摘要:

A high‐pressure viscometer based on a diamond‐anvil pressure cell is described and its use demonstrated. A sphere (typical diameter 50 &mgr;m) rolls down a diamond surface under the force of gravity and its position is tracked electronically. The resulting velocity is converted to viscosity through a modified Stokes equation that includes a term describing the increased drag due to the close proximity of the walls within the pressure chamber. A model for this wall correction is described and that the drag coefficient is constant throughout the data collection is demonstrated. The precision of the resulting viscosity data averages 4%. The accessible viscosity range is 0.5–107cP using a diamond cell that is capable of pressures exceeding 10 GPa. However, most fluids exceed 107cP at far lower pressures. The accuracy of the device is demonstrated by comparing the presented data on the pressure dependence of the viscosity for five fluids [chloroform, water, octamethyltrisiloxane, 1‐pentanol, and bis(2‐ethylhexyl)sebacate] with previous measurements.

ISSN:0021-8979    

DOI:10.1063/1.351157

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The pressure‐induced frequency shift of the Sm:YAGY1 peak at elevated temperature is calibrated against the temperature‐corrected Raman shift of the nitrogen vibron and, at temperatures less than 673 K, theR1 shift of ruby. The results presented here indicate that pressure can be determined from theY1 andY2 peak frequencies, without temperature correction, from 6 to 820 K and from 1 bar to 25 GPa by using the equations:P(GPa) =−0.12204 (&ohgr;Y1obs−16187.2) andP(GPa)=−0.15188 (&ohgr;Y2obs−16232.2). However, pressure determinations based onY2 are less accurate, especially at high temperature. At elevated temperature, the Sm:YAGY1 andY2 peak frequencies are most accurately determined by curve fitting a spectral window at least 400 cm−1wide. The spectral range was chosen in order to include the decay of the intensity of the LorentzianY1 peak to a background value and incorporate a third peak at 16360 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.351158

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The acceleration of test electrons in a relativistic electron plasma wave is computed. The plasma wave is either due to the beating of two laser beams or constitutes the wake of an ultrashort laser pulse. The plasma wave geometry reflects the geometry of the laser. The electron beam has a finite emittance. The parameters of the calculation correspond to experiments now in progress. This 3Dmodel is able to predict the corresponding electron spectrum.

ISSN:0021-8979    

DOI:10.1063/1.351159

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A new technique forQswitching lasers by inducing reflectivity changes in one of the cavity’s multilayer‐coated mirrors is discussed. This mirror is such that, under normal operation, the constructive interference condition is not satisfied, and it therefore has a low reflectivity. Towards the end of the laser gain‐pumping pulse, suitably tuned radiation from a short‐pulse visible/uv laser is shone upon the mirror, resulting in changes in layer thicknesses and layer indices of refraction, such that the interference condition is satisfied. In this manner, the cavityQmay be switched in a short time interval (picoseconds) from a low to a high value. The mechanisms by which the induced changes occur are discussed, and calculations of reflectivity as a function of layer thicknesses and indices of refraction are presented. A general design criterion for successful implementation of the technique is given, along with some specific examples. Estimates of the required characteristics of the external, change‐inducing laser are also provided. In principle, the method can be applied to any laser cavity, including the recently demonstrated soft‐x‐ray multipass laser cavities, and would find use in many other situations where currently available techniques do not work. The lower bound on the wavelength of the laser line which can beQswitched in this manner is found to be between 200 and 400 A˚.

ISSN:0021-8979    

DOI:10.1063/1.351160

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Pulsewidth dependence of time‐resolved two‐photon absorption with picosecond pump–probe excitation is investigated. It is shown theoretically that the time‐resolved probe transmittance change by two‐photon absorption almost follows a temporal autocorrelation of the pump and probe pulses. Time‐resolved two‐photon absorption measurements of 1.064 &mgr;m, 29 ps pulses with semi‐insulating undoped GaAs are also performed. The comparison of the experimental results with second harmonic generation autocorrelation measurements confirms the theory.

ISSN:0021-8979    

DOI:10.1063/1.351131

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The temperature dependence of the complex refractive indices of SbSI, MoS2, and BiVO4have been investigated using spectroscopic ellipsometry. The wavelength of maximum temperature dependence was identified for an in depth study of the sensitivity of the reflectance coefficient to a small ac temperature modulation. It was found feasible to detect temperature changes of 10−3, 10−2, and 10−2 °C by monitoring the reflectance coefficient of SbSI, MoS2, and BiVO4, respectively, using null ellipsometry. It is thus possible to use the pyro‐optic phenomena for long wavelength infrared imaging, analogous to pyroelectrics.

ISSN:0021-8979    

DOI:10.1063/1.351132

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A new two‐dimensional model for transverse discharge‐excited gas lasers is described. The model can compute accurate electric fields for complex geometries. This facility is exploited to investigate the effect of the discharge electrode profile on XeCl laser performance. It is found that any of the stock electrode designs is satisfactory for high‐efficiency lasers with excitation pulses lasting for up to 200 ns. For longer‐pulse lasers, the pulse length and energy output are sensitive to the choice of electrode and there are grounds for preferring the Stappaerts profile over the other available designs.

ISSN:0021-8979    

DOI:10.1063/1.351133

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Bistability in the reflected light from an InSb surface has been observed without the use of a resonance cavity, and is explained by an increasing absorption positive feedback mechanism. Relying on this mechanism, a novel InSb optical switch is demonstrated that uses only the surface reflection and operates at room temperature using a pulsed CO2laser as the pump beam. The polarity of the switched‐out signal, the probe beam, was found to depend on the wavelength of the probe beam in the visible and near‐infrared range. This novel spectral dependence was used to demonstrate several optical logic elements.

ISSN:0021-8979    

DOI:10.1063/1.351134

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The detection of resonances of various targets immersed in water is investigated by means of a new signal processing. The fast Fourier transform analysis of the reflected signal allows to obtain the evolution of the ratio between the imaginary part and the real part of the complex number obtained. This ratio strongly depends on the resonant character of the target. Special attention is given to the problem of the time origin in order to have a spectrum well interpretable in term of resonances. Results about a single plane plate, a coated plane plate and a solid cylinder are shown. The method proposed gives at least as much information as the classical ones in the case of an elastic target, and more information in the case of an elastic target coated with an absorbent.  

ISSN:0021-8979    

DOI:10.1063/1.351135

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

This paper presents a theoretical analysis of the surface acoustic wave (SAW) propagation in a GaAs/AlGaAs heterostructure covered with a piezoelectric film such as ZnO, BaTiO3, PbTiO3or PZT. We calculate the SAW potential profile, SAW velocity, and effective piezoelectric coupling constant as functions of the film thickness using a Laguerre polynomial technique. The effective material constants such as the characteristic admittance and equivalent dielectric constant are also obtained. We concentrate on the suitable choice of the film for a heterojunction acoustic charge transport device. A ZnO film of 0.3–0.5 acoustic wavelength thickness seems to be the most suitable choice.

ISSN:0021-8979    

DOI:10.1063/1.351136

出版商:AIP    

年代:1992

数据来源: AIP