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Journal of Applied Physics

Issue 1 • Date Jul 1991

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Displaying Results 1 - 25 of 93
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Backscattering of electromagnetic missile by a perfectly conducting elliptical cylinder

    Page(s): 1 - 3
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    Recently the backscattering of an electromagnetic (EM) missile with an energy decreasing as r by a perfectly conducting plate has been studied and the analysis shows that the energy backscattered varies as r with the same exponent as the incident EM missile. Scattering of an EM missile by other shapes is left as an open question. In this paper we study the backscattering of an EM missile with energy decreasing as r by a perfectly conducting elliptical cylinder. It has been shown that the rate of the energy decay for the backscattered field far from the cylinder is proportional to r-(1+ϵ), which is faster than r for plate scattering but still slower than the ordinary energy decay r-2 for a two‐dimensional backscattered field. View full abstract»

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  • Beam divergence from sharp emitters in a general longitudinal magnetic field

    Page(s): 4 - 12
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    We calculate the trajectories of electrons emitted from a sharp protrusion with corners, where the electric field is intense and the radius of curvature tends to zero. The calculations include the effects of a longitudinal magnetic field but ignore the space‐charge effects. We find that the arbitrarily large electric field at a mathematically sharp corner does not necessarily impart to the electrons an excessive amount of transverse momentum, whether or not the external magnetic field is present. Scaling laws are derived for the beam’s transverse displacement in terms of macroscopic quantities, such as anode‐cathode voltage drop, gap separation, magnetic field, and the protrusion dimensions. The implication of these findings on the electron sources generated from microtips is addressed. View full abstract»

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  • Enhanced polarization‐dependent coupling between an optical waveguide and a laterally coupled photodetector

    Page(s): 13 - 16
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    We have calculated the parameters for an optimal matching layer between a waveguide and a laterally coupled photodetector for TE and TM modes. We find a difference on the order of a few percent between the classical and exact calculations based on optical modes concerning the optimal matching layer thickness. Also, the thickness of the matching layer depends critically on the thickness of the absorbing layer. Comparing TE and TM modes, the absorption maxima for TE(TM) and the minima for TM(TE) coincide approximately, and the maxima and minima depend on the matching layer thickness. View full abstract»

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  • New symmetrical pi‐conjugated molecules having large third‐order optical nonlinearities

    Page(s): 17 - 19
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    New symmetrical pi‐conjugated molecules having large third‐order nonlinear susceptibilities, χ(3), are reported. The χ(3) spectra of these molecules have been measured by a THG Maker‐fringe method. The magnitudes of the three‐photon resonant χ(3) reach 10-10 esu. The positions of the three‐photon resonance lie at the lowest‐energy peaks in the vibrational structure of each absorption spectra for these molecules. View full abstract»

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  • Origin of picosecond‐pulse‐induced, degenerate four‐wave‐mixing signals in KTa1-xNbxO3 crystals

    Page(s): 20 - 28
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    Transient gratings have been produced in KTa1-xNbxO3 by picosecond‐pulse, two‐photon excitation using degenerate four‐wave‐mixing techniques. The excitation process has been characterized through fluorescence studies, and the fluorescence is attributed to the transition between an excited state and the ground state of B4+ ions that are produced in ABO3 perovskite crystals. Strong electron‐phonon coupling gives rise to the luminescence quenching. The observed degenerate four‐wave‐mixing signal is shown to be due predominantly to a phase grating caused by a change in the susceptibility associated with the formation of Nb4+ or Ta4+ ions in the peak region of the grating. The grating signal intensity was found to be dependent on the concentration of niobium ions and the crossing angle of the two laser write beams. View full abstract»

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  • Reflectance modulator utilizing an electro‐optic polymer film

    Page(s): 29 - 32
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    Device fabrication and experimental results are reported for a reflectance modulator in which coupling between a 632.8 nm laser beam reflected from the base of a prism and a guided mode in an electro‐optic polymer is electrically varied. The beam intensity was varied ±31% by application of 46 V rms across the polymer. The response is independent of frequency up to at least 5 MHz. View full abstract»

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  • Growth and self‐pumped phase conjugation of Ce‐doped KNa(Sr0.61Ba0.39)0.9Nb2O6 crystal

    Page(s): 33 - 35
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    Self‐pumped phase conjugate in trapezoid‐cut Ce‐doped KNa(Sr0.61Ba0.39)0.9Nb2O6 crystal was demonstrated without external reflection mirrors and applied field, phase conjugation reflectivities as high as 25% were measured. The threshold power was lower than 0.03 W/cm2. The device operated by one‐region four‐wave mixing using the photorefractive effect. View full abstract»

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  • Temperature dependence of degenerate four wave mixing in azo dye doped polymer films

    Page(s): 36 - 40
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    Experimental results of the temperature dependence of the nonlinear optical response of disperse red 1 and methyl red doped in polystyrene film in the range 90–290 K are reported. It is found that the phase conjugate signal intensity resulting from degenerate four wave mixing is decreased by an order of magnitude at low temperature. The decay time of the recorded grating at fixed pump intensity also decreases with decreasing temperature. View full abstract»

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  • Investigation of electron density and velocity distribution in high‐pressure discharges for excimer laser pumping using laser Thomson scattering

    Page(s): 41 - 45
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    Thomson scattering from plasmas in high‐pressure discharges for rare‐gas halide excimer laser pumping was observed using an yttrium‐aluminum‐garnet laser with a second harmonic generator, combined with a double‐monochromator to reject large signal from Rayleigh scattering. The results showed the electron velocity distribution to be Maxwellian which was different from prediction of computer simulations, while variation of electron density and temperature with time and gas composition was consistent with results of computer simulations. View full abstract»

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  • Grating‐assisted surface acoustic wave directional couplers

    Page(s): 46 - 52
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    Physical properties of novel grating‐assisted Y directional couplers are examined using the coupled‐mode theory. A general formalism for the analysis of the lateral perturbed directional coupler properties is presented. Explicit expressions for waveguide key parameters such as coupling length, grating period, and other structural characterizations, are obtained. The influence of other physical properties such as time and frequency response or cutoff conditions are also analyzed. A plane grating‐assisted directional coupler is presented and examined as a basic component in the integrated acoustic technology. View full abstract»

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  • Propagation of shear horizontal surface acoustic waves parallel to the grooves of a random grating

    Page(s): 53 - 62
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    An elastic medium that occupies the region x3≳ζ(x1), where the surface‐profile function ζ(x1) is assumed to be a stationary, stochastic process, is studied. The displacement field u(x;t) in this region is assumed to satisfy stress‐free boundary conditions on the surface x3=ζ(x1). First the boundary conditions satisfied by the mean displacement field 〈u(x;t)〉 on the plane x3=0 are obtained, where the angle brackets 〈 〉 denote an average over the ensemble of realizations of ζ(x1). The results are then used to obtain the dispersion relation for surface acoustic waves of shear horizontal polarization propagating in the x2 direction, i.e., parallel to the grooves of the random grating defined by the surface‐profile function ζ(x1). It is found that the surface acoustic waves of this polarization, which cannot exist on a planar, stress‐free surface of an elastic medium, are trapped by the random roughness, but are bound to the surface for only a finite range of values of the wave number k2 characterizing their propagation in the x2 direction. The attenuation of these roughness‐trapped surface acoustic waves is also determined. View full abstract»

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  • Finite element analysis of nonlinear, coupled thermal‐electric problems: The memory switch

    Page(s): 63 - 68
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    The finite element method is applied to heat conduction problems where the heat generation is due to electric current flowing through the body. The electric and temperature fields are, in general, coupled. The electric field causes Joule heating and both the electric and thermal properties are temperature dependent. The time scale for changes in the electric field is short compared to that for the temperature field. Therefore, the electric field is assumed to be quasistatic and the temperature field transient. The formulation is applied to memory switching devices which rely on Joule heating to cause a thermally induced transition from a state of low, to one of high electric conductivity. View full abstract»

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  • Optical properties of oxygen‐deficient centers in silica glasses fabricated in H2 or vacuum ambient

    Page(s): 69 - 74
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    Silica soots were sintered under a reducing ambient to study the generation mechanism of the oxygen‐deficient‐type defects in silica glasses. Glasses sintered under H2/He gas mixtures or vacuum were prepared with an intent to produce oxygen‐deficient centers. Two optical‐absorption bands were generated in the silica glasses sintered under the H2/He gas mixtures. One was 5.17‐eV absorption band and the other was an absorption tail near the Urbach tail of silica glasses. Both absorptions were attributed to the silicon lone pair center (SLPC). On the other hand, the other type of reduced silica glasses were prepared in the vacuum ambient. Three kinds of absorption bands could be detected clearly: one was peaking at 5.17 eV, another showed only its shoulder near the Urbach tail, (both were also attributed to the SLPC) and the third was peaking at 6.7 eV and was attributed to a SiSiSi structure. Fabricated in higher volatile ambient under vacuum, the color of the sample showed a smoky black and generated broad absorption band peaking at 5.1 eV. This band is tentatively attributed to the optical absorption due to excitation from the inner shell of silicon. View full abstract»

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  • Interaction of CO2 laser pulses of microsecond duration with Al2O3 ceramic substrates

    Page(s): 75 - 81
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    The working efficiency and other properties of laser‐induced surface plasmas have been compared for the case where two kinds of CO2 laser pulse interact with alumina substrates. Pulses of several times 10‐μs duration are able to drill hole patterns while producing negligible mechanical stresses in the material. However, because of the formation of a laser‐induced plasma which shields the material against the incoming laser beam, the drilling efficiency is rather low. A simple vaporization model shows that 30%–100% of the laser‐pulse energy is used for material processing. There is in addition a threshold energy of about 50 mJ, below which no material processing occurs. Plasma diagnosis verifies characteristic times of absorptive modulation in the range of 5–10 μs. Therefore, laser pulses of high repetition rate (≳10 kHz) and of a pulse duration below 5 μs have been used in order to overcome the above‐mentioned disadvantages. An important role with respect to the shielding effect is played by the electron density inside the laser‐induced plasmas, because the absorption coefficient of the laser light in these surface plasmas is proportional to the square of the electron density. The spatial and temporal distribution of the electron density during material processing is therefore continuously monitored by a beam‐deflection technique. View full abstract»

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  • Ion energy distributions in radio‐frequency discharges

    Page(s): 82 - 92
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    Using a theoretical description of potentials in radio‐frequency (rf) discharges reported elsewhere [Song, Field, and Klemperer, J. Phys. D: Appl. Phys. 23, 673 (1990)] we derive the equations of motion for ions from the plasma region striking the cathode and anode in the absence of collisions in the sheath. We compute ion energy distributions (IEDs) which we find to be in excellent agreement with recent experimental data for Ar, Ar/H2 mixtures, O2, and CF4 parent gases. We also present a method of estimating IEDs which does not involve extensive computation, a development of particular value in the design of reactive ion etching processes. View full abstract»

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  • A model for steady‐state breakdown plasmas in microwave transmit‐receive tubes

    Page(s): 93 - 98
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    A model is proposed for the consistent description of the steady‐state interaction between a high‐power microwave and a breakdown plasma in transmit‐receive (TR) tubes often used as switches in radar microwave systems. The electron‐density equilibrium in the TR tube is established as a balance between the ionization produced by the wave and various electron‐loss processes. In the currently proposed model, it is shown that diffusive losses of electrons in the direction of wave propagation is the most important loss process. An approximate analytical solution is derived from the model and compared with the numerically obtained full solution. Finally, the predictions obtained from the model concerning the scaling of absorbed and reflected microwave power are shown to be in good agreement with experimental results. View full abstract»

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  • Probe diagnostics of anisotropic plasma in a hollow cathode arc

    Page(s): 99 - 105
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    Measuring the second derivatives of electron retarded current characteristics of a plane probe at different angle positions, the coefficients f*i(ϵ) of the velocity distribution function developed as a Legendre polynomial are determined for the anisotropic plasma of a hollow cathode arc discharge. The integral values deduced from this for the electron concentration ne and the mean electron energy ϵ¯, as well as the drift velocity vd, show only a limited dependence on the number of angle positions used for measurement. Even for measurements at a single angle position (Druyvesteyn method), the values obtained for ne, ϵ¯, and vd differ by less than 10% from the corresponding values measured with the multiposition method, so long as vdrift ≤ vthermal. On the contrary, the shape of f*i(ϵ), like the angle dependence f*(θ)ϵ=const, is clearly affected by the number of angle positions used. A Maxwell distribution superimposed by a group of directed fast electrons proves to be a sufficient model for the plasma investigated here. View full abstract»

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  • A high‐power two stage traveling‐wave tube amplifier

    Page(s): 106 - 113
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    Results are presented on the development of a two stage high‐efficiency, high‐power 8.76‐GHz traveling‐wave tube amplifier. The work presented augments previously reported data on a single stage amplifier and presents new data on the operational characteristics of two identical amplifiers operated in series and separated from each other by a sever. Peak powers of 410 MW have been obtained over the complete pulse duration of the device, with a conversion efficiency from the electron beam to microwave energy of 45%. In all operating conditions the severed amplifier showed a ‘‘sideband’’‐like structure in the frequency spectrum of the microwave radiation. A similar structure was apparent at output powers in excess of 70 MW in the single stage device. The frequencies of the ‘‘sidebands’’ are not symmetric with respect to the center frequency. The maximum, single frequency, average output power was 210 MW corresponding to an amplifier efficiency of 24%. Simulation data is also presented that indicates that the short amplifiers used in this work exhibit significant differences in behavior from conventional low‐power amplifiers. These include finite length effects on the gain characteristics, which may account for the observed narrow bandwidth of the amplifiers and for the appearance of the sidebands. It is also found that the bunching length for the beam may be a significant fraction of the total amplifier length. View full abstract»

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  • Theoretical studies of high‐power Cerenkov amplifiers

    Page(s): 114 - 124
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    The main theoretical aspects of the experiments performed and reported recently are discussed here. First the one stage amplifier is considered. As a preliminary step the behavior of the electrons is followed in the phase space at different points along the interaction region. This analysis reveals that about 30% of the interaction region is utilized for construction of the electron bunches. It is shown that although the average energy of the electrons remains unchanged along most of the amplifier, their energy spread increases substantially. Since the system consists of two long tapered sections, it is suggested that the effective length of the interaction region might be significantly longer than the physical length of the uniform structure. It is further suggested that the electrostatic periodic potential induced by the beam may also improve the interaction process. The next subject addressed here is the bandwidth of a single stage amplifier. It is shown that the reason for the narrow measured bandwidth is the gain of the system. In fact the output signal from a short amplifier is narrowed by exactly the same amount the amplitude of the electromagnetic wave is increased. This result is general as long as part of the radiation field is reflected from both ends of the amplifier. In the second part of this paper the two stage amplifier is analyzed. As in the case of the single stage amplifier the behavior of the electrons is followed in phase space at various locations along the system. This discussion leads to an analysis of the development of ‘‘sidebands’’ which are not symmetrically located around the initial frequency at power levels that do not correspond to a nonlinear process. It is suggested that these sidebands are amplified noise-produced basically in the first stage. The sharp selection of frequencies is due to the constructive interference of the waves bouncing between the two ends of the second stage. This selection is no- t symmetric relative to the initial frequency. Some considerations regarding the bandwidth of the system are also presented. View full abstract»

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  • Identification of infrared absorption peaks of amorphous silicon carbon hydrogen alloy prepared using ethylene

    Page(s): 125 - 130
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    Amorphous silicon‐carbon hydrogen alloy was prepared by radio frequency glow discharge decomposition of silane‐ethylene mixture. The infrared absorption spectra were measured at various stages of thermal annealing. By observing the change of relative intensities between these peaks the molecular bonding responsible for the absorption peaks could be assigned. For example, in addition to the CH3 radical commonly found in films prepared by silane‐methane mixture, other carbon hydrogen radicals such as CH2 and C2H5 were also unambiguously identified. At same gas phase flow ratio (Xg = 0.8), the CHx contents of ethylene‐based film is about 6.6 times larger than that of the methane‐based film. Hence, to grow a‐SiC:H with a larger optical gap, the ethylene will be a good choice as the deposition source. View full abstract»

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  • Phase separation in ion bombarded FeNi Invar alloys

    Page(s): 131 - 134
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    Studies of the effects of ion bombardment on the phase equilibria and ordering of FeNi alloys with 30.6 and 39.5 at. % Ni were performed. The FeNi samples were bombarded with Ne, Ar, and Kr at different doses and temperatures, and analyzed using the conversion electron Mössbauer spectroscopy (CEMS) technique with 57Fe. The changes in the CEMS spectra of Fe69.4Ni30.6 and Fe60.5Ni39.5 alloys, bombarded with Ne and Ar, indicate a phase separation, but no evidence of ordering. The Kr bombardment does not produce any change, however samples of 39.5 at. % Ni already showing phase separation returned to the single phase state after Kr irradiation. Our results give strong evidences of a prevalence of displacement mixing for bombardment with heavier ions, and of radiation enhanced diffusion for bombardment with lighter ions. View full abstract»

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  • Nematic director orientation in a liquid‐crystal‐dispersed polymer: A deuterium nuclear‐magnetic‐resonance approach

    Page(s): 135 - 143
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    The nuclear‐magnetic‐resonance (NMR) technique is employed to liquid‐crystal‐dispersed‐polymer (LCDP) systems. Deuterium NMR is used to determine director distributions in LCDP cells in the presence of a magnetic field. The samples consist of a nematic liquid crystal mixed with small percentages of a monomer that is polymerized under different conditions after dissolving it in the liquid crystal. NMR spectra of the deuterated liquid‐crystal molecules give information on orientation and order in such systems. The orientation of the polymer skeleton formed during the polymerization is found to be stable. It determines the orientation of the nematic director even in the presence of a strong external magnetic field. Simple models for the director field are presented. View full abstract»

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  • Radiation damage and optical properties of Ar+‐implanted GaP

    Page(s): 144 - 149
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    The defect production in GaP during Ar+ ion implantation at 300 and 80 K is investigated by means of Rutherford backscattering spectrometry and optical‐absorption spectroscopy. It is shown that the transformation to the amorphous state in the medium dose region is different for the two temperatures and differs remarkably from the behavior observed in GaAs. An attempt is made to understand the amorphization process in the framework of previously predicted overlap damage models. Contrary to the situation in GaAs, in GaP the number of overlaps of primarily produced clusters necessary to amorphize a certain region increases with decreasing temperature connected with an increasing area damaged by one ion. A possible explanation is a modified change of the primarily produced damage regions in GaP compared to GaAs. Generally, however, the primarily damaged regions are more stable in GaP than in GaAs. View full abstract»

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  • Determination of the elastic constants of anisotropic materials using laser‐generated ultrasonic signals

    Page(s): 150 - 157
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    This paper presents the solution of the materials characterization problem in which the elastic constants of an anisotropic material are determined from ultrasonic wavespeed measurements made in nonprincipal directions of a specimen. The ultrasonic waves were generated via the point‐source/point‐receiver technique using a pulsed laser as a source and a miniature, point‐like transducer as a receiver. Data were acquired during a scan of the source along one of the principal acoustic axes of symmetry of the material. In each waveform the arrivals of the quasi‐longitudinal and the two quasi‐shear bulk modes were measured and the elastic constants of the material were then recovered using an optimization algorithm. Experimental results are presented for a transversely isotropic, unidirectional fiberglass/polyester and a single crystal specimen of silicon. It was found that the nonlinear fit between the measured and the recovered longitudinal slowness values is excellent. Some discrepancies are observed in the data for the two shear modes. These are shown to be related to the complexity of the detected signals. 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