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15 Mar 2011

Volume 109, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 109, 061301 (2011); http://dx.doi.org/10.1063/1.3552291 (56 pages)

S. Preu, G. H. Döhler, S. Malzer, L. J. Wang, and A. C. Gossard
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Tunable, continuous-wave Terahertz photomixer sources and applications

S. Preu, G. H. Döhler, S. Malzer, L. J. Wang, and A. C. Gossard

J. Appl. Phys. 109, 061301 (2011); http://dx.doi.org/10.1063/1.3552291 (56 pages) | Cited 9 times

Online Publication Date: 22 March 2011

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This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.
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84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
85.60.Dw Photodiodes; phototransistors; photoresistors
84.40.Ba Antennas: theory, components and accessories
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back to top Lasers, Optics, and Optoelectronics

Analysis of electronic structures of quantum dots using meshless Fourier transform k·p method

Qiuji Zhao and Ting Mei

J. Appl. Phys. 109, 063101 (2011); http://dx.doi.org/10.1063/1.3549686 (13 pages) | Cited 1 time

Online Publication Date: 16 March 2011

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We develop a complete Fourier transform k·p method and present its application for a theoretical investigation on electronic structures of quantum dots with consideration of the built-in strain effects. The Fourier transform technique is applied to the periodic position-dependent Hamiltonian, and a simple and neat expression of the Hamiltonian matrix in the Fourier domain is formulated due to the orthogonality of exponential functions. Spurious solutions can be avoided due to the truncation of high Fourier frequencies. A kinetic Hamiltonian matrix in momentum domain is formulated by entering the analytical Fourier transform of the quantum-dot shape function into the neat Hamiltonian matrix easily, which allows meshless numerical implementation. The formulation of strain Hamiltonian matrix is done by convolution of Fourier series of strain components and Fourier series of the quantum-dot shape functions. Therefore, an original Fourier transform-based k·p approach is developed by combining the kinetic Hamiltonian matrix and the strain Hamiltonian. This approach is adopted to study the dimension effect and strain effect on the ground states of electrons and holes of pyramidal quantum dots that are truncated to different heights. The ground-state energy variation shows that the electron state is the most sensitive to these effects and the strain effect on E1, LH1, and HH1 is more prominent for sharperquantum dots. This investigation shows that band mixing between the conduction band and valence band, and band mixing between heavy-hole and light-hole bands are reduced due to the strain effect, whereas this effect is more prominent for nontruncated pyramidal quantum dots due to the stress concentration. Among the three ground states, light-hole states are more weakly confined in the nonpyramidal quantum dot and shift to the tip of the pyramid due to the strain.
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73.21.La Quantum dots
73.20.At Surface states, band structure, electron density of states
71.15.-m Methods of electronic structure calculations

Principles and promise of Fabry–Perot resonators at terahertz frequencies

R. Braakman and G. A. Blake

J. Appl. Phys. 109, 063102 (2011); http://dx.doi.org/10.1063/1.3560771 (11 pages)

Online Publication Date: 17 March 2011

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Fabry–Perot resonators have tremendous potential to enhance the sensitivity of spectroscopic systems at terahertz (THz) frequencies. Increasing sensitivity will be of benefit in compensating for the relatively low power of current high resolution continuous wave THz radiation techniques, and to fully express the potential of THz spectroscopy as source power increases. Improved sensitivities, and thus scanning speeds, will allow detailed studies of the complex vibration-rotation-tunneling dynamics that large molecules show at THz wavelengths, and will be especially important in studying more elusive, transient species such as those present in planetary atmospheres and the interstellar medium. Coupling radiation into the cavity presents unique challenges at THz frequencies, however, meaning that the cavity configurations common in neighboring frequency domains cannot simply be translated. Instead, novel constructions are needed. Here we present a resonator design in which wire-grid polarizers serve as the input and output coupling mirrors. Using this configuration, Q-factors of a few times 105 are achieved near 0.3 THz. To aid future investigations, the parameter space that limits the quality of the cavity is explored and paths to improved performance highlighted. Lastly, the performance of polarizer cavity-based Fourier transform (FT) THz spectrometers is discussed, in particular those design optimizations that should allow for the construction of THz instrumentation that rivals and eventually surpasses the sensitivities achieved with modern FT-microwave cavity spectrometers.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
07.60.Rd Visible and ultraviolet spectrometers

Visible-light-driven photocatalysts: (La/Bi + N)-codoped NaNbO3 by first principles

Guodong Liu, Shulin Ji, Liangliang Yin, Guoping Xu, Guangtao Fei, and Changhui Ye

J. Appl. Phys. 109, 063103 (2011); http://dx.doi.org/10.1063/1.3554697 (6 pages) | Cited 1 time

Online Publication Date: 17 March 2011

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To improve the photocatalytic activity of NaNbO3 for water splitting, the bandgap and the band edges of NaNbO3 should be tailored to match the visible part of the solar spectrum and hydrogen and oxygen redox potentials. By analyzing the band structures of La/Bi-doped and (La/Bi + N)-codoped NaNbO3, we found that the pseudointermediate band (PIB) was formed in the bandgap in all the doped systems because of the orbital splitting of the Nb 4d induced by the dramatically enlarged O-Nb-O angles. The PIB could make the wide bandgap semiconductors absorb visible-light photons as long as it was degenerate or partially degenerate. Considering that the appropriate band edges and absorption properties, we believe that (La/Bi + N)-codoped NaNbO3 materials are promising photocatalysts for hydrogen production through water splitting under visible-light irradiation without other modifications.
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71.20.Nr Semiconductor compounds
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.50.-m Photochemistry
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Ha Other nonmetallic inorganics

Oxide phosphors for light upconversion; Yb3+ and Tm3+ co-doped Y2BaZnO5

Isabelle Etchart, Ignacio Hernández, Arnaud Huignard, Mathieu Bérard, Marine Laroche, William P. Gillin, Richard J. Curry, and Anthony K. Cheetham

J. Appl. Phys. 109, 063104 (2011); http://dx.doi.org/10.1063/1.3549634 (7 pages) | Cited 3 times

Online Publication Date: 18 March 2011

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The optical properties of Yb3+ and Tm3+ co-doped Y2BaZnO5, synthesized by solid-state reaction, are investigated in detail. Three main emission bands centered around 479 nm (blue), 654 nm (red), and 796 nm (near-infrared) are observed under near-infrared laser excitation via an upconversion process. Detailed studies of the upconversion properties as a function of dopant concentrations are described and upconversion efficiencies quantified precisely. Maximum efficiencies of ∼ 1.53% in the 730-870 nm near-infrared emission range and of ∼ 0.09% in the 420-530 nm blue range are obtained. The results of power dependence studies and concentration dependent lifetime measurements are presented. This in-depth spectroscopic study allows us, for the first time, to identify the dominant processes involved in the upconversion mechanism of Yb3+, Tm3+ co-doped Y2BaZnO5 oxides.
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78.55.Hx Other solid inorganic materials
78.30.Hv Other nonmetallic inorganics

Spherical metallic nanoparticle arrays for super-resolution imaging

Chang Chun Yan, Dao Hua Zhang, and Dong Dong Li

J. Appl. Phys. 109, 063105 (2011); http://dx.doi.org/10.1063/1.3553875 (5 pages)

Online Publication Date: 21 March 2011

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We report super-resolution imaging in a metamaterial system comprising spherical silver nanoparticle chain arrays, where each chain consists of nanoparticles with a smaller particle added to the end. Our simulations reveal that silver nanoparticale chains have subwavelength resolution capability at visible wavelengths and that the field intensity in the imaging plane varies with the number of layers of nanoparticles, their polarization, and their coupling. By adding a smaller nanoparticle at the end of each chain, the resolution capability is significantly enhanced, and high-quality super-resolution imaging can be realized for incident waves polarized along the chain direction.
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42.79.-e Optical elements, devices, and systems

InP/InAlGaAs light-emitting transistors and transistor lasers with a carbon-doped base layer

Yong Huang, Jae-Hyun Ryou, Russell D. Dupuis, Forest Dixon, Milton Feng, and Nick Holonyak, Jr.

J. Appl. Phys. 109, 063106 (2011); http://dx.doi.org/10.1063/1.3561368 (6 pages) | Cited 2 times

Online Publication Date: 21 March 2011

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Characteristics of InP/InAlGaAs light-emitting transistors (LETs) and transistor lasers (TLs) using carbon (C) for p-type doping of the base region were investigated. The N-InP/p-In0.52(Al0.4Ga0.6)0.48As/N-In0.52Al0.48As LETs show a current gain of 0.22 and light emission at wavelength of λ ∼ 1610 nm. The low current gain is attributed to the short minority carrier lifetime in the C-doped base with a quantum well. The TL demonstrates continuous-wave operation at −190 °C with a threshold current of IB = 35 mA. By comparing the optical output characteristics of the TL and a laser diode with similar structure, it is suggested that the low differential quantum efficiency and the high threshold current density in the TL is related to the strong inter-valence band absorption in the heavily doped base layer.
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85.60.Jb Light-emitting devices
85.60.Dw Photodiodes; phototransistors; photoresistors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
61.72.uj III-V and II-VI semiconductors
42.55.Px Semiconductor lasers; laser diodes

Stable optical-signal emitter based on a semiconductor photonic dot

Yongyou Zhang, Bingsuo Zou, and Guojun Jin

J. Appl. Phys. 109, 063107 (2011); http://dx.doi.org/10.1063/1.3559297 (6 pages)

Online Publication Date: 22 March 2011

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We propose a polariton hyperparametric oscillator (PHO) based on a semiconductor photonic dot at the micro/nano scale. By theoretical derivations and numerical calculations, we find that the PHO not only amplify weak signals like general large-planar polariton amplifiers, but also depress strong signals unusually. The coexistence of such signal amplification and depression can cause a stable signal emission being almost independent of the excitation instabilities in the strong-excitation regime. It has been verified that the instability of the signal emission, increasing with the increase of the excitation instabilities, is only about one to two percent deviation from its average intensity even under strong instable excitations. Hence, the PHO can serve as a stable optical-signal emitter in micro/nano optical systems.
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42.65.Yj Optical parametric oscillators and amplifiers
42.79.-e Optical elements, devices, and systems
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Linear and nonlinear optical absorption coefficients and refractive index changes in a two-electron quantum dot

Liangliang Lu, Wenfang Xie, and Hassan Hassanabadi

J. Appl. Phys. 109, 063108 (2011); http://dx.doi.org/10.1063/1.3560541 (5 pages) | Cited 4 times

Online Publication Date: 22 March 2011

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Analytical expressions of the optical absorption coefficients and refractive index changes in a two-electron quantum dot with oscillating and linear confining terms are obtained by using compact-density matrix approach and exact analytical method. Numerical results on typical GaAs/AlGaAs materials show that, an increase of the confinements blueshifts the peak positions of absorption coefficients and refractive index changes. Additionally, an increase of the optical intensity and relaxation time considerably changes the absorption coefficients as well as the refractive index changes.
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78.67.Hc Quantum dots
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Intrinsic optical bistability in a two-subband system in a semiconductor quantum well: Analytical results

Spyridon G. Kosionis, Andreas F. Terzis, Constantinos Simserides, and Emmanuel Paspalakis

J. Appl. Phys. 109, 063109 (2011); http://dx.doi.org/10.1063/1.3553871 (5 pages) | Cited 2 times

Online Publication Date: 24 March 2011

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We study theoretically the conditions under which optical bistability is achievable in a two-subband system in a semiconductor quantum well. We consider the interaction of the two-subband system with a continuous wave electromagnetic field, which induces intersubband transitions. For the description of the system dynamics we use the effective nonlinear density matrix equations. We solve these equations analytically, in the steady state, for a GaAs/AlGaAs quantum well structure. For several combinations of the values of the parameters three real solutions of the population inversion arise and the phenomenon of optical bistability prevails.
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78.67.De Quantum wells
81.05.Ea III-V semiconductors
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Extraction of physical parameters from photoacoustic spectroscopy using wavelet transform

Jianghua Li, Changshui Chen, Chujun Zheng, Songhao Liu, and Zhilie Tang

J. Appl. Phys. 109, 063110 (2011); http://dx.doi.org/10.1063/1.3554832 (6 pages)

Online Publication Date: 29 March 2011

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Photoacoustic (PA) signals carry much information related to the physical parameters of a sample, such as its optical, thermal, acoustic, and elastic properties. These physical parameters synchronously make their own contributions to PA signals. It is difficult to distinguish the contributions of different physical parameters to the mixed PA signals. To separate these parameters and to extract the featured information, we report herein a means of dissecting the transient PA signal using time-frequency analysis in a forward detection model. The simulated and experimental results indicate that the features of different physical parameters related to the sample can be qualitatively extracted from the mixed PA signals.
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78.20.Pa Photoacoustic effects
43.60.Hj Time-frequency signal processing, wavelets
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
back to top Plasmas and Electrical Discharges

Transient operation of the relativistic S-band magnetron with radial output

A. Sayapin and A. Shlapakovski

J. Appl. Phys. 109, 063301 (2011); http://dx.doi.org/10.1063/1.3553839 (5 pages)

Online Publication Date: 16 March 2011

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The influence of the time-varying accelerating voltage and electromagnetic waves reflected from the output window of the antenna on the operation of a relativistic S-band magnetron with radial output is analyzed. It is shown that within the range of radiation frequencies generated by the magnetron, the antenna with the dielectric window and feed waveguide can be considered as a resonant load. Depending on the phase relations between the reflected electromagnetic waves and those traveling in the magnetron, minimal coupling between the magnetron cavities adjacent to the place of microwave extraction can be realized. In this regime, the magnetron represents an open-end chain of coupled cavities and maintains the effective interaction of the electron flow and generated electromagnetic wave at a decreasing voltage. As the drift velocity of electrons decreases and its difference from the electromagnetic wave phase velocity increases, the microwave frequency changes. The changed frequencies do not necessarily belong to and are not limited by the discrete spectrum of closed resonant ring waveguide structures.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
41.20.Jb Electromagnetic wave propagation; radiowave propagation
84.40.Az Waveguides, transmission lines, striplines

Departures from local thermodynamic equilibrium in cutting arc plasmas derived from electron and gas density measurements using a two-wavelength quantitative Schlieren technique

L. Prevosto, G. Artana, H. Kelly, and B. Mancinelli

J. Appl. Phys. 109, 063302 (2011); http://dx.doi.org/10.1063/1.3552304 (6 pages)

Online Publication Date: 16 March 2011

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A two-wavelength quantitative Schlieren technique that allows inferring the electron and gas densities of axisymmetric arc plasmas without imposing any assumption regarding statistical equilibrium models is reported. This technique was applied to the study of local thermodynamic equilibrium (LTE) departures within the core of a 30 A high-energy density cutting arc. In order to derive the electron and heavy particle temperatures from the inferred density profiles, a generalized two-temperature Saha equation together with the plasma equation of state and the quasineutrality condition were employed. Factors such as arc fluctuations that influence the accuracy of the measurements and the validity of the assumptions used to derive the plasma species temperature were considered. Significant deviations from chemical equilibrium as well as kinetic equilibrium were found at elevated electron temperatures and gas densities toward the arc core edge. An electron temperature profile nearly constant through the arc core with a value of about 14000–15000 K, well decoupled from the heavy particle temperature of about 1500 K at the arc core edge, was inferred.
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52.25.Kn Thermodynamics of plasmas
52.25.Gj Fluctuation and chaos phenomena
52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Wireless communication capability of a reconfigurable plasma antenna

Rajneesh Kumar and Dhiraj Bora

J. Appl. Phys. 109, 063303 (2011); http://dx.doi.org/10.1063/1.3564937 (9 pages)

Online Publication Date: 21 March 2011

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A 30 cm long plasma column is excited by a surface wave, which acts as a plasma antenna. Using plasma properties (pattern formation/striations in plasmas) single plasma antenna can be transformed into array, helical, and spiral plasma antenna. Experiments are carried out to study the power patterns, directivity, and half power beam width of such different plasma antennas. Moreover, field properties of plasma and copper antenna are studied. Further, wireless communication and jamming capability of plasma antenna are tested. Findings of this study suggest that directivity and communication range can be increased by converting single plasma antenna in to array/helical/spiral plasma antenna. Field frequencies of plasma antenna determine the communication and jamming of radio frequency waves. Therefore, this study invokes applications of pattern formation or striations of plasmas in plasma antenna technology.
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52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.80.Pi High-frequency and RF discharges
52.77.-j Plasma applications
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

Magnetic control of breakdown: Toward energy-efficient hollow-cathode magnetron discharges

O. Baranov, M. Romanov, S. Kumar, X. X. Zhong, and K. Ostrikov

J. Appl. Phys. 109, 063304 (2011); http://dx.doi.org/10.1063/1.3553853 (8 pages) | Cited 1 time

Online Publication Date: 22 March 2011

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Show Abstract
Characteristics of electrical breakdown of a planar magnetron enhanced with an electromagnet and a hollow-cathode structure, are studied experimentally and numerically. At lower pressures the breakdown voltage shows a dependence on the applied magnetic field, and the voltage necessary to achieve the self-sustained discharge regime can be significantly reduced. At higher pressures, the dependence is less sensitive to the magnetic field magnitude and shows a tendency of increased breakdown voltage at the stronger magnetic fields. A model of the magnetron discharge breakdown is developed with the background gas pressure and the magnetic field used as parameters. The model describes the motion of electrons, which gain energy by passing the electric field across the magnetic field and undergo collisions with neutrals, thus generating new bulk electrons. The electrons are in turn accelerated in the electric field and effectively ionize a sufficient amount of neutrals to enable the discharge self-sustainment regime. The model is based on the assumption about the combined classical and near-wall mechanisms of electron conductivity across the magnetic field, and is consistent with the experimental results. The obtained results represent a significant advance toward energy-efficient multipurpose magnetron discharges.
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52.80.Pi High-frequency and RF discharges
52.25.Fi Transport properties
52.80.Hc Glow; corona

Propagation of terahertz waves in an atmospheric pressure microplasma with Epstein electron density profile

Chengxun Yuan, Zhongxiang Zhou, Jingwen W. Zhang, Xiaoli Xiang, Hongguo Sun, He Wang, and Yanwei Du

J. Appl. Phys. 109, 063305 (2011); http://dx.doi.org/10.1063/1.3561834 (6 pages)

Online Publication Date: 23 March 2011

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Propagation properties of terahertz (THz) waves in a bounded atmospheric-pressure microplasma (AMP) are analyzed in this study. A modified Epstein profile model is used to simulate the electron density distribution caused by the plasma sheaths. By introducing the dielectric constant of a Drude–Lorentz model and using the method of dividing the plasma into a series of subslabs with uniform electron density, the coefficients of power reflection, transmission, and absorption are derived for a bounded microplasma structure. The effects of size of microplasma, electron density profile, and collision frequency on the propagation of THz waves are analyzed numerically. The results indicate that the propagation of THz waves in AMPs depend greatly on the above three parameters. It is demonstrated that the THz wave can play an important role in AMPs diagnostics; meanwhile, the AMP can be used as a novel potential tool to control THz wave propagation.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.25.Mq Dielectric properties
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.40.Kh Plasma sheaths
52.40.Hf Plasma-material interactions; boundary layer effects

The effect of excitation wavelength on dynamics of laser-produced tin plasma

S. S. Harilal, T. Sizyuk, A. Hassanein, D. Campos, P. Hough, and V. Sizyuk

J. Appl. Phys. 109, 063306 (2011); http://dx.doi.org/10.1063/1.3562143 (9 pages) | Cited 10 times

Online Publication Date: 24 March 2011

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We investigated the effect of the excitation wavelength on the density evolution of laser-produced tin plasmas, both experimentally and numerically. For producing plasmas, Sn targets were excited with either 10.6 μm CO2 laser or 1.06 μm Nd:yttrium aluminum garnet laser; both are considered to be potential excitation lasers for extreme ultraviolet lithography laser-produced plasma light sources. The electron density of the plasma during the isothermal expansion regime was estimated using an interferometric technique. The Stark broadening of isolated singly-ionized emission was employed for deducing the density during the plasma adiabatic expansion regime. Our results indicate that the excitation source wavelength determines the initial density of the plasma, as well the plume expansion dynamics. Numerical simulation using HEIGHTS simulation package agrees well with the experimentally measured density profile.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.65.-y Plasma simulation
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.30.-q Plasma dynamics and flow

Combined experimental and modeling studies of microwave activated CH4/H2/Ar plasmas for microcrystalline, nanocrystalline, and ultrananocrystalline diamond deposition

James C. Richley, Oliver J. L. Fox, Michael N. R. Ashfold, and Yuri A. Mankelevich

J. Appl. Phys. 109, 063307 (2011); http://dx.doi.org/10.1063/1.3562185 (14 pages)

Online Publication Date: 25 March 2011

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A comprehensive study of microwave (MW) activated CH4/H2/Ar plasmas used for diamond chemical vapor deposition is reported, focusing particularly on the effects of gross variations in the H2/Ar ratio in the input gas mixture (from H2/Ar mole fraction ratios of > 10:1, through to ∼1:99). Absolute column densities of C2(a) and CH(X) radicals and of H(n = 2) atoms have been determined by cavity ringdown spectroscopy, as functions of height (z) above a substrate and of process conditions (CH4, H2, and Ar input mole fractions, total pressure, p, and input microwave power, P). Optical emission spectroscopy has also been used to explore the relative densities of electronically excited H atoms, and CH, C2, and C3 radicals, as functions of these same process conditions. These experimental data are complemented by extensive 2D (r, z) modeling of the plasma chemistry, which provides a quantitative rationale for all of the experimental observations. Progressive replacement of H2 by Ar (at constant p and P) leads to an expanded plasma volume. Under H2-rich conditions, > 90% of the input MW power is absorbed through rovibrational excitation of H2. Reducing the H2 content (as in an Ar-rich plasma) leads to a reduction in the absorbed power density; the plasma necessarily expands in order to accommodate a given input power. The average power density in an Ar-rich plasma is much lower than that in an H2-rich plasma operating at the same p and P. Progressive replacement of H2 by Ar is shown also to result in an increased electron temperature, an increased [H]/[H2] number density ratio, but little change in the maximum gas temperature in the plasma core (which is consistently ∼3000 K). Given the increased [H]/[H2] ratio, the fast H-shifting (CyHx + H ↔ CyHx−1 + H2; y = 1−3) reactions ensure that the core of Ar-rich plasma contains much higher relative abundances of “product” species like C atoms, and C2, and C3 radicals. The effects of Ar dilution on the absorbed power dissipation pathways and the various species concentrations just above the growing diamond film are also investigated and discussed.
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81.05.ug Diamond
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
52.77.Dq Plasma-based ion implantation and deposition
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Generation of an amorphous graphite substrate by cumulative deuterium bombardment using molecular dynamics with full nonbonded interactions

Jaime Marian

J. Appl. Phys. 109, 063501 (2011); http://dx.doi.org/10.1063/1.3553848 (4 pages)

Online Publication Date: 16 March 2011

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A deuterated amorphous carbon target is generated from a fresh graphite layer via hyperthermal irradiation with deuterium ions using molecular dynamics (MD) simulations. We use interatomic potentials that include nonbonded (long-range) interactions for maximum accuracy and simulate cumulative bombardment up to doses of 5.80 × 1016 ions cm−2 at 1000 K. The graphite target goes through several stages of erosion and swelling, leading to complete amorphization and significant density loss. The calculations show a transition from graphitelike hybridization to a mixture of diamondlike and linear hybridizations with dose. It is concluded that the current sample sizes obtained directly by cumulative irradiation affordable with MD are not sufficiently large to be used for sputtering calculations under steady-state conditions.
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61.80.Jh Ion radiation effects
61.82.Ms Insulators
61.43.Bn Structural modeling: serial-addition models, computer simulation
61.43.-j Disordered solids

A generalized 3ω method for extraction of thermal conductivity in thin films

Zhao-Xiang Zong, Zhi-Jun Qiu, Shi-Li Zhang, Reinhard Streiter, and Ran Liu

J. Appl. Phys. 109, 063502 (2011); http://dx.doi.org/10.1063/1.3559299 (8 pages)

Online Publication Date: 16 March 2011

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It has been found that in the low-frequency regime, the experimentally determined thermal conductivity of thin dielectric films deviates significantly from the theoretical expectation based on Cahill’s 3ω thermal model. It is shown in the present work that this deviation is mainly caused by the heat transport in the heater strip, which is neglected in Cahill’s one-dimensional model. By taking this mechanism into account, an analytical model is developed to simultaneously consider the heat transport in the heater strip and the heat flow into the underlying substrate. The validity of this two-dimensional model is confirmed by experiments using specially designed test structures as well as by numerical simulation. The results show that the heat transport along the heater strip, originated from a nonuniform temperature, becomes comparable to that in the substrate at low frequencies. This effect of a nonuniform temperature distribution can also be exploited for extraction of the thermal conductivity of the metallic strip itself.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
77.55.-g Dielectric thin films
68.60.Dv Thermal stability; thermal effects

Bulk diffusion induced structural modifications of carbon-transition metal nanocomposite films

M. Berndt, G. Abrasonis, Gy. J. Kovács, M. Krause, F. Munnik, R. Heller, A. Kolitsch, and W. Möller

J. Appl. Phys. 109, 063503 (2011); http://dx.doi.org/10.1063/1.3559302 (11 pages) | Cited 2 times

Online Publication Date: 16 March 2011

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The influence of transition metal (TM = V,Co,Cu) type on the bulk diffusion induced structural changes in carbon:TM nanocomposite films is investigated. The TMs have been incorporated into the carbon matrix via ion beam co-sputtering, and subsequently the films have been vacuum annealed in the temperature range of 300 – 700 °C. The structure of both the dispersed metal rich and the carbon matrix phases has been determined by a combination of elastic recoil detection analysis, x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The as-grown films consist of carbidic (V and Co) and metallic (Cu) nanoparticles dispersed in the carbon matrix. Thermal annealing induces surface segregation of Co and Cu starting at ≥ 500 °C, preceded by the carbide-metal transformation of Co-carbide nanoparticles at ∼ 300 °C. No considerable morphological changes occur in C:V films. In contrast to the surface diffusion dominated regime where all the metals enhance the six-fold ring clustering of C, in the bulk diffusion controlled regime only Co acts as a catalyst for the carbon graphitization. These results are consistent with the metal-induced crystallization mechanism in the C:Co films. The results are discussed on the basis of the metal-carbide phase stability, carbon solubility in metals or their carbides, and interface species.
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68.35.Fx Diffusion; interface formation
81.15.Cd Deposition by sputtering
68.35.Dv Composition, segregation; defects and impurities
81.15.Jj Ion and electron beam-assisted deposition; ion plating
85.40.Sz Deposition technology
68.55.at Other materials
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation

Twin interaction and large magnetoelasticity in Ni-Mn-Ga single crystals

L. Straka, H. Hänninen, N. Lanska, and A. Sozinov

J. Appl. Phys. 109, 063504 (2011); http://dx.doi.org/10.1063/1.3552292 (7 pages) | Cited 1 time

Online Publication Date: 16 March 2011

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We demonstrate experimentally the existence of triple twins in Ni-Mn-Ga magnetic shape memory single crystals with a modulated five-layered martensite structure using optical observations andx-ray diffraction. Subsequently, we investigate the response of the crystals with triple-twin segments to compressive loading up to several MPa. Such loading typically resulted in an abrupt rearrangement of the twin microstructure to a configuration with many fine twins (1–10 µm in size) ending at a twin boundary. This type of twin microstructure exhibited recoverable deformation with up to 0.3% macroscopic strain and an estimated 2.5% local strain, while the recoverable strain was much smaller for other studied microstructure configurations. The results indicate that by the creation of a suitable twin microstructure, the originally pseudoplastic or magnetoplastic material can be made rubberlike elastic or magnetoelastic with the macroscopic recoverable strain comparable to 2.5%.
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75.80.+q Magnetomechanical effects, magnetostriction
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.fg Shape-memory effect; yield stress; superelasticity
62.20.D- Elasticity
61.72.Mm Grain and twin boundaries

Mesogenic linear azobenzene polymer-stabilized nematic liquid crystals

Arutyun Bagramyan, Olivier Thibault-Maheu, Tigran Galstian, André Bessette, and Yue Zhao

J. Appl. Phys. 109, 063505 (2011); http://dx.doi.org/10.1063/1.3553854 (6 pages) | Cited 2 times

Online Publication Date: 16 March 2011

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We describe the detailed study of a polymer stabilized liquid crystal compound, which was created by using a reactive (monofunctional) azobenzene mesogenic guest and a nematic liquid crystal host. The resonant interaction of light with the azobenzene segment of the guest and the mesogenic nature of the latter enable the optical alignment of host molecules and the permanent fixing of that orientation by means of UV polymerization of the guest. We use dynamic spectral, polarimetric, and scattering techniques to study the orientational ordering and interaction of the guest–host system. We show that the uniform UV polymerization of this compound results in a low scattering material system with dielectric and elastic properties that are relatively close to those of the host, while still providing the capacity for optical configuration of its morphology.
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61.30.Vx Polymer liquid crystals
42.70.Df Liquid crystals
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
87.15.rp Polymerization
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Silicide formation process of Pt added Ni at low temperature: Control of NiSi2 formation

Nobuyuki Ikarashi and Koji Masuzaki

J. Appl. Phys. 109, 063506 (2011); http://dx.doi.org/10.1063/1.3560532 (5 pages)

Online Publication Date: 17 March 2011

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Transmission electron microscopy (TEM) and ab initio calculations revealed that the Ni-Si reaction around 300 °C is significantly changed by adding Pt to Ni. TEM analysis clarified that NiSi2 was formed in a reaction between Ni thin film (∼1 nm) and Si substrate, while NiSi was formed when Pt was added to the Ni film. We also found that the Ni-adamantane structure, which acts as a precursor for NiSi2 formation around the reaction temperature, was formed in the former reaction but was significantly suppressed in the latter reaction. Theoretical calculations indicated that Pt addition increased stress at the Ni-adamantane structure/Si-substrate interface. The increase in interface stress caused by Pt addition should raise the interface energy to suppress the Ni-adamantane structure formation, leading to NiSi2 formation being suppressed.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.37.Lp Transmission electron microscopy (TEM)
68.35.Md Surface thermodynamics, surface energies
68.35.Ct Interface structure and roughness

Structural templating and growth behavior of copper phthalocyanine thin films deposited on a polycrystalline perylenetetracarboxylic dianhydride layer

Dajung Hong, Young Rag Do, Hyon Tae Kwak, and Sanggyu Yim

J. Appl. Phys. 109, 063507 (2011); http://dx.doi.org/10.1063/1.3553884 (4 pages)

Online Publication Date: 17 March 2011

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Structural templating and the growth behavior of copper phthalocyanine (CuPc) thin films deposited on a polycrystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) layer were examined using x-ray diffraction (XRD) and atomic force microscopy. The templated CuPc films were found to grow on both the (110) and (102) plane of the α-PTCDA layer, indicating the intermolecular π-π interaction is strong enough to induce templating even on the underlying layer which is tilted at an angle of 25° from the substrate surface as well as on the surface-parallel layer. In contrast to the large growth exponent (β) values for other single layer molecular thin films, a significantly small β value of 0.17 ± 0.06 was obtained on the PTCDA/CuPc heterolayer thin film system. The XRD and scaling behavior studies suggest that this relatively slow surface roughening can be rationalized by the lack of surface parallel crystalline ordering at the initial stage of CuPc film growth on the PTCDA layer.
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68.55.am Polymers and organics
68.60.-p Physical properties of thin films, nonelectronic
68.35.bm Polymers, organics
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