摘要:

Finite‐time thermodynamics is used to investigate theoretically the wall gain and product loads on a refrigerator based on an endoreversible Carnot cycle. Explicit formulas are obtained for the operating time, coefficient of performance, and the cooling rate and the procedure is illustrated numerically for typical refrigerators.

ISSN:0021-8979    

DOI:10.1063/1.354748

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Under certain experimental conditions a semiconductor‐discharge gap structure can be used as detector for spatiotemporal resolved measurements on IR radiation. With a streak camera system and a semiconductor laser diode (&lgr;=1.3 &mgr;m), we investigate experimentally the speed properties of this kind of converter. The experimental results are compared with the predictions of a simple theoretical model.

ISSN:0021-8979    

DOI:10.1063/1.354749

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This work investigates the temperature rise and heating mechanisms at the facets of quantum well lasers. An analytical solution of the heat conduction equation yields the temperature distribution in the laser and the temperature rise at the laser facets. The heat generation mechanisms are discussed and modeled through a one‐dimensional carrier diffusion equation. The normalized results from the models agree well with available experimental data but the absolute value of the maximum temperature rise is about 5 times lower than that of the measurement. This discrepancy is explained by the reduction of thermal conductivity caused by phonon reflection and transmission at the GaAs/AlGaAs interfaces. Averaging the calculated results over a probe diameter around 1.5 &mgr;m, as is often used in the microprobe Raman spectroscopy measurement of the facet‐temperature rise, reveals that the actual peak temperature at the facet is only 2–5 times higher than the measured value. This is a surprising result considering that the probe diameter is about two orders of magnitude larger than the active region thickness. A detailed examination of the calculation and the existing experimental data suggests a new explanation for the thermal runaway process in quantum well lasers. It is the onset of the absorption by the cladding media, rather than by the active region itself as is commonly believed, that provides the driving force for the thermal runaway process.

ISSN:0021-8979    

DOI:10.1063/1.354750

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Up‐converted beams of a conventional quadrupled Nd:YAG laser by anti‐Stokes Raman scattering in hydrogen are characterized as a vacuum ultraviolet light source. The beams diverge with the increase of the hydrogen pressure and also with the increase of the order of anti‐Stokes scattering. Although the profiles of anti‐Stokes beams vary from a Gaussian‐like shape to a typical ring shape, the maximum energy conversion is always obtained with a near‐Gaussian beam profile. The output energy ranges from 5.9 mJ for the first‐order anti‐Stokes wave (240 nm) to 8 &mgr;J for the ninth‐order anti‐Stokes wave (133 nm). The energy of the sixth‐order anti‐Stokes wave (160 nm) fluctuates ±45% of the average output energy. The beam characteristics are described well by a model on the basis of phase matching between four waves related to each Raman process.

ISSN:0021-8979    

DOI:10.1063/1.354751

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A direct measurement of a key material parameter in photorefractivity, the charge carrier mobility, has been achieved only recently by means of a holographic time‐of‐flight technique. In this paper we report, as far as we know, the first successful direct determination of electron mobility using the classical time‐of‐flight method. This consists in measuring the velocity of a sheet of excess carriers, created by a short‐duration excitation pulse of strongly absorbed photons close to the surface. These carriers drift through the sample under the action of an applied electric field. However, the technique could not be used in its original configuration. A constant background illumination was required in order to saturate traps and to prevent space charge build up. Optimal illumination conditions (wavelength below 550 nm, suitable integrated number of photons) were found under which a quasifree, nondispersive, charge transport was observed. The mobility is limited by interaction with a shallow trap, the population of which can be modulated by the additional monochromatic illumination. An unexpected high value was found as compared to results published earlier. This value is probably very close to the microscopic collision‐limited mobility. This explains the relatively small spread of the results obtained with different nominally undoped Bi12GeO20(BGO) samples. The values lie in the range 0.2–1.0 cm2 V−1 s−1. They are in excellent agreement with that measured elsewhere in BSO using the holographic time‐of‐flight technique.

ISSN:0021-8979    

DOI:10.1063/1.354752

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Two normalized parameters (i.e.,s=&lgr;/gandh=a/g, wheregis the grating period andais the width of metal dot) are introduced to characterize the two‐dimensional (2D) double periodic reflection metal grating coupler for a multiquantum well infrared photodetector (QWIP). The ‘‘method of moments’’ is used to analyze this square metal grating structure for coupling the normal incident infrared (IR) radiation into the quantum wells. The main advantage for such a grating structure is that the coupling of the normal incident IR radiation is independent of its polarization direction. Two universal graphs relating the higher‐order diffracted power and diffracted angle to the normalized wavelength have been constructed for the design of 2D metal grating coupled QWIPs. By using a simple scaling rule, the coupling quantum efficiency versus wavelength over a broad IR spectrum can be obtained for the grating coupled QWIPs.  

ISSN:0021-8979    

DOI:10.1063/1.354725

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A novel combination of a quasioptical antenna and a blazed grating which has been applied to gyrotron investigations at millimeter and submillimeter wavelengths is described. In this system the quasioptical antenna converts the conical beam from the gyrotron into a well‐collimated, linear‐polarized beam and the blazed grating acts as a frequency‐selective demultiplexer. The system is very effective in separating the submillimeter wavelength output from GYROTRON V at the second harmonic of the electron cyclotron frequency from the nearby strong fundamental. This has enabled the study of mode competition between the second harmonic and the fundamental processes.

ISSN:0021-8979    

DOI:10.1063/1.354726

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The results of a parametric investigation of the high‐pressure fission‐fragment excited atomic neon laser operating on the 703.2‐nm (3p[1/2]1→3s[3/2]20) and 724.5‐nm (3p[1/2]1→3s[3/2]10) transitions in He/Ne/Ar and He/Ne/Kr gas mixtures are reported. The highest measured power efficiency was 0.12% using a 1550‐Torr He/Ne/Kr (17/75/8) gas mixture at a pump power of 1 kW/cm3. The power efficiency and the ratio of the 703.2‐ to the 724.5‐nm laser output as a function of total pressure, He/Ne ratio, and percent of Ar or Kr will be discussed.

ISSN:0021-8979    

DOI:10.1063/1.354727

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Acoustic propagation through thick composites has become a subject of intensive study due to their application to nondestructive evaluation. The anisotropic multilayered media are now usually studied by the propagator matrix formalism. Though this formalism is very convenient, it leads to numerical instabilities for thick composites at high frequencies. These numerical instabilities come from the combination of very high exponential terms which reduces the dynamics of the calculation. A very interesting case is the one of anisotropic periodically multilayered media. The method developed in this paper uses Floquet waves which correspond to the modes of an infinite periodically multilayered medium. They are linear combinations of the real waves propagating in each layer of the medium. The Floquet wave numbers are the eigenvalues of the propagation matrix of one period of the medium. The anisotropic periodically multilayered medium can then be considered as a dummy medium in which the Floquet waves propagate. High exponential terms can be avoided through a judicious choice of reference of the Floquet waves’ amplitudes. This method enabled us to calculate reflection coefficients up until 40 MHz, of thick composites of carbone/epoxy placed in water. Furthermore, it has permitted us to not have a limitation for a single layer of any given material, at any given frequency.

ISSN:0021-8979    

DOI:10.1063/1.355324

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We calculate the efficiency of an endoreversible Carnot‐type cycle in the maximum power regime by using a nonlinear heat transfer law (the so‐called Dulong and Petit’s law of cooling). The results obtained from this model compare well (around 99% in some cases) with observed efficiencies for several power plants. The considered law of cooling includes conductive‐ convective and radiative contributions to the heat exchange between the working fluid and its surroundings. Our calculations improve considerably those obtained by means of a linear heat transfer law for the same power sources. We also analyze a nuclear power plant using an ecological optimization criterion for finite‐time heat engines.

ISSN:0021-8979    

DOI:10.1063/1.354728

出版商:AIP    

年代:1993

数据来源: AIP