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Journal of Applied Physics

Issue 8 • Date Oct 2011

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Displaying Results 1 - 25 of 180
  • Photoluminescence of deep defects involving transition metals in Si: New insights from highly enriched 28Si

    Page(s): 081301 - 081301-25
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    Deep luminescence centers in Si associated with transition metals have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. They are among the most ubiquitous luminescence centers observed in Si, and have served as testbeds for elucidating the physics of isoelectronic bound excitons, and for testing ab-initio calculations of defect properties. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in the recently available highly enriched 28Si enabled the extension of the established technique of isotope shifts to the measurement of isotopic fingerprints, which reveal not only the presence of a given element in a luminescence center, but also the number of atoms of that element. This has resulted in many surprises regarding the actual constituents of what were thought to be well-understood deep luminescence centers. Here we summarize the available information for four families of centers containing either four or five atoms chosen from (Li, Cu, Ag, Au, Pt). The no-phonon transition energies, their isotope shifts, and the local vibrational mode energies presented here for these deep centers should prove useful for the still-needed theoretical explanations of their formation, stability and properties. View full abstract»

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  • Effect of excitation pulse width on thermoacoustic signal characteristics and the corresponding algorithm for optimization of imaging resolution

    Page(s): 083101 - 083101-6
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    Thermoacoustic (TA) signal amplitude, waveform, and spatial resolution related to width of the excitation pulse were theoretically explained and simulated by finite-difference time-domain (FDTD) method. TA signals and the reconstructed TA images were compared under variable microwave pulse widths tunable from 400 to 1200 ns to analyze the relationships. The peak-to-peak interval of TA signals increase and the spatial resolution of TA images decrease with the increment of the excitation pulse width in the long pulse region. Also, the experimental results clearly reveal that the TA conversion efficiency decrease with increasing microwave pulse duration under the same peak power, which have a good agreement with the theoretical predication and FDTD simulation. View full abstract»

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  • Ion-implantation induced nano distortion layer and its influence on nonlinear optical properties of ZnO single crystals

    Page(s): 083102 - 083102-6
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    Second harmonic generation (SHG) and X-ray diffraction rocking curves of high-quality ZnO single crystals implanted by different ions (He, Cu, and Zn) were investigated. Interestingly, it was found that both He- and Zn-implanted samples show a convinced increment in SHG efficiency while the Cu-implanted one does not. X-ray diffraction rocking curves of the samples show satellite structures, and the simulations firmly reveal the formation of quasi-interfaces inside He- and Zn-implanted crystals. These quasi-interfaces lead to SHG improvement in the two samples. Polarization dependence of SHG of the samples on the excitation light also evidences this conclusion. View full abstract»

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  • Auger recombination rates in ZnMgO from first principles

    Page(s): 083103 - 083103-4
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    We investigate direct electron-electron-hole interband Auger recombination for wurtzite Zn1-xMgxO alloys in the range 0 ≤ x ≤ 1. Recombination rates are computed by interpolating the band structure and transition matrix elements from ab initio calculations of bulk ZnO, Zn0.5Mgn0.5O, and MgO primitive cells. We find that interband Auger recombination is most probable for Mg concentrations around 50%, where ZnMgO does not exist in a stable wurtzite phase. Since, for low Mg concentrations, the calculated Auger coefficients are far below 10-32 cm6/s, we do not expect significant nonradiative loss through direct interband recombination in wurtzite ZnMgO. View full abstract»

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  • Tuning open-circuit voltage in organic solar cells by magnesium modified Alq3

    Page(s): 083104 - 083104-5
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    The low molecular weight tris-(8-hydroxyquinoline) aluminum (Alq3) has been incorporated with magnesium (Mg) that altered the nature of its opto-electronic characteristics. The lowering of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in Mg:Alq3, compared to pure Alq3, creates a stronger field (exceeding the exciton binding energy) at the donor-acceptor junction to dissociate the photo-generated exciton and also provides a low barrier for electron transport across the device. In an electron-only device (described in the text), a current enhancement in excess of 103, with respect to pure Alq3, could be observed at 10 V applied bias. Optimized Mg:Alq3 layer, when introduced in the photovoltaic device, improves the power conversion efficiencies significantly to 0.15% compared to the pure Alq3 device. The improvement in the photovoltaic performance has been attributed to the superior exciton dissociation and carrier transport. View full abstract»

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  • Optical gain in short period Si/Ge superlattices on [001]-SiGe substrates

    Page(s): 083105 - 083105-8
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    Results are here presented for the electronic band structure and the material gain of selected Si/Ge superlattices grown on Ge and SiGe buffers along the [001] direction. The chosen superlattices were proposed in the literature as promising direct-gap candidates based on group IV materials. A sp3d5s* tight-binding model has been adopted for the evaluation of the bands and the material gain in the presence of realistic charge injection levels and for different polarizations of the radiation associated with the direct transitions. For the superlattices studied here, we show that in the most favorable case (Si2/Ge14) the peak gain values are only a factor of 6 weaker than the corresponding value obtained for a typical III-V direct gap bulk crystal. View full abstract»

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  • Carrier-induced modulation of radiation by a gated graphene

    Page(s): 083106 - 083106-5
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    The modulation of the transmitted (reflected) radiation due to change of interband transitions under variation of carriers concentration by the gate voltage is studied theoretically. The calculations were performed for strongly doped graphene on high-κ (Al2O3, HfO2, AlN, and ZrO2) or SiO2 substrates under normal propagation of radiation. We have obtained the modulation depth above 10% depending on wavelength, gate voltage (i.e., carriers concentration), and parameters of substrate. The graphene—dielectric substrate—doped Si (as gate) structures can be used as an effective electrooptical modulator of near-IR and mid-IR radiation for the cases of high-κ and SiO2 substrates, respectively. View full abstract»

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  • Mass sensing based on a circuit cavity electromechanical system

    Page(s): 083107 - 083107-5
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    We present a scheme for mass sensing based on a circuit cavity electromechanical system where a free-standing, flexible aluminium membrane is capacitively coupled to a superconducting microwave cavity. Integration with the microwave cavity enables capacitive readout of the mechanical resonance directly on the chip. A microwave pump field and a second probe field are simultaneously applied to the cavity. The accreted mass landing on the membrane can be measured conveniently by tracking the mechanical resonance frequency shifts due to mass changes in the probe transmission spectrum. The mass responsivity for the membrane is 0.72 Hz/ag and we demonstrate that frequency shifts induced by adsorption of one hundred 1587 bp DNA molecules can be well resolved in the probe transmission spectrum. View full abstract»

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  • Analysis of energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses

    Page(s): 083108 - 083108-5
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    Energy transfer processes in Yb3+-Tb3+ co-doped, low-silica calcium aluminosilicate glasses were analyzed. Luminescence and time-resolved measurements were used to study upconversion processes, such as Yb-Tb cooperative sensitization, Yb-Yb cooperative luminescence, and Yb-Tb cross relaxation. The quantum cross relaxation efficiency was evaluated as a function on the Yb3+ concentration, and the maximum estimated value was approximately 51%. In addition, the intensity of the upconversion luminescence from the Tb3+:5D4 level decreased by two orders of magnitude comparing the value at room temperature with that at 123 K. As a consequence, Yb-Yb cooperative luminescence around 500 nm became comparable with the intensity of upconversion from the Tb3+:5D4 level. Furthermore, a dependence of the upconversion kinetics luminescence on temperature was observed. The upconversion rise time was constant and equal to 65 μs for temperatures between 296 to 473 K and decreased from 65 to 19 μs, without variation in the decay part, when the temperature was lowered from 296 to 123 K. These results were explained by a phonon-assisted cooperative sensitization process for the population of the Tb3+:5D4 level. View full abstract»

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  • Study of plasmonic crystals using Fourier-plane images obtained with plasmon tomography far-field superlenses

    Page(s): 083109 - 083109-4
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    We explore the use of surface plasmon polariton (SPP) tomography far-field superlenses for quantitative characterization of plasmonic crystals with sub-wavelength features. Essential information concerning the dependence of the effective refractive index on the hole diameter and the filling factor was obtained from the Fourier-plane images of the fabricated plasmonic crystals. We also provide a comprehensive discussion on the influence of hole diameters on the formation of directional stop-bands in plasmonic crystals. View full abstract»

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  • Third-order nonlinear optical response and photoluminescence characterization of tellurite glasses with different alkali metal oxides as network modifiers

    Page(s): 083110 - 083110-7
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    Studies of the third-order nonlinear optical properties in TeO2-MO-R2O glasses with three different alkali metal oxides R2O (R = Li, Na, K) as network modifiers and two network intermediates MO (M = Zn, Mg) are reported. The influence of such modifiers and intermediates on the nonlinear optical properties of these glasses was investigated using the standard Z-scan and the thermally managed Z-scan techniques under femtosecond pulse excitation at 800 nm. For different modifiers and intermediates, the nonlinear refraction indices n2 of these glasses varied in the range 1.31–2.81 (×10–15 cm2/W). It was found that n2 increases as the ionic radius of both network modifiers and intermediates decreases. Furthermore, the measurements show that the contribution from thermo-optical effects to the nonlinear refraction index is negligible for all of the studied glass compositions. In addition, the effect of modifiers and intermediates in the formation of localized states in the vicinity of the optical bandgap was also studied through photoluminescence experiments. These experiments revealed the presence of two emission bands (red and blue) originating from these localized states that can be populated after optical excitation and subsequent relaxation. View full abstract»

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  • Energy level decay and excited state absorption processes in erbium-doped tellurite glass

    Page(s): 083111 - 083111-10
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    The fundamental excited state decay processes relating to the 4I11/2 → 4I13/2 transition in singly Er3+-doped tellurite (TZNL) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 4I11/2 energy level at 970 nm and selective laser excitation of the 4I13/2 energy level at 1485 nm has established that energy transfer upconversion by way of a dipole-dipole interaction between two excited erbium ions in the 4I13/2 level populates the 4I11/2 upper laser level of the 3 μm transition. This upconversion has been analyzed for Er2O3 concentrations between 0.5 mol. % and 2.2 mol. %. The 4I13/2 and 4I11/2 energy levels emit luminescence with peaks located at 1532 nm and 2734 nm, respectively, with radiative decay efficiencies of 65% and 6.8% for the higher (2.2 mol. %) concentration sample. The low 2.7 μm emission efficiency is due to the non-radiative decay bridging the 4I11/2 → 4I13/2 transition and energy transfer to the OH- groups in the glass. Excited state absorption was observed to occur from the 4I13/2 and 4I11/2 levels with peak absorptions occurring at 1550 nm and 971 nm, respectively. The decay time of the 4I11/2 excited state decreased with an increase in the Er3+ concentration, which related to energy transfer to OH- ions that had a measured concentration of 6.6 × 1018 cm-3. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼80 kW cm-2 for a cw laser pump at 976 nm i- - f [Er3+] ≥ 1.2 × 1021 cm-3 (or [Er2O3] ≥ 2.65 mol. %) without OH- impurities being present. View full abstract»

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  • Population inversion of molecular nitrogen in an Ar: N2 mixture by selective resonance-enhanced multiphoton ionization

    Page(s): 083112 - 083112-7
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    Resonance-enhanced multiphoton ionization (REMPI) is shown to offer an attractive strategy for population inversion of molecular nitrogen in an Ar: N2 gas mixture. We present a detailed analysis of the key processes leading to a population inversion of molecular nitrogen in a REMPI-pumped Ar: N2 gas mixture, including a (3 + 1) REMPI of argon atoms, conversion of the REMPI-generated atomic argon ions into molecular ions, and generation of long-lived metastable excited-state argon atoms through dissociative recombination, populating the C3πu states of molecular nitrogen. Population inversion achieved for the second-positive-band laser transitions of molecular nitrogen enables stimulated emission of ultraviolet radiation at 337 nm. A high selectivity of the REMPI process helps to radically reduce the depletion of the working medium through the ionization of N2, providing a pump mechanism that is ideally suited for the creation of a new type of a highly efficient nitrogen laser. View full abstract»

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  • InGaN metal-semiconductor-metal photodetectors with triethylgallium precursor and unactivated Mg-doped GaN cap layers

    Page(s): 083113 - 083113-9
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    InGaN epitaxial films grown by metalorganic chemical vapor deposition using trimethylgallium and triethylgallium as precursors exhibited different optical and electrical properties. The films were characterized by x-ray diffraction, photoluminescence, secondary ion mass spectroscopy, and atomic force microscopy. Impacts of unactivated Mg-doped GaN in situ grown cap layers on InGaN and GaN films were further investigated. Current-voltage and spectral response measurements combined with Hall-effect measurement and analytical modeling have been used to assess possible current transport mechanisms of reverse dark and photo current flow in metal-semiconductor-metal photodetectors fabricated from InGaN and GaN. Unlike the dominant thermionic emission, which can be blocked by higher and thicker potential barrier in GaN, the trap-assisted tunneling is more pronounced in InGaN. The passivation effect on high density surface states in InGaN is proposed to explain the improved device performances after the incorporation of Mg-doped GaN. View full abstract»

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  • Mapping of femtosecond laser-induced collateral damage by electron backscatter diffraction

    Page(s): 083114 - 083114-5
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    The distribution of the collateral damage beneath craters produced by ultrafast (femtosecond) laser single pulses in a nickel base superalloy single crystal has been analyzed using electron backscatter diffraction. The procedure is based on measurements of the localized crystal rotations caused by the dislocations generated by the shock wave that is produced as an effect of laser pulse interaction with a material. Damage in the form of lattice misorientation created by excess dislocations was negligible at laser fluences up to 2 J/cm2. At higher fluences (5.1-41 J/cm2), the depth of the damage zone beneath the crater increased linearly with laser fluence. Based on lattice misorientation, excess dislocation densities as high as 3 × 1010/cm2 are calculated to be present below the ablated surface in the high fluence regime. View full abstract»

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  • Efficient evanescent wave coupling conditions for waveguide-integrated thin-film Si/Ge photodetectors on silicon-on-insulator/germanium-on-insulator substrates

    Page(s): 083115 - 083115-9
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    We studied evanescent wave coupling behavior between low index-contrast upper-level waveguides and thin-film Si and Ge photodetectors on SOI and germanium-on-insulator (GOI) substrates, respectively. We present a simple and intuitive leaky-mode phase-matching model using a ray-optics approach to determine the conditions for efficient coupling, both in 2D and 3D structures. It is shown that the presence of leaky modes that are phase-matched between the waveguide and the Si or Ge photodetector layer is the key condition for efficient coupling. Our approach was compared to other methods, such as finite-difference time domain (FDTD)/beam propagation method (BPM) and mode analysis. We report that, depending on the way a waveguide photodetector device is designed, waveguide-to-photodetector coupling efficiency may or may not be critically sensitive to design parameters, such as the photodetector layer thickness. As an example, the stark contrast of coupling behavior in the two most popular Ge photodetector structures integrated with Si rib waveguide versus channel waveguide is shown. The device design factors and the trends that affect such coupling sensitivity are identified and explained in the paper. View full abstract»

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  • Analysis of indium zinc oxide thin films by laser-induced breakdown spectroscopy

    Page(s): 083116 - 083116-8
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    We have performed spectroscopic analysis of the plasma generated by Nd:YAG (λ = 266 nm) laser irradiation of thin indium zinc oxide films with variable In content deposited by combinatorial pulsed laser deposition on glass substrates. The samples were irradiated in 5 × 104 Pa argon using laser pulses of 5 ns duration and 10 mJ energy. The plasma emission spectra were recorded with an Echelle spectrometer coupled to a gated detector with different delays with respect to the laser pulse. The relative concentrations of indium and zinc were evaluated by comparing the measured spectra to the spectral radiance computed for a plasma in local thermal equilibrium. Plasma temperature and electron density were deduced from the relative intensities and Stark broadening of spectral lines of atomic zinc. Analyses at different locations on the deposited thin films revealed that the In/(In + Zn) concentration ratio significantly varies over the sample surface, from 0.4 at the borders to about 0.5 in the center of the film. The results demonstrate that laser-induced breakdown spectroscopy allows for precise and fast characterization of thin films with variable composition. View full abstract»

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  • Opportunities of deoxyribonucleic acid complexes composites for nonlinear optical applications

    Page(s): 083117 - 083117-4
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    In this paper, we illustrate new functionalities for nonlinear optical applications of bio-molecular systems. This study presents DNA complex with new ionic surfactants. These surfactants enabled DNA solubility in solvents other than alcohols, like aromatic and chlorinated ones. Composites with two nonlinear optical (NLO) active dyes are subjects of the second and third harmonic generation experiments. The found effective nonlinear susceptibilities values are much higher than that for standard fused silica. We also demonstrate any influence of the surfactant on NLO properties. View full abstract»

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  • Efficiency enhancement of top-emitting organic light-emitting diodes using conversion dyes

    Page(s): 083118 - 083118-6
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    We report recent results on top-emitting organic light-emitting diodes (OLEDs) using color conversion layers (CCLs) embedded into the electron transport layer of the OLED structure. The method of color conversion provides the possibility to generate a color stable emission with operating lifetime. Due to a constant ratio between absorbed blue emission and converter emission, the spectral shape remains for all time. This guarantees constant color coordinates of the OLED, which is essential for lighting applications. It is shown that OLEDs using conversion layers reach external quantum efficiencies (EQE) which can be higher than the corresponding blue top-emitting OLED. The used conversion layer thickness is below 100 nm, reaching Commission Internationale de l’Éclairage (CIE) coordinates of (0.23; 0.27) close to the Planckian locus at a maximum EQE of 3.16% using a blue fluorescent emitter system. Furthermore, we show that the excitation mechanism of the conversion layer is caused by absorption and no parasitic electrical excitation is taking place. Investigations on the emission color over the lifetime show color-stability over a period of up to 2200 h. View full abstract»

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  • Effect of indirect interband absorption in Ge/SiGe quantum wells

    Page(s): 083119 - 083119-10
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    The effect of the indirect interband absorption both below and above the direct gap in Ge/SiGe quantum-well (QW) structures is theoretically investigated in comparison to the direct interband absorption. The theoretical formula for the indirect interband absorption is derived based on the second-order time-dependent perturbation theory. The direct interband absorption is calculated by summing the bound-state exciton contribution based on the variational method and the QW continuum-state contribution in the consideration of the valence band mixing effect and a Sommerfeld excitonic enhancement factor. The calculated indirect interband absorption parabolically increases below the direct bandgap and shows a stair-step-like gradual increase above the direct bandgap. The calculated optical absorption spectra at room temperature are compared with the experimental results measured by Kuo etal [Nature 437, 1334 (2005)]. Although the indirect interband absorption is very small below the direct bandgap, the contribution of the gradually increasing indirect interband absorption is significant at the high transition energy above the direct bandgap. View full abstract»

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  • Spectroscopic characteristics of Dy3+ doped Y3Al5O12 transparent ceramics

    Page(s): 083120 - 083120-8
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    The spectroscopic characteristics of Dy3+ in YAG (Y3Al5O12) transparent ceramics prepared by solid state synthesis were investigated, with special attention to the visible (yellow, blue) emissions that could be pumped directly by the new developed blue–violet or near UV laser diodes. The absorption and emission spectra, recorded at different temperatures from 10 to 300 K, were analyzed and relevant new parameters, improved energy level scheme, temperature effects, intensity parameters based on Judd-Ofelt theory and radiative transition rates, branching ratios, etc., were estimated and the data are compared with previous reports on garnet single crystals. The potential of the Dy3+: YAG transparent ceramics for yellow or blue laser emission is discussed. View full abstract»

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  • Complete energy transfer due to rare-earth phase segregation in optical fiber preform glasses

    Page(s): 083121 - 083121-6
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    An Yb3+ to Tm3+ energy-transfer quantum yield close to one has been found in phase-separated yttrium–alumina silicate optical fiber preform glasses. Optical absorption, luminescence, lifetime measurements, and rare-earth concentration dependence have been performed to investigate the feasibility of efficient blue upconversion fiber lasers through convenient Yb3+ sensitation. Luminescence decay measurements have demonstrated the co-existence of two phases. One of the phases is characterized by an yttrium-rich composition. Most of the RE ions partition into the yttrium-rich phase and produce the intense upconversion emission. View full abstract»

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  • Helium corona-assisted air discharge

    Page(s): 083301 - 083301-4
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    Operation of atmospheric discharge of electronegative gases including air at low voltages yet without consuming any inert gas will enormously promote the application of non-thermal plasmas. By taking advantage of the low onset voltage for helium corona, air discharge was successfully launched at much reduced voltages with a needle-plate system partly contained in a helium-filled glass bulb—for a needle-plate distance of 12 mm, 1.0 kV suffices. Ultraviolet emission from helium corona facilitates the discharging of air, and the discharge current manifests distinct features such as relatively broad Trichel pulses in both half periods. This design allows safe and economic implementation of atmospheric discharge of electronegative gases, which will find a broad palette of applications in surface modification, plasma medicine and gas treatment, etc. View full abstract»

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  • On the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon

    Page(s): 083302 - 083302-5
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    Experimental observations on the plasma dynamics inside the nozzle of a 30 A oxygen cutting torch operated at conditions close to the double arcing are reported. It is employed a technique previously developed in our laboratory consisting in using the nozzle as a large-sized Langmuir probe. Based on the behavior of the ion current signal and simple estimations, it is concluded that (1) the non-equilibrium plasma inside the nozzle is far from the steady state in time, in contrast to what is frequently assumed. The power supply ripple was identified as the main fluctuations source and (2) large-scale plasma fluctuations inside the nozzle could cause transient (total duration of the order of 100 μs) Townsend avalanches developing in the space-charge layer located between the arc plasma and the nozzle wall. Such events trigger the so called non-destructive double-arcing phenomena without appealing to the presence of insulating films deposited inside the nozzle orifice, as was previously proposed in the literature. View full abstract»

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  • Enhancements of extreme ultraviolet emission using prepulsed Sn laser-produced plasmas for advanced lithography applications

    Page(s): 083303 - 083303-6
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    Laser-produced plasmas (LPP) from Sn targets are seriously considered to be the light source for extreme ultraviolet (EUV) next generation lithography, and optimization of such a source will lead to improved efficiency and reduced cost of ownership of the entire lithography system. We investigated the role of reheating a prepulsed plasma and its effect on EUV conversion efficiency (CE). A 6 ns, 1.06 μm Nd:yttrium aluminum garnet laser was used to generate the initial plasma that was then reheated by a 40 ns, 10.6 μm CO2 laser to generate enhanced EUV emission from a planar Sn target. The effects of prepulsed laser intensity and delay timings between the prepulsed and the pumping pulse were investigated to find the optimal pre-plasma conditions before the pumping pulse. The initial optimization of these parameters resulted in 25% increase in CE from the tin LPP. The cause of increased EUV emission was identified from EUV emission spectra and ion signal data. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory