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15 Nov 2008

Volume 104, Issue 10, Articles (10xxxx)

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Comparison of resonant-phonon-assisted terahertz quantum-cascade lasers with one-well injector and three-well module

H. Li, J. C. Cao, Z. Y. Tan, and S. L. Feng

J. Appl. Phys. 104, 103101 (2008); http://dx.doi.org/10.1063/1.3021060 (5 pages) | Cited 3 times

Online Publication Date: 17 November 2008

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The authors compare resonant-phonon-assisted terahertz quantum-cascade lasers with different active region designs, i.e., one-well injector and three-well module, using an ensemble Monte Carlo method. Carrier transport under injection anticrossing, parasitic anticrossing, and collection anticrossing couplings are investigated for both structures. The results show that the main parasitic current channel is the resonance of the injector 1′ with the lower lasing level 3 and one-well injector design benefits a high injection selectivity because of a better Δ1′4/Δ1′3 ratio. Due to the diagonal radiative transition, the one-well injector design sacrifices the oscillator strength and modal gain to reduce the parasitic coupling. Temperature evaluation shows that longitudinal-optical phonon emission is the dominant scattering mechanism contributing to lasing and the calculated modal gain in the one-well design shows a slower decreasing rate with temperature.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
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Mechanisms of the ultrasonic modulation of fluorescence in turbid media

Baohong Yuan, John Gamelin, and Quing Zhu

J. Appl. Phys. 104, 103102 (2008); http://dx.doi.org/10.1063/1.3021088 (13 pages) | Cited 8 times

Online Publication Date: 17 November 2008

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To understand the modulation mechanisms of fluorescence emission induced by ultrasonic waves in turbid media, a mathematical model is proposed and compared with the recent experimental observations of Kobayashi et al. [Appl. Phys. Lett. 89, 181102 (2006) ]. Modulation of fluorophore concentration is considered as the source of the oscillation of fluorescence signals when fluorophore concentration is low enough so that quenching effects can be ignored. By solving the rate equation and photon diffusion equation, quantitative solutions are given to quantify the modulation strength. Our calculations predict that the modulation depth (the ratio of the modulated signal strength to the unmodulated signal strength) can reach 10−4 when ultrasonic pressure with the order of magnitude of megapascals is applied in the ultrasound focal zone. Our model explains the relationship between the modulation strength and the average fluorophore concentration and also predicts a method to measure or image fluorescence lifetime in the turbid medium. When fluorophore concentration is high enough so that fluorescence quenching occurs, the fluorescence modulation is attributed to the modulation of quenching efficiency. Quenching caused by fluorescence resonance energy transfer can lead to a nonlinear relationship between the modulation fluorescence strength and the applied ultrasound strength.
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87.63.L- Visual imaging
87.63.D- Ultrasonography
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Laser annealing characteristics of multiple bulk defect populations within DKDP crystals

Paul DeMange, Christopher W. Carr, Raluca A. Negres, Harry B. Radousky, and Stavros G. Demos

J. Appl. Phys. 104, 103103 (2008); http://dx.doi.org/10.1063/1.3000460 (7 pages) | Cited 3 times

Online Publication Date: 17 November 2008

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Laser annealing by pre-exposure to subdamage threshold laser pulses is a well-established method to increase the damage performance of bulk KH2PO4 and KDxH2−xPO4 nonlinear optical materials. The origin of laser-induced damage is believed to be localized absorption by a defect structure, either a light-absorbing foreign nanoparticle or a cluster of stoichiometric defects. It has been recently shown that there are at least three populations of such defect structures in these materials in which pre-exposure to laser pulses of specific fluence and wavelength results in a measurable reduction in their number density or decrease in their susceptibility to damage. In this work, we investigate the annealing characteristics of these three populations of defect structures in DKDP under variable irradiation conditions. The aim is to understand the similarities and differences between these populations in the way they interact with laser light. The results depict distinct behaviors that reveal information on the relationship between the defect populations and their modifications responsible for annealing.
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61.72.Cc Kinetics of defect formation and annealing
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.66.Bi Elemental solids
61.66.Dk Alloys
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Ferromagnetic Cu doped ZnO as an electron injector in heterojunction light emitting diodes

T. S. Herng, S. P. Lau, S. F. Yu, S. H. Tsang, K. S. Teng, and J. S. Chen

J. Appl. Phys. 104, 103104 (2008); http://dx.doi.org/10.1063/1.3021142 (6 pages) | Cited 4 times

Online Publication Date: 18 November 2008

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Ferromagnetic and highly conductive copper doped ZnO (ZnO:Cu) films were prepared by filtered cathodic vacuum arc technique. By employing a biasing technique during growth, the electron concentration and resistivity of the ZnO:Cu films can be as high as 1020 cm−3 and 5.2×10−3 Ω cm, respectively. The ferromagnetic behavior is observed in all the conductive films, but its magnetization is quenched with an increment in carrier concentration, suggesting that carrier induced exchange is not directly responsible for the ferromagnetism. Heterojunction light emitting diodes have been fabricated using the conductive ZnO:Cu layer as an electron injector and a p-type GaN as hole injector. Electroluminescence can be detected from the devices.
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75.70.Ak Magnetic properties of monolayers and thin films
73.61.Ga II-VI semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.ag Semiconductors
85.60.Jb Light-emitting devices
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Optical emission from a high-refractive-index waveguide excited by a traveling electron beam

Yuji Kuwamura, Minoru Yamada, Ryuichi Okamoto, Takeshi Kanai, and Hesham Fares

J. Appl. Phys. 104, 103105 (2008); http://dx.doi.org/10.1063/1.3021150 (10 pages) | Cited 5 times

Online Publication Date: 18 November 2008

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An optical emission scheme was demonstrated, in which a high-refractive-index waveguide is excited by a traveling electron beam in a vacuum environment. The waveguide was made of Si–SiO2 layers. The velocity of light propagating in the waveguide was slowed down to 1/3 of that in free space due to the high refractive index of Si. The light penetrated partly into the vacuum in the form of a surface wave. The electron beam was emitted from an electron gun and propagated along the surface of the waveguide. When the velocity of the electron coincided with that of the light, optical emission was observed. This emission is a type of Cherenkov radiation and is not conventional cathode luminescence from the waveguide materials because Si and SiO2 are transparent to light at the emitted wavelength. This type of emission was observed in an optical wavelength range from 1.2 to 1.6 μm with an electron acceleration voltage of 32–42 kV. The characteristics of the emitted light, such as the polarization direction and the relation between the acceleration voltage of the electron beam and the optical wavelength, coincided well with the theoretical results. The coherent length of an electron wave in the vacuum was confirmed to be equal to the electron spacing, as found by measuring the spectral profile of the emitted light.
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42.79.Gn Optical waveguides and couplers
41.75.Fr Electron and positron beams
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
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Alternative to classic annealing treatments for fractally patterned TiO2 thin films

O. Van Overschelde, G. Guisbiers, F. Hamadi, A. Hemberg, R. Snyders, and M. Wautelet

J. Appl. Phys. 104, 103106 (2008); http://dx.doi.org/10.1063/1.3021161 (8 pages) | Cited 4 times

Online Publication Date: 18 November 2008

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Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass and subsequently irradiated by UV radiation using a KrF excimer laser. The influence of the laser fluence (F) on the constitution and microstructure of the deposited films is studied for 0.05<F<0.40 J/cm2. The diffraction data reveal that as deposited films are amorphous, while irradiated films present an anatase structure. Additional Raman spectroscopy study shows better crystal quality for the films irradiated with F<0.13 J/cm2. The film morphology appears to be strongly modified after laser treatment. Atomic force microscopy and scanning electron microscopy measurements reveal fractally textured films presenting characteristics of high porosity and high specific surface area. Finally, contact angle analysis suggests hydrophobic or wetting behavior depending on F. In order to explain the laser-induced structuration mechanisms, we have successfully applied a fractal as well as the nucleation theories. We propose that electronics effects could be responsible for the observed crystallization.
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61.72.Cc Kinetics of defect formation and annealing
81.15.Cd Deposition by sputtering
68.55.ag Semiconductors
68.55.J- Morphology of films
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.30.Hv Other nonmetallic inorganics
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Simulation of the bulk and surface modes supported by a diamond lattice of metal wires

M. A. Shapiro, K. R. Samokhvalova, J. R. Sirigiri, R. J. Temkin, and G. Shvets

J. Appl. Phys. 104, 103107 (2008); http://dx.doi.org/10.1063/1.3021310 (8 pages) | Cited 1 time

Online Publication Date: 18 November 2008

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We present a numerical study of the electromagnetic properties of the three-dimensional metallic wire lattices operating at microwave frequencies with applications to advanced accelerating structures and microwave sources. The metallic lattices can be considered as “artificial plasmas” because they demonstrate the properties of plasmas with a negative dielectric constant. Bulk modes in a diamond lattice of metal wires and surface modes on its interface are calculated. It is shown that the lattice can be modeled as an anisotropic medium with spatial dispersion. In contrast to a simple cubic lattice, the diamond lattice allows the existence of three different interfaces—one isotropic and two anisotropic. The surface modes supported by these interfaces are affected by spatial dispersion, in sharp contrast with the surface mode on an isotropic vacuum/plasma interface. For particle accelerator applications, we identify the electromagnetic mode confined by a plasmonic waveguide formed as a defect in a diamond lattice. All deleterious higher order modes excited as wakefields from the accelerating particle are found to be leaky. The diamond lattice is also useful as a research tool for studying particle radiation in media with spatial dispersion.
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42.70.Qs Photonic bandgap materials
84.40.-x Radiowave and microwave (including millimeter wave) technology
42.79.Gn Optical waveguides and couplers
77.22.Ch Permittivity (dielectric function)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.66.Bz Metals and metallic alloys
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Spectroscopic characterization of laser ablation brass plasma

Nek M. Shaikh, Sarwat Hafeez, M. A. Kalyar, R. Ali, and M. A. Baig

J. Appl. Phys. 104, 103108 (2008); http://dx.doi.org/10.1063/1.3021466 (7 pages) | Cited 8 times

Online Publication Date: 18 November 2008

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We present optical emission studies of the laser ablation brass plasma generated by the fundamental, second, and third harmonics of a neodymium doped yttrium aluminum garnet laser. The spectra predominantly reveal the spectral lines of the neutral and singly ionized copper and zinc. The excitation temperatures are determined by the Boltzmann plot method, whereas the electron number densities have been extracted from the Stark broadened line profiles. The spatial variations in the spectral line intensities and the plasma parameters at 1000, 500, and 100 mbar air pressures have been evaluated. Besides, the effect of the ambient gases (He, Ne, and Ar), the laser irradiance, and the laser wavelengths on the plasma parameters have been investigated.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.-b Plasma properties
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
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Subwavelength imaging based on frequency scanning

Stanislav Maslovski, Pekka Alitalo, and Sergei Tretyakov

J. Appl. Phys. 104, 103109 (2008); http://dx.doi.org/10.1063/1.3021056 (5 pages) | Cited 1 time

Online Publication Date: 19 November 2008

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A new principle of subwavelength imaging based on frequency scanning is considered. It is shown that it is possible to reconstruct the spatial profile of an external field exciting an array (or coupled arrays) of subwavelength-sized resonant particles with a frequency scan over the whole band of resonating array modes. During the scan it is enough to measure and store the values of the near field at one or at most two points. After the scan the distribution of the near field can be reconstructed with simple postprocessing. The proposed near-field microscope has no moving parts.
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07.79.Fc Near-field scanning optical microscopes
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Observation of coherent acoustic and optical phonons in bismuth nanowires by a femtosecond pump-probe technique

Alexandre A. Kolomenskii, Sergey N. Jerebtsov, Haidong Liu, Hong Zhang, Zuxin Ye, Zhiping Luo, Wenhao Wu, and Hans A. Schuessler

J. Appl. Phys. 104, 103110 (2008); http://dx.doi.org/10.1063/1.3021101 (6 pages) | Cited 6 times

Online Publication Date: 19 November 2008

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Coherent acoustic and optical phonon oscillations in Bi nanowire samples were studied with a femtosecond pump-probe technique. Laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the terahertz range. The transmission signal of nanowires on a glass substrate and the signal of light scattered from freestanding nanowires were measured. The acoustic velocity in nanowires was found to be close to that of bulk polycrystalline material. The changes in the optical phonon frequency at different laser fluences were simulated taking into account excitation inhomogeneity, lattice anharmonicity, diffusion, and recombination of the carriers and gave good agreement with experimental results.
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63.22.Gh Nanotubes and nanowires
78.47.-p Spectroscopy of solid state dynamics
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Dependence on chirp rate and spectral resolution of the terahertz field pulse waveform measured by electro-optic detection using a chirped optical pulse and a spectrometer and its effect on terahertz spectroscopy

H. Murakami, K. Shimizu, M. Katsurada, and S. Nashima

J. Appl. Phys. 104, 103111 (2008); http://dx.doi.org/10.1063/1.3021151 (7 pages) | Cited 1 time

Online Publication Date: 20 November 2008

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Single-shot measurement of a terahertz field pulse waveform by electro-optic sampling using a chirped optical pulse and a spectrometer was demonstrated by and Jiang and Zhang [Appl. Phys. Lett. 72, 1945 (1998) ]. We have performed an experimental and theoretical investigation into the dependence of the waveform thus measured on the chirp rate and spectral resolution. It was found that the waveform exhibits multicyclic behavior at a chirp rate of −0.24 THz2, which corresponds to a chirped-pulse width of over 10 ps, for the monocyclic original terahertz field, while it approaches the monocyclic behavior with decreasing pulse width. Further, broadening of the spectral resolution of the spectrometer gives rise to a monocyclic waveform in the chirp rate range where the waveform is expected to be multicyclic. In addition, we have derived an analytical expression for the terahertz field pulse waveform thus measured without using the method of stationary phase. The theoretical results were found to be consistent with measured ones. Finally, we examined the spectral bandwidth and resolution of terahertz spectroscopy using this method.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
78.70.Gq Microwave and radio-frequency interactions
78.47.J- Ultrafast spectroscopy (<1 psec)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
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Optical emission and energy disposal characterization of the laser ablation process of CaF2, BaF2, and NaCl at 1064 nm

M. Villagran-Muñiz, H. Sobral, C. A. Rinaldi, I. Cabanillas-Vidosa, and J. C. Ferrero

J. Appl. Phys. 104, 103112 (2008); http://dx.doi.org/10.1063/1.3021352 (7 pages) | Cited 2 times

Online Publication Date: 20 November 2008

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A method to simultaneously measure the fraction of incident energy spent as absorbed, scattered, and consumed energy by the plasma generated in the ablation process is presented. The results obtained allowed to make an energy balance in the ablation process of halogenated earth alkaline metal crystals (NaCl, BaF2, and CaF2) and to relate the nature of the crystal structure to the absorbed energy. The latter could be determined by monitoring the magnitude of the breakdown induced by the discharge between the plates of a capacitor. Simultaneously, time resolved optical emission spectroscopy was used to analyze the ion emission intensity and as a function of the energy absorbed by the sample. This plot shows the usual sigmoid behavior for the ablation process with the three typical regions, corresponding to the vaporization, screening, and ablation regimes. A heuristic equation has been used to analyze this process in terms of the nature of the crystal structure. From this equation the critical absorbed energy for each of the studied samples could also be determined.
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61.66.Fn Inorganic compounds
79.20.Ds Laser-beam impact phenomena
78.47.D- Time resolved spectroscopy (>1 psec)
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Molecular second harmonic generation induced at a metallic tip

I. Berline, C. Fiorini-Debuisschert, C. Royal, L. Douillard, and F. Charra

J. Appl. Phys. 104, 103113 (2008); http://dx.doi.org/10.1063/1.3028265 (6 pages)

Online Publication Date: 21 November 2008

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We report the measurement of efficient second harmonic generation at a metallic tip immersed in a molecular solution of highly nonlinear molecules. The dependence of the signal either with the tip-to-substrate applied voltage or with the tip-to-substrate distance is studied. We show that the signal originates from the orientation of a limited number of molecules following local centrosymmetry breaking under the tip. The results are discussed within the context of scanning probe microscopy.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
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Growth and characterization of Er-doped single crystal lithium niobate fibers

Andrea Arcangeli, Daniela Parisi, Alessandra Toncelli, Mauro Tonelli, Marco Bazzan, Nicola Argiolas, Maria Vittoria Ciampolillo, and Cinzia Sada

J. Appl. Phys. 104, 103114 (2008); http://dx.doi.org/10.1063/1.3021292 (6 pages) | Cited 1 time

Online Publication Date: 21 November 2008

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Single crystal lithium niobate fibers with diameters ranging from 1 to 3 mm have successfully been grown by the micro-pulling-down technique. The fibers are doped with various Er doping levels from 0 to 0.7 mol % and appear to be transparent and crack-free. The study includes x-ray structural investigation and complete spectroscopic characterization. The analysis shows a good homogeneity of composition, and the comparison with bulk crystals demonstrates the good optical quality of the samples. Particular attention has been devoted to the measurements of the fluorescence decay time of the 4I13/2 Er level, which has been carried out with a particular technique in order to avoid radiation trapping effects. Results are in agreement with the literature both in the case of bulk crystals and of fibers. Reliable polarized emission cross section in the 1.5 μm region are presented.
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81.10.Jt Growth from solid phases (including multiphase diffusion and recrystallization)
61.66.Fn Inorganic compounds
78.55.Hx Other solid inorganic materials
61.72.up Other materials
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Quasi-in-plane leaky modes in lasing cholesteric liquid crystal cells

Lev M. Blinov, Gabriella Cipparrone, Alfredo Mazzulla, Pasquale Pagliusi, Vladimir V. Lazarev, and Serguei P. Palto

J. Appl. Phys. 104, 103115 (2008); http://dx.doi.org/10.1063/1.2975971 (7 pages) | Cited 2 times

Online Publication Date: 21 November 2008

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Using a planar cell consisting of a prism and a flat glass, we have carried out precise measurements of the angles at which quasi-in-plane leaky (QIPL) laser modes propagate within a thin layer of a cholesteric liquid crystal (CLC) doped with a laser dye. Both the prism and the glass have refraction indices higher than relevant CLC indices, therefore, the waveguide effect in CLC is excluded. For this type of almost thresholdless lasing neither mirrors nor distributed feedback is necessary. The modes are generated in the amplifying CLC layer due to strong Fresnel reflections from the glass boundaries at propagation angles very close to 90° with respect to the cell normal. In the experiment, two modes polarized differently (s- and p-) have been found outgoing from the prism. Using equivalency between optical properties of a CLC and optically negative nematic liquid crystal at the propagation angles close to 90°, the analytical approach known for a uniform nematic liquid crystal was used for calculations of the threshold gain of the QIPL modes in a helical CLC. The gain has been found for the particular eigenmodes whose propagation angles have been measured. The experimental and calculated data on the propagation angles, polarization and threshold gain are in quantitative agreement.
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42.55.Mv Dye lasers
42.70.Df Liquid crystals
42.79.Gn Optical waveguides and couplers
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Testing the effect of surface coatings on alkali atom polarization lifetimes

S. J. Seltzer, D. M. Rampulla, S. Rivillon-Amy, Y. J. Chabal, S. L. Bernasek, and M. V. Romalis

J. Appl. Phys. 104, 103116 (2008); http://dx.doi.org/10.1063/1.2985913 (7 pages) | Cited 9 times

Online Publication Date: 24 November 2008

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The evaluation of different surface coatings used in alkali metal atomic magnetometers is necessary for the improvement of sensitivity of these devices. A method to measure the polarization lifetime of alkali atoms in the region between substrates with different coatings was developed to determine the effectiveness of the coating at preserving alkali spin polarization as well as chemical compatibility and high-temperature stability. Multiple coatings can be compared under identical experimental conditions, using an experimental geometry that allows surface characterization before and after evaluation of the polarization lifetime. Multilayered, cross-linked octadecyltrichlorosilane films, alkyltrichlorosilane monolayers, and octadecylphosphonic acid monolayers were evaluated using this approach.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
81.65.-b Surface treatments
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Efficient formation of surface relief grating on azopolymer films by gold nanoparticles

Sung-Kwan Na, Jung-Sung Kim, Seok-Ho Song, Cha-Hwan Oh, Yang-Kyoo Han, Young-Ho Lee, and Seong-Geun Oh

J. Appl. Phys. 104, 103117 (2008); http://dx.doi.org/10.1063/1.3031278 (5 pages)

Online Publication Date: 25 November 2008

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This paper reports a method for the efficient formation of surface relief grating (SRG) on azopolymer films in which gold nanoparticles are dispersed. The height and shape of the SRG, which were formed by a two-beam interference technique, were found to be dependent on the content of gold nanoparticles. The optimum concentration of gold nanoparticles for the efficient formation of SRG was confirmed by atomic force microscopy (AFM), and the height of SRG at the content of 0.06 wt % was about 2.2 times higher than that of pure azopolymer film without gold nanoparticles. This is due to the field enhancement effect derived from the localized plasmon excitations of gold nanoparticles dispersed in the azopolymer film.
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42.79.Dj Gratings
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
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Modification of emission of CdTe nanocrystals by the local field of Langmuir–Blodgett colloidal photonic crystals

Sergei G. Romanov, Maria Bardosova, Ian M. Povey, C. M. Sotomayor Torres, Martyn E. Pemble, Nikolai Gaponik, and Alexander Eychmüller

J. Appl. Phys. 104, 103118 (2008); http://dx.doi.org/10.1063/1.2981087 (8 pages) | Cited 8 times

Online Publication Date: 26 November 2008

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A light source on the surface of a slab of 2+1-dimensional photonic crystal has been prepared by the Langmuir–Blodgett deposition of a colloidal crystal on top of a thin film containing CdTe nanocrystals. The directional enhancement of the light emission intensity in the spectral range of the photonic bandgap has been revealed through the comparative examination of the angle-resolved transmission, diffraction, and photoluminescence spectra of the prepared structures. Changes in the emission spectrum have been tentatively explained in terms of the acceleration of the radiative recombination due to the increase in the local field strength at photonic bandgap resonance and changes in the emission diagram—as arising from the wavelength dependence of the topology of the local field pattern.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.70.Qs Photonic bandgap materials
82.70.Dd Colloids
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
81.16.-c Methods of micro- and nanofabrication and processing
78.55.Et II-VI semiconductors
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Three-dimensional modeling of the plasma arc in arc welding

G. Xu, J. Hu, and H. L. Tsai

J. Appl. Phys. 104, 103301 (2008); http://dx.doi.org/10.1063/1.2998907 (9 pages) | Cited 11 times

Online Publication Date: 17 November 2008

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Most previous three-dimensional modeling on gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) focuses on the weld pool dynamics and assumes the two-dimensional axisymmetric Gaussian distributions for plasma arc pressure and heat flux. In this article, a three-dimensional plasma arc model is developed, and the distributions of velocity, pressure, temperature, current density, and magnetic field of the plasma arc are calculated by solving the conservation equations of mass, momentum, and energy, as well as part of the Maxwell’s equations. This three-dimensional model can be used to study the nonaxisymmetric plasma arc caused by external perturbations such as an external magnetic field. It also provides more accurate boundary conditions when modeling the weld pool dynamics. The present work lays a foundation for true three-dimensional comprehensive modeling of GTAW and GMAW including the plasma arc, weld pool, and/or electrode.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.25.Fi Transport properties
52.77.Fv High-pressure, high-current plasmas (plasma spray, arc welding, etc.)
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Cathode effects in cylindrical Hall thrusters

E. M. Granstedt, Y. Raitses, and N. J. Fisch

J. Appl. Phys. 104, 103302 (2008); http://dx.doi.org/10.1063/1.2999343 (5 pages) | Cited 7 times

Online Publication Date: 17 November 2008

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Stable operation of a cylindrical Hall thruster has been achieved using a hot wire cathode, which functions as a controllable electron emission source. It is shown that as the electron emission from the cathode increases with wire heating, the discharge current increases, the plasma plume angle reduces, and the ion energy distribution function shifts toward higher energies. The observed effect of cathode electron emission on thruster parameters extends and clarifies performance improvements previously obtained for the overrun discharge current regime of the same type of thruster, but using a hollow cathode neutralizer. Once thruster discharge current saturates with wire heating, further filament heating does not affect other discharge parameters. The saturated values of thruster discharge parameters can be further enhanced by optimal placement of the cathode wire with respect to the magnetic field.
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52.75.Di Ion and plasma propulsion
52.80.Hc Glow; corona
52.50.-b Plasma production and heating
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Ultraviolet light in glow discharges

Loic Ledernez, Fethi Olcaytug, Hirotsugu Yasuda, and Gerald Urban

J. Appl. Phys. 104, 103303 (2008); http://dx.doi.org/10.1063/1.3021106 (4 pages)

Online Publication Date: 18 November 2008

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The use of ultraviolet light has prevailed in investigation of gas breakdown phenomena since Townsend developed the model of gas breakdown discharge carrying his name because ultraviolet light creates the supposedly necessary electrons in the interelectrode space. However, the ultraviolet light influences the gas breakdown characteristics. Therefore, the extent of the influence of the ultraviolet light on the prebreakdown current, the breakdown current, the breakdown voltage, and the Townsend’s coefficients is studied.
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52.80.Hc Glow; corona
52.80.Dy Low-field and Townsend discharges
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Characterization of a controlled plasma expansion in vacuum for laser driven ion acceleration

A. Flacco, A. Guemnie-Tafo, R. Nuter, M. Veltcheva, D. Batani, E. Lefebvre, and V. Malka

J. Appl. Phys. 104, 103304 (2008); http://dx.doi.org/10.1063/1.3021316 (5 pages) | Cited 3 times

Online Publication Date: 18 November 2008

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We present experimental and numerical results on the formation of a controlled plasma density gradient in front of a solid target irradiated with a subpicosecond, moderate intensity laser pulse. Interferometry with femtosecond probe is used to map the temporal evolution of the spatial density distribution of the generated plasma. Experimental results are found to be in good agreement with 1D1/2 hydrodynamic simulations. Moreover, these numerical simulations enable us to determine the impact of such a heating beam on the target rear surface and to correlate the plasma gradient that can be produced on the illuminated surface with the position of the shock wave in the bulk.
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52.25.-b Plasma properties
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.35.Tc Shock waves and discontinuities
52.70.-m Plasma diagnostic techniques and instrumentation
52.65.-y Plasma simulation
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Probing the plasma chemistry in a microwave reactor used for diamond chemical vapor deposition by cavity ring down spectroscopy

Jie Ma, James C. Richley, Michael N. R. Ashfold, and Yuri A. Mankelevich

J. Appl. Phys. 104, 103305 (2008); http://dx.doi.org/10.1063/1.3021095 (9 pages) | Cited 4 times

Online Publication Date: 19 November 2008

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Absolute column densities of C2(a) and CH radicals and H(n = 2) atoms have been measured in a diamond growing microwave reactor operating with hydrocarbon/Ar/H2 gas mixtures as functions of height (z) above the substrate surface and process conditions. The monitored species are each localized in the hot plasma region, where Tgas ∼ 3000 K, and their respective column densities are each reproduced, quantitatively, by two-dimensional (r,z) modeling of the plasma chemistry. The H(n = 2) distribution is seen to peak nearer the substrate, reflecting its sensitivity both to thermal chemistry (which drives the formation of ground state H atoms) and the distributions of electron density (ne) and temperature (Te). All three column densities are found to be sensitively dependent on the C/H ratio in the process gas mixture but insensitive to the particular choice of hydrocarbon (CH4 and C2H2). The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions investigated, encourages confidence in the predicted number densities of other of the more abundant radical species adjacent to the growing diamond surface which, in turn, reinforces the view that CH3 radicals are the dominant growth species in microwave activated hydrocarbon/Ar/H2 gas mixtures used in the chemical vapor deposition of microcrystalline and single crystal diamond samples.
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52.70.Ds Electric and magnetic measurements
52.25.-b Plasma properties
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
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Decomposition of methylene blue in water using a dielectric barrier discharge: Optimization of the operating parameters

Monica Magureanu, Daniela Piroi, Nicolae Bogdan Mandache, and Vasile Parvulescu

J. Appl. Phys. 104, 103306 (2008); http://dx.doi.org/10.1063/1.3021452 (7 pages) | Cited 4 times

Online Publication Date: 20 November 2008

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The decomposition of methylene blue (MB) in aqueous solution was investigated using a dielectric barrier discharge in coaxial configuration operated in pulsed regime. The MB solution (volume = 300 ml, concentration = 50 mg/l) contained in a reservoir was circulated by a pump and made to flow as a film over the surface of the inner electrode of the plasma reactor. The best results were obtained when the discharge was operated in oxygen. The conversion of MB reached 95% after 30 min of plasma treatment and the corresponding yield was 57 g/kW h. Increasing the amplitude of the voltage pulses, and implicitly, the power dissipated in the discharge led to faster decomposition of the dye; however, the yield for MB degradation was lower. Better results were obtained for negative polarity of the applied voltage as compared to positive polarity, but after 30 min treatment the conversion had similar values for both cases. The gas flow rate did not influence the decomposition of MB in the range investigated, 300–900 ml/min. The solution flow rate had an effect on the dye degradation only for short treatment times, where a lower flow rate led to improved results, while for long treatment times the conversion was similar for 30 and 90 ml/min.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
52.80.Wq Discharge in liquids and solids
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Influence of a preplasma on electron heating and proton acceleration in ultraintense laser-foil interaction

R. Nuter, L. Gremillet, P. Combis, M. Drouin, E. Lefebvre, A. Flacco, and V. Malka

J. Appl. Phys. 104, 103307 (2008); http://dx.doi.org/10.1063/1.3028274 (7 pages) | Cited 8 times

Online Publication Date: 21 November 2008

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Two-dimensional particle-in-cell simulations are performed to study laser-induced proton acceleration from solid-density targets in the presence of laser-generated preformed plasma. The preplasma generation and hydrodynamics are described using a one-dimensional Lagrangian code. The electron acceleration mechanism is shown to depend on the plasma scale length, exhibiting a transition from math×math heating to standing wave heating as smoother and smoother profiles are considered. Accordingly, the relativistic electron temperature and the cutoff proton energy are found to increase with the preplasma characteristic length.
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52.38.Kd Laser-plasma acceleration of electrons and ions
29.20.Ej Linear accelerators
52.65.Rr Particle-in-cell method
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
41.75.Ak Positive-ion beams
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