摘要:

We report on a scanning tunneling microscope with a photoconductive gate in the tunneling current circuit. The tunneling tip is attached to a coplanar transmission line with an integrated photoconductive switch. The switch is illuminated through a fiber which is rigidly attached to the switch substrate. By using a firmly attached fiber we achieve an excellent reproducibility and unconstrained positioning of the tip. We observe a transient signal with 2.9 ps pulse width in tunneling mode and 5 ps in contact mode. The instrument is applied to investigating the mode structure on a coplanar waveguide. The measurements show that the probe works as a transient voltage detector in contact and a capacitively coupled transient field detector in tunneling mode. We do not measure the transient voltage change in the ohmic tunneling current. In this sense, the spatial resolution for propagating electrical pulses is better in contact mode than in tunneling mode. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364322

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this study we demonstrate the feasibility of generation of the electric charge and electric current in systems with rapidly varying parameters. It is shown that the electric current is excited spontaneously in the systems where electric resistance of their elements changes. Remarkably, effective damping resistance of the layered conductors may become negative, although ohmic resistance of its layers grows. Similar effects occur in electric circuits with different conductors connected in parallel. The effective resistance of such a system can decrease in spite of the increase of the ohmic resistance of the conductors. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364323

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated the decay processes of theEr:4S3/2and4F9/2states and the mechanisms of frequency upconversion under 800 nm excitation ofEr3+in ternary ZrF4–BaF2–LaF3glasses. The decay processes of theEr:4S3/2and4F9/2states are discussed on the basis of the Er concentration dependence of the fluorescence lifetime. The quantum efficiencies of emission of the4S3/2and4F9/2states are 73&percent; and 25&percent;, respectively. The difference in the quantum efficiencies between these states can be explained from the difference in the multiphonon decay rates: the multiphonon decay rate of the4S3/2state is much smaller than that of the4F9/2state. The multiphonon relaxation is a dominant nonradiative decay process of the4F9/2state. A dominant nonradiative decay process of the4S3/2state is self-quenching at high Er concentrations. The upconversion mechanisms under 800 nm excitation are also investigated from the dependence of upconversion luminescence on excitation wavelengths (800 and 980 nm), excitation laser power, and Er concentration. The mechanism of upconversion from 800 to 660 nm is mainly due to the energy transfer between the4I9/2→4I13/2and4F9/2←4I11/2transitions. We also show that the upconversion from 800 to 550 nm is due to excited-state absorption. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364324

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Using a single beamz-scan technique with a 30 ps laser pulse at 640 and 1064 nm, the real and imaginary parts of the third-order nonlinear optical susceptibilities of porphyrin oligomers for different numbers of repetition units in solutions were measured. At a probe wavelength 640 nm, the real parts of&khgr;(3) (Re &khgr;(3))of oligomers which consist of porphyrins bridged by 1,4-phenylene moiety were found to be negatively enhanced significantly compared with that of the monomer. WhileRe &khgr;(3)of the oligomers increased with increasing the porphyrin unit, the value per monomer unit remains almost constant; that is,Re &khgr;(3)is proportional to the number of monomer units. The oligomers in which the porphyrin rings are directly attached together at the meso position exhibited enhanced imaginary parts of&khgr;(3)at 640 and 1064 nm. The enhanced real part was also detected at 1064 nm. These unique properties are discussed in terms of the two photon resonance process. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364325

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Waveguiding properties of InGaN/GaN/AlGaN double heterostructures were modeled. A comparison between double heterostructures (DHs) and separate confinement heterostructures (SCHs) has been made. This is an attempt to consider influence of material absorption in passive layers of nitride laser structures on a threshold current. This absorption significantly increases the threshold current, and is especially important in the case of quantum well SCHs. The threshold current density in nitride-based lasers has been estimated to be about 10 kA/cm2. Due to high absorption of light in passive layers, no decrease in the threshold current of single quantum well SCH lasers in comparison with DH lasers is expected. The threshold current density can be decreased in multiquantum well lasers. In these lasers, the threshold current density is expected to be about several kA/cm2. The optimal thickness of active and adjacent layers as well as alloy compositions in cladding layers have been estimated, and as our simulation has shown, no more than 8&percent;–10&percent; of AlN in claddings are needed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364326

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A model is presented of the dependence of acoustic harmonic generation in polycrystalline solids on the coherency strains resulting from the lattice mismatch at the interface between the matrix material and a precipitated second phase in the matrix material. The acoustic nonlinearity parameter (a quantitative measure of acoustic nonlinearity) is shown to depend on the second, third, and fourth order elastic constants of the material, the precipitate-matrix lattice misfit parameter, and the volume fraction of precipitates. The model is applied to the artificial aging of aluminum alloy 2024 from the T4 to the T6 temper. Experimental measurements on samples of Al 2024 taken at various heat treatment times not only confirm the predictions of the model but together with the model provide a basis for assessing the influence of precipitate phase transformations on the aging process.

ISSN:0021-8979    

DOI:10.1063/1.364327

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We investigate the intrinsic error sensitivity of photoacoustic experiments in obtaining reliable and accurate information on the inhomogeneous thermal conductivity below the surface. The analysis is based on thermal wave properties and does not depend on any approximate inversion technique. As an example, the theory is applied to the important class of monotonic profiles. The theory can help us in choosing the lower frequency limit in an actual experiment. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364328

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A photothermal microscope that provides micrometer lateral and submicrometer depth resolution was designed. Thermal conductivity measurements with modulation frequencies up to 12 MHz on single grains in polycrystalline diamond demonstrate its lateral resolution power even for a highly conducting material. Measured conductivities strongly depend on the averaged volume and values up to 2200 W/mK are found in the high frequency limit where the properties inside a grain are sampled. The capability of the instrument to measure thermal parameters on thin films is demonstrated for gold films evaporated on quartz with a thickness ranging from 20 to 1500 nm. Measurements reveal a strong thickness dependence for both thin film conductivity and the contact resistance between film and substrate. Thermal conductivity decreases monotonically from 230 to 30 W/mK whereas the contact resistance rises from 2×10−7to 8×10−6m2K/W with decreasing film thickness. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364329

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this work we propose that endoreversible Carnot–type heat engines have a general property independent of the heat transfer law used to describe heat exchanges between the working fluid and its thermal reservoirs. This property has to do with the so-called ecological function [F. Angulo–Brown, J. Appl. Phys.69, 7465 (1991)]. According to this property, the efficiency at the maximum of the ecological function is the semisum of the Carnot and the maximum power efficiencies for any heat transfer law. This result is obtained by using the quasiparabolic behavior of power versus efficiency. From this property, we obtain a corollary over a general quantitative relation between the power (and also the entropy production) of both maximum power and maximum ecological regimes. We also discuss a criterion to find the best ecological function. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364090

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The magnetic buoyancy force acting on a bubble in a one-dimensional magnetic field can be represented asF=(&khgr;G−&khgr;L)∫H(dH/dx)dVolB,where&khgr;Gand&khgr;Lare the volume magnetic susceptibilities of the gas and liquid, respectively, andHis the magnetic field strength. Since|&khgr;L|≫|&khgr;G|and most liquids are diamagnetic, this expression indicates that the magnetic buoyancy forces act in the direction of increasing magnetic field strength. Because the magnetic buoyancy force in a diamagnetic fluid is small, the motion of bubbles under normal gravity is difficult to study, but microgravity offers the possibility of detailed observations. Using a compact permanent magnet under microgravity conditions,N2bubbles in pure water (0.01 dyne s/cm2)and in a 69:31 glycerol/water mixture (0.21 dyne s/cm2)were found to move in the direction of increasingH, and to be held stationary at the point of maximumH. The motion of the bubbles was also simulated with a theoretical model and was found to agree with measurements made under microgravity conditions. These results indicate that magnetic buoyancy can be used to control bubble motion. Since most fluids are diamagnetic, magnetic buoyancy can be used to control bubbles in many fluidic devices used in space applications.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.364330

出版商:AIP    

年代:1997

数据来源: AIP