摘要:

This review of electrical resistance standards begins with a description of classical standard resistors and their limitations. Methods of comparing resistance are described; these include bridges based on cryogenic current comparators capable of achieving statistical uncertainties approaching one part in1010in the measurement of resistance ratios. Such reproducibility is nearly two orders of magnitude smaller than the overall uncertainty of the most accurate determinations of the ohm from its SI definition via the calculable capacitor. The quantum Hall effect can provide an invariable reference standard of resistance linked to the fundamental physical constants. Many factors, however, limit the accuracy of practical realizations of quantized Hall resistance standards. Ultimately, the accuracy of a specific realization must be confirmed by comparison with similar standards; methods for doing this and the resulting agreement are presented. The ac techniques used in the determinations of the SI ohm by means of the calculable capacitor are now being applied to accurately link the quantized Hall resistance to the impedance of standard capacitors and thereby to provide a new reference standard of capacitance. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149062

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Results are presented on development of processes for fabricating linear and circular Bragg–Fresnel lenses (BFLs) on Si and III–V compound material substrates, and on x-ray characterization of linear BFLs at the Advanced Photon Source (APS). Processes were developed for fabricating long (zone length >5 mm) linear BFLs on Si with enhanced capability for focusing high-energy x rays. By stitching together 20 sequentially exposed 400-&mgr;m-long linear BFLs, we were able to fabricate 8-mm-long linear BFLs with 0.5-&mgr;m finest zone width. BFLs were also fabricated on III–V compound semiconductor substrates GaAs and InP, with improved process control due to the substantially reduced zone thickness required (∼50&percent; less than Si). Reduction of the zone aspect ratio (thickness/width) lessens the demand on the process technology, and may lead to higher lens resolution and pattern transfer accuracy. A process was explored to fabricate BFLs on a GaAs/AlGaAs heterostructure incorporating a built-in “etch stop” layer to ensure uniform zone thickness. Experimental characterization of the focusing properties of a field-stitched 8-mm-long linear BFL on Si (zone aperture=150 &mgr;m) was conducted at APS undulator beamline 1-ID-C using 10-keV x rays. Based on measured focal plane intensity distribution, the focus was estimated to be 1.2 &mgr;m, comparable to the geometrically demagnified source size of ∼1 &mgr;m. Lens efficiency was estimated to be ∼30&percent;. Work is underway to incorporate the BFL-microprobe in x-ray microdiffraction and fluorescence microscopy experiments to study spatially confined complex materials. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149022

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Anin situcalibration experiment of a soft x-ray spectrograph for x-ray laser research has been performed. The calibrated spectrograph was a flat-field grazing incidence spectrometer with an x-ray charge-coupled device camera. The absolute sensitivity of the spectrograph was obtained for the wavelength range from 2.6 to 4.0 nm using a transmission grating spectrograph as a reference spectrograph set along the axial direction of a line-focused laser-produced plasma. The absolute sensitivity determined in this work shows nearly the same value as that deduced from the calibration experiment using synchrotron radiation. ©1998 American Institute of Physics. 

ISSN:0034-6748    

DOI:10.1063/1.1149023

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A white-light Michelson interferometer in the visible and near infrared regions is described which can obtain the cross-correlation interferogram between the incident light and the transmitted light through samples or optical components. By comparing the autocorrelation interferogram of the incident white light with the cross-correlation interferogram between the incident light and the transmitted light, we can obtain linear response functions of samples or optical components with femtosecond time resolution. Fourier analysis of the interferograms gives also the complex optical constants of the samples without relying on the Kramers–Kronig relation. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149024

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This article describes an imaging spectrometer developed for remote sensing applications. The instrument consists of an acousto-optic tunable filter (AOTF), a high sensitive charged coupled device camera and optic lens. The selected AOTF was designed to have a large entrance aperture and to diffract light between 600 and 900 nm with a high spectral resolution. The signal was digitized by the camera and acquired with a developedWINDOWSexecutable software which drives the instrument. The spectrometer has been operated in a classical mode and in an imaging mode for its evaluation. In the imaging mode, images including different types of targets were acquired in reflection and transmission mode and were used to investigate image homogeneity, spatial resolution, chromatic and geometric aberrations, sensitivity and reliability. Spectra have been extracted from images and were successfully compared with spectra from the same sample area obtained with classical nonimaging spectrometers. This confirms the ability of this instrument to clear up many problems in surface analysis, particularly in the case of laboratory and field measurements used in remote sensing studies. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149025

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A simple technique for investigating whispering gallery modes in optical fibers is described. Unlike the conventional technique of winding a fiber around a mandrel, a bent fiber is produced simply by inserting it inside a commercial glass tube. The fiber naturally assumes a semicircular shape at the bend. It is shown that the loss spectrum due to whispering gallery modes is insensitive to small variations in the glass tube diameter. The glass tube with the bent fiber inside is used to fabricate a rugged interferometric temperature sensor which can be moved around even during measurements. Application of the technique for quality control is also demonstrated by measuring the variation of fiber buffer properties along the length of fiber. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149026

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

An experimental technique was developed for studying the optical Kerr effect induced by a cwAr+laser with 500 mW output. A Pockels cell was used for modulating the pump laser so that the polarization changed periodically between the vertical and horizontal plane. The dielectric anisotropy thus induced in a sample liquid was probed in the ellipsometry with a He–Ne laser. The system was applied to nitrobenzene andp-n-hexylp′-cyanobiphenyl (6CB), a nematogen in the isotropic phase. The observed birefringence showed a linear dependence on the pump laser intensity and the gradient yielded the Kerr constant:B=5.75×10−14 V−2 mat 30 °C in nitrobenzene andB=6.72×10−12(T−T*)−1 V−2 min 6CB,T*being the virtual transition temperature to the nematic phase. This technique can be used for investigating the relaxation of molecular orientation in frequency domain. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149027

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Recent advances in the transient dc photocurrent technique for measuring excited state dipole moments, developed in our group, are discussed. A variety of approaches with detailed analyses of their advantages and disadvantages including cell design, circuit construction tricks, the data acquisition procedure, calibration, and the theoretical treatment of different conditions, are presented. Sensitivity, time resolution limitations, and newly developed features, such as the signal’s dependence on light polarization as well as charge separation at interfaces are outlined. Dipole moments of a few molecules (diphenylcyclopropenone, bianthryl, dimethylaminonitrostilbene, Coumarin 153, and fluoroprobe) suitable for calibration purpose are reported—some of them for the first time. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149028

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A novel gas nozzle design that allows for collinear laser and molecular beams is described. Unlike conventional nozzles that open and close to limit the gas flow into a vacuum chamber, this nozzle opens to release a small volume of gas into the chamber. Gas pulses of 300 &mgr;s duration and 65±5 &mgr;s opening times have been obtained. When room-temperatureH2is used as the source gas, the rotational temperature was found to be 175±12 K, whereas when room-temperatureD2was used, the rotational temperature was found to be 104±7 K. The collinear optical and molecular beam path provided in this design may be useful in photodissociation experiments, the study of atomic and molecular collisions, Rydberg state lifetime measurements, and vacuum ultraviolet generation by nonlinear mixing in gases. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149029

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We have developed off-line experimental techniques and apparatuses that permit direct measurement of effusive-flow delay times and ionization efficiencies for nearly any chemically reactive element in high-temperature target/ion sources (TIS) commonly used for on-line radioactive ion-beam (RIB) generation. The apparatuses include a hot Ta valve for effusive-flow delay-time measurements, a cooled molecular injection system for determination of ionization efficiencies, and a gas flow measurement/control system for introducing very low, well-defined molecular flows into the TIS. Measurements are performed on a test stand using molecular feed compounds containing stable complements of the radioactive nuclei of interest delivered to the TIS at flow rates commensurate with on-line RIB generation. In this article, the general techniques are described and effusive-flow delay times and ionization efficiency measurements are reported for fluorine in an electron-beam plasma target/ion source developed for RIB generation and operated in both positive- and negative-ion extraction modes. ©1998 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1149225

出版商:AIP    

年代:1998

数据来源: AIP