摘要:

We have constructed two types of high‐temperature superconducting (HTSC) bolometers, whose performance is essentiallywavelengthindependentfrom &lgr;∼0.6 to 450 &mgr;m: The first is a microbridge, of dimensions 20×20 &mgr;m, and the second is a meander, covering a region of ∼1 mm2. Both were fabricated by photolithography of a superconducting YBaCuO thin film on SrTiO3. The bolometers are current biased, and the ac voltage induced by the chopped radiation is measured using lock‐in detection. Operating optimally in the vicinity of the transition temperature (90 K), the first bolometer shows responsivityS∼1 mV/(W/cm2), while the second givesS∼800 V/W. The response of the first bolometer for various chopping frequencies is basicallyS∼1/&sqrt;f, which makes it usable at frequencies up to ≥50 kHz, while the second bolometer has a chopping‐frequency response that is basicallyS∼1/f, with a cutoff frequency near 0.01 Hz.

ISSN:0021-8979    

DOI:10.1063/1.352289

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A finite difference approximation is used to predict the temperature profile on an oxidized silicon substrate resulting from the biasing of a thin‐film resistor fabricated on the surface of the wafer. The wafer is coated with a polymer whose deposition rate depends on the substrate temperature and the resulting thickness variation is compared to that predicted by theory, as a means of comparing the expected temperature distribution to the actual distribution.

ISSN:0021-8979    

DOI:10.1063/1.352290

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A stationary process of secondary particle production due to impact ionization of neutral gas by an electron beam and their escape in the presence of an axial focusing magnetic field is studied analytically. It is shown that a region of net positive space charge can be created for strong beam focusing within a short biased auxiliary tube placed inside a drift tube. It is also found that there is a threshold of the magnetic field magnitude after which ion focusing of a continuous beam is not possible because no balance between the production and collisionless escape of ions exists. The dynamics of the secondary electrons is investigated to determine the possibility of reaching a stationary state while keeping the secondary electron concentration smaller than the beam’s concentration thereby allowing to avoid various electron–electron instabilities. Axial escape of the secondary electrons due to the influence of the beam’s fields, ions’ electric field and external magnetic field is assumed. An expression for the secondary electron concentration is obtained.

ISSN:0021-8979    

DOI:10.1063/1.352291

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The coupling of the accelerating cavities to dummy cavities was recently found to reduce beam breakup growth. This article analyzes a more sophisticated model that includes the time delay between coupled cavities and covers the possibilities of wave cutoff and resonance in the coupling path. A dispersion relation is obtained. We show that the peak spatial exponentiation rate of the dominant beam breakup mode is reduced by as much as a factor of 2, and this reduction is insensitive to the coupling path length. Other modes are destabilized, however, but they have lower growth rates, in general.

ISSN:0021-8979    

DOI:10.1063/1.352263

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Plastic hollow waveguides (used as fibers) for infrared (IR) transmission were made from plastic tubes covered, on the internal wall, with a metal layer (Ag) and growing a dielectric thin (AgI) overlayer by direct iodination on it. The existence of several absorption lines at given wavelengths in the middle infrared (mid‐IR) region is predicted theoretically and measured experimentally. From the wavelengths of absorption lines the thickness of the AgI film has been computed. The average thickness of the AgI in the hollow waveguide increased with the iodination time and with the concentration of the iodine solution. The crystal size of the AgI was increased with the increase of the AgI thickness. By controlling the iodination process it was possible to make waveguides which can be employed as filters for various wavelengths of the transmitted mid‐IR radiation.

ISSN:0021-8979    

DOI:10.1063/1.352264

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The temperature of single quantum well semiconductor laser facets increases during operation, eventually reaching a critical temperature, thermal runaway, and catastrophic optical damage. A study of changes in composition of the near‐surface region of facets which accompany heating has been carried out for continuously operated, uncoated AlGaAs‐GaAs‐AlGaAs graded index separately confined heterostructure single quantum well lasers. High resolution depth profiles by scanning Auger microscopy show that the laser facets can be quite variable in initial composition, and undergo pronounced stoichiometry changes even during the first few minutes of operation. At longer times a continuing out‐migration of the group III elements is observed. Unlike the double heterojunction lasers, facet oxidation is not pronounced and is not responsible for diffusion of Ga and Al. There are indications, however, that a slow leakage of oxygen into the crystal may occur. Spatially resolved analyses provide evidence that carrier‐mediated elemental redistribution is an important factor in facet degradation. The progressive accumulation of defects which may act as non‐radiative recombination centers provides a simple means of facet heating. Analyses of lasers which have suffered catastrophic damage indicate that the facets are not always melted, and that there is no typical chemical state which distinguishes them from facets of lasers which are fully operational. These results are compared to studies of facet degradation in double heterojunction lasers. Implications of the data for models of catastrophic optical damage are discussed.

ISSN:0021-8979    

DOI:10.1063/1.352363

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Thermo‐optical switching in Si based etalons has been demonstrated in two device structures. In one experiment the switching time of a Si etalon has been reduced from milliseconds to microseconds by choosing a probe beam of shorter wavelength in an external switching configuration we reported previously [Appl. Phys. Lett.58, 2073 (1991)]. The switching time has further been improved to the ns range by the use of a 1.06 &mgr;m Nd:YAG laser pump which is presumed to give rise to a thermo‐refractive change in the Si etalon and at the same time the switching threshold energy has been reduced to ∼1 &mgr;J as compared to ∼1 mJ for a CO2laser pump. In comparing Si etalons with thicknesses of 400, 72 and 1.5 &mgr;m, we find that the 72 &mgr;m etalon exhibits the best behavior in terms of low threshold power, high speed and contrast. In addition, effects of the pump beam intensity on the signal pulse shape has been investigated which indicates a multiple interference fringe shift and transverse thermal relaxation dynamics. The second experiment used a Si/Si3N4film structure in which the index of the Si3N4is approximately the square root of the index of the Si, and the optical thickness of the Si3N4is an odd quarter wavelength multiple of the probe beam. This yielded a minimum in surface reflection. Based on an increase of the optical thickness with pump beam heating, a probe beam reflection has been switched out with a high contrast ratio (switched on/off) of 61:1. Different structures using different pump beams are discussed and the restriction of the film optical thickness has been investigated.

ISSN:0021-8979    

DOI:10.1063/1.352265

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An exact solution is derived to the one‐dimensional, time‐dependent, heat‐conduction equation for a two‐layer, semi‐infinite, composite solid with uniform heat generation in the surface layer, no heat transfer through the surface plane, and uniform initial temperature. The interface between the two layers is assumed to have no thermal contact resistance. This solution enables a discussion of the ideality with which a step‐function electric current in a metallic foil can generate a step‐function heat flux into a contacting semi‐infinite solid. Previous measurements of thermal diffusivity (based on the above conditions) have relied on the idealized constant‐flux solution for data reduction. It is shown here that the temperature errors in the substrate arising from nonideality of the constant‐flux boundary condition increase with depth into the substrate, foil thickness, and decreasing thermal conductivity/diffusivity of the substrate.

ISSN:0021-8979    

DOI:10.1063/1.352266

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Based on molecular field theory, including the effect of the field‐dependent nature of Ne´el temperature, the entropy, and the magnetocaloric effect in terbium (Tb) have been calculated with the external field between 0 and 7 T (Tesla). The calculated results are compared with the existing experimental measurements. The maximum magnetocaloric temperature change (&Dgr;T) is approximately 14 K near the zero‐field Ne´el temperature (230 K) for an external field at 7 T field. This relatively large magnetocaloric effect indicates that Tb could be an attractive candidate for magnetic heat pump application. The results are then used to study the performance of heat pumps for the Carnot cycle, the constant field cycle, and the ideal regenerative cycle. The performances of these three cycles are discussed and compared.

ISSN:0021-8979    

DOI:10.1063/1.352267

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An investigation of streaming potential and electrical noise generated during capillary flow of aqueous solutions of poly(ethylene oxide) showed that these effects do not depend on each other. While the former effect arises in the electrical double layer at the capillary wall, the latter is a bulk effect produced by turbulent movement of the flowing liquid.

ISSN:0021-8979    

DOI:10.1063/1.352268

出版商:AIP    

年代:1992

数据来源: AIP