摘要:

The article begins with an account of prewar German work, particularly that of von Ardenne, who established the theoretical basis of a scanning electron microscope and constructed an instrument which was primarily intended to overcome chromatic aberration when relatively thick specimens were examined by transmission. Neither this microscope nor a different one built a few years later in the U. S. A. attained sufficient resolution to gain acceptance and the reasons for this are examined. The remainder of the article deals with work carried out in the Cambridge University Engineering Department over the years from 1948 to about 1965, when the first successful commercial instrument was produced. The contributions made by successive research students are explained, as are also the nonscientific factors which influenced the course of the development.

ISSN:0021-8979    

DOI:10.1063/1.331666

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The time response of the aximuthal magnetic field due to a vertical electric dipole over a two‐layer medium is obtained by application of the double deformation technique. The solutions are shown to obey the principle of casuality. Numerical results are illustrated with the complete time response for the cases of a slab of nonconducting dielectric and a conducting two‐layer medium.

ISSN:0021-8979    

DOI:10.1063/1.330586

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Submicron area Nb edge tunnel junctions have been fabricated yielding devices withRCtimes of 10−13s. Tested as quasiparticle mixers at 55 GHz, they yielded conversion loss of 6.4 db SSB and a noise temperature ?27 K. A gain of 1.4 db with a noise temperature of 145 K was also achieved but was attributed to the Josephson effect.

ISSN:0021-8979    

DOI:10.1063/1.330587

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Molecular mean excitation energies for ionic bonded molecules calculated according to the local plasma approximation are compared to the Bragg rule. Adjustments of 15% are calculated for LiF in agreement with experiments while 6% adjustments are predicted for HF and 3% for LiH.

ISSN:0021-8979    

DOI:10.1063/1.330588

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The general nonlinear self‐consistent equations describing the FEL in the steady‐state amplifying configuration is derived with the space‐charge effects and all the efficiency enhancement schemes: (i) a spatially varying wiggler amplitude and/or period and (ii) a dc electric fieldEdc(z) = [−∂&fgr;dc/∂z]e⁁z. All the efficiency enhancement schemes are shown to be somewhat equivalent. For a variable period magnetic wiggler, the FEL equations can be appropriately normalized to a three‐parameter set of coupled equations. All operating regimes are covered by these normalized equations. The condition for space charge to be unimportant is given. In the absence of the space‐charge effects, the three‐parameter set of equations reduces to a two‐parameter set. The inclusion of additional efficiency enhancement schemes increases the number of parameters to describe the FEL. Curves showing the efficiency, radiation amplitude, and growth rate will be presented.

ISSN:0021-8979    

DOI:10.1063/1.330589

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The stimulated emission profiles of oxide‐isolated double‐heterostructure diode lasers with 8‐&mgr;m stripes are predicted using a dielectric profile completely determined by measurement. The profiles inferred from the spontaneous emission vary as 1 − (‖x‖/w)&agr;with &agr;≅4. The current‐induced changes in gain and index are determined from the Fabry‐Perot mode spectrum. A simple model with no adjustable parameters is used to predict near‐ and far‐field widths to within 6%.

ISSN:0021-8979    

DOI:10.1063/1.330582

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Simple closed‐form expressions are derived for the modal propagation constant and field intensity of stripe‐geometry double‐heterostructure laser diodes with dielectric profiles of the form &egr;(x) = &egr;(0)−&Dgr;&egr;‖x/w‖&agr;. These expressions are useful for analyzing near‐field and far‐field data when the parabolic approximation is not sufficiently accurate.

ISSN:0021-8979    

DOI:10.1063/1.330590

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Surface cracks introduced in soda lime glass and MgO single crystal have been examined with a 200‐ and 420‐MHz scanning acoustic microscope. A fringe pattern was observed around the cracking line on the surface which could not be observed with an optical microscope. A simplified model is proposed in which fringe formation is due to the interference between the leaky surface wave reflected by the crack and the specularly reflected wave at the surface or the leaky surface wave that did not reach the crack. By monitoring these fringes, a Hertzian crack in soda lime glass with a depth as small as 25 &mgr;m was detected. The direction of crack orientation relative to the surface normal was also obtained from the asymmetrical contrast distribution of the fringes.

ISSN:0021-8979    

DOI:10.1063/1.330591

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The characteristics of laser‐induced breakdown in N2and O2at a wavelength of 1.064 &mgr; were investigated using 10‐nsec pulses for pressures ranging from 1 to 50 atm. The pressure dependence of the experimentally determined threshold field intensities for these two gases is essentially the same except at the lowest pressures where the O2values are slightly lower than those for N2. Comparison of this data with existing microwave data can be accomplished most directly by means of a ’’universal plot’’ of the type used effectively in the latter case. The general trends of the microwave and optical data are essentially the same; however, actual numerical values for the optical data tend to be lower than the corresponding microwave values by a constant factor of roughly 0.5. The fact that the two sets of data correspond so closely in spite of the large difference in frequencies has to be regarded as providing substantial evidence supporting the validity at optical frequencies of the cascade theory of breakdown used to interpret microwave breakdown measurements. Time‐resolved and time‐integrated emission spectra were analyzed, from which the electron density and temperature were obtained for the initial stage of spark development. The spectral characteristics, the electron densities, and temperatures are similar to those determined previously by other investigators for air.

ISSN:0021-8979    

DOI:10.1063/1.330592

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The relationship between current density, applied voltage, and pressure in dc diode discharges used for sputter deposition has been calculated using a model which considers the electron and ion currents within the cathode dark space. Electrons emitted from the target cause ionization primarily at the interface between the dark space and negative glow and ions crossing the dark space suffer symmetric charge exchange collisions with neutrals. The resulting expression for the characteristic is of the formJ=B−2/3(Va−V0)3/2, whereV0is the minimum voltage at which the discharge can be maintained. The values ofBandV0have been obtained by comparison with experimental data for argon and oxygen discharges with a Ta sputtering target. They vary approximately as (pressure)−4/3and (pressure)−1, respectively. Values ofBcalculated from the experimental data forJandVain argon and oxygen discharges agree with the values obtained from the model and measured values for the dark space distance. Comparison of the model with experimental data gives an estimate of the average value of Townsend’s ionization coefficient.

ISSN:0021-8979    

DOI:10.1063/1.330550

出版商:AIP    

年代:1982

数据来源: AIP