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Journal of Applied Physics

Issue 5 • Date Mar 2011

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Displaying Results 1 - 25 of 132
  • Effect of Coulomb interaction on nonlinear (intensity-dependent) optical processes and intrinsic bistability in a quantum well under the electric and magnetic fields

    Page(s): 053101 - 053101-6
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    The effect of the electrostatic Coulomb interaction on the line shape related to the intensity-dependent intersubband optical processes in a Si δ-doped quantum well is studied using the density matrix formalism. The electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger–Poisson equations. The line shape function is considerably modified by the optical intensity and the electric and magnetic fields. Moreover, we demonstrate the existence of the optical bistability for appropriate values of the optical intensity and also the control of the optical bistability with the electric and magnetic fields. It is also found that the intersubband relaxation time plays an important role in determining the optical bistability region. View full abstract»

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  • A two-dimensional analytical model for the gate–source and gate–drain capacitances of ion-implanted short-channel GaAs metal-semiconductor-field effect transistor under dark and illuminated conditions

    Page(s): 053102 - 053102-10
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    This paper presents an analytical model for the internal capacitances of short-channel ion-implanted GaAs MESFETs under dark and illuminated conditions. The device structure considered in this study is a normally-on self-aligned GaAs MESFET with indium tin oxide (ITO) as the Schottky metal for the gate electrode of the device. The gate area of the device is illuminated by an optical radiation of 0.87 μm from an external source which is coupled into the device through the semitransparent ITO metal at Schottky-gate to modulate the electrical and microwave characteristics of the device. The nonanalytic Gaussian doping profile commonly considered for the channel doping of an ion-implanted GaAs MESFET has been replaced by an analytic Gaussianlike function for the simplicity of the present model. The two-dimensional (2D) potential distribution obtained by solving the 2D Poisson’s equation by including the effect of photo-generated carriers due to the incident optical radiation has been utilized to model the depletion region width below the gate of the short-channel GaAs MESFETs. The depletion width model has then been used to model the internal gate-source and gate-drain capacitances of the device operating under both the linear and saturation regions. The effect of illumination on the Schottky junction at the gate of the MESFET has been modeled by considering an induced photo-voltage developed across the junction that is superimposed on the applied gate bias voltage. The proposed model has been verified by comparing the theoretically predicted results with simulated data obtained by using the commercially available ATLASTM 2D device simulator. View full abstract»

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  • Optimizing facet coating of quantum cascade lasers for low power consumption

    Page(s): 053103 - 053103-4
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    Typical high power consumption (∼10 W) of mid-infrared quantum cascade lasers (QCLs) has been a serious limitation for applications in battery powered systems. A partial high-reflection (PHR) coating technique is introduced for power downscaling with shorter cavity lengths. The PHR coating consists of a double layer dielectric of SiO2 and Ge. With this technique, a 4.6 μm QCL with an ultra low threshold power consumption of less than a watt (0.83 W) is demonstrated in room temperature continuous wave operation. At 25°C, the maximum output power and wall plug efficiency are 192 mW and 8.6%, respectively. View full abstract»

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  • Thermally tunable and omnidirectional terahertz photonic bandgap in the one-dimensional photonic crystals containing semiconductor InSb

    Page(s): 053104 - 053104-6
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    Thermally tunable and omnidirectional terahertz (THz) photonic bandgaps in the one-dimensional photonic crystals composed of alternating layers of semiconductor material InSb and dielectric material SiO2 are studied theoretically. This photonic bandgap is strongly dependent on the lattice constants and the thickness ratio of the constituent InSb and SiO2 layers. It is found that the lower-order gap is invariant on the lattice constants, but the higher-order gaps are all sensitive to the lattice constants. Moreover, the band-edges of the higher-order gaps shift to lower frequency as the increasing thickness ratio. Additionally, the Gap II is also angle-independent, and the omnidirectional bandgap can occur in the terahertz range. Omnidirectional bandwidth for both polarizations is about 1.5 THz at 220 K. Most important of all, the omnidirectional bandgap can be tuned by controlling the external temperature, which results from the dependence of the semiconductor plasma frequency on the temperature. As the temperature increases, the upper and lower frequency limits of the Gap II shift to higher frequency and the bandwidth becomes narrow. View full abstract»

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  • Reshaped Terahertz waveforms from a large-aperture photoconductive antenna with millimeter scale metal hole and grid combination

    Page(s): 053105 - 053105-5
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    Time-domain THz transmission of a metal wire grid is investigated experimentally. The transmission depends on the relative angle (θ) between the polarization of the THz wave and the direction of the wires. When the polarization is parallel to the wires (θ = 0°), the grid works as a high-pass filter with a cutoff frequency at 0.3 THz. Meanwhile, when the polarization is perpendicular to the wires (θ = 90°), the waveform of THz pulses is only slightly altered after passing through the grid. These results are reproduced accurately in computational simulations of the electromagnetic field. Simulation results indicate that strongly excited surface waves between two adjacent metal rods are responsible for the transmission peak at 0.81 THz, while surface waves slightly excited on the front surface of the grid are responsible for the low transmission at 0.3 THz. Transmission spectra of θ = 0° are interpreted qualitatively by applying the concept of effective surface plasmons. View full abstract»

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  • Photopolymerizable glasses incorporating high refractive index species and ionic liquid: A comparative study

    Page(s): 053106 - 053106-10
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    Three different holographic photomaterials belonging to the class of photopolymerizable glasses have been synthesized using sol-gel techniques, and characterized with the purpose of a comparative study. Their behavior is analyzed in terms of achieved refractive index modulation, dark diffusion mechanism, diffraction efficiency and optical quality; in order to determine their suitability for different holographic applications. View full abstract»

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  • Optical design of organic light emitting diodes

    Page(s): 053107 - 053107-7
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    Out-coupling of light from organic light emitting diodes (OLEDs) is a significant challenge for the application of OLEDs in solid state lighting. Most of the light is trapped in the stratified thin film structure and the glass substrate. In this study, an optical model is developed to simulate the optical electrical field for OLEDs with a stratified structure based on the dipole source term and transfer matrix approach. The exciton distribution is also considered in the proposed model. OLEDs with weak microcavity are selected to evaluate the model. Calculation of the electroluminescence spectrum, device efficiency as well as the angular dependence is shown to have a good agreement with the experimental data. Moreover, by using the weak microcavity design, an OLED of more than 70% improved efficiency is achieved. View full abstract»

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  • The study of thermal silicon dioxide electrets formed by corona discharge and rapid-thermal annealing

    Page(s): 053108 - 053108-6
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    A silicon dioxide (SiO2) electret passivates the surface of crystalline silicon (Si) in two ways: (i) when annealed and hydrogenated, the SiO2–Si interface has a low density of interface states, offering few energy levels through which electrons and holes can recombine; and (ii) the electret’s quasipermanent charge repels carriers of the same polarity, preventing most from reaching the SiO2–Si interface and thereby limiting interface recombination. In this work, we engineer a charged thermal SiO2 electret on Si by depositing corona charge onto the surface of an oxide-coated Si wafer and subjecting the wafer to a rapid thermal anneal (RTA). We show that the surface-located corona charge is redistributed deeper into the oxide by the RTA. With 80 s of charging, and an RTA at 380 °C for 60 s, we measure an electret charge density of 5 × 1012 cm–2, above which no further benefit to surface passivation is attained. The procedure leads to a surface recombination velocity of less than 20 cm/s on 1 Ω-cm n-type Si, which is commensurate with the best passivation schemes employed on high-efficiency Si solar cells. In this paper, we introduce the method of SiO2 electret formation, analyze the relationship between charge density and interface recombination, and assess the redistribution of charge by the RTA. View full abstract»

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  • Photoluminescence properties and energy transfer between Eu3+ and Nd3+ in polyborate BaGdB9O16: Eu3+, Nd3+

    Page(s): 053109 - 053109-5
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    The photoluminescence properties of BaGdB9O16: Eu3+, Nd3+ were investigated under ultraviolet excitation. For the samples BaGdB9O16: xEu3+, yNd3+, the emission spectra include the visible and near infrared region, which correspond to the emission spectra of Eu3+ and Nd3+. Moreover, based on the luminescence spectra and decay lifetimes of BaGdB9O16: 0.01Eu3+, xNd3+, the effective energy transfer was demonstrated from Eu3+ to Nd3+. The energy transfer efficiency was calculated with the measured decay lifetimes of Eu3+. The possible energy transfer mechanism was proposed to rationalize the experimental results. View full abstract»

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  • Computations of the optical transitions and absorption spectra in a set of realistic, elongated InAs/GaAs quantum boxes having a Gaussian distribution

    Page(s): 053110 - 053110-5
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    InAs/GaAs quantum dots (QDs) grown by various methods do not have the same dimensions in the three axes. This paper reports on expressions for computations of the optical transitions and absorption spectra of InAs/GaAs QDs that have a square base and the variation of the height is Gaussian. The dots were considered to be elongated quantum boxes with square bases having finite potentials at the boundaries. The results are in excellent agreement with reported experimental data of photoluminescence and absorption. The expressions could be successfully applied to short quantum wires. View full abstract»

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  • Electro-optic tunable optical isolator in periodically poled LiNbO3

    Page(s): 053111 - 053111-5
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    We proposed a design of a tunable optical isolator. The electro-optic (EO) effects in periodically poled lithium niobate (PPLN) with a defect were studied. EO effects and second harmonic generation (SHG) allow three-wave coupling in this nonlinear optics process. A defect is inserted in an asymmetrical position in PPLN to break the reciprocity of the fundamental-wave transmission in two reversed propagating directions. We numerically demonstrated that the contrast of this optical isolator can be tuned from -1 to 1 by simply changing the external dc electric field. With EO effects, the position of the defect can be designed more flexibly maintaining a high contrast of the optical isolation. View full abstract»

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  • The effect of microcavity laser recombination lifetime on microwave bandwidth and eye-diagram signal integrity

    Page(s): 053112 - 053112-9
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    Vertical microcavity surface-emitting lasers employing quantum wells and small aperture buried-oxide current and field confinement are demonstrated with wider mode spacing and faster spontaneous carrier recombination (enhanced Purcell factor), lower threshold current, larger side mode suppression ratio (SMSR), and higher photon density and temperature insensitivity. The result is a microcavity laser that achieves higher microwave modulation bandwidth (f-3dB = 15.8 GHz) at ultra-low power consumption (1.5 mW) with a slope for the modulation current efficiency factor (MCEF) = 17.47 GHz/mA-1/2, as well as a better quality eye diagram in high-speed data transmission. The microwave behavior model for the microcavity laser is used to estimate the enhanced recombination and reduced lifetime. View full abstract»

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  • Time-resolved shadowgraphic study of femtosecond laser ablation of aluminum under different ambient air pressures

    Page(s): 053113 - 053113-8
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    Femtosecond pulse laser ablation of aluminum under different ambient air pressures between 1 atm and 4 × 10-4 Pa is investigated using a femtosecond time-resolved shadowgraphic method. It is observed that as the ambient air pressure decreases, the contact front becomes more and more distinct for a certain pressure range, demonstrating that the confinement effect of the ambient air to the ablated target material can play a critically important role in the laser ablation process. It is also found that the concentric and semicircular stripe pattern, which results from the diffraction of the probe beam by the expanding plume of a specific material state and is typically observed in the shadowgraphs for 1–2 ns delay time, gradually blurs and disappears while the ambient air pressure decreases from 1 atm to 7000 Pa. If a prepulse or a relatively large pulse pedestal exists before the main pulse, however, the stripe pattern can still be observed even though the ambient air pressure is 5 × 10-4 Pa. It is thus inferred that what contributes to the formation of the unique stripe pattern is a mixture of the ejected target material and ionized background gas induced by the femtosecond laser ablation. View full abstract»

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  • Tuning etch selectivity of fused silica irradiated by femtosecond laser pulses by controlling polarization of the writing pulses

    Page(s): 053114 - 053114-5
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    We report on experimental study on chemical etch selectivity of fused silica irradiated by femtosecond laser with either linear or circular polarization in a wide range of pulse energies. The relationships between the etch rates and pulse energies are obtained for different polarization states, which can be divided into three different regions. A drop of the etch rate for high pulse energy region is observed and the underlying mechanism is discussed. The advantage of using circularly polarized laser is justified owing to its unique capability of providing a 3D isotropic etch rate. View full abstract»

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  • Tuning optical modes in slab photonic crystal by atomic layer deposition and laser-assisted oxidation

    Page(s): 053115 - 053115-6
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    The authors experimentally investigate the effects of atomic layer deposition (ALD) and laser-assisted oxidation on the optical modes in GaAs L3 photonic crystal air-bridge cavities, using layers of InAs quantum dots as internal light source. Four distinct optical mode peaks are observed in the photonic bandgap and they show different wavelength-redshifts (0–6.5 nm) as the photonic crystal surface is coated with an Al2O3 layer (0–5.4 nm thick). Numerical finite-difference time-domain (FDTD) simulations can well-reproduce the experimental result and give insight into the origin of the shifts of modes with different spatial profiles. By combining the ALD coating with in situ laser-assisted oxidation, we are able to both redshift and blueshift the optical modes and we attribute the blueshift to the formation of a GaAs-oxide at the expense of GaAs at the interface between GaAs and the Al2O3 layer. This result can be quantitatively reproduced by including a GaAs-oxide layer into the FDTD model. Selective etching experiments, confirm that this GaAs-oxide layer is mainly at the interface between GaAs and Al2O3 layers. View full abstract»

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  • Studies of concentration dependences in the luminescence of Ti-doped Al2O3

    Page(s): 053116 - 053116-6
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    The variation of luminescence and excitation spectra of titanium doped Al2O3 for the concentration of Ti ranging from 10 to 1000 ppm was investigated using synchrotron radiation. In the lightly doped Al2O3–Ti (<100 ppm) samples we identified several emission bands. These are the emission of the excitons localized at Ti (290 nm), the emission due to F+ centers (325 nm), the band around 420 nm traditionally attributed to F center emission, and the luminescence of Ti3+ ions at 720 nm. The emphasis in this study is on the clarification of the nature of the blue emission band in the samples with high concentration of Ti (≥100 ppm), where the luminescence and excitation spectra of the blue emission exhibit noticeable variability. This is explained by a model of the luminescence process of Ti4+–F centers that includes the photoionization of Ti3+, the subsequent capture of electrons at F+-centers, formation of excited F-centers and, finally, the emission of a blue photon. The quenching of the blue emission with increasing Ti concentration is interpreted in terms of competition between oxygen vacancies and Ti4+ centers in the capture of the electron. View full abstract»

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  • X-ray enhancement in a nanohole target irradiated by intense ultrashort laser pulses

    Page(s): 053301 - 053301-7
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    In this paper, we present a comparative study of the laser energy absorption, soft x-ray emission (in the water window region: 2.3–4.4 nm) and hard x-ray emission (in the 2–20 keV range) from planar aluminum and nanohole alumina of 40 nm average diameter, when irradiated by Ti:sapphire laser pulses. The laser pulse duration was varied from 45 to 500 fs, and the focused intensity on the target ranged from ∼3 × 1016 W/cm2 to 3×1017 W/cm2. The x-ray yield enhancement from the nanoholes shows an increased coupling of the laser energy to the target. The effect of laser pulse duration on the x-ray emission was also studied, where a resonance like phenomenon was observed. The laser energy absorption measurements in the nanoholes showed a marginal enhancement in absorption as compared to planar Al. The integrated keV x-ray yield, from nanohole alumina and planar Al, at an intensity of 3 × 1017 W/cm2, was 25 and 3.5 μJ, respectively. The results can be explained by considering the hydrodynamic expansion of the laser irradiated structure and field enhancement in the nanoholes. View full abstract»

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  • Dynamics of C2 formation in laser-produced carbon plasma in helium environment

    Page(s): 053302 - 053302-6
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    We investigated the role of helium ambient gas on the dynamics of C2 species formation in laser-produced carbon plasma. The plasma was produced by focusing 1064 nm pulses from an Nd:YAG laser onto a carbon target. The emission from the C2 species was studied using optical emission spectroscopy, and spectrally resolved and integrated fast imaging. Our results indicate that the formation of C2 in the plasma plume is strongly affected by the pressure of the He gas. In vacuum, the C2 emission zone was located near the target and C2 intensity oscillations were observed both in axial and radial directions with increasing the He pressure. The oscillations in C2 intensity at higher pressures in the expanding plume could be caused by various formation zones of carbon dimers. View full abstract»

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  • Electron density and temperature of gas-temperature-dependent cryoplasma jet

    Page(s): 053303 - 053303-6
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    A microsize cryoplasma jet was developed and analyzed at plasma gas temperatures ranging from room temperature down to 5 K. Experimental results obtained from optical emission spectroscopy and current–voltage measurements indicate that the average electron density and electron temperature of the cryoplasma jet depend on the gas temperature. In particular, the electron temperature in the cryoplasma starts to decrease rapidly near 60 K from about 13 eV at 60 K to 2 eV at 5 K, while the electron density increases from about 109 to approximately 1012 cm-3 from room temperature to 5 K. This phenomenon induces an increase in the Coulomb interaction between electrons, which can be explained by the virial equation of state. View full abstract»

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  • Observation and numerical analysis of plasma parameters in a capillary discharge-produced plasma channel waveguide

    Page(s): 053304 - 053304-7
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    We observed the parameters of the discharge-produced plasma in cylindrical capillary. Plasma parameters of the waveguide were investigated by use of both a Normarski laser interferometer and a hydrogen plasma line spectrum. A space-averaged maximum temperature of 3.3 eV with electron densities of the order of 1017 cm-3 was observed at a discharge time of 150 ns and a maximum discharge current of 200 A. One-dimensional dissipative magnetohydrodynamic (MHD) code was used to analyze the discharge dynamics in the gas-filled capillary discharge waveguide for high-intensity laser pulses. Simulations were performed for the conditions of the experiment. We compared the temporal behavior of the electron temperature and the radial electron density profiles, measured in the experiment with the results of the numerical simulations. They occurred to be in a good agreement. An ultrashort, intense laser pulse was guided by use of this plasma channel. View full abstract»

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  • Deactivation of A549 cancer cells in vitro by a dielectric barrier discharge plasma needle

    Page(s): 053305 - 053305-6
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    An inactivation mechanism study on A549 cancer cells by means of a dielectric barrier discharge plasma needle is presented. The neutral red uptake assay provides a quantitative estimation of cell viability after plasma treatment. Experimental results show that the efficiency of argon plasma for the inactivation process is very dependent on power and treatment time. A 27 W power and 120 s treatment time along with 900 standard cubic centimeter per minute Ar flow and a nozzle-to-sample separation of 3 mm are the best parameters of the process. According to the argon emission spectra of the plasma jet and the optical microscope images of the A549 cells after plasma treatment, it is concluded that the reactive species (for example, OH and O) in the argon plasma play a major role in the cell deactivation. View full abstract»

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  • Chemical response of lithiated graphite with deuterium irradiation

    Page(s): 053306 - 053306-6
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    Lithium wall conditioning has been found to enhance plasma performance for graphite walled fusion devices such as TFTR, CDX-U, T-11M, TJ-II and NSTX. Among observed plasma enhancements is a reduction in edge density and reduced deuterium recycling. The mechanism by which lithiated graphite retains deuterium is largely unknown. Under controlled laboratory conditions, X-ray photoelectron spectroscopy (XPS) is used to observe the chemical changes that occur on ATJ graphite after lithium deposition. The chemical state of lithiated graphite is found to change upon deuterium irradiation indicating the formation Li-O-D, manifest at 532.9 ± 0.6 eV. Lithium-deuterium interactions are also manifest in the C 1s photoelectron energy range and show Li-C-D interactions at 291.2 ± 0.6 eV. Post-mortem NSTX tiles that have been exposed to air upon extraction are cleaned and examined, revealing the chemical archaeology that formed during NSTX operations. XPS spectra show strong correlation (± 0.3 eV) in Li-O-D and Li-O peaks from post-mortem and control experiments, thus validating offline experiments. We report findings that show that deuterium is found to interact with lithium after lithium has already reacted with carbon and oxygen. View full abstract»

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  • Time resolved metal line profile by near-ultraviolet tunable diode laser absorption spectroscopy

    Page(s): 053307 - 053307-6
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    Pulsed systems are extensively used to produce active species such as atoms, radicals, excited states, etc. The tunable diode laser absorption spectroscopy (TD-LAS) is successfully used to quantify the density of absorbing species, but especially for stationary or slow changing systems. The time resolved-direct absorption profile (TR-DAP) measurement method by TD-LAS, with time resolution of μs is proposed here as an extension of the regular use of diode laser absorption spectroscopy. The spectral narrowness of laser diodes, especially in the blue range (∼0.01 pm), combined with the nanosecond fast trigger of the magnetron pulsed plasma and long trace recording on the oscilloscope (period of second scale) permit the detection of the sputtered titanium metal evolution in the afterglow (∼ms). TR-DAP method can follow the time-dependence of the temperature (Doppler profile) and the density (deduced from the absorbance) of any medium and heavy species in a pulsed system. View full abstract»

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  • Ultrastiffness and metallicity of rhenium nitrides

    Page(s): 053501 - 053501-6
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    The equation of states, mechanical properties and electronic structures of the recently synthesized rhenium nitrides (Re3N and Re2N) and the pure metal Re have been investigated by the density functional theory calculations considering the effect of spin-orbit coupling. Our results not only indicate Re3N and Re3N to be ultrastiff and hard materials but also reveal that they exhibit mechanical stability and metallic character. Furthermore, the mechanical behaviors for Re, Re3N, and Re3N can be qualitatively clarified from their crystal and electronic structures. The metallic, ultrastiff, and hard Re3N and Re2N may find their promising applications as cutting tools and hard conductors at the extreme conditions. View full abstract»

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  • Correlation between structural and luminescent properties of Eu3+-doped ZnO epitaxial layers

    Page(s): 053502 - 053502-7
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    We have studied the epitaxial growth and photoluminescent (PL) properties of Eu3+-doped ZnO layers in correlation with structural analyses. Incorporation of Eu3+ ions into a ZnO host resulted in deterioration in crystal quality and lattice distortion. The spectral structure of Eu3+ emission revealed that Eu3+ ions were located at site symmetries lower than the original C3v of the Zn2+ ion sites in a hexagonal wurtzite structure of ZnO, which allowed for efficient red luminescence based on the electric dipole transitions. Moreover, we have found an anticorrelation of PL intensity between the excitonic and Eu3+ emissions, related to energy transfer from the ZnO host to the Eu3+ ions. The excitonic structure at the band edge was obscured upon doping with Eu3+ ions due to the formation of a band tail around 3.2 eV. This correlated with a reduction in excitonic emissions, leading to enhancement of Eu3+ emission. Strong PL emission from Eu3+ ions was only observed at low temperatures, and was remarkably suppressed with increasing temperature due to two types of nonradiative energy transfer paths. For the energy transfer process to Eu3+ ions, we suggest that the recombination energy of an electron–hole pair at the band tail near the band edge is close to that of high-lying excited levels in the Eu3+ 4f shell, providing an efficient energy pathway for the excitation of Eu3+ ions in the ZnO system. 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