摘要:

The elastic scattering cross sections for proton scattering from4Hewere determined in the proton energy region of 1.4–6.0 MeV. The measurements were performed at laboratory angles of 110°–170° for proton energies of 1.4–2.7 MeV and at angles of 140° and 170° for proton energies of 2.7–6.0 MeV. The scattering cross sections for1Hrecoils by4Heion bombardment for helium energies of 5.6–24 MeV at recoil angles of 4°–28° were calculated by kinematically reversing the reaction. The helium targets were prepared by implanting 60 keV4Heions into thin tantalum foils. The helium concentration was determined by transmission elastic recoil detection analysis employing 10 MeV28Siions. The obtained cross section data are compared with the available literature values. Differences up to 10&percent; are found in the values at the cross section curve maximum in comparison to earlier data. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366008

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Influence of the initial energy modulation caused by the self-potential depression on the premodulated electron-beam propagation through a drift tube is investigated. The potential depression &kgr; can significantly vary because of the initial current modulation. Thus, beam’s kinetic energy at the injection varies accordingly. A self-consistent nonlinear theory of current modulation of the premodulated electron beam is developed. It is shown that the initial energy modulation caused by the self-potential depression at injection plays a significant role in the current modulation for long range propagation. It is also found from a small signal theory that reduction of the beam’s kinetic energy due to its potential depression accelerates debunching process of the initial current modulation. Although the initial current modulation is debunched quickly for high current beam, amplitude of the current modulation never becomes zero because of the initial energy modulation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366272

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Closed form expressions for the electromagnetic fields in homogeneous anisotropic media are derived in terms of the circular cylindrical vector wave functions for isotropic media. An analytical solution is obtained for the problem of scattering by an infinite homogeneous anisotropic circular cylinder for an obliquely incident plane wave of an arbitrary linear polarization. Numerical implementation and convergence behavior of this solution is discussed. Numerical results for some special cases are compared with existing data. We also present numerical results for the more general anisotropic material case in both graphical and tabular forms. The formulation of this article can be easily generalized to the cases of more complex cases, such as other complex media cases whose eigenwave numbers and eigenwave vectors can be determined, layered structures cases, and multiscatterers cases. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366009

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A simple optical setup that uses a single pulse width laser is proposed to identify different kinds of nonlinear optical absorption mechanisms. It can easily distinguish reverse saturable absorption from two photon absorption, and direct three photon absorption from two photon absorption followed by one photon absorption from the excited level. Both theoretical and experimental results are reported. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366010

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An efficient matrix formulation combining a discrete Bessel transform and a beam propagation method for the modeling of vertical-cavity surface-emitting lasers is presented. Devices with circular symmetry are considered. The theory generalizes the concepts of field reflection and transmission coefficients, and generates a round-trip matrix that characterizes the entire structure. The model yields, among other things, the threshold current and intensity distribution at threshold and higher-order mode profiles. We apply the model to the threshold analysis of a gain-guided device. Our theoretical results agree very well with experimental observations on the temperature-dependent threshold characteristics. The optical modal reflection behavior of distributed Bragg reflectors based on our discrete Bessel transform approach is also analyzed and compared to other models. The importance of coupling loss for devices with oxide confinement layers is also discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366011

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Three anisotropic diffraction cones in doped(K0.5Na0.5)0.2(Sr0.61Ba0.39)0.9Nb2O6crystals with two extraordinary polarized incident beams can be observed for the experimental configuration where thecaxis is parallel or perpendicular to the incident plane. These cones are attributed to anisotropic conical self-diffraction and cross-diffraction process, respectively. Photorefractive multiwave interaction caused by the incident beams and forward scattered beams satisfying the phase matching condition is proposed to explain the experimental results. The coupled wave equations and its numerical solution including anisotropic self-diffraction and cross diffraction are also presented. Theoretical analysis and experimental results show that anisotropic conical cross-diffracted light arising from the contribution of both incident beams. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366012

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Competition between carrier concentration and effective temperature effects on the dielectric function of a degenerate semiconductor in a spectral range near its fundamental absorption edge is suggested as a mechanism for regenerative pulsations in a stationary pumped optical etalon. The origin of self-pulsations is similar to that in a bistable etalon with competing concentration and thermal optical nonlinearities, but, due to its purely electronic nature, the proposed mechanism provides a way for a much higher repetition frequency, probably in the GHz range. The model of the phenomenon and modeling examples are all related to a Fabry–Perot resonator filled with the bulkn+-In0.53Ga0.47Asas an active medium. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366099

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electron-phonon scattering rates and intersubband optical gain spectra were calculated, including the optical phonon confinement effect in AlGaAs/GaAs/AlGaAs quantum well heterostructures. Comparison of the calculated gain spectra with those calculated using the bulk phonon approximation shows that details of the phonon spectrum have a strong influence on the intersubband optical gain. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366013

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Depletion of the metal vapor density from the central region of metal vapor laser tubes has recently been identified as an important factor limiting laser output. We have studied the mechanisms that deplete the ground-state metal density profile by measuring radially resolved population histories of neutral and ionic species during a “burst” (i.e., a pulse train of 20–30 excitation pulses) in a barium vapor laser (BVL). The observed spatiotemporal density behavior during the afterglow of the first shot in the burst agrees well with a simple model for diffusion and recombination, which we have used to show that the primary depletion mechanism during the establishment of steady-state conditions arises from the ambipolar diffusion of ions to the tube wall, and that gas-heating effects are secondary. Analysis of the steady-state (i.e., late-burst) afterglow behavior further reveals that the depletion is significantly greater than that expected from the measured barium ionization, particularly when operating at low barium densities at the wall; this we attribute to a large radial ambipolar field induced by the presence of ionized buffer–gas atoms on axis. The results show that it is important to reduce the time-averaged ionization in order to minimize ground-state depletion. The implications for power scaling of the BVL and other metal vapor lasers are discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366014

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The aim of this paper is to show that Lamb waves may be effectively generated using piezoelectric transducers embedded inside a composite plate, for nondestructive evaluation and health monitoring applications. A cylindrical transducer embedded in a composite host plate is considered. The electrical impedance of the transducer alone in vacuum and then of the embedded transducer, which allows the identification of the resonance modes, have been obtained by the finite element method (FEM). Moreover, the displacement fields in the plate, which allow the identification of the types of Lamb waves, have been computed at the resonance frequencies. Comparison between the FEM results and the Lamb wave dispersion curves of the host material are in good agreement. Experimental results (electrical impedance, frequency response, and phase velocities) concerning a composite plate specimen containing the same piezoelectric transducer inside it are shown. A good agreement is generally obtained between numerical and experimental results. In addition, it has been shown that the radial mode of the embedded transducer, which has a high coupling coefficient (around 50&percent;), can be used to generateS0Lamb waves. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366015

出版商:AIP    

年代:1997

数据来源: AIP