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Journal of Applied Physics

Issue 5 • Date Sep 2009

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Displaying Results 1 - 25 of 149
  • Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications

    Page(s): 051101 - 051101-14
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    Femtosecond laser fabrication of three-dimensional structures for photonics applications is reviewed. Fabrication of photonic crystal structures by direct laser writing and holographic recording by multiple beam interference techniques are discussed. The physical mechanisms associated with structure formation and postfabrication are described. The advantages and limitations of various femtosecond laser microfabrication techniques for the preparation of photonic crystals and elements of microelectromechanical and micro-optofluidic systems are discussed. View full abstract»

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  • Low-temperature grown near surface semiconductor saturable absorber mirror: Design, growth conditions, characterization, and mode-locked operation

    Page(s): 053101 - 053101-8
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    We have developed a mode-locked diode-pumped Yb:KY(WO4)2 laser generating nearly bandwidth limited pulses as short as 101 fs. At 1.1 W of absorbed power and for 3% transmission output coupler, the laser delivers 150 mW in pulses of 110 fs duration, which corresponds to the efficiency of 14%. This has been achieved using semiconductor saturable absorber mirror (SESAM) grown by molecular beam epitaxy. The low-temperature (LT) absorbers were crystallized under the carefully optimized growth conditions. The resonantlike type structures ensured relatively high enhancement factor and in consequence high absorption modulation. The main device parameters such as group delay dispersion (GDD) and enhancement factor were chosen to be wavelength independent. The optimization of the growth conditions resulted in a reduction in the nonsaturable absorption in as-grown LT-InGaAs absorbing layer and ensured the fast carrier trapping and recombination. We assume that the nonsaturable losses of the annealed LT layers result from the absorption connected with defects generated in the crystal during LT growth. Moreover, the annealing deteriorates the interface sharpness and the crystal quality of LT pseudomorphic, nonstoichiometric InGaAs layer. On the other hand, higher growth temperature and lower ratio of group V to group III beam equivalent pressure (V/III ratio) ensure lower defect densities and high crystal quality but suffer from the absorption related to AsGa0 conduction band transitions. The careful balancing of these contradictory tendencies allowed for optimization of the absorber properties. The InGaAs quantum well absorbing layer was grown at the temperature as high as 420 °C, under the V/III ratio as low as 10. No postgrowth annealing was performed. The - recovery time of the SESAM structure characterized by the pump-probe measurements was equal to 9.6 ps. The nonsaturable losses of 1.94% decreased the modulation depth to 1.48% but still self-starting and stable mode locking was observed. The saturation fluence of 27 μJ/cm2 did not suffer significantly from a little bit too high nonsaturable losses. View full abstract»

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  • The reduction of Eu3+ to Eu2+ in BaMgAl10O17:Eu and the photoluminescence properties of BaMgAl10O17:Eu2+ phosphor

    Page(s): 053102 - 053102-5
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    The variations in emission intensities of AlO4-tetrahedron-containing compound BaMgAl10O17:Eu (BAM) phosphor prepared in different atmospheres is discussed in view of the structure of host compounds. A reduction phenomenon of Eu3+Eu2+ was observed by emission and excitation spectra measurement in BAM for the first time. This reduction was explained by a charge compensation mechanism and the photoluminescence properties were discussed. By comparison, it was found that Eu2+ ions occupy three different lattice sites by substitution for Ba2+ ions in BaMgAl10O17:Eu2+. BaMgAl10O17:Eu2+ phosphors exhibited more excellent thermal stability when Eu2+ ions occupied more Eu (1) sites, and gave a dominating contribution for the photoluminescence when Eu2+ ions occupied Eu (2) sites. View full abstract»

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  • Modal characteristics of terahertz surface-emitting distributed-feedback lasers with a second-order concentric-circular metal grating

    Page(s): 053103 - 053103-11
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    A theoretical model is developed to study the modal characteristics of a second-order concentric-circular metal grating surface-emitting distributed-feedback (DFB) laser operating at terahertz regime. A series of high-order diffracted fields, which can be expressed as a Floquet–Bloch expansion of Hankel functions, is assumed to be generated from the concentric-circular metal grating. The resonant frequencies and transverse profiles of all the diffracted fields can be deduced from the related eigenequations established through the boundary conditions of the interfaces of the metal-dielectric-metal waveguide. The results show that the interference of the diffracted cylindrical waves can form two types of resonant modes, namely, quasisymmetric and quasiantisymmetric modes. Surface radiation is excited mainly by the influence of quasisymmetric modes, which exhibit constructive interference with the grating geometry. Furthermore, the resultant intensities of the diffracted waves decay exponentially from the center of the circular grating, indicating that the proposed grating geometry has the potential to realize surface terahertz radiation with excellent beam quality. The influence of grating duty cycle on the resonant conditions and transverse distributions of the diffracted fields are also investigated. View full abstract»

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  • Comparison of SiO2, Si3N4, As2S3, and Ge0.25Se0.75 dielectric layers for InP- and GaAs-based material systems for midinfrared quantum cascade laser waveguides

    Page(s): 053104 - 053104-6
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    We present in this paper a direct comparison of the optical properties of InP- and GaAs-based quantum cascade laser waveguides operating in the midinfrared wavelength range. The absorption loss and optical confinement were calculated using a two-dimensional electromagnetic finite-element method. The effect on the waveguide properties of SiO2, Si3N4, As2S3, and Ge0.25Se0.75 used as electrical insulation layers is investigated. The results indicate that a careful choice of this particular layer according to the geometrical structure and the emission wavelength should enhance the laser performance. View full abstract»

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  • Photo-induced charge state conversion of Eu2+ in Ca2ZnSi2O7

    Page(s): 053105 - 053105-7
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    Eu2+ doped sorosilicate Ca2ZnSi2O7 (melilite) exhibits a broad band emission peaking at ∼600 nm (∼2 eV) due to the electric dipole allowed transition of 4f65d1 to 4f7 of Eu2+ by an excitation with blue light (460 nm). Strong O2- ligand field with low symmetry due to the layered tetragonal crystallographic structure of the melilite may play a dominant role in lowering the emission band energy to ∼2 eV. In addition, line emissions attributable to the transitions from 5D0 to 7FJ of Eu3+ ions are detected by an excitation with deep UV light with sub-360 nm wavelengths. This is due to the formation of transient Eu3+ ions via charge transfer from Eu2+ to the matrix. The lifetime of the transient Eu3+ ion is found to be 58 ms by a pump-probe measurement, in which UV pulse laser and green continuous wave laser are employed as pump and probe lights, respectively. Based on these results, the energy diagram of Eu2+ in Ca2ZnSi2O7 is proposed. View full abstract»

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  • On the enhanced Raman scattering of the nanosize semiconductor: A couple of cylinders (silicon and silver)

    Page(s): 053106 - 053106-8
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    Raman enhancement in a couple of nanocylinders (one made of silicon and one made of silver) has been calculated in our work. A formulation for enhanced Raman scattering of semiconductor cylinders is presented. The numerical calculation result based on the formulation indicates that the couple of nanocylinders (one made of silicon and one made of silver) can enhance the electric field in the silicon cylinder greatly and shows strong enhancement for volume average compared with bulk silicon. The largest enhancements per unit volume approach 107 times as compared with bulk crystalline silicon. The silicon and silver nanocylinder structures can combine useful features of the resonant excitations of silicon cylinder and the effect of resonant interaction between silicon cylinder and silver cylinder. This leads to the giant enhancement in the field inside the silicon cylinder and hence an increased Raman scattering. Moreover the results indicate that the interaction between the silicon cylinder and the silver cylinder is very strong under proper conditions. View full abstract»

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  • Submicrosecond fluorescence dynamics in erbium-doped silicon-rich silicon oxide multilayers

    Page(s): 053107 - 053107-5
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    The energy transfer process between amorphous silicon nanoparticles and erbium ions in Er-doped silicon-rich silicon oxide is investigated by fluorescence dynamics measurements. A fast decay is observed in the wavelength range of the radiative relaxation of erbium excited ions at 1.53 μm. Alternatively to a previous interpretation, we assign this fast decay to emission of deep traps induced by Si-based sensitizers of Er3+ ions, which emit in the visible and the infrared region. View full abstract»

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  • Grafted polybutadiene for fast retrieval of optical information

    Page(s): 053108 - 053108-4
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    The goal of the investigations was to measure the dynamics of molecular reorientation in polybutadiene backbone based polymer grafted with photochromic thiols. Measurements were performed in a typical degenerate two- and four-wave mixing experiments, in which time constants of dynamic diffraction grating formation were determined. Holographic gratings which were inscribed in polymeric liquid crystalline films show switching times (build-up and decay) within the 20–70 ms range. An excellent reversibility of the recording-erasure process is reported. View full abstract»

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  • Single- and double-negative refractive indices of combined metamaterial structure

    Page(s): 053109 - 053109-5
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    We experimentally and numerically studied the transmission spectra of a combined metamaterial structure whose negative refractive index was supposed to be achievable only with simultaneously negative permittivity and permeability. However, it was found that the negativity of the refractive index of such a structure could be obtained not only when both parameters were negative but also when only one parameter, more specifically the permittivity was negative. These characteristics of combined structure were analyzed in detail by using the standard retrieval effective-medium method. According to the analyses, it can be concluded that the negativity originates from the complex permittivity and permeability. The interplay among the real and the imaginary parts of those parameters is the key to the negative behavior of refractive index. View full abstract»

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  • Influence of wall impedance and self-fields on the cyclotron maser instability

    Page(s): 053110 - 053110-8
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    The compound influence of wall impedance and self-fields on the cyclotron maser instability is investigated for a hollow electron beam. A stability analysis is carried out using the linearized Vlasov–Maxwell equations, under the assumption that the beam thickness is small compared to the beam radius. A dispersion relation is derived and solved numerically to study the effects of the wall impedance and self-fields on the cyclotron maser instability. These effects lead to the elliptical motion of the equilibrium configuration. The growth rate decreases due to the wall resistivity and self-fields. It has been shown that the interaction between the self-field and impedance effects is in the lower reduction in the growth rate when they are both present compared to their separate effects added together. The instability bandwidth increases due to the wall impedance and decreases due to the self fields. In the presence of self-fields, a very small increase in the wall impedance causes an increase in the instability bandwidth. This shows that the widening effect of the bandwidth due to the wall impedance is dominant and prevails over the narrowing effect of the self-field. View full abstract»

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  • Anisotropic composite right/left-handed-material switch

    Page(s): 053111 - 053111-4
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    It is shown that an anisotropic composite structure, which consists of alternating layers with either negative permittivity or negative permeability, can switch between optical responses characteristic of right-handed and left-handed materials depending on the polarization (TE or TM) of the incident electromagnetic field. The switching possibility is maintained for a small angular range around normal incidence, the behavior of the structure for larger incidence angles showing superluminal group velocities characteristic for anisotropic metamaterials. View full abstract»

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  • Addressing the plasma formation on the surface of a ferroelectric sample

    Page(s): 053301 - 053301-5
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    A description of plasma formation on the surface of a ferroelectric sample is presented, based on simple assumptions about the electrodynamic evolution of the system along with the continuity equation for the current and the conservation of the ion flow impulse. Two models starting with these assumptions are shown capable of predicting, with satisfactory agreement to experimental data, the plasma density and temperature, and plasma propagation velocity. Also, one of the models allows one to obtain the distributions of the potential, surface charge density, and current density along the ferroelectric surface during plasma propagation. View full abstract»

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  • Achieving multifrequency transparency with cylindrical plasmonic Cloak

    Page(s): 053302 - 053302-6
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    In this work, we have theoretically investigated the possibility to achieve multifrequency transparency and cloaking for a cylindrical object by employing a multilayered plasmonic shell. The frequency dispersion of plasmonic material and the size effect of the cylindrical object have been considered. The analytical transparency condition in the quasistatic case has not only been given, an exact numerical calculation based on the Mie scattering theory has also been performed. Our results show that the multifrequency transparency and cloaking for any dielectric or metal cylinder can always be realized by a suitable adjustment of the structure parameter, permittivity, and permeability of the multilayered shell. Such a transparency is not only for a certain polarized wave, it can also be realized for both polarizations of electromagnetic wave at the same time. View full abstract»

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  • An efficient mode conversion configuration in relativistic magnetron with axial diffraction output

    Page(s): 053303 - 053303-3
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    Relativistic magnetron with diffraction output, also known as magnetron with axial extraction, is a favorite in exploring the most compact narrow band high power microwave source. For further improving power conversion efficiency and radiation pattern without diminishing the compactness, this paper proposed an efficient mode conversion configuration in an S-band π-mode operating relativistic magnetron with diffraction output. The whole device has a total length of 290 mm and a maximum diameter of 260 mm. In three-dimensional particle-in-cell (PIC) simulation, an optimized outcome that the power conversion efficiency can reach 43% corresponding to 4.2 GW output and the radiation pattern is similar to TE11 mode is achieved. View full abstract»

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  • Measuring the pressure in ultrahigh-pressure mercury arcs

    Page(s): 053304 - 053304-14
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    Ultrahigh-pressure (UHP) mercury lamps are important as high-brightness light sources for digital projection. Hg pressures are usually above 20 MPa and difficult to measure. We have built special UHP lamps with a liquid Hg condensate in a temperature-controlled reservoir, allowing us to tune the Hg vapor pressure p between 14 and 30 MPa. As a simple measure for p, we recorded the width Δλ of the 546 nmHg line while varying p and also the lamp current I and voltage U. The data define a function p(Δλ,I,U) that will deliver p to better than 3% from simple measurements of Δλ, I, and U for most UHP lamps in the important 100–200 W power range. The method is applied to sample lamps, yielding pressures up to 26 MPa and demonstrating how filled Hg amount, burning position, arc gap, and lamp power affect the pressure. The effective temperature of typical UHP lamps is found to be 2400 K. We also derive an improved characteristic U(d,p,I) for the dependence of the arc voltage on arc gap, pressure, and current for electrode-stabilized Hg discharges in the UHP regime. Some aspects of the experiment are of general interest in the field of discharge lamps, such as a model for the heat balance of the Hg condensate under conductive, radiative, and evaporative cooling/heating, a short discussion of high-temperature vapor-pressure data for Hg, and an improved Hg equation of state for UHP conditions. View full abstract»

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  • Sheath expansion and plasma dynamics in the presence of electrode evaporation: Application to a vacuum circuit breaker

    Page(s): 053305 - 053305-12
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    During the postarc dielectric recovery phase in a vacuum circuit breaker, a cathode sheath forms and expels the plasma from the electrode gap. The success or failure of current breaking depends on how efficiently the plasma is expelled from the electrode gap. The sheath expansion in the postarc phase can be compared to sheath expansion in plasma immersion ion implantation except that collisions between charged particles and atoms generated by electrode evaporation may become important in a vacuum circuit breaker. In this paper, we show that electrode evaporation plays a significant role in the dynamics of the sheath expansion in this context not only because charged particle transport is no longer collisionless but also because the neutral flow due to evaporation and temperature gradients may push the plasma toward one of the electrodes. Using a hybrid model of the nonequilibrium postarc plasma and cathode sheath coupled with a direct simulation Monte Carlo method to describe collisions between heavy species, we present a parametric study of the sheath and plasma dynamics and of the time needed for the sheath to expel the plasma from the gap for different values of plasma density and electrode temperatures at the beginning of the postarc phase. This work constitutes a preliminary step toward understanding and quantifying the risk of current breaking failure of a vacuum arc. View full abstract»

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  • Development of atomic radical monitoring probe and its application to spatial distribution measurements of H and O atomic radical densities in radical-based plasma processing

    Page(s): 053306 - 053306-4
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    Atomic radicals such as hydrogen (H) and oxygen (O) play important roles in process plasmas. In a previous study, we developed a system for measuring the absolute density of H, O, nitrogen, and carbon atoms in plasmas using vacuum ultraviolet absorption spectroscopy (VUVAS) with a compact light source using an atmospheric pressure microplasma [microdischarge hollow cathode lamp (MHCL)]. In this study, we developed a monitoring probe for atomic radicals employing the VUVAS with the MHCL. The probe size was 2.7 mm in diameter. Using this probe, only a single port needs to be accessed for radical density measurements. We successfully measured the spatial distribution of the absolute densities of H and O atomic radicals in a radical-based plasma processing system by moving the probe along the radial direction of the chamber. This probe allows convenient analysis of atomic radical densities to be carried out for any type of process plasma at any time. We refer to this probe as a ubiquitous monitoring probe for atomic radicals. View full abstract»

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  • Effect of the electron energy distribution function in plasma on the Bohm criterion and on the drop voltage through the sheath: Case of microwave expanding plasma

    Page(s): 053307 - 053307-9
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    This work is devoted to the study of the sheath expansion above a charged species collector immersed into the plasma in the general case of the electron energy distribution function (EEDF) (not necessarily a Boltzmann distributed function). In the first part, considering the general form of the electron kinetic energy distribution function (EEDF) in the plasma and assuming a one dimensional system, we propose an equation describing the limit value of the ion velocity at the sheath edge. Then, we extend this equation to the three dimensional problem. These results are discussed assuming Boltzmann distribution for the one dimensional system and Maxwell–Boltzmann distribution in the case of the three dimensional system. Both one and two temperature distributions are considered. Then, the method is applied to experimental results obtained in expanding microwave discharge sustained in argon. In the second part, we study the drop voltage through the sheath that is produced around a cylindrical collector biased at the floating potential. A theoretical model is given, and results are compared with experimental values measured by means of a cylindrical Langmuir probe immersed in microwave expanding plasma. The model agrees with experiments and points out the role of the EEDF profile on the ion velocity at the sheath edge and, consequently, on the drop voltage through the sheath. View full abstract»

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  • Determination of plasma velocity from light fluctuations in a cutting torch

    Page(s): 053308 - 053308-4
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    Measurements of plasma velocities in a 30 A high energy density cutting torch are reported. The velocity diagnostic is based on the analysis of the light fluctuations emitted by the arc which are assumed to propagate with the flow velocity. These light fluctuations originate from plasma temperature and plasma density fluctuations mainly due to hydrodynamic instabilities. Fast photodiodes are employed as the light sensors. The arc core velocity was obtained from spectrally filtered light fluctuations measurements using a band-pass filter to detect light emission fluctuations emitted only from the arc axis. Maximum plasma jet velocities of 5000 ms-1 close to the nozzle exit and about 2000 ms-1 close to the anode were found. The obtained velocity values are in good agreement with those values predicted by a numerical code for a similar torch to that employed in this work. View full abstract»

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  • Resolving the coverage puzzle of the
    Pb/Si(111)-
     7
    ×
     3
    phase

    Page(s): 053501 - 053501-3
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    The dense

    Pb/Si(111)-α-
     3
    ×
     3
    has been extensively studied over the last 20 years with scanning tunneling microscopy (STM) and surface diffraction to determine its structure and its phase transformations with temperature T and coverage θ. Two apparently incompatible models have been proposed for the structure and have been debated in the literature. One model was based on a coverage assignment θ=1 ML. The other model was based on assigning coverage in the range 1.2 ML≪θ≪4/3 ML. The current experiments show that there are two different
     7
    ×
     3
    phases with the same unit cell symmetry but different stoichiometry. This hopefully answers the debated question. View full abstract»

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  • Extraordinary magnetic field induced suppression of luminescence in Er3+-doped nano-glass-ceramics

    Page(s): 053502 - 053502-5
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    The effect of magnetic field on luminescence intensity is generally expected to be rather weak. Contrary to this expectation, we report here an extraordinary strong intensity suppression (by two orders of magnitude) of the Er3+ 4S3/24I15/2 green emission line in Er3+-doped nano-glass-ceramics by applying magnetic field up to 50 T. We argue that this effect arises from different site geometries and, consequently, strongly differing Judd–Ofelt intensities of transitions for the two Kramers doublets of the emitting 4S3/2 multiplet. View full abstract»

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  • The orientation dependent electromigration induced healing on the surface cracks and roughness caused by the uniaxial compressive stresses in single crystal metallic thin films

    Page(s): 053503 - 053503-12
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    The first order unified linear instability analysis (ULISA) of the governing equation for evolutions of surfaces and interfaces under the capillary, electromigration, and elastostatic forces including the thermomigration (Soret effect) is developed very recently by the author. In the present application of the theory, the concurrent effects of uniaxial applied stresses and the electrostatic field on the sidewall morphological evolution of a single crystal thin metallic film are explored by dynamic computer simulations by taking the surface drift diffusion anisotropy fully into account. These computer experiments, which are supported by ULISA, clearly show that only the applied elastic compressive stresses are primary agents responsible for the morphological instability of the surface undulations through the elastic dipole tensor interactions but not the uniaxial tension loading in thin solid films. It is also demonstrated that these morphological instabilities manifested themselves as formations of the surface cracks and thus one may fully control the roughness. To do that, one needs to select crystal orientations properly with respect to the applied field so that a counteraction of the applied electrostatic fields (healing effect) is created above well defined threshold levels of electromigration. On the contrary to the healing effects, the improper selection of crystal orientations may drastically enhance the instability and eventually may cause catastrophic interconnect failure. At large normalized surface undulation amplitudes (a≥0.20), the drastic reductions in the decay rate constants (i.e., the strain relaxation rate) are detected in the nonlinear uniaxial tension regime compared to the ULISA theory regardless of the intensity of the normalized stress by analyzing the data obtained from the computer simulations. This situation is contrary to the results deduced from the low to moderate normalized amplitude (a≤0.10) measurements, where one finds that the decay rate constant closely obeys the prediction of the ULISA theory even for very high stress intensities. View full abstract»

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  • Poissonian statistics of excitonic complexes in quantum dots

    Page(s): 053504 - 053504-6
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    We report a detailed experimental investigation of the power dependence of excitonic complexes (neutral exciton, neutral biexciton, and charged exciton) confined in single self-assembled GaAs/AlGaAs strain-free quantum dots grown by droplet epitaxy. By using the random population theory we show that, under stationary excitation, the power dependence of the excitonic complexes precisely follows the Poissonian statistics. This result allows us to determine with great accuracy the state filling condition of the quantum dots (QDs) and therefore to estimate the capture volume of the QDs. View full abstract»

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  • Determination of second-order elastic constants of cyclotetramethylene tetranitramine -HMX) using impulsive stimulated thermal scattering

    Page(s): 053505 - 053505-5
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    The second-order elastic constants for cyclotetramethylene tetranitramine -HMX) single crystals were determined using the impulsive stimulated thermal scattering (ISTS) method. Despite the low symmetry of these crystals, the complete set of 13 elastic constants were determined accurately from acoustic velocity measurements using samples cut parallel to three different crystal planes. Our acoustic velocities are consistent with the limited sound speed data available from ultrasonic measurements. However, significant differences are observed between the elastic constants determined from our experiments and those obtained previously using Brillouin scattering. Our results demonstrate the usefulness and efficiency of the ISTS method for determining the full set of elastic constants of low-symmetry molecular crystals, including energetic crystals. 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