Index of content:
Volume 56, Issue 1, 01 July 1984

A numerical solution of cylindrical coordinate Laplace’s equation with mixed boundary conditions along the axis of symmetry: Application to intracerebral stimulating electrodes
View Description Hide DescriptionA numerical solution method of Laplace’s equation with cylindrical symmetry and mixed boundary conditions along the Z coordinate is presented. The method is based on an iterative process. It is applied to the evaluation of current density distribution in the region surrounding electrodes used for intracerebral electrical stimulations. The procedure converges quickly and after only twelve iterations the boundary conditions are satisfied within an accuracy of 0.1%. The convergence criterion is discussed and the results obtained on the current density distribution are presented.

High sensitivity apparatus for low optical absorption measurements
View Description Hide DescriptionThe paper describes a laser calorimetric apparatus constructed and calibrated for measuring very low absorption coefficients of transparent materials. By using miniature thermistors as temperature sensors the sensitivity has been increased by at least two orders of magnitude compared with thermocouplecalorimetry. Such a high sensitivity opens new experimental possibilities, like performing absorption measurements on highly transparent materials with low power cw lasers or with a single laser shot in the pulsed regime.

Scattering of electromagnetic waves from a randomly perturbed quasiperiodic surface
View Description Hide DescriptionThe scattering of electromagnetic waves from a randomly perturbed quasiperiodic surface is studied for active remote sensing of plowed fields. The Kirchoff approximation is used. The narrow‐band Gaussian random variation around the spatial frequency of the sinusoidal variation is used to introduce the quasiperiodicity. The physical optics integral is evaluated to obtain closed form solutions for coherent and incoherent bistatic scattering coefficients. In the geometrical optics limit, it is shown that the bistatic scattering coefficients are proportional to the probability of the occurrence of the slopes which will specularly reflect the incident wave to the observation direction. The theoretical results are illustrated for the various cases by plotting backscattering cross sections as a function of the angle of incidence. It is shown that there is a large difference between the cases where the incident wave vector is parallel or perpendicular to the row direction. When the incident wave vector is perpendicular to the row direction, the maximum value of the backscattering cross section does not necessary occur at normal incidence. The scattering coefficients can be interpreted as a convolution of the scatteringpatterns for the sinusoidal and the random rough surfaces. For the backscattering cross sections we observe the occurrence of peaks whose relative magnitudes and the locations are explained in terms of the scatteringpatterns for sinusoidal surfaces.

Neutral beam line ion beam raster scanning with a dipole magnet
View Description Hide DescriptionAn ion beam raster scanning method utilizing a wide gap dipole magnet has been developed at the Oak Ridge National Laboratory that can reduce the time‐averaged amplitude of the ion beam power density on the ion beam dump by factors of 3 to 6. Time‐varying in‐phase and out‐of‐phase modulation current components are used in the dipole magnet coils in order to generate a time‐varying magnetic field shape and magnitude whose magnetostatic central surface position oscillates between the two magnetiron poles. This time‐varying three‐dimensional (3‐D) magnetic field shape deflects the ion beam onto an ion dump and can be programmed to produce a raster scan pattern that depends on the coil current modulation pattern. The 3‐D magnetic field produced by an iron yoke system has been computed for different coil current ratios using a finite‐element integral method. Ion orbit equations do not need to include the small space‐charge forces present in the beam used for these purposes. The time‐averaged amplitude of beam dump power densities should be lowered by a factor of 6 by sharing the beam impact pattern over a larger area with a short raster scan period compared to the ion dump thermal time constant.

A quantum model of the orotron free‐electron laser
View Description Hide DescriptionA quantum model of the orotron (Smith–Purcell) free‐electron laser is formulated in which the classical electron current density derived from a linearization of the equation of motion exhibits hybrid properties between the current densities of the Čerenkov and the wiggler free‐electron lasers. Here, consistent with the (four‐dimensional) current‐density conservation law, the current density is proportional to the average change of the electron velocity Δv in the interaction region. From the interaction of the electron current with the quantized radiation field in an interaction volume of finite extent, we arrive at the multiphotondistribution function, which in turn yields the full ‘‘quantum‐mechanical’’ gain after quantum recoil is taken into account. In an example where the radiation wavelength is 0.4 cm and the electron beam velocity is 0.1 c [corresponding to the Harry Diamond Laboratories (HDL) orotron experiment], we estimate that the maximum gain can easily be 8% or larger although the interaction length is chosen to be only 4 cm.

Efficient XeF(C→A) laser oscillation using electron‐beam excitation
View Description Hide DescriptionSignificantly improved XeF(C→A) laser energy density and efficiency have been obtained using electron‐beam excited Ar‐Xe gas mixtures at pressures up to 10 atm which contain both NF_{3} and F_{2}. Maximum blue‐green laser pulse energy density in excess of 1.0 J/liter was obtained, corresponding to an intrinsic electrical‐optical energy conversion efficiency estimated to be in the 0.5%–1.0% range. Comprehensive, time‐resolved absolute measurements of XeF(C→A) fluorescence, laser energy, and gain were carried out for a wide variety of experimental conditions. Analysis of these data has resulted in identification of the dominant transient absorbing species in the laser medium. For the laser mixtures investigated in this work, the primary blue/green absorption processes have been identified as photoionization of the 4p, 3d, and higher lying states of Ar, and of the Xe 6p and 5d states, and photodissociation of Ar_{2}(^{3}∑^{+} _{ u }) and Ar^{+} _{3}.

Theory of thermal behavior of laser operation in In_{0.53}Ga_{0.47}As
View Description Hide DescriptionA theory is given of thermal behavior of laser operation in In_{0.53}Ga_{0.47}As through rigorous calculation of laser parameters concerning the intervalence‐band absorption, the Auger recombination, the radiative recombination, and the gain. The theory is based on a realistic band model, where anisotropic nonparabolic bands are considered. We use the Green’s function formalism, into which a phonon scattering effect is incorporated in order to take into account the electronic level broadening from the first principle. Experimental results on the laser parameters are well explained by the present theory. It is found that strong temperature dependences of the threshold currents and the differential quantum efficiencies, which are observed especially above room temperature, can be explained neither by the intervalence‐band absorption nor the Auger recombination. Some possible models are proposed as alternatives to the previous theory based on the intervalence‐band absorption.

Power characteristics of single‐mode semiconductor lasers
View Description Hide DescriptionThe basic aspects of power calculations for high‐gain semiconductor lasers are briefly reviewed, and a straightforward one‐dimensional model is described. The relative importance of spontaneous emission, distributed losses, band‐to‐band absorption, and high single‐pass gain are investigated in detail.

Effect of stress on the polarization of stimulated emission from injection lasers
View Description Hide DescriptionThe role of stress on the polarization of stimulated emission is discussed in this paper. The stimulated emission from injection lasers is usually polarized in the TE mode. However, in the presence of a sufficiently high stress in the active region, TM polarized stimulated emissiona t t h r e s h o l d can be observed. We find that for some InGaAsP buried heterostructure lasers with a kink in the light‐current characteristic, the kink represents the onset of a TM polarized stimulated emission. X‐ray diffraction measurements show that the active region of these devices are under stress. We show that the change in polarizationcharacteristics associated with the light‐current kinks can be due to (i) higher optical gain of the TM emission than TE in the presence of stress and (ii) larger reflectivity of the higher‐order TM modes. This mechanism is supported by the observation of higher‐order TM modes than TE and the observation that the TM gain is unsaturated above TE threshold. We also find that the TM kinks occur at lower currents as the external pressure is increased. In the absence of stress in the active region, stimulated emission in higher‐order TE modes associated with light‐current kinks is observed in buried heterostructure lasers with large active region. Our results show that the observation of stimulated TM emission in real index guided laser structures is indicative of internal stress in the active region of these devices.

Frequency control of an optically pumped Na_{2} laser
View Description Hide DescriptionThe electronic control of a Na_{2} dimer laser and of its argon‐ion pump is described. This leads to an extremely narrow emission linewidth (less than 200 Hz) as well as a good amplitude stability and a tunability of a few GHz around each line center.

Linear electro‐optic effects in zincblende‐type semiconductors: Key properties of InGaAsP relevant to device design
View Description Hide DescriptionGeneral expressions are derived for the dispersion of linear electro‐optic coefficients in terms of electric‐field‐induced modulations of the electronic energy‐band structures. The theoretical predictions are compared with existing linear electro‐optic dispersion data in materials of a number of the II‐VI and III‐V binary compounds below the direct‐band edge. It is found that the calculations show a quite good agreement with these experimental data. This model is applied to In_{1−x } Ga_{ x } As_{ y } P_{1−y } quaternaries lattice matched to InP. The dispersion of the nonlinear optical (optical rectification) coefficients is also obtained on the basis of this model. The present results allow us to design a wide variety of optoelectronic devices.

Characteristics of coupled‐mode optical filters
View Description Hide DescriptionWe provide a quantitative picture of the characteristics of a CdS coupled‐mode band‐pass optical filter. The operating principle of these coupled mode filters (CMF) is the energy exchange between the ordinary and extraordinary modes when the sample, inserted between two crossed polarizers ∥ and ⊥C, is strained by uniaxial stress at 45° from the C axis. The filter operates at the isotropic wavelength λ_{ i } where n _{ e }=n _{0} ensures phase matching. The transmission rate and the selectivity are investigated as functions of the stress magnitude T and the sample thickness e. The best results are obtained by adjusting T so that the coherence length L _{ c }=2e. For a 1‐mm‐thick crystal, the half‐height passband is ∼4 Å, while the transmission rate is ∼70% at 77 °K and 20% at 300 °K. The large field of view, in practice ∼45°, constitutes a significant advantage over interference filters. We compare the CMF with zero crossing birefringent filters (ZCBF). Around λ_{ i }, the characteristics of both filter types are rather similar, but the single transmission window of CMF gives the latter a distinct advantage over ZCBF. Transmission control by variation of applied stress is an added advantage of CMF.

A ferroelectric domain layer surface wave
View Description Hide DescriptionA theoretical analysis is made of the effect of the 180° ferroelectric domain structure on the dispersion of the surface wave at an oscillatory domain boundary. From the general dispersion equation, the equation of Meerfeld–Tournois (or Privorotskii) mode, the equation of Bleustein–Gulyeav mode, and the dispersion expression of the ferroelectric domain layer mode are derived with the oscillatory boundary.

Diffraction characteristics of laser‐induced acoustic waves in liquids
View Description Hide DescriptionThe effect of diffraction on the temporal shape of laser‐excited plane acoustic waves propagating in liquids is studied both theoretically and experimentally. Two different boundary conditions corresponding to a constrained and a free surface of the liquid are considered. The main features are the development of a rarefaction zone behind the pure compression pulse in the first case and the appearance of a second compression zone following the original compression/rarefaction signal in the second case. Experiments have been performed in distilled water with a hybrid CO_{2} laser and piezoelectric detection of the acoustic transients. A good agreement between theory and experiment is found.

Characterization of a multipole ion source for ion implantation
View Description Hide DescriptionA design for a single‐slit multipole source for ion implantation was investigated. Some of the general physical processes involved are reviewed. Argon plasma characteristics are described for four multipole configurations. Langmuir probe data was found to agree in form with continuity theory. In addition, we present scaling relations for secondary electron temperature and density. Superior performance was observed for gas and arc efficiencies compared to a similar Bernas‐type source.

Limitation of accelerating process in the partly neutralized relativistic electron hollow beam
View Description Hide DescriptionA fluid‐Maxwell thoery of the diocotron instability is developed for a relativistic electron hollow beam which is assumed in rigid‐rotor and cold laminar flow equilibria. Stability analysis is performed for a sharp boundary electron density profile including the influence of positive ions which can accumulate in a long pulse device, and which form a partially neutralizing background. In the case of the strong magnetic field and tenuous electron beam (plasma frequency ω_{ p b } ≪gyrofrequency ω_{ c }) a partially neutralizing ion background staying uniformly within the beam annulus (R _{1}<r<R _{2}) has a stabilizing effect on the diocotron instability,R _{1} and R _{2} are the inner and outer radius of the annular hollow beam, respectively. However, the ions accumulating in the center of the beam (0<r<R _{1}) have a destabilizing effect on the diocotron instability. Most importantly the kink mode becomes unstable with a growth rate several tenths of the diocotron frequency ω_{ D } ≡ ω^{2} _{ p b }/2γ^{2}ω_{ c }, where γ is the relativistic scaling factor.

Hydrodynamics of thermal self‐focusing in laser plasmas
View Description Hide DescriptionTwo‐dimensional Eulerian hydrodynamics simulations have been carried out to investigate the thermal self‐focusing of laser beams in plasmas with long scale lengths. Realistic modeling of the plasma dynamics and the time dependence of the plasma heating is found to be necessary to follow the fully nonlinear development of self‐focusing. The laser propagation is treated using a self‐consistent ray‐tracing model. The simulations presented illustrate the dependence of thermal self‐focusing on beam size, plasma scale length and initial temperature, laser wavelength and intensity, and flux limiter. For the interaction of a finite‐sized beam with a preformed long‐scale‐length plasma, whole‐beam self‐focusing is found to occur when the beam diameter is smaller than the plasma scale length. For beams with a hot spot imposed on a uniform background, self‐focusing occurs only for hot spots of sufficient amplitude (≳a few percent in typical cases). Self‐focusing is less likely to occur in an initially hot plasma. Low‐density self‐focusing channels formed in the underdense plasma by the expulsion of plasma from laser‐heated regions may collimate the reflected light back towards the laser. Small‐scale self‐focusing has also been simulated, along with a subsequent evolution into whole‐beam self‐focusing.

Frequency dependence of the pulsed high‐pressure sodium arc spectrum
View Description Hide DescriptionSpectral energy distributions are calculated and measured for a wall‐stabilized high‐pressure Na‐Hg‐Xe arc in pulsed operation. These and other quantities are studied as a function of the pulsing frequency over the range 500–5000 Hz, and as a function of duty cycle over the range 25%–100% for a fixed power input of 150 W. The arc is contained in a 5.5‐mm‐diam channel with an electrode separation of 39 mm containing a gas mixture of 100‐Torr Na, 470‐Torr Hg, and 190‐Torr Xe. The fully time‐dependent energy balance model assumes local thermodynamic equilibrium and one‐dimensional cylindrical symmetry. The radiation transport calculation includes the effects of 28 Na lines in the visible and infrared, 20 of which are treated as optically thick. The amount of nonresonance Naradiation in the spectrum is found to be maximum at the lowest frequency and duty cycle. Under these conditions the model shows the arc center temperature to be lowest prior to the pulse, but to reach the highest peak values well before the end of the power pulse.

Collapse of multiple gas bubbles by a shock wave and induced impulsive pressure
View Description Hide DescriptionThe problem of bubble‐bubbleinteraction is studied experimentally. The motions of multiple gas bubbles attached to a solid wall by a shock wave are observed by using a high speed camera, and the induced impulsive pressures are measured. On the basis of these results, the effects of number and configuration of gas bubbles on the collapsing process and the impulsive pressure are clarified. The maximum impulsive pressure quickly decreases with reducing the interval beteween bubbles due to significant interaction. The direction of a liquid jet formed within a bubble is determined as a resultant of effects such as shock direction, shock strength, and interactions between bubbles and between a bubble and a solid wall.

Multiple scattering formalism: Application to scattering by two spheres
View Description Hide DescriptionWe present a multiple scattering formalism that was developed in order to calculate elastic wave scattering by complex defects. The formalism is based on an expansion of the T matrix in terms of single scatterer T matrices; we include all single and double scattering processes. The method has been numerically implemented for the case of two spherical cavities; the interaction terms were found to be small, which indicates that other problems of interest, such as a subsurface spherical cavity or crack can also be treated.