摘要:

Optical losses in pillar microcavities have been measured directly by means of microreflectance spectroscopy. It was shown that optical losses of the cavities larger than about 3 &mgr;m were constant irrespective of the diameters. The lasing thresholds were also measured, and they were proportional to the cavity volume. This result is consistent with the measured constant optical losses. A drastic increase in optical losses has been observed when reducing the cavity diameters below 2 &mgr;m. The scattering losses due to sidewall roughness were considered mainly responsible for the optical losses. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361283

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A significant attenuation of the specular echo in a wide frequency domain occurs in scattering of waves by Galois gratings. Diffraction of ultrasonic waves by immersed one‐dimensional elastic gratings constructed from the finite Galois fieldsGF(pm) is investigated. We develop a theoretical model taking into account the elasticity of the structure, its finite thickness, the boundary conditions at the fluid–solid and solid–fluid interfaces, and the periodicity of the gratings. In addition to the typical attenuation of the specular echo predicted by discrete Fourier transform, such a model predicts minima in the spectrum of the reflection coefficient corresponding to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) by mode conversion and to the excitation of Lamb‐type modes. The attenuation as well as the minima are experimentally observed and these results are in good agreement with theoretically predicted ones. Galois gratings could constitute an advance in the search for new anechoic devices. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361218

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The photoacoustic resonance method has been applied to study molecular relaxation in a cylindrical cavity. The profile of the acoustic resonance, excited by the vibrational mode &ugr;3of gas sample SF6with a CO2laser, was measured as a function of pressure between 1 and 800 Torr. A general theoretical treatment of vibrational and rotational relaxation on the acoustically resonant characteristics, such as the resonant frequency and the broadening of half‐width, taking into account the contributions of surface and volume losses, and the nonideality of gas, was worked out in detail and was used to frame the discussion of the relaxation processes following the vibrational excitation. The analysis of the experimental data yields a value of vibrational relaxation time (p&tgr;)V‐T=0.21±0.01 &mgr;s atm, and a value of (p&tgr;)R‐T=0.09±0.06 ns atm for the rotational relaxation time of SF6atT=295 K. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361284

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The total yield of H−ions,Y(Ein), produced in backscattering of low‐energy H+and H+2ions from polycrystalline gold, tungsten, and molybdenum converter surfaces was measured at normal incidence in the energy rangeEin=2–30 eV per nucleus. The yield per nucleus is independent of the ion mass. This indicates that the molecular ions are dissociated before colliding with the converter surface. A universal expression forY(Ein) was developed by combining the electron tunneling theory with atomic scattering theory. This expression agrees well with measurements. The yield is completely characterized by two parameters,Eth/REandRN&eegr;0, which can be determined experimentally:Y=0 forEin=Eth/RE, andYapproaches the maximum yieldR&eegr;0asEinincreases. These parameters were determined from measured H−yields in ion beam backscattering experiments, as well as for backscattering of thermal distributions of hydrogen atoms. For beam experiments, the maximum yield of 0.3 per nucleus was obtained for Mo/Cs converters with 1.5 eV work function. A higher maximum yield of 0.42 was obtained from experiments on backscattering thermal distributions of H atoms. This is attributed to high extraction fields. The universal yield formula made it possible to compare the results of the two different types of experiments. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361285

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The behavior of electron swarms in gases in time‐varying ramp electric fields is investigated using a Monte Carlo simulation. In particular, we consider the case of swarms in chlorine gas. Swarm parameters as a function of instantaneousE/N(Eis the electric field andNis the gas number density) for differentdE/dtare determined. At higherdE/dt, all the parameters as a function ofE/Nare shifted to largerE/Nwhen compared to the steady‐state case. The drift velocity curve develops a peak which shifts to higherE/Nand increases in magnitude asdE/dtis increased. Next, we pay attention to the definition of the swarm parameters, where the parameters derived from kinetic theory and by analogy to experiment are compared. For the highdE/dtcases, differences in the swarm parameters at the sameE/Nvalues are caused by the inertia of the electrons and their transient beam‐like high drift velocity, while discrepancies due to the difference between the definitions in the kinetic theory and in the analogy to experiment appear significantly. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361286

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Radiative recombination from states located in the band gap of single‐crystal &agr;‐alumina has been observed by electron‐induced x‐ray emission spectrometry. The variation of intensity as a function of the incident electron beam current has been determined. From this variation, we show that trapping of thermalized incident electrons takes place in the defect sites associated with the observed states. From the binding energies of the states, the sites have been identified as oxygen vacancies. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361221

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The stray field, magnetic microstructure, and switching behavior of high‐resolution electron beam fabricated thin film tips for magnetic force microscopy (MFM) are investigated with different imaging modes in a transmission electron microscope (TEM). As the tiny smooth carbon needles covered with a thermally evaporated magnetic thin film are transparent to the electron energies used in these TEMs it is possible to observe both the external stray field emanating from the tips as well as their internal domain structure. The experiments confirm the basic features of electron beam fabricated thin film tips concluded from various MFM observations using these tips. Only a weak but highly concentrated stray field is observed emanating from the immediate apex region of the tip, consistent with their capability for high resolution. It also supports the negligible perturbation of the magnetization sample due to the tip stray field observed in MFM experiments. Investigation of the magnetization distributions within the tips, as well as preliminary magnetizing experiments, confirm a preferred single domain state of the high aspect ratio tips. To exclude artefacts of the observation techniques both nonmagnetic tips and those supporting different magnetization states are used for comparison. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361287

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A resonant cavity technique was used to measure the dielectric constant and loss tangent of ceramic insulators at a frequency near 100 MHz during pulsed fission reactor irradiation near room temperature. Tests were performed on single crystal and several different grades of polycrystalline Al2O3, MgAl2O4, AlN, and Si3N4. Lead shielding experiments were performed for some of the irradiations in order to examine the importance of gamma ray versus neutron irradiation effects. With the exception of AlN, the dielectric constant of all of the ceramics decreased slightly (<0.2% change) during the pulsed fission reactor irradiation. The dielectric constant of AlN was observed to slightly increase during irradiation. Significant transient increases in the loss tangent to values as high as 6×10−3occurred during pulsed reactor irradiation with peak ionizing and displacements per atom (dpa) radiation fields of 4.2×104Gy/s and 2.4×10−6dpa/s, respectively. The loss tangent measured during irradiation for the different ceramics did not show any correlation with the preirradiation or postirradiation values. Analysis of the results indicates that the transient increases in loss tangent are due to radiation induced increases in the electrical conductivity. The loss tangent increases were proportional to the ionizing dose rate in all materials except for AlN, which exhibited a dose rate exponent of ∼1.6. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361288

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Argon incorporation and the formation of silicon carbide in Si(100) by low energy Ar+ion bombardment have been studied by angle‐resolved x‐ray photoelectron spectroscopy (XPS). The bombardment was performed at ion energies of 1, 1.5, and 2 keV and various ion fluences in an ultrahigh vacuum chamber equipped with XPS. The XPS measurements showed that the incorporated Ar concentrations achieved saturation in the near‐surface region at ion bombardment fluences ≳1016cm−2. The surface Ar concentrations decreased with increasing bombardment energy. No Ar bubbles on the surface of Ar+‐bombarded samples were observed by atomic force microscopy under these experimental conditions suggesting that Ar bubble formation was not the main Ar trapping mechanism in our study. The SiC formation was confirmed by characteristic XPS peaks of Si 2pand C 1sfor SiC. The carbide formed at lower ion fluence was of a metastable structure as inferred by XPS. Bombardment at higher ion fluence yielded a stable carbide phase through continuous ion beam mixing. No strong dependence of carbide depth distribution on bombardment energy was observed suggesting that the carbide phase is probably dispersed inside the bombarded layer and that carbon is bonded to silicon at localized defect sites. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361289

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have investigated the growth of three‐dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi‐one‐dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361290

出版商:AIP    

年代:1996

数据来源: AIP