摘要:

A new approach to laser particle acceleration in vacuum is proposed that incorporates three ingredients: (i) a radially polarized laser beam; (ii) an aligned axicon focus; and (iii) the approximate matching of particle and laser phase velocities. This combination has several important advantages which are discussed. The critical issue in any vacuum acceleration scheme is the inherent backward slippage of the particle relative to the laser beam phase, causing the particles to experience successive accelerations and decelerations. According to the Lawson–Woodward theorem, which applies for fairly general conditions, the net result is no energy gain. It is shown that this theorem does not apply to the present laser particle acceleration scheme, and this is verified by applying an exact solution for the electric field. Indeed, acceleration gradients approaching 1 GeV/m over an interaction length of a few centimeters are predicted for 1013W laser peak power at 10.6 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.351442

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The optical absorption spectrum of ions oscillating in a three‐dimensional harmonic trap is calculated for laser excitation. The effect of the mode structure of the laser beam employed, in particular the case of the TEM00mode used in most experiments is considered. The spectrum contains many sidebands, besides the central transition frequency, generated byallcombinations of the ion oscillation frequencies in the three orthogonal directions of motion, even when the laser is directed along the trap axis or is incident in the radial plane, and even when perfect symmetry of the trapping fields is assumed. It is shown that this coupling between the axial and radial frequencies is due to the Gaussian mode structure of the laser beam. In laser spectroscopy of ions trapped in a real radio frequency (rf) trap, energy transfer between the ion motional degrees of freedom is therefore caused by the mode structure of the laser beam and, as previously assumed, by asymmetries of the pseudopotential well and for more than one ion in the trap by ion space charge fields. The ion motion coupling due to the laser mode structure is of particular significance during the initial phases of laser cooling experiments when comparatively hot ions sample a distance comparable to the waist of the collimated laser beams and when this distance is of the order of the optical wavelength.

ISSN:0021-8979    

DOI:10.1063/1.351443

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Lithium niobate single crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 45.5–58 mol % Li2O. Their optical damage resistance was characterized by measurement of the photoinduced birefringence change. Their crystal quality was characterized by x‐ray topography and x‐ray rocking curves. The photoinduced birefringence change increased with an increase in the Li content of the crystal. The Li‐poor crystals, which were richer in Nb content than the crystal with the congruent composition, showed the smallest birefringence change, in spite of the fact that the x‐ray rocking curve width of these Li‐poor crystals was much wider than the congruent crystal. It is concluded that the optical damage resistance of undoped LiNbO3is increased with an increase in the Nb content of the crystal, and that the crystal quality, as represented by x‐ray rocking curve peak width, has no correlation with the optical damage resistance.

ISSN:0021-8979    

DOI:10.1063/1.351444

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We investigate the reliability of numerical analysis techniques for the perturbed nonlinear Schro¨dinger equation that predicts soliton propagation in optical fibers. We use the propagating‐beam method and the split‐step finite‐element method for this purpose, and offer useful data on the parameters, such as the number of descretizing points and the step size along the propagating direction.

ISSN:0021-8979    

DOI:10.1063/1.351445

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The eleventh anti‐Stokes order at 116 nm of the KrF excimer laser radiation was obtained in H2gas at room temperature. The effects of the depletion of the first Stokes order by the S1‐S2 Raman conversion on the efficient vacuum ultraviolet anti‐Stokes generation was studied, and it determined the optimal Raman gain coefficient in this experiment. Ne gas was added into hydrogen gas to suppress the S1‐S2 conversion, and the significant increase in the conversion efficiency for the lower anti‐Stokes orders was observed in certain circumstances.

ISSN:0021-8979    

DOI:10.1063/1.351446

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

It is shown here that ultrasonic measurements can provide accurate information on an adhesive layer whose thickness is negligible compared with the wavelength. In this case, the adhesive layer can be considered as a simple interface between two elastic solids. This kind of contact enables small discontinuities of stresses or displacements between both substrates. A simple formulation is introduced to take into account these discontinuities expressed in terms of imperfect boundary conditions. It is shown that this formulation is consistent with exact calculation in layered media. The range of applications of this model and its limits is also investigated. This model is used to infer quantitative data on the changes in mechanical properties of a thin bonding layer (about 5 &mgr;m) from reflection coefficient measurements during its hardening process. The results obtained are in excellent agreement with ultrasonic measurement in bulk.

ISSN:0021-8979    

DOI:10.1063/1.351447

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Surface‐acoustic‐wave (SAW) devices with a low sensitivity to both temperature and stresses effects are presented and experimentally tested. Devices are built on SAW quartz cuts with zero first‐order temperature coefficient and zero sensitivity to planar isotropic stresses. In comparison with similar devices built on conventional (ST, X) cuts, two kinds of experiments are reported. First, devices built on circular quartz plates are submitted to in‐plane diametrical compressions. The force‐frequency effect is measured and results are compared to theoretical predictions either by analytical models or using finite element analysis. In a second experiment, the same devices are submitted to in‐plane thermoelastic stresses in a dynamical thermal regime. It is shown that the sensitivity of the new cuts to dynamical thermal effects is one order of magnitude lower than the conventional (ST, X) cut, an attractive potentiality for fast‐warm‐up high‐stability SAW oscillators in severe environmental conditions.

ISSN:0021-8979    

DOI:10.1063/1.351448

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Many important classes of surface reactions exhibit both high heats of reaction and large, positive activation energies. In addition, many surface reactions often occur in thermally isolated environments. As a result, significant autothermic effects are possible. In part I of this article, a generalized model of these effects is presented which describes the enhancement in reaction rate as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Reactant concentration and reaction order effects are also considered. Part II of this work presents the application of this model to numerous experimental plasma etching data.

ISSN:0021-8979    

DOI:10.1063/1.351449

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Part I of this paper presented a generalized model of heat effects operative during surface reactions. The enhancement in reaction rate due to autothermic effects was analyzed as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Application of this model to experimental plasma etching data is presented in part II of this work. Characteristic temperatures calculated from experimental data in numerous plasma etching systems agree closely with the critical characteristic temperature predicted by the heat of the reaction model. Possible reasons for this consistency are given. Further, the autothermic enhancement in the Ta‐CF4/O2etching system is accurately predicted as a function of reactant concentration by a heat of reaction model.

ISSN:0021-8979    

DOI:10.1063/1.351450

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The problem of adhesion at the interface between two polymers is investigated using a fracture mechanics approach. The effect of viscoelastic deformation of the bulk polymers and the micromechanical behavior of the interface on the fracture toughness of the interface was investigated by analyzing the problem of a steadily growing interface crack under the condition of small scale yielding. This condition assumes that the region of displacement discontinuity across the interface (the adhesive zone) due to loss of adhesion is small compared with typical specimen dimensions. It is found that the amount of inelastic or viscous energy dissipation is strongly coupled to the interface behavior and that the measured interface fracture toughness can be much higher than the intrinsic interface fracture toughness. For sufficiently high crack growth rates, a simple expression is obtained which relates the rate of viscous energy dissipation and the intrinsic interface fracture toughness to the viscoelastic properties of the bulk polymer. The dependence of the stress field inside the adhesive zone and the length of the adhesive zone on the viscoelastic properties of the polymer, the applied loading, the interface micromechanical model and the crack growth ratea˙is obtained by numerically solving an integral equation. The size of the region of the viscoelastic energy dissipation is found to scale witha˙&tgr;r , where &tgr;ris the retardation time of the bulk polymer.

ISSN:0021-8979    

DOI:10.1063/1.351451

出版商:AIP    

年代:1992

数据来源: AIP