bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-photon-absorption-induced nonlinear photoresponse in GaAs∕AlGaAs quantum-well infrared photodetectors
Authors: Jiang J. , Fu Y. , Li Ning , Chen X. S. , Zhen H. L. , Lu W. , Wang M. K. , Yang X. P. , Wu G. , Fan Y. H. , Li Y. G.
Published: October 2004

Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.
Target diagnostics for the future AWE Orion laser facility
Authors: Oades Kevin , Evans Andrew , Slark Gary , Foster John , Eagleton Richard , Clark Eugene
Published: October 2004

The Atomic Weapons Establishment has proposed building a new laser facility in the United Kingdom. This will use 10 ns-class beams in conjunction with two, subpicosecond, petawatt-class beams to access plasma conditions inaccessible to even the largest megajoule-class facilities. Diagnostic techniques for the long pulse regime are fairly mature, whereas techniques in the short pulse regime are still evolving. This article describes the development of a suite of target diagnostics to exploit the high temperature, high density plasma conditions that will be achievable on the Orion laser, and discusses some of the opportunities and problems that will be encountered in attempting to combine the two sets of techniques.
Multi-tier approach for data acquisition programming in the TJ-II remote participation system
Authors: Vega J. , Sánchez E. , Portas A. , Ruiz M. , Barrera E. , López S.
Published: October 2004

Programming software to setup acquisition channels during device operation has been developed for the TJ-II remote participation system. The software follows a three-tier model. A first tier (client tier) groups client software containing only user interface code. A second tier (middle tier) includes code for authorization, authentication, and query processing. A third tier (data tier) consists of a relational database server for managing configurations. Multi-platform characteristics are provided by web browsers (client tier) and web servers (middle tier). This architecture avoids that data acquisition system controllers provide access control, database support, or graphic user interface resources. Therefore, computation capabilities of these systems can mainly be devoted to data handling. LabView (from National Instruments) has been used as programming language in the acquisition systems. This design allows a very transparent management of signals, independently on hardware modules and systems.
Ponderomotive barrier as a Maxwell demon
Authors: Dodin I. Y. , Fisch N. J. , Rax J. M.
Published: October 2004

The possibility of efficient ponderomotive current drive in a magnetized plasma was reported recently in [N. J. Fisch, J. M. Rax, and I. Y. Dodin, Phys. Rev. Lett. 91, 205004 (2003)]. Precise limitations on the efficiency are now given through a comprehensive analytical and numerical study of single-particle dynamics under the action of a cyclotron-resonant rf drive in various field configurations. Expressions for the particle energy gain and acceleration along the dc magnetic field are obtained. The fundamental correlation between the two effects is described. A second fundamental quantity, namely, the ratio of the potential barrier to the energy gain, can be changed by altering the field configuration. The asymmetric ponderomotive current drive effect can be optimized, by minimizing the transverse heating.
Integrating diagnostic data analysis for W7-AS using Bayesian graphical models
Authors: Svensson J. , Dinklage A. , Geiger J. , Werner A. , Fischer R
Published: October 2004

Analysis of diagnostic data in fusion experiments is usually dealt with separately for each diagnostic, in spite of the existence of a large number of interdependencies between global physics parameters and measurements from different diagnostics. In this article, we demonstrate an integrated data analysis model, applied to the W7-AS stellarator, where diagnostic interdependencies have been modeled in a novel way by using so called Bayesian graphical models. A Thomson scattering system, interferometer, diamagnetic loop, and neutral particle analyzer are combined with an equilibrium reconstruction, forming together one single model for the determination of quantities such as density and temperature profiles, directly in magnetic coordinates. The magnetic coordinate transformation is itself inferred from the measurements. Influence of both statistical and systematic uncertainties on quantities from equilibrium calculations, such as position of flux surfaces, can therefore be readily estimated together with uncertainties of profile estimates. The model allows for modular addition of further diagnostics. A software architecture for such integrated analysis where possibly large number of diagnostic and theoretical codes need to be combined, will also be discussed.
Development of an integrated pellet diagnostic system for fueling pellets
Authors: Kubota Y. , Yoshikawa M. , Nakashima Y. , Yamada H. , Sakamoto R.
Published: October 2004

We developed a new diagnostic system of the hydrogen ice pellets for fueling into plasma devices. The system consists of three parts. They are: a light gate system, a microwave mass detector, and a shadowgraph system. This system provides the pellet velocity, mass and shape, simultaneously. In particular, by using multiple mirror system, the light gate system can achieve high detective probability with high signal to noise ratio for submillimeter pellets. At the results of the bench tests carried out on GAMMA 10 and the Large Helical Device, the light gate system completely detected the pellets used in the bench tests and the pellet velocity was estimated by using this system. Similarly, the shadowgraph system was available for the pellet. The microwave mass detector was found to be applicable. These results show the capability of simultaneous measurement of the pellet parameters using this diagnostic system.
PCI data acquisition and signal processing hardware modules for long pulse operation
Authors: Sousa J. , Batista A. J. N. , Combo A. , Pereira R. , Correia Miguel , Cruz N. , Carvalho P. , Correia Carlos , Varandas C. A. F.
Published: October 2004

A set of PCI instrumentation modules was developed at the EURATOM/IST Association. The modules were engineered around a reconfigurable hardware core which permits one to reduce the development time of instrument for new applications, provide support for long time or even continuous operation, and is able to perform real-time digital signal processing. The core was engineered at low cost and the modules incorporate a high number of channels, which contribute to reduce the total cost per channel. Field results are as expected in terms of performance both in data throughput and input characteristics. Currently, a 2 MSPS, 14-bit, eight channel galvanic isolated transient recorder; a 200 MSPS, 8-bit, four channel pulse digitizer; an eight channel time-to-digital-converter with a resolution of 0.4 ns, and a reconfigurable hardware expandable board, are implemented.
On the use of MHD mode analysis as a technique for determination of q-profiles in JET plasmas
Authors: Nave M. F. F. , Borba D. , Galvão R. , Hacquin S. , Alper B. , Challis C. , Gerasimov S. , Hawkes N. , Mailloux J. , Sharapov S. , Boswell C. , Brix M. , Joffrin E. , de la Luna E. , Smeulders P. ,   EFDA-JET Contributors
Published: October 2004

JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis, and determination of mode radial position has the potential as a diagnostic for the plasma q profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pickup signals. Cross correlation of magnetic and multiarray signals from ECE, SXR, and reflectometer systems provide the mode localization. The present status of the diagnostic systems used for fluctuation analysis is summarized, with particular emphasis on the fast magnetic pickup system. A new data collection system allows the magnetic signals to be recorded at 2 MHz for up to 32 s in each pulse. An example of q-profile validation from the observation of Alfven eigenmode cascades and a snake is given. At JET, Alfven cascades are used to determine qmin in optimized shear discharges, while a variety of low frequency modes (such as the snake) allow the determination of the radial position of q integer values in monotonic and reversed shear q profiles.
Hopf maps as static solutions of the complex eikonal equation
Authors: Adam C.
Published: October 2004

We demonstrate that a class of torus-shaped Hopf maps with arbitrary linking number obeys the static complex eikonal equation. Further, we explore the geometric structure behind these solutions, explaining thereby the reason for their existence. As this equation shows up as an integrability condition in certain nonlinear field theories, the existence of such solutions is of some interest.
On perturbations of Dirac operators with variable magnetic field of constant direction
Authors: Richard Serge , Tiedra de Aldecoa Rafael
Published: October 2004

We carry out the spectral analysis of matrix valued perturbations of three-dimensional Dirac operators with variable magnetic field of constant direction. Under suitable assumptions on the magnetic field and on the pertubations, we obtain a limiting absorption principle, we prove the absence of singular continuous spectrum in certain intervals and state properties of the point spectrum. Various situations, for example, when the magnetic field is constant, periodic or diverging at infinity, are covered. The importance of an internal-type operator (a two-dimensional Dirac operator) is also revealed in our study. The proofs rely on commutator methods.
Inhomogeneous quantum groups for particle algebras
Authors: Arik M. , Baykal A.
Published: October 2004

We construct the inhomogeneous quantum groups FIO(d) and BISp(2d) and investigate their Hopf algebra structure. FIO(d) and BISp(2d) leave the algebra of fermion and boson creation/annihilation operators invariant, respectively. We also present the corresponding R-matrices.
Ballooning mode stability in high β tokamaks with a q=1 magnetic island
Authors: Kleva Robert G. , Guzdar Parvez N.
Published: October 2004

The secondary stability of ballooning modes in a high β tokamak plasma with a q=1 magnetic island is calculated numerically. As the island grows, the temperature gradient around the X point of the island steepens. Resistive ballooning modes grow more rapidly in the presence of the island, but ballooning modes are unstable even in the absence of the island. When the safety factor at the magnetic axis q0=0.8 and the width of the island is equal to the radius of the q=1 flux surface, the ballooning mode growth rate is twice as large as the growth rate in the absence of the island. The connection length along a magnetic field line from the outside of the torus, where the magnetic curvature is unfavorable for stability, to the inside of the torus, where the curvature is favorable, is shorter when q is smaller. The shorter connection length at smaller q provides greater stability at q=1 than at larger q, and somewhat mitigates the effect of the steep temperature gradient generated at the X point of the magnetic island.
Ion-hose instability growth and saturation for counterstreaming electron and ion beams in an applied magnetic field
Authors: Rose D. V. , Genoni T. C. , Welch D. R.
Published: October 2004

The growth and saturation of the ion-hose instability in an applied axial magnetic field is examined. This dipole streaming instability is an important factor in determining the time-integrated electron beam spot size for high-power electron beam accelerators and diodes. Three-dimensional particle-in-cell simulations are used to characterize the growth and saturation of this instability for the case of counterstreaming electron and ion beams in an applied axial magnetic field. The simulation results are compared with a detailed spread-mass model. For the parameters considered here, the simulations show that the ion beam distribution evolves as the beam propagates, deviating significantly from the injected Gaussian profile. The combined modeling results elucidate the critical role of this evolving ion beam distribution on the saturation amplitude of the instability.
Asymptotic upper bounds for the entropy of orthogonal polynomials in the Szegő class
Authors: Beckermann B. , Martínez-Finkelshtein A. , Rakhmanov E. A. , Wielonsky F.
Published: October 2004

We give an asymptotic upper bound as n→∞ for the entropy integral, En(w)=−∫pn2(x)log(pn2(x))w(x)dx, where pn is the nth degree orthonormal polynomial with respect to a weight w(x) on [−1,1] which belongs to the Szegő class. We also study two functionals closely related to the entropy integral. First, their asymptotic behavior is completely described for weights w in the Bernstein class. Then, as for the entropy, we obtain asymptotic upper bounds for these two functionals when w(x) belongs to the Szegő class. In each case, we give conditions for these upper bounds to be attained.
A Wronskian of Jost solutions
Authors: Corona-Corona Gulmaro
Published: October 2004

Based on the standard fact that any matrix potential u=u(x) determines a family of Jost solutions whose parameter runs analytically (continuously) on the (closed) half planes, respectively, the zeros of a suitable matrix valued Wronskian of a Jost solution pair are explored.
Using the surface spontaneous depolarization field of ferroelectrics to direct the assembly of virus particles
Authors: Dunn Steve , Cullen Dave , Abad-Garcia Estefania , Bertoni Cristina , Carter Richard , Howorth Dave , Whatmore Roger W.
Published: October 2004

Tobacco mosaic virus (TMV) particles have been assembled at predetermined locations on the surface of a ferroelectric thin film with the composition PbZr0.3Ti0.7O3. The domain structure, hence the surface depolarization field for the ferroelectric associated with the bending of the ferroelectric band structure, was modified at length scales of around 2μm. A suspension of TMV particles was deposited onto the surface of the ferroelectric over the region where domain modification had been performed. SPM images (intermittent contact) revealed that the virus particles were attracted to regions of the surface that had either been positively modified or were nominally neutral. Regions of modified negative surface charge effectively repelled the virus particles.
Improved model for multipactor in low pressure gas
Authors: Udiljak R. , Anderson D. , Lisak M. , Semenov V. E. , Puech J.
Published: October 2004

The effect of electron-neutral collisions on the multipactor threshold in a low pressure gas is analyzed using a model that takes into account the friction force from the electron impacts with the neutrals, the thermalization of the electrons, as well as the contribution from impact ionization to the total number of electrons. It is found that the friction force is most important for first-order multipactor for materials with a low first cross-over energy and leads to an increased threshold. For higher-order modes with the same material, the thermalization effect dominates and results in a lowered threshold. The contribution to the total number of electrons from impact ionization for such materials is insignificant, but for materials with a high first cross-over energy this contribution dominates the behavior and a decreased threshold is obtained for all orders of multipactor.
Theory of three-wave parametric instabilities in inhomogeneous plasmas revisited
Authors: Short R. W. , Simon A.
Published: October 2004

The well-known theory of three-wave parametric instabilities in an inhomogeneous medium is reexamined. An analytic form is obtained for the Green’s function response to an initial δ-function pulse, which allows a more refined assessment of its properties and greater facility in employing it to model realistic initial conditions. Most significantly, it is found that physically realistic initial pulses behave quite differently than would be expected on the basis of the singular δ-function result, and more in accord with what would be expected from the theory of convective instabilities in homogeneous plasmas.
Compression and acceleration of dense electron bunches by ultraintense laser pulses with sharp rising edge
Authors: Kulagin Victor V. , Cherepenin Vladimir A. , Suk Hyyong
Published: October 2004

In this paper, the generation of a single ultrashort and coherent relativistic electron bunch (relativistic electron mirror) during interaction of an ultraintense femtosecond laser pulse having a sharp enough rising edge (nonadiabatic laser pulse) with a thin plasma layer is considered. It is shown that due to the action of the radiation reaction forces the Coulomb repulsion among the bunch electrons is partially compensated and the initial geometry of the bunch is supported in the acceleration process. Besides, the bunch can be compressed by many times in the longitudinal direction at the initial stage of interaction with the front of the nonadiabatic laser pulse. As a result, all of the bunch electrons can be synchronously accelerated to ultrarelativistic velocities during the first several half periods of the external electromagnetic field that can correspond to time intervals of hundreds of femtoseconds in the laboratory frame. The characteristics of the accelerated electron bunches for different laser-plasma parameters (shape of laser pulse, initial density of electrons in the target, and initial target thickness) are investigated. One possible application for relativistic electron mirrors is considered. It is shown that the reflection of a probe counter-propagating laser pulse from such a mirror can produce intense, ultrashort, and coherent hard x-ray pulse. The spectrum of the frequency up-shifted radiation is investigated.
Self-consistent determination of Rayleigh–Taylor growth rates and ablation-front density in planar targets accelerated by laser light
Authors: Smalyuk V. A. , Goncharov V. N. , Boehly T. R. , Knauer J. P. , Meyerhofer D. D. , Sangster T. C.
Published: October 2004

A technique to provide a self-consistent determination of Rayleigh–Taylor (RT) growth rates along with ablation-front density from measured optical-depth growth of preimposed, two-dimensional sinusoidal modulations is presented. The RT growth rates of ablation-front amplitude along with ablation-front density were determined using the optical-depth modulation ratios of the fundamental wavelength to the second-harmonic amplitudes.
Forced Couette flow simulations using direct simulation Monte Carlo method
Authors: Liou William W. , Fang Yichuan
Published: October 2004

Three-dimensional unsteady flows between two infinite walls are simulated by using the direct simulation Monte Carlo (DSMC) method. An artificial forcing that mimics the centrifugal force in the Taylor problem has been applied to the flow. The sampled behaviors of the resulting flow, including the long time average and the disturbance components, are studied. The computations have been preformed using parallel computer clusters. The results presented are for two different channel heights with various values for the forcing coefficient. The change in the channel height, which also results in changes in the flow Knudsen number and Reynolds number, affects the development of both the mean flows and the disturbances. Spatially coherent mean flow patterns, which are dominated by a hierarchy of harmonic modes, can be identified in the DSMC solutions. Temporally, the evolution of the Fourier amplitudes of the harmonic modes shows that these modes grow in a sequential manner. Disturbances with energy spectra that are significantly higher than the statistical noises are resolved. Their pathline patterns indicate that the disturbance flow fields are three dimensional and spatially coherent. These results suggest that the discrete DSMC approach is capable of capturing unsteady, three-dimensional flow disturbances that evolve around a stationary mean flow.
Microstability in a “MAST-like” high confinement mode spherical tokamak equilibrium
Authors: Applegate D. J. , Roach C. M. , Cowley S. C. , Dorland W. D. , Joiner N. , Akers R. J. , Conway N. J. , Field A. R. , Patel A. , Valovic M. , Walsh M. J.
Published: October 2004

Gyrokinetic microstability analyses, with and without electromagnetic effects, are presented for a spherical tokamak plasma equilibrium closely resembling that from a high confinement mode (H mode) discharge in the mega-ampere spherical tokamak (MAST) [A. Sykes et al., Nucl. Fusion 41, 1423 (2001)]. Electrostatic ion temperature gradient driven modes (ITG modes) were found to be unstable on all surfaces, though they are likely to be substantially stabilized by equilibrium E×B flow shear. Electron temperature gradient driven modes (ETG modes) have stronger growth rates that substantially exceed the equilibrium flow shearing rates. Mixing length arguments suggest that ITG modes would give rise to significant transport if they are not stabilized by sheared flows, and predict weak transport from ETG turbulence. Significant plasma flows have been neglected in this first analysis, and are probably important in the delicate balance between ITG growth rates and flow shear, and in the formation of internal transport barriers on MAST. Electromagnetic effects are found to be important even in this low β discharge, especially for longer length-scale modes with k⊥ρi<O(1) on the inner surfaces, where tearing parity modes are found to be the fastest growing modes, with growth rates that are sensitive to the electron collision frequency. These tearing parity microinstabilities are highly extended along the magnetic field, and have been reported in a number of spherical tokamak equilibria.
Role of particle size and gas pressure on the nonlinear oscillatory behavior of a dust particle in a direct current discharge
Authors: Park S. , Seon C. R. , Choe W.
Published: October 2004

Experimental and theoretical studies were conducted to simultaneously study the role of particle size and gas pressure on nonlinear behavior of dust oscillations in a plasma. Oscillation spectra were experimentally obtained by using four different sized (1.5, 2, 3, and 4 μm) particles at 250 mTorr, which is high pressure compared to previously reported works. The measured results were in good agreement with theoretical calculations based on a self-consistent collisional plasma model and a parametric dust oscillation model. In addition, particle size and gas pressure dependence of force profile and oscillation spectrum was investigated by numerical calculation in order to understand the role of particle size and pressure separately. It is concluded that occurrence of the subharmonic resonance and the net trapping force profile are mainly determined by particle size while gas pressure is mostly responsible for the superharmonic resonance and hysteresis.
Effect of nonuniform dust charge on dust-acoustic soliton in an inhomogeneous plasma
Authors: Li Yang-fang , Ma J. X. , Xiao De-long
Published: October 2004

The propagation of dust-acoustic solitons in an inhomogeneous plasma is studied for the case in which the equilibrium dust density is a constant but the equilibrium dust charge is spatially nonuniform and determined by the self-consistent charging equation. It is shown that the amplitude of the solitons is inversely proportional to the one-fourth power of the equilibrium dust charge number and only compressive solitons exist in this case. When the soliton propagates from high to low density regions, its amplitude and width increases with distance while its velocity shows complex variations. The spatial distribution of the relative amplitude of the dust density perturbation exhibits a transition from decrease to increase, which differs significantly from that of the potential perturbation.
Experimental evidence of parametric decay processes in the variable specific impulse magnetoplasma rocket (VASIMR) helicon plasma source
Authors: Boswell R. W. , Sutherland O. , Charles C. , Squire J. P. , Chang Díaz F. R. , Glover T. W. , Jacobson V. T. , Chavers D. G. , Bengtson R. D. , Bering E. A. , Goulding R. H. , Light M.
Published: October 2004

Decay waves have been observed in the megahertz range in the helium plasma generated by the variable specific impulse magnetoplasma rocket magnetoplasma thruster. They are measured using one of the tips of a triple probe connected to a 50 Ω input of a spectrum analyzer via a dc block (a small capacitor). The maximum amplitude of all waves is in the center of the plasma and does not appear correlated to the radial electron density or temperature profiles. The waves seem to be generated close to the helicon antenna that was 91 cm “upstream” from the measuring Langmuir probe. A possible explanation is parametric decay of the large amplitude helicon wave that also generates the plasma.
Non-null Lie quadratics in E3
Authors: Noakes Lyle
Published: October 2004

Interpolation problems in the space SO(3) of rotations of Euclidean 3-space E3 are reviewed in Secs. I s2 as background and motivation to a study of curves in E3 called Lie quadratics. Except for a special class called null, Lie quadratics have resisted analysis until now. The rest of the present paper is devoted to new results showing non-null Lie quadratics have rich analytical, geometrical, and asymptotic structures: rates of growth are studied using differential equations and inequalities, Lie quadratics are proved to be extendible over the whole of R, and existence of axes is proved under fairly general conditions. Examples show sharpness of many results.
q
-deformation of z→(αz+β)/(γz+δ)
Authors: Klimčík Ctirad
Published: October 2004

We construct the action of the quantum double of Uq(su(2)) on the standard Podleś sphere and interpret it as the quantum projective formula generalizing to the q-deformed setting the action of the Lorentz group of global conformal transformations on the ordinary Riemann sphere.
Upper quantum Lyapunov exponent and parametric oscillators
Authors: Jauslin H. R. , Sapin O. , Guérin S. , Wreszinski W. F.
Published: October 2004

We introduce a definition of upper Lyapunov exponent for quantum systems in the Heisenberg representation, and apply it to parametric quantum oscillators. We provide a simple proof that the upper quantum Lyapunov exponent ranges from zero to a positive value, as the parameters range from the classical system’s region of stability to the instability region. It is also proved that in the instability region the parametric quantum oscillator satisfies the discrete quantum Anosov relations defined by Emch, Narnhofer, Sewell, and Thirring.
Fluctuation statistics in networks: A stochastic path integral approach
Authors: Jordan Andrew N. , Sukhorukov Eugene V. , Pilgram Sebastian
Published: October 2004

We investigate the statistics of fluctuations in a classical stochastic network of nodes joined by connectors. The nodes carry generalized charge that may be randomly transferred from one node to another. Our goal is to find the time evolution of the probability distribution of charges in the network. The building blocks of our theoretical approach are (1) known probability distributions for the connector currents, (2) physical constraints such as local charge conservation, and (3) a time scale separation between the slow charge dynamics of the nodes and the fast current fluctuations of the connectors. We integrate out fast current fluctuations and derive a stochastic path integral representation of the evolution operator for the slow charges. The statistics of charge fluctuations may be found from the saddle-point approximation of the action. Once the probability distributions on the discrete network have been studied, the continuum limit is taken to obtain a statistical field theory. We find a correspondence between the diffusive field theory and a Langevin equation with Gaussian noise sources, leading nevertheless to nontrivial fluctuation statistics. To complete our theory, we demonstrate that the cascade diagrammatics, recently introduced by Nagaev, naturally follows from the stochastic path integral. By generalizing the principle of minimal correlations, we extend the diagrammatics to calculate current correlation functions for an arbitrary network. One primary application of this formalism is that of full counting statistics (FCS), the motivation for why it was developed in the first place. We stress however, that the formalism is suitable for general classical stochastic problems as an alternative approach to the traditional master equation or Doi–Peliti technique. The formalism is illustrated with several examples: Both instantaneous and time averaged charge fluctuation statistics in a mesoscopic chaotic cavity, as well as the FCS and new results for a generalized diffusive wire.
Phonon-polaritons in quasiperiodic piezoelectric superlattices
Authors: Zhang Xue-jin , Lu Yan-qing , Zhu Yong-yuan , Chen Yan-feng , Zhu Shi-ning
Published: October 2004

Phonon-polaritons are studied both theoretically and experimentally in a one-dimensional two-component generalized quasiperiodic piezoelectric superlattice. The experimental observation of phonon-polaritons through dielectric abnormality is carried out at the microwave region. Some potential applications are discussed.
Current scaling of radiated power for 40-mm diameter single wire arrays on Z
Authors: Nash T. J. , Cuneo M. E. , Spielman R. B. , Chandler G. A. , Leeper R. J. , Seaman J. F. , McGurn J. , Lazier S. , Torres J. , Jobe D. , Gilliland T. , Nielsen D. , Hawn R. , Bailey J. E. , Lake P. , Carlson A. L. , Seamen H. , Moore T. , Smelser R. , Pyle J. , Wagoner T. C. , LePell P. D. , Deeney C. , Douglas M. R. , McDaniel D. , Struve K. , Mazarakis M. , Stygar W. A.
Published: October 2004

In order to estimate the radiated power that can be expected from the next-generation Z-pinch driver such as ZR at 28 MA, current-scaling experiments have been conducted on the 20 MA driver Z. We report on the current scaling of single 40 mm diameter tungsten 240 wire arrays with a fixed 110 ns implosion time. The wire diameter is decreased in proportion to the load current. Reducing the charge voltage on the Marx banks reduces the load current. On one shot, firing only three of the four levels of the Z machine further reduced the load current. The radiated energy scaled as the current squared as expected but the radiated power scaled as the current to the 3.52±0.42 power due to increased x-ray pulse width at lower current. As the current is reduced, the rise time of the x-ray pulse increases and at the lowest current value of 10.4 MA, a shoulder appears on the leading edge of the x-ray pulse. In order to determine the nature of the plasma producing the leading edge of the x-ray pulse at low currents further shots were taken with an on-axis aperture to view on-axis precursor plasma. This aperture appeared to perturb the pinch in a favorable manner such that with the aperture in place there was no leading edge to the x-ray pulses at lower currents and the radiated power scaled as the current squared ±0.75. For a full-current shot we will present x-ray images that show precursor plasma emitting on-axis 77 ns before the main x-ray burst.
Ellipticity criterion for flowing two-fluid high β equilibria
Authors: Ishida Akio , Harahap Caesar O. , Steinhauer Loren C. , Peng Y.-K. Martin
Published: October 2004

An improved formalism for flowing two-fluid equilibria with nonuniform density is developed in the absence of dissipation. The system for axisymmetric equilibria can be expressed using second-order partial differential equations for the stream functions of the generalized vorticities of the electron and ion fluids. It is found that the equilibrium equations are elliptic when the poloidal ion flow speed is less than the sound speed. This property differs markedly from the single-fluid model. The two-fluid result is compared with that for the single-fluid model. It is important to determine if well-confined flowing plasmas satisfy this ellipiticity criterion throughout the plasma. Measurements of plasma poloidal flow velocities in high β plasmas are therefore encouraged.
Noise-driven multistability in a plasma diode
Authors: Seo J. T. , Pae K. H. , Hahn S. J.
Published: October 2004

Recently, a brand-new phenomenon of competing multistability in plasma diode systems has been observed by our group [S. J. Hahn and K. H. Pae, Phys. Plasmas 10, 314 (2003)]. By employing one-dimensional particle-in-cell simulations with proper control of the inherent numerical noise, the noise-driven multistability and dynamical transitions between regular and multistable solutions have been investigated. Throughout this work, it is verified that the competing multistability is, in principle, a noise-driven dynamical switch between attractors and occurs due to noise fluctuations inside a system near the fractal basin boundaries.
Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor
Authors: Fujikake Hideo , Murashige Takeshi , Sugibayashi Makiko , Ohta Kazuchika
Published: October 2004

We used a time-of-flight method to study the charge carrier mobility properties of a molecular-aligned discotic liquid crystal semiconductor based on Cu-phthalocyanine. The heated isotropic-phase semiconductor material was sandwiched between transparent electrodes coated onto glass substrates without conventional alignment layers. This was then cooled, and a discotic liquid crystal semiconductor cell was obtained, which we used to make mobility measurements. The material had a fixed molecular alignment due to the supercooling of the hexagonal columnar mesophase. It was clarified that the carrier mobility for electrons was as high as it was for holes at room temperature. The maximum value of negative charge mobility reached 2.60×10−3cm2∕Vs, although negative carrier mobility is often much lower than positive carrier mobility in other organic semiconductors, including conventional Cu-phthalocyanine vacuum-deposited films.
Gallium diffusion into self-assembled InAs quantum dots grown on indium phosphide substrates
Authors: Raz T. , Shuall N. , Bahir G. , Ritter D. , Gershoni D. , Chu S. N. G.
Published: October 2004

The photoluminescence spectrum of small self-assembled In(Ga)As quantum dots grown on InP substrates is composed of distinct spectral lines. These lines correspond to monolayer variations in the dots smallest dimension: their heights. We use this phenomenon in order to study the diffusion of gallium atoms into the self-assembled quantum dots. We demonstrate that substantial amounts of gallium atoms diffuse from a strained GaInP layer underneath the quantum dots into the quantum dots.
Optical properties of AlN and GaN in elevated temperatures
Authors: Nam K. B. , Li J. , Lin J. Y. , Jiang H. X.
Published: October 2004

Deep ultraviolet photoluminescence spectroscopy has been employed to study the optical transitions in AlN and GaN epilayers at temperatures from 10 to 800K, from which the parameters that describe the temperature variation of the energy band gap (α and β or aB and θ) and linewidth broadening have been obtained. These parameters are compared with the previously reported values in AlN and GaN obtained by different methods in narrower temperature ranges. Our experimental results demonstrate that the broader temperature range of measurements is necessary to obtain accurate values of these parameters, particularly for AlN. These results, together with other well-known physical properties of AlN, may expand future prospects for the application of III-nitride materials.
Temperature-dependent photovoltaic effects in the manganite-based heterojunction
Authors: Sun J. R. , Shen B. G. , Sheng Z. G. , Sun Y. P.
Published: October 2004

Strong temperature-dependent photovoltaic effects have been observed in the heterojunction composed of a La0.32Pr0.35Ca0.33MnO3 (LPCM) film and a 0.5 wt % Nb-doped SrTiO3 substrate. The photovoltage shows a monotonic increase with the decrease of temperature, and its relative change can be as large as ∼7000% for a modest light intensity of 20 mW (wavelength=632nm) when cooled from room temperature down to 17 K. The synchronous variation of photovoltage and the magnetization of LPCM indicates the magnetic origin of the temperature dependence of the photovoltaic effect. It is suggested that the temperature affects the photovoltaic effect by modifying the magnetic order, then the band structure of LPCM. An enhancement of the photovoltaic effect under strong light illumination is also observed, which is probably a result of illumination-induced change of the band structure of LPCM.
Prism coupling to on-chip silicon based bragg cladding waveguide
Authors: Yi Yasha , Sandland Jessica G. , Akiyama Shoji , Duan Xiaoman , Kimerling Lionel C.
Published: October 2004

We developed a silicon based asymmetric Bragg cladding waveguide, which is composed of high index contrast Si and Si3N4 clad layers and has omnidirectional reflectivity. Prism coupling was used to demonstrate the guiding mechanism by Bragg reflection. The effective index of the propagation mode was measured directly. The measured effective mode index is less than either the Si or Si3N4 cladding layers, which is a clear demonstration of the Bragg waveguiding principle. Low loss of the Si Bragg cladding waveguide around 0.5dB∕cm for both TE and TM polarizations is achieved. Potential applications include high power transmission, low dispersion, thin cladding thickness, and nonlinear properties engineering on silicon chip.
Experimental study of the influence of edge roughness on magnetization switching in Permalloy nanostructures
Authors: Bryan M. T. , Atkinson D. , Cowburn R. P.
Published: October 2004

Rectangular Permalloy nanostructures with different edge roughness, but the same average width, were fabricated by controlled variations in the exposure conditions of an electron-beam lithography process. The average widths that were studied ranged from 200 to 640 nm. Structures were characterized individually using magneto-optical Kerr effect single-structure nanomagnetometry and scanning electron microscopy on the same structure. Increased edge roughness caused increased coercivity, except for 200-nm-wide structures. Large edge roughness doubled the coercivity of wide (>600nm) structures. The coercivity of nominally identical structures (same roughness amplitude) differed by up to 20 Oe, suggesting that the specific edge profile influences magnetic switching.
Shape-and-solder-directed self-assembly to package semiconductor device segments
Authors: Zheng Wei , Jacobs Heiko O.
Published: October 2004

The self-assembly and packaging of integrated semiconductor device segments have been accomplished by combining geometrical shape recognition with site specific wetting and binding involving liquid solder. Components with complementary shapes were fabricated to recognize and encapsulate functional semiconductor devices. The components were suspended in water and agitated using a pulsating liquid flow. Two hundred AlGaN∕GaN light-emitting diodes with a chip size of 380×330 micrometers were assembled and packaged with a yield of 95% in 2min. The self-assembly procedure forms electrical interconnects between three-dimensionally shaped objects and provides a route to parallel assembly of hybrid microsystems.
Metal heterowaveguides for nanometric focusing of light
Authors: Wang Bing , Wang Guo Ping
Published: October 2004

Based on the different propagation characteristics of surface plasmon polaritons (SPPs) excited on different metallic surfaces and on the tendency of SPPs to the region in which SPPs possess lower phase velocity, we design a kind of metal heterowaveguide for nanometric focusing of light. Finite-difference time-domain simulations demonstrate that, by converting light into SPPs, such heterowaveguides constructed with both Ag and Al can focus incident light into about a 20nm×30nm (λ∕27×λ∕18≈0.002λ2) domain with higher than 15% coupling efficiency.
Kerr nonlinear effects in AlGaAs multimode waveguides
Authors: Magaña-Cervantes Marco A. , De La Rue Richard M. , Modotto Daniele , De Angelis Costantino , Morandotti Roberto , Linden Stefan , Mondia Jessica P. , van Driel Henry M. , Aitchison J. Stewart
Published: October 2004

We have experimentally investigated the linear and nonlinear characteristics of AlGaAs multimode waveguides. By varying the angle of incidence and∕or the intensity of the input beam, it is possible to change the multipeak intensity distributions at the output facet in a controllable fashion. Numerical simulations have reproduced the main features of the intensity profiles observed at the waveguide output over a range of intensities and angles.
Triple quantum dot charging rectifier
Authors: Vidan A. , Westervelt R. M. , Stopa M. , Hanson M. , Gossard A. C.
Published: October 2004

Three tunnel-coupled quantum dots in the Coulomb blockade regime act as a molecular rectifier. We have realized this device in a GaAs∕Al0.3Ga0.7As heterostructure containing a two-dimensional electron gas using lithographically patterned gates. The current through two tunnel-coupled dots in series is recorded versus applied voltage. Ratchet behavior is created by a third dot, with one lead, tunnel-coupled to one of the two dots. An electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the device. The current–voltage characteristics show rectification and negative resistance arising from charging of the third dot.
Thermophotovoltaic generation with selective radiators based on tungsten surface gratings
Authors: Sai Hitoshi , Yugami Hiroo
Published: October 2004

Two-dimensional surface-relief gratings with a period of 1.0–0.2μm composed of rectangular microcavities were fabricated on single crystalline W substrates to develop spectrally selective radiators for thermophotovoltaic generation. The radiators displayed strong emission in the near-infrared region where narrow-band-gap photovoltaic cells could convert photons into electricity. The enhancement of thermal emission was attributed to the microcavity effect. Power generation tests were carried out and the W gratings showed more than two times higher generation efficiency, compared to a SiC radiator. The results showed that the microstructured W radiators behave as good selective radiator, with both high efficiency and high power density.
Two-stream stability for a focusing charged particle beam
Authors: Genoni T. C. , Rose D. V. , Welch D. R. , Lee E. P.
Published: October 2004

The growth of the two-stream instability in one-dimensonal (1D) spherical geometry is analyzed using Laplace and Fourier transforms. An analytic expression for the asymptotic growth is obtained and compared to the well-known planar formula. Stronger linear growth is predicted for a converging beam than for its corresponding 1D planar counterpart at the same initial density. Calculations based on the analytic model are compared to 1D particle-in-cell simulations and excellent agreement is obtained.
Formation of highly transmissive liquid metal contacts to carbon nanotubes
Authors: Yazdanpanah Mehdi M. , Chakraborty S. , Harfenist S. A. , Cohn R. W. , Alphenaar B. W.
Published: October 2004

We have developed a method to produce liquid metal contacts to carbon nanotubes that allows direct measurement of the influence of the contact on the nanotube conductance. Gallium is deposited onto standard gold nanotube contacts, where it gradually spreads to coat the contact region. The two-terminal multiwall nanotube conductance increases by as much as 1.2e2∕h during the transition from gold to gallium contacts, and approaches 2e2∕h at room temperature, with a current density of 2×108A∕cm2. Surprisingly, the conductance is independent of the contact area or contact separation, providing evidence that transport is ballistic in multiwall nanotubes.
Ultrafast-pulse diagnostic using third-order frequency-resolved optical gating in organic films
Authors: Ramos-Ortiz G. , Cha M. , Thayumanavan S. , Mendez J. , Marder S. R. , Kippelen B.
Published: October 2004

We report on the diagnostic of ultrafast pulses by frequency-resolved optical gating (FROG) based on strong third-harmonic generation (THG) in amorphous organic thin films. The high THG conversion efficiency of these films allows for the characterization of sub-nanojoule short pulses emitting at telecommunication wavelengths using a low cost portable fiber spectrometer.
Integrated optical waveguides with liquid cores
Authors: Yin D. , Deamer D. W. , Schmidt H. , Barber J. P. , Hawkins A. R.
Published: October 2004

We report the design, fabrication, and demonstration of single-mode integrated optical waveguides with liquid cores. The principle of the device is based on antiresonant reflecting optical (ARROW) waveguides with hollow cores. We describe design principles for waveguide loss optimization down to 0.1∕cm. Using a fabrication process based on conventional silicon microfabrication and sacrificial core layers, waveguides of varying widths and lengths with volumes covering the pico- to nanoliter range were fabricated. We observe confined mode propagation, measure waveguide losses of 2.4∕cm, and demonstrate that the waveguides possess tailorable wavelength selectivity. The potential for highly integrated, sensitive devices based on these properties of the ARROW waveguides is discussed.
Third-order optical nonlinearities of carbon nanotubes in the femtosecond regime
Authors: Lauret J-S. , Voisin C. , Cassabois G. , Tignon J. , Delalande C. , Roussignol Ph. , Jost O. , Capes L.
Published: October 2004

Femtosecond pump–probe experiments have been carried out on an ensemble of single-wall carbon nanotubes deposited on a glass substrate. Measurements of transient changes of transmission and reflection provide an estimate of the real and imaginary parts of the second-order hyperpolarizability of carbon nanotubes. These values are compared with previous measurements and are discussed in the light of a simple model of the optical nonlinearities near the optical band-gap.
Ultrafast optical switching at the telecommunication wavelength by single electron transfer between fluorene-containing redox polymer and its counter anion
Authors: Nagamura Toshihiko , Nagai Yasuyuki , Furube Akihiro , Murata Shigeo
Published: October 2004

Ultrafast and large absorption change in the optical telecommunication wavelength region was achieved in solid films by making transitory expansion of a π-electronic system with photoinduced electron transfer and reverse reactions between redox-active ion pairs. Femtosecond dynamics of absorption changes at about 500–1550 nm due to single electron transfer was studied for a poly(tetramethyleneoxide) film containing 2,7-bis(4-pyridiniumyl)fluorene unit as part of the main chain. It showed ON response in less than 100 fs and fastest OFF response in 240 fs at the telecommunication wavelength. This polymer film may be applied to tetrahertz all optical information processing in the telecommunication system.