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Volume 65, Issue 4, 15 February 1989

Electromagnetic radiation from an AT‐cut quartz plate under lateral‐field excitation
View Description Hide DescriptionEquations of linear piezoelectricity (with the quasistatic approximation) for the quartz plate and Maxwell’sequations for the electromagnetic field in the surrounding vacuum are solved for the thickness‐shear vibrations of rotated Y cut of quartz plate. For an AT‐cut plate vibrating near the thickness‐shear resonance excited by a uniform, lateral electric field of magnitude 1 V/m, the electromagnetic energy radiated from each face is about 0.13 μW/cm^{2}. The radiated power is about 0.1 μW/cm^{2} if the plate is excited by a shearing face traction which produces a strain of 10^{−} ^{5}. Present solution is compared in detail with Mindlin’s [Int. J. Solids Struct. 9, 697 (1972)] solution of equations of piezoelectromagnetism (without the quasistatic approximation) for the thickness‐shear vibrations excited by shearing face traction. It is found that the percent difference in radiated powers computed from these two solutions, due to the quasistatic approximation, is in the order of β^{2}(=v̂^{2}/ĉ^{2}), where v̂ is the velocity of the x _{1}‐thickness‐shear wave, ĉ is the velocity of electromagnetic wave of E _{3} propagating in the x _{2} direction in quartz, and β≊10^{−} ^{5}.

Bragg resonances in a doubly periodic planar waveguide
View Description Hide DescriptionThe dispersion relation for a doubly periodic electromagnetic waveguide is determined by means of the null‐field and modal approaches. The waveguide walls are assumed to be perfectly conducting and to have the form of corrugated parallel plates. Bragg resonances appear in the form of stopbands or cross‐over resonances. The field inside the waveguide is obtained by means of modal theory. Numerical problems that emanate from mode degeneracy and large matrix sizes are dealt with by means of the Muller algorithm and singular value decomposition. The computed Poynting vector and the group velocity are discussed.

Transient reflection from a lossy magnetized Lorentz medium (longitudinal propagation)
View Description Hide DescriptionThe transient response of a lossy magnetized Lorentz medium illuminated by an impulsive electromagnetic plane wave is obtained. The propagation vector of incident pulse and the vector of external magnetic field were colinear and both perpendicular to the sharp boundary between free space and the Lorentz medium. The transient response is expressed as a convergent infinite series of modified Bessel functions of the first kind with the complex argument. The transient responses are computed for different values of the electron cyclotron and Lorentz medium resonant frequencies ω_{ c } and ω_{0}. The obtained results might have a possible use as a diagnostic tool for the evaluation of characteristic parameters of magnetized Lorentz medium and in special cases, (ω_{0}=0) for diagnosing the magnetized plasma.

Ion kinetics study of a high‐efficiency, high‐energy hydrogen fluoride chain chemical laser initiated by an intense electron beam
View Description Hide DescriptionThe operational characteristics of intense electron‐beam (e‐beam) initiated hydrogen fluoride (HF) chain chemical lasers were theoreticallyanalyzed with a new computer code including both neutral and ionic reactions. The gas mixture used is F_{2}/H_{2}/SF_{6}/O_{2}. Both theoretical HF laser output energy and pulse width were in good agreement with those of the experiment. Using this code, we calculated the performance characteristics as a function of e‐beam current pulse width, initial total gas pressure, H_{2} partial pressure, O_{2} partial pressure, and gas temperature. In order to optimize the total gas pressure, we introduced a new parameter which shows the effect of the pressure rise by the exothermic chemical reactions. The optimum total gas pressure of 850 Torr was predicted in our system. Moreover, by lowering the initial gas temperature from 300 to 220 K both the output energy and the chemical efficiency were improved.

Optical bistability in small metallic particle composites
View Description Hide DescriptionWe predict that a composite dielectric material composed of silver particles can have an optical bistable transmission without cavity feedback. Our model includes propagation and exhibits an inhomogeneous absorption characteristic that can be useful for device applications. Results are presented for quantum‐size silver particles embedded in silica glass.

Scattering of Hermite–Gaussian beams from an irregular surface
View Description Hide DescriptionThe scattering of two‐dimensional Hermite–Gaussian beams of arbitrary order from a conducting irregular surface is analyzed by using the Kirchhoff or physical optics approximation and the plane‐wave spectrum representation. Both coherent and incoherent components of the mean scattered power are derived. It is shown that, for the normal incidence of a fundamental beam, the decrease of the beam radius or spot size gives rise to the decrease of the ratio of the coherent component to the incoherent one in the backscattering. However, for the obliquely incident case, the ratio is increased by the decrease of the beam radius. For the first‐order beam incidence, the deep dip appearing in the specular direction in the mean scattered power pattern rapidly disappears as the surface roughness is increased.

Optimization of the Čerenkov sum‐frequency generation in proton‐exchanged Mg:LiNbO_{3} channel waveguides
View Description Hide DescriptionOptical sum‐frequency generation has been investigated in channel waveguides produced by proton exchange in Mg‐doped as well as standard LiNbO_{3}. Pyrophosphoric acid was used as an exchange media and Ta mask were patterned on z‐cut plates by reactive‐ion etching to form diffusionapertures ranging in width from 1 to 3 μm running parallel to the y axis. Samples were made by direct immersion in acid held at a temperature of 240 °C for times ranging from 10 to 28 min. End‐fire coupling at 1.06 μm revealed that the Čerenkov radiation output at the second harmonic was optimized for the narrowest channel with a diffusion time of 24 min. A coupled throughput of 0.57 mW would produce a Čerenkov second‐harmonic radiation mode due to the TM_{0} _{0} fundamental of 0.031 μW for a sample 4 mm in length. Diffusion times differing by 1 min around the optimum time would result in a reduction in harmonic generation efficiency by approximately a factor of 5. Samples pumped at 0.833 μm showed peak efficiency for a correspondingly shorter diffusion time. Severe drift in both fundamental throughput as well as harmonic generation is observed if pump throughput exceeds 2 mW in specimens formed in either Mg‐doped or undoped LiNbO_{3}. In addition, diffusion times for optimized nonlinear generation varied from batch to batch. The Čerenkov generation efficiency has been modeled using a step‐index approximation for channel waveguides generalized to the z‐cut y‐propagating orientation used. The nonlinear source polarization at the sum frequency is considered to be due to only d _{3} _{3}. Using values for the increase in extraordinary index estimated from fitting mode indices to the step‐index approximation, we find optimum second‐harmonic generation
due to the TM_{0} _{0} takes place for the mode close to cutoff at either 1.06 or or 0.833 μm. The magnitude of the peak SHG efficiency is found to agree reasonably well with experiment. The variation in guide depth over which the optimum SHG is observed is roughly 0.05 μm. The model predicts this should be larger by a factor of 3. We attribute the discrepancy to the inability of the channel waveguide model to accurately calculate the propagation and decay constants of the pump mode near cutoff. Our effective index measurements further indicate the dispersion in the increase of the extraordinary index (Δn _{ e }) under the ion‐exchange process is essentially equivalent to results published for guides made using benzoic acid when the exchange is performed at 249 °C.

Resonance radiation trapping effects in CuCl laser
View Description Hide DescriptionThe effects of resonance radiation trapping of 3248 and 3274 Å upon the corresponding laser fluorescent lines of 5106, 5700 and 5782 Å of copper have been investigated as a function of temperature in a CuCl laser with and without helium buffer gas. The experimental values of the laser starting temperature at 315 °C and the dissociation level at about 10% in CuCl at resonance radiation trapping threshold are found to be in good agreement with the reported values from calculations.

Transient generation by a wide thermoelastic source at a solid surface
View Description Hide DescriptionA method for computing the radiation from a thermal expansion source, generated at a solid surface by a laser pulse of finite width, is evaluated for two radial beam energy distributions. A theoretical expression for the impulse response is obtained for two components of the displacement at any point within the solid, and may be used to compute the field for an arbitrary time excitation. Numerical results are presented, and are compared to experimental results for generation by a pulsed laser.

An interferometric calorimeter for thin‐film thermal diffusivity measurements
View Description Hide DescriptionWe demonstrate that the thermal diffusivity of thin‐film samples bonded to transparent substrates can be measured from the phase lag of thermal waves propagating from the sample surface into the substrate. The thermal waves are detected from the substrate’s interferometrically modulated reflectance. This technique is used to determine the thermal diffusivity of 5‐μm‐thick polymer films.

Nonlinear analyses for optimized short‐period‐wiggler free‐electron laser oscillators
View Description Hide DescriptionProcedures to determine the parameters of optimized (for maximum efficiency) experimental short‐period‐wiggler free‐electron laser (FEL)oscillators are presented. The calculations are based on a normalized set of nonlinear equations which describe a one‐dimensional FELoscillator. Scaling formulas relating the normalized theoretical parameters to dimensional experimental quantities are obtained. The physics of such realistic effects as waveguide dispersion, wiggler field dependence on magnet gap, beam energy spread, and the frequency tunability of tapered wigglers are considered. Finally, specific examples for both tapered and untapered 300‐GHz, 1.0‐MW FELoscillators are calculated and discussed.

A spectroscopic study using line ratios of lithiumlike ions in a laser‐produced plasma
View Description Hide DescriptionSpectra of highly ionized titanium and calcium in the extreme ultraviolet region were observed in laser‐produced plasmas using the OMEGA 24 beam (351 nm) laser system at the University of Rochester. The plasmas were produced using glass microballoon targets coated with a layer of a medium Z element and a layer of parylene (CH). Time‐integrated electron temperatures and densities were obtained by comparing measured line intensity ratios of lithiumlike charge states of Ti and Ca to numerical calculations from a collisional‐radiative model. The variation of line intensity ratios with electron density and temperature using the collisional‐radiative model is discussed.

Action of a static magnetic field on an argon discharge produced by a traveling wave
View Description Hide DescriptionThis paper deals with a low‐pressure argon microwavedischarge (5–100 mTorr), created and sustained by a traveling wave (390 MHz), in the presence of an axial static magnetic field (up to 550 G). First, the propagation characteristics of the wave(wave number, wave collisional damping, radial distribution of the electromagnetic energy) have been computed from the numerical resolution of the dispersion equation. It has been emphasized that the plasma is sustained either by a surface wave, or by a volume wave, according to the electron density and magnetic field values. Second, the electron density, the effective electron‐neutral collision frequency ν for momentum transfer, the mean power θ required to maintain an electron in the discharge, and the effective electric field E _{eff} for the discharge maintaining have been experimentally determined, versus the pressure and the magnetic field. At a fixed pressure, the power required to maintain the discharge decreases as the magnetic field increases. It has been shown that this experimental result is consistent with simple theoretical modeling, describing the decrease of the electron loss by diffusion and recombination on the discharge tube walls.

Excitation of whistler modes in a sheath‐helix loaded waveguide
View Description Hide DescriptionThe excitation of electromagnetic modes in a cylindrical plasma column by an electron beam is considerably improved by the introduction of a sheath helix inside the vacuum chamber. The helix slows down the whistler mode and relaxes the requirements on the beam energy for the generation of a given frequency.

A two‐dimensional computer simulation for dry etching using Monte Carlo techniques
View Description Hide DescriptionThis work introduces a novel two‐dimensional dry‐etching simulation technique and presents experimental verifications of etching profiles based on an algorithm that calculates the transport of charged particles across a plasma sheath, the etch rate at each node, and the overall time evolution of the simulated structure. Monte Carlo techniques are applied to solve for the transport of ions across the plasma sheath to obtain the angular ion‐distribution functions at the wafer surface. The etch rates are calculated from the particle fluxes and energies on the wafer during each time step. These fluxes depend on the relative position of the node with respect to the general geometry of the structure. Different etching mechanisms are decoupled into a chemical component proportional to the reactive neutral concentration in the gas and an ion‐enhancement component proportional to the energy flux deposited on the wafer. The chemical component is assumed to be uniform and perpendicular to the surface being etched; the ion‐enhancement component is calculated from the angular ion‐distribution function. These mechanisms are incorporated into a two‐dimensional computer‐simulation program that computes their interaction as a function of time. The simulation program allows the addition of secondary effects as input options, such as energy and flux thresholds, glancing ions, and inhibitor layers. The predictions based on the simulations agree well with experimental results obtained on extreme cases of chemical and ion‐enhanced etching.

Structural characteristics of Sr_{1−x }La_{ x }Ti_{3+δ} as a function of oxygen partial pressure at 1400 °C
View Description Hide DescriptionThe structural characteristics of Sr_{1−x } La_{ x } TiO_{3+δ} (0≤x≤0.4) at 1400 °C have been investigated as a function of ambient oxygen partial pressure. A modified Rietveld pattern‐fitting structure‐refinement program [H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969)] was used to determine the nature of the distortions of the fundamental perovskite unit cell, the degree of lattice perfection, and the cation vacancy concentrations. Specimens equilibrated in forming gas displayed a linear relation between x, the lattice parameters, and the degree of lattice perfection while those samples annealed in air and oxygen deviated significantly from linearity. A monoclinic distortion of the perovskitestructure was seen in the samples at low oxygen partial pressures while a second phase or layer type of distortion appeared in samples with x>0.2 under oxidizing conditions.

Mechanisms of phase transformation in low‐temperature irradiated NiAl
View Description Hide DescriptionPhase transformations induced by low‐temperature (15 K), heavy‐ (Xe) and light‐ (D) ion irradiation of a monocrystalline NiAl intermetallic compound are investigated via i n s i t uRutherford backscattering and channeling experiments. Total amorphization of the irradiated layer is observed, whatever the irradiation ion mass, as soon as a mean number of displacements per atom of 0.3–0.4 is reached. On the contrary, the mechanisms of phase transformation appear to be strongly dependent on the cascade regime. Light‐ion irradiation leads to a sigmoidal shape of the amorphization kinetics, suggesting that amorphization takes place by a defect accumulation process. Heavy‐ion irradiation shows a more complex behavior with the formation of a premartensitic phase prior to amorphization. An interpretation of this new type of phase transformation is presented in terms of defect cascade disordering and damage‐induced stresses.

SiGe/Si superlattices on implanted buried‐oxide structures
View Description Hide DescriptionIn an attempt to reduce the number of threading dislocations propagating through a siliconepitaxial layer grown on an implanted buried‐oxide structure, a SiGe/Si superlattice was placed between the initial siliconsurface layer and the subsequently grownsiliconepitaxial layer. Both the superlattice and the siliconepitaxial layer were formed by molecular‐beam epitaxy. Some of the threading dislocations are bent parallel to the superlattice as they propagate through the structure. Some of these are annihilated by interacting with other dislocations while others bend toward the surface again after propagating parallel to the superlattice for some distance. The effect of the superlattice is limited, and many of the dislocations continue propagating through the superlattice toward the surface of the siliconepitaxial layer.

Planar channeling in GaAs/In_{ x }Ga_{1−x }As/GaAs strained‐layer structures
View Description Hide DescriptionPlanar {110} channeling measurements with 1–2‐MeV He^{+}ions have been used to investigate strained‐layer structures of (100) GaAs/In_{ x }Ga_{1−x }As/GaAs with x=0.07–0.17. Beam‐steering effects in the surfaceGaAs layer are observed to have a major influence on {110} planar angular scans carried out in the In_{ x }Ga_{1−x }As layers resulting generally in a double or more complex‐shaped angular dip. A simple computer simulation has been developed which determines the main features of the experimental angular scan data. Comparison of simulation and experimental data allows a reasonable determination of the kink angle θ_{ K } between the {110} planes for the surfaceGaAs and underlying strained layer. Only in limiting cases is it possible to unequivocally determine θ_{ K } from the experimental data alone. They are as follows: (i) when θ_{ K }=0; (ii) θ_{ K }>2ψ^{ p } _{ c } (ψ^{ p } _{ c } is the critical angle for {110} planar channeling); and (iii) when the thickness t of the top layer is thin (t≪2d/ψ^{ p } _{ c }, where d is the {110} interplanar spacing).

A quantitative study of the creation of EL2 defects in GaAs by plastic deformation
View Description Hide DescriptionOptical absorption quenching measurements have been carried out on a lot of plastically deformedn ^{+} samples (either Si or Se doped), which were EL2 free before deformation. It is shown that the density of EL2 defects created during the deformation increases linearly first with the dislocation density and then saturates for higher strains. The introduction rate of EL2 is also higher in the Se‐ than in the Si‐doped material. Photoluminescence characterization of these same samples shows the appearance of the 0.93‐eV band only after deformation; this band being associated with the divacancy. A possible model of EL2 creation during deformation is finally proposed.