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Journal of Applied Physics

Issue 10 • Date May 2008

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Displaying Results 1 - 25 of 154
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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  • Small-scale energy harvesting through thermoelectric, vibration, and radiofrequency power conversion

    Page(s): 101301 - 101301-24
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    As sensors for a wide array of applications continue to shrink and become integrated, increasing attention has been focused on creating autonomous devices with long-lasting power supplies. To achieve this, energy will have to be harvested from the sensors’ environment. An energy harvesting device can power the sensor either directly or in conjunction with a battery. Presented herein is a review of three types of energy harvesting with focus on devices at or below the cm3 scale. The harvesting technologies discussed are based on the conversion of temperature gradients, mechanical vibrations, and radiofrequency waves. Operation principles, current state of the art, and materials issues are presented. In addition, requirements and recent developments in power conditioning for such devices are discussed. Future challenges specific to miniaturization are outlined from both the materials and device perspectives. View full abstract»

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  • Measurement of optical loss variation on thickness of InGaN optical confinement layers of blue-violet-emitting laser diodes

    Page(s): 103101 - 103101-5
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    An optical loss of GaN-based blue-violet laser diodes (BV-LDs) was measured by taking the intensity decay of edge emitting luminescence with respect to the distance from cleaved edge of a wafer to the position where an excitation laser was focused. Amplified spontaneous emission (ASE) was also investigated by tuning the power of an excitation laser on BV-LD wafers. Measurements were performed on wafers with different thicknesses of InGaN optical confinement layers (OCLs). The threshold power of ASE intensity was minimized at an optimum thickness of InGaN OCL. We also found that optical loss of wafers was determined by absorption of an InGaN layer in thicker OCL structure. From experimental data and fittings, we obtained 40 cm-1 for InGaN absorption at 405 nm. The optical field confined in OCL region was reasonably high enough to affect the overall modal loss in devices. Therefore, the optical losses still remained even though the Mg-doped GaN regions are far enough from the active layers. The crystal quality of an InGaN layer should be an important aspect to improve the performance of BV-LDs. View full abstract»

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  • Low saturation fluence in a semiconductor saturable electroabsorber mirror operated in a self-biased regime

    Page(s): 103102 - 103102-4
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    A semiconductor saturable absorber mirror utilizing the electroabsorption effect on a self-biased stack of extremely shallow quantum wells is proposed and analyzed theoretically and numerically. The saturation flux and recovery time of the proposed device when operated with picosecond incident pulses are shown to compare very favorably with existing all-optical constructions. View full abstract»

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  • Nonequilibrium population distribution in excited electronic states of Tb3+ in Y3Al5O12

    Page(s): 103103 - 103103-6
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    The Zeeman effect has been studied within the “green” range of luminescence due to the radiative 4f→4f transitions 5D47F5 for the rare earth ion Tb3+ in yttrium aluminum garnet (Y3Al5O12) at a sample temperature of 85 K. The special features associated with the magnetic field dependence of the Zeeman splitting have been explained for transitions observed in the geometry of the longitudinal and transverse Zeeman effect. An analysis of the results obtained from the magneto-optical data indicates that in a magnetic field a nonequilibrium population distribution occurs between the sublevels of the quasidoublet state 14) in the 5D4 manifold. The energy of the quasidoublet state is approximately 20 585 cm-1. View full abstract»

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  • Investigation of neodymium-diffused yttrium vanadate waveguides by confocal microluminescence

    Page(s): 103104 - 103104-6
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    We report on the confocal microspectroscopic characterization of planar optical waveguides fabricated in yttrium vanadate (YVO4) crystals by thermal diffusion of neodymium ions. We have found that the waveguide formation is accompanied by Nd3+ fluorescence quenching and by a redshift of the Nd3+ sub-Stark transitions. These effects have been explained in terms of the high Nd3+ concentrations achieved close to the diffusion surface, which lead to the appearance of nonradiative processes, as well as to a local compression in the surroundings of the indiffused Nd3+ ions. By comparing with previous works, we have estimated an equivalent residual stress of 0.1 GPa. The possible mechanisms at the origin of waveguide formation have been also discussed. View full abstract»

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  • Stimulated Brillouin scattering pulse compression to 175 ps in a fused quartz at 1064 nm

    Page(s): 103105 - 103105-4
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    Stimulated Brillouin scattering pulse compression of a 2.5 ns laser into a 175 ps pulse using a fused quartz is demonstrated without optical damage. The synchronization and the time jitter between the initial and the compressed pulses were measured (σ≪80 ps) and analyzed numerically. View full abstract»

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  • Terahertz wave localization at a three-dimensional ceramic fractal cavity in photonic crystals

    Page(s): 103106 - 103106-5
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    A three-dimensional microphotonic crystal with diamond structure, in which a cube cavity of the stage 2 Menger sponge fractal is embedded, was designed in order to localize effectively terahertz waves. The crystal was fabricated from alumina ceramics by a computer aided design/computar aided manufacturing microstereolithography and sintering process. The localization frequency was in good agreement with both the simulated frequency determined by the transmission line modeling method and the calculated frequency by using the empirical equation. The simulated amplitude distribution of electric field oscillations showed that three different resonance modes exist in substructures of the sponge with each half wavelength oscillation. The wave leakage from the single sponge fractal was effectively blocked by the outer diamond structure. View full abstract»

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  • Effects of thermal treatment under different atmospheres on the spectroscopic properties of nanocrystalline TiO2

    Page(s): 103107 - 103107-4
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    The anatase nanocrystalline TiO2 samples were synthesized by a sol-gel method with a thermal treatment under air and oxygen atmospheres at 500 °C and investigated by x-ray powder diffraction, ultraviolet-visible diffuse reflectance spectra (UV-vis DRSs), Raman spectra, and photoluminescence (PL) spectra, respectively. Compared to the sample calcined in air, there were slight differences in the lattice parameter and Raman line shape for the sample calcined in oxygen. UV-vis DRSs exhibited a visible redshift of the absorption edge, which could be understood by Burstein–Moss effect. Different calcination atmospheres led the obvious variations of intensity, position, and linewidth in the PL spectra, which could be attributed to the surface oxygen defect changes. View full abstract»

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  • Quantitative broadband chemical sensing in air-suspended solid-core fibers

    Page(s): 103108 - 103108-7
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    We demonstrate a quantitative broadband fiber sensor based on evanescent-field sensing in the cladding holes of an air-suspended solid-core photonic crystal fiber. We discuss the fabrication process, together with the structural and optical characterization of a range of different fibers. Measured mode profiles are in good agreement with finite element method calculations made without free parameters. The fraction of the light in the hollow cladding can be tuned via the core diameter of the fiber. Dispersion measurements are in excellent agreement with theory and demonstrate tuning of the zero dispersion wavelength via the core diameter. Optimum design parameters for absorption sensors are discussed using a general parameter diagram. From our analysis, we estimate that a sensitivity increase of three orders of magnitude is feasible compared to standard cuvette measurements. Our study applies to both liquid and gas fiber sensors. We demonstrate the applicability of our results to liquid chemical sensing by measuring the broad absorption peak of an aqueous NiCl2 solution. We find excellent agreement with the reference spectrum measured in a standard cuvette, even though the sample volume has decreased by three orders of magnitude. Our results demonstrate that air-suspended solid-core photonic crystal fibers can be used in quantitative broadband chemical-sensing measurements. View full abstract»

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  • Optical properties of simple hexagonal and rhombohedral few-layer graphenes in an electric field

    Page(s): 103109 - 103109-8
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    The influence of a perpendicular electric field (F) on the optical properties of simple hexagonal and rhombohedral few-layer graphenes is studied through the tight-binding model. The electric-field-modulated absorption spectra depend on the stacking sequence. The low-energy absorption spectra of simple hexagonal few-layer graphenes exhibit the jumping structures in the absence or presence of an electric field. On the other hand, absorption spectra of rhombohedral few-layer graphenes show discontinuities and sharp peaks at F=0. Besides, the application of F affects the absorption spectra, generates new peaks, and changes peak position and peak height. The frequency of the peak is predicted to be closely associated with the stacking sequences and the field strength. Above all, the predicted absorption spectra could be verified by optical measurements. View full abstract»

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  • From beamforming to diffraction tomography

    Page(s): 103110 - 103110-7
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    Recent progress in sensor array technology has boosted the use of beamforming (BF) in a number of applications. The same progress has generated interest in other imaging modalities such as diffraction tomography (DT). This paper demonstrates the existence of a linear mapping between BF and DT, which is represented by a linear filter in the spatial frequency domain. The filter is used to study and compare the information content of images obtained with the two imaging modalities. In particular, it is demonstrated that a BF image is a distorted version of the corresponding DT image. Moreover, the analytical expression of the filter is derived leading to an algorithm for DT which, in contrast to currently available algorithms, does not require the use of data interpolation techniques. View full abstract»

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  • Generation of controlled radiation sources in the atmosphere using a dual femtosecond /nanosecond laser pulse

    Page(s): 103111 - 103111-6
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    The generation of powerful radiation sources at predetermined remote locations in the atmosphere using dual femtosecond/nanosecond laser pulses is proposed. The plasma channel generated in the wake of the collapsed beam formed during intense ultrashort pulse propagation in the atmosphere serves as seed target plasma to absorb energy delivered by a long pulse laser, inducing further ionization and significantly enhancing the energy density locally deposited in the plasma. Energy equipartition on time scales shorter than 1 μs results in large energy density in the heated air and is followed by several orders of magnitude enhancement of the radiation emitted by the plasma channel. The effect is demonstrated using a one-dimensional computational study. A proof-of-principle experiment is suggested. View full abstract»

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  • Enhanced efficiency in near-infrared inorganic/organic hybrid optical upconverter with an embedded mirror

    Page(s): 103112 - 103112-5
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    We report a hybrid organic-inorganic optical upconverter with an embedded mirror, which converts 1.5 μm infrared light to visible light. The device was fabricated through direct tandem integration of an organic light-emitting diode with an inorganic InGaAs/InP photodetector. It was found that the device with an embedded mirror exhibited a low turn-on voltage (∼3.2 V) and an enhanced efficiency. The ratio of photocurrent-induced light with an input power density of 0.67 mW/mm2 versus dark-current-induced visible light was over 500 at a device bias of 6 V at room temperature. The results show that the embedded mirror at the inorganic-organic interface plays a vital role in the performance enhancement of a hybrid upconverter. View full abstract»

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  • Monte Carlo simulation of carrier transport and output characteristics of terahertz quantum cascade lasers

    Page(s): 103113 - 103113-4
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    An ensemble Monte Carlo method, including electron-phonon, electron-electron, electron-impurity scatterings and the hot phonon effect, is used to simulate the carrier transport and output characteristics of a lasing terahertz quantum cascade laser. A simulated I-V curve fits well with the measurement at the lasing domain. Extracted output characteristics, e.g., the gain, threshold current density, and threshold bias, are in good agreement with the experimental results. All the above indicate that the proposed Monte Carlo model is a useful tool for investigating the physical characteristics of terahertz quantum cascade lasers, as well as for analyzing and optimizing device performances. View full abstract»

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  • On the correlation between the photoexcitation pathways and the critical energies required for ablation of poly(methyl methacrylate): A molecular dynamics study

    Page(s): 103114 - 103114-8
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    The energetics initiating ablation in poly(methyl methacrylate) (PMMA) are studied using molecular dynamics (MD) simulation. The critical energy to initiate ablation in PMMA following the absorption of photons is investigated for two penetration depths along a range of fluences using a coarse-grained, hybrid Monte Carlo-MD scheme. Both heating and direct bond scission are simulated separately after photon absorption with additional transformation of material occurring via chemical reactions following the photochemical bond cleavage. For a given type of absorption and reaction channel, a critical energy can well describe the amount of energy required to initiate ablation. The simulations show a decrease in the critical energy when a greater amount of photochemistry is introduced in the system. The simulations complement experimental studies and elucidate how enhanced photochemistry lowers ablation thresholds in polymer substrates. View full abstract»

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  • Polarization-dependent optical characteristics of violet InGaN laser diodes

    Page(s): 103115 - 103115-6
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    The polarization-dependent optical characteristics of violet InGaN laser diodes, such as band diagrams, emission wavelength, and threshold current, under different operation temperatures have been investigated numerically. Specifically, the normal and reversed polarizations are presented when the laser diodes with wurtzite structure are grown along Ga-face and N-face orientations, respectively. The simulation results show that the lowest threshold current is obtained for the double-quantum-well laser diode with normal polarization, while it is obtained for the single-quantum-well laser diode with reversed polarization. The main physical explanation for the phenomenon is due to effectively reduced electron leakage current, increased hole current density, and reduced Shockley–Read–Hall recombination rate within the active region as the idea of reversed polarization is considered. View full abstract»

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  • Intersubband resonant enhancement of the nonlinear optical properties in compositionally asymmetric and interdiffused quantum wells

    Page(s): 103116 - 103116-7
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    We report the resonant enhancement of the second- and third-order optical nonlinearities in compositionally asymmetric quantum wells with finite confining potential and interdiffused quantum wells. The energy levels and the envelope wave functions in quantum wells are obtained by solving numerically the Schrödinger equation. The optical rectification, second and third harmonic generation coefficients are calculated within the framework of the density matrix formalism. The effect of the structure parameters such as the well width and the barrier height on the nonlinear optical properties is investigated in detail. The resulting nonlinear susceptibilities obtained in both quantum wells are considerably larger than those of bulk GaAs. View full abstract»

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  • Conversion efficiencies of electron beam energy to vacuum ultraviolet light for Ne, Ar, Kr, and Xe excited with continuous electron beams

    Page(s): 103301 - 103301-8
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    Continuous 8–15 keV electron beams were used to excite Ne, Ar, Kr, and Xe at pressures ranging from 75 to 1400 hPa. Conversion efficiencies of the energy deposited by the electron beam in the gas to vacuum ultraviolet light emitted from excimers are obtained. At pressures above 500 hPa, the efficiencies show constant values of 0.31±0.06 for Ne, 0.33±0.04 for Ar, 0.42±0.05 for Kr, and 0.42±0.05 for Xe. Apparent energy conversion efficiencies for practical devices utilizing this excitation technique are also reported. View full abstract»

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  • Reaction mechanisms during plasma-assisted atomic layer deposition of metal oxides: A case study for Al2O3

    Page(s): 103302 - 103302-14
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    Plasma-assisted atomic layer deposition (ALD) of metal oxide films is increasingly gaining interest, however, the underlying reaction mechanisms have rarely been addressed. In this work, a case study is presented for the plasma-assisted ALD process of Al2O3 based on Al(CH3)3 dosing and O2 plasma exposure. A complementary set of time-resolved in situ diagnostics was employed, including spectroscopic ellipsometry, quartz crystal microbalance, mass spectrometry, and optical emission spectroscopy. The saturation of the Al(CH3)3 adsorption reactions was investigated, as well as the reaction products created during both the precursor dosing and the plasma exposure step. The generality of the observations was cross-checked on a second commercial ALD reactor. The main observations are as follows: (i) during the precursor dosing, the Al(CH3)3 predominantly binds bifunctionally to the surface at 70 °C through a reaction in which H is abstracted from the surface and CH4 is released into the gas phase; (ii) during the plasma exposure, O radicals in the plasma are consumed at the surface by combustionlike reactions with the surface -CH3 ligands, producing mainly H2O, CO2, and CO; (iii) small gas phase densities of CH4 and higher hydrocarbons (C2Hx) are also present during the O2 plasma exposure step indicating complementary surface reactions including a secondary thermal ALD-like reaction by the H2O produced at the surface; (iv) the plasma and its optical emission are strongly affected by the surface reaction products released in the plasma. In the latter respect, optical emission spectroscopy proved to be a valuable tool to study the surface reaction products during the plasma exposure as well as the saturation of the surface reactions. The implications of the experimental observations are addressed and it is discussed that the reaction mechanisms are generic for plasma-assisted ALD processes based on metal organic precursors and O2 plasma as oxidant source. View full abstract»

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  • Force generation due to three-dimensional plasma discharge on a conical forebody using pulsed direct current actuators

    Page(s): 103303 - 103303-7
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    Understanding the behavior of three-dimensional plasmas around a pulsed dc actuator can be useful for its efficient operation in many applications. The effect of such actuators is studied using a self-consistent multibody system of neutral oxygen species and its plasma. The equations governing the motion of charged species are solved with the drift diffusion approximation. The electrostatic potential is computed from Poisson’s equation. The electric field and ionization level are the highest close to the junction of electrodes and dielectric. The plasma body force thus generated also follows a similar characteristic. Results also show some dc corona instabilities. The temporal average of such force shows mostly acceleration from anode to cathode above the actuator. View full abstract»

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  • Spatial structure of a slot-antenna excited microwave N2Ar plasma source

    Page(s): 103304 - 103304-12
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    The spatial structure of a large-scale, slot-antenna excited (2.45 GHz) surface wave plasma source operating in N2Ar mixtures is investigated. A self-consistent theoretical model is developed in the local approximation to investigate the entire spatial structure of the system, including the discharge zone sustained by the field of the TM140 surface mode and the remote plasma zone. Maxwell’s equations and the rate balance equations for the most important excited species—vibrationally and electronically excited states, ions, and N(4S) atoms—and the electron Boltzmann are consistently solved. The pumping of the higher νth levels of N2(X 1Σg+,ν) molecules is shown to be very effective and to strongly influence the remote plasma kinetics. Collisions of N2(X 1Σg+,ν) molecules with N(4S) atoms are responsible for the increase in the number densities of electrons and electronically excited states N2(A 3Σu+,B 3Πg,C 3Πu,a 1Σu-) in the “far” remote plasma zone. View full abstract»

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  • Transversely isotropic elastic properties of single-walled carbon nanotubes by a rectangular beam model for the CC bonds

    Page(s): 103501 - 103501-11
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    Continuum mechanics modeling of carbon nanotubes has long been an attractive issue, but how to reflect exactly the physics essential of the atomic bonds still remains to be a challenging problem. To capture the distinguishing in-plane σ-σ and out-of-plane σ-π bond angle bending rigidities of CC bonds in carbon nanotubes, an equivalent beam element with rectangular section is proposed and a corresponding frame structure model for a single-walled carbon nanotube (SWNT) is developed. By using the model, the five independent elastic moduli of SWNTs with arbitrary chirality and diameter are evaluated systematically. It is found that the elastic properties of the SWNTs are transversely isotropic when the tube diameter is small. The smaller the tube diameter is, the stronger the dependence of the elastic properties on the tube size and chirality is, while when the tube diameter is large enough, the SWNTs degenerate from transversely isotropic to isotropic and the elastic moduli tend to that of a graphite sheet. The present model can be incorporated into any standard finite element software directly, providing an extremely versatile and powerful tool for the study of nanostructures that beyond the computational capability of current atomistic approaches. View full abstract»

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  • Band gap narrowing and radiative efficiency of silicon doped GaN

    Page(s): 103502 - 103502-5
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    Radiative efficiency, band gap narrowing, and band filling are studied in Si-doped GaN films as a function of carrier concentration (n), using room and low temperature cathodoluminescence (CL). Using the Kane model, a band gap narrowing ΔEg of -(3.6±0.6)×10-8 and -(2.6±0.6)×10-8 n1/3 eV n1/3 is obtained for epitaxially strained and relaxed material, respectively. Band-edge CL time response and absolute external photon yield are measured. The internal radiation efficiency is deduced. Its monotonic increase as n increases is explained by the increase in the spontaneous radiative rate with a radiative free carrier band-to-band recombination coefficient B=(1.2±0.3)×10-11 cm3s-1. 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