonlinear evolution of stimulated Raman scattering (SRS) near the quarter-critical density is studied using one-dimensional (1D) and two-dimensional (2D) particle-in-cell simulations in homogeneous plasmas. In 1D configuration, with two-plasmon decay (TPD) instability excluded, the system evolves into two regimes distinguished by whether density cavities have been formed or not. At low temperatures, a cavity regime characterised by high absorption and low reflection, and at high temperatures nonlinear frequency shift regime due to particle trapping, are observed. Furthermore, a competition between SRS and TPD in 2D simulations reveals that the nonlinear SRS does play a significant role near the quarter-critical density, whose influences were mostly neglected before.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Simultaneous evaluation of acoustic nonlinearity parameter and attenuation coefficients using the finite amplitude method

A novel method to determine acoustic parameters involved in measuring the nonlinearity parameter of fluids or solids is proposed. The approach is based on the measurement of fundamental and second harmonic pressures with a calibrated receiver, and on a nonlinear least squares data-fitting to multi-Gaussian beam (MGB) equations which explicitly define the attenuation and diffraction effects in the quasilinear regime. Results obtained in water validate the proposed method. The choice of suitable source pressure is discussed with regard to the quasilinear approximation involved. The attenuation coefficients are also acquired in nonlinear regime and their relations are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Linear and nonlinear evolution of the ion resonance instability in cylindrical traps: A numerical study

Numerical experiments have been performed to investigate the linear and nonlinear dynamics, and energetics of the ion resonance instability in cylindrically confined nonneutral plasma. The instability is excited on a set of parametrically different unstable equilibria of a cylindrical nonneutral cloud, composed of electrons partially neutralized by a much heavier ion species of single ionization. A particle-in-cell code has been developed and employed to carry out these simulations. The results obtained from the initial exponential growth phase of the instability in these numerical experiments are in agreement with the linearised analytical model of the ion resonance instability. As the simulations delve much further in time beyond the exponential growth phase, very interesting nonlinear phenomena of the ion resonance instability are revealed, such as a process of simultaneous wave breaking of the excited poloidal mode on the ion cloud and pinching of the poloidal perturbations on the electron cloud. This simultaneous nonlinear dynamics of the two components is associated with an energy transfer process from the electrons to the ions. At later stages there is heating induced cross-field transport of the heavier ions and tearing across the pinches on the electron cloud followed by an inverse cascade of the torn sections.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonlinear optical conductivity of bilayer graphene with Rashba spin-orbit interaction in the terahertz regime

The effect of Rashba spin-orbit coupling on the nonlinear optical conductivity in a bilayer graphene is investigated. We demonstrate the very different role played by the Rashba term and interlayer hopping; in some cases, the two roles can be quite opposite. It is found that the Rashba term can either enhance or suppress the nonlinear effect in a bilayer graphene, depending on the strength of the interlayer hopping. For a weak interlayer hopping, the Rashba term can significantly enhance the nonlinear effect. An analytical result was derived, showing the interplay of the Rashba effect and the interlayer hopping effect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of Sn doping on nonlinear optical properties of quaternary Se-Sn-(Bi,Te) chalcogenide thin films

The aim of this work is to report the effect of Sn doping on the third order nonlinear optical properties of chalcogenide Se{sub 84-x}Te{sub 15}Bi{sub 1.0}Sn{sub x} thin films. Melt quenching technique has been used for the preparation of bulk chalcogenide glasses. Thin films of the studied composition are deposited on cleaned glass substrate by thermal evaporation technique. Optical band gap (E{sub g}) is calculated by using Tauc extrapolation method and is found to increase from 1.27 eV to 1.64 eV with the incorporation of Sn content. Stryland approach is utilized for the calculation of two photon absorption coefficient (β{sub 2}). The nonlinear refractive index (n{sub 2}) and third order susceptibility (χ{sup (3}) are calculated using Tichy and Ticha approach. The result shows that nonlinear refractive index (n{sub 2}) follows the same trend as that of linear refractive index (n). The values of n{sub 2} of studied composition as compared to pure silica are 1000-5000 times higher.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Nonlinear terahertz response of HgTe/CdTe quantum wells

Without breaking the topological order, HgTe/CdTe quantum wells can have two types of bulk band structure: direct gap type (type I) and indirect gap type (type II). We report that the strong nonlinear optical responses exist in both types of bulk states under a moderate electric field in the terahertz regime. Interestingly, for the type II band structure, the third order conductivity changes sign when chemical potentials lies below 10 meV due to the significant response of the hole excitation close to the bottom of conduction band. Negative nonlinear conductivities suggest that HgTe/CdTe quantum wells can find application in the gain medium of a laser for terahertz radiation. The thermal influences on nonlinear optical responses of HgTe/CdTe quantum wells are also studied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Nonlinear upper hybrid waves and the induced density irregularities

Upper hybrid waves are excited parametrically by the O-mode high-frequency heater waves in the ionospheric heating experiments. These waves grow to large amplitudes and self-induced density perturbations provide nonlinear feedback. The lower hybrid resonance modifies the nonlinear feedback driven by the ponderomotive force; the nonlinear equation governing the envelope of the upper hybrid waves is derived. Solutions in symmetric alternating functions, in non-alternating periodic functions, as well as in solitary functions are shown. The impact of lower hybrid resonance on the envelope of the upper hybrid waves is explored; the results show that both the spatial period and amplitude are enlarged. The average fluctuation level of induced density irregularities is also enhanced. In the soliton form, the induced density cavity is widened considerably.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Linear and nonlinear heavy ion-acoustic waves in a strongly coupled plasma

A theoretical study on the propagation of linear and nonlinear heavy ion-acoustic (HIA) waves in an unmagnetized, collisionless, strongly coupled plasma system has been carried out. The plasma system is assumed to contain adiabatic positively charged inertial heavy ion fluids, nonextensive distributed electrons, and Maxwellian light ions. The normal mode analysis is used to study the linear behaviour. On the other hand, the well-known reductive perturbation technique is used to derive the nonlinear dynamical equations, namely, Burgers equation and Korteweg-de Vries (K-dV) equation. They are also numerically analyzed in order to investigate the basic features of shock and solitary waves. The adiabatic effects on the HIA shock and solitary waves propagating in such a strongly coupled plasma are taken into account. It has been observed that the roles of the adiabatic positively charged heavy ions, nonextensivity of electrons, and other plasma parameters arised in this investigation have significantly modified the basic features (viz., polarity, amplitude, width, etc.) of the HIA solitary/shock waves. The findings of our results obtained from this theoretical investigation may be useful in understanding the linear as well as nonlinear phenomena associated with the HIA waves both in space and laboratory plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonlinear shearing modes approach to the diocotron instability of a planar electron strip

The nonlinear evolution of the diocotron instability of a planar electron strip is investigated analytically by means of the nonlinear shearing mode for the solution of the initial and boundary value problems. The method is based on the sheared spatial coordinates which account for the motion of electron flow in the electrostatic field of the unstable diocotron modes in addition to the unperturbed sheared motion of the electron flow on the transformed shear coordinates. The time evolutions are studied by the solution of the initial and boundary value problems. The obtained solutions for the perturbed electrostatic potential include two nonlinear effects—the effect of the distortion of the boundaries of the planar electron strip and the effect of the coupling of the sheared nonmodal diocotron modes. It was proved by a two-dimensional particle-in-cell simulation that the developed theory is valid as long as the distortion of the boundaries of the basic shear flow does not change the frequency and growth rate of the linear diocotron instability in the transformed coordinates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Nonlinear aspects of acoustic radiation force in biomedical applications

In the past decade acoustic radiation force (ARF) became a powerful tool in numerous biomedical applications. ARF from a focused ultrasound beam acts as a virtual “finger” for remote probing of internal anatomical structures and obtaining diagnostic information. This presentation deals with generation of shear waves by nonlinear focused beams. Albeit the ARF has intrinsically nonlinear origin, in most cases the primary ultrasonic wave was considered in the linear approximation. In this presentation, we consider the effects of nonlinearly distorted beams on generation of shear waves by such beams.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

GPU simulation of nonlinear propagation of dual band ultrasound pulse complexes

In a new method of ultrasound imaging, called SURF imaging, dual band pulse complexes composed of overlapping low frequency (LF) and high frequency (HF) pulses are transmitted, where the frequency ratio LF:HF ∼ 1 : 20, and the relative bandwidth of both pulses are ∼ 50 − 70%. The LF pulse length is hence ∼ 20 times the HF pulse length. The LF pulse is used to nonlinearly manipulate the material elasticity observed by the co-propagating HF pulse. This produces nonlinear interaction effects that give more information on the propagation of the pulse complex. Due to the large difference in frequency and pulse length between the LF and the HF pulses, we have developed a dual level simulation where the LF pulse propagation is first simulated independent of the HF pulse, using a temporal sampling frequency matched to the LF pulse. A separate equation for the HF pulse is developed, where the the presimulated LF pulse modifies the propagation velocity. The equations are adapted to parallel processing in a GPU, where nonlinear simulations of a typical HF beam of 10 MHz down to 40 mm is done in ∼ 2 secs in a standard GPU. This simulation is hence very useful for studying the manipulation effect of the LF pulse on the HF pulse.

42 ENGINEERING↗