摘要:

Entropy generation minimization (finite time thermodynamics, or thermodynamic optimization) is the method that combines into simple models the most basic concepts of heat transfer, fluid mechanics, and thermodynamics. These simple models are used in the optimization of real (irreversible) devices and processes, subject to finite‐size and finite‐time constraints. The review traces the development and adoption of the method in several sectors of mainstream thermal engineering and science: cryogenics, heat transfer, education, storage systems, solar power plants, nuclear and fossil power plants, and refrigerators. Emphasis is placed on the fundamental and technological importance of the optimization method and its results, the pedagogical merits of the method, and the chronological development of the field. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362674

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Results of the investigation into the suitability of the shear‐force distance regulation mechanism for scanning near‐field optical microscopy (SNOM) are presented. It is shown that there is a range of relative sizes of surface features to tip size that gives rise to malfunctions of the shear‐force distance regulation mechanism. If the size of the tip is comparable to the size of any depressions on the sample, the corresponding shear‐force image may show contrast reversal. It is also shown that the resolution obtained when imaging a surface with the shear‐force scanning microscope may differ for the two perpendicular lateral directions. The resolution along the oscillation axis is usually lower than the one perpendicular to it. This has implications for SNOM images. Therefore, the interpretation of shear force, and hence SNOM images, may become complex when imaging any sample with significant surface roughness. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361014

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A formula for predicting the stress effect on the electromagnetic resonances of dielectric resonators is obtained by applying a perturbation method to the three‐dimensional Maxwell’s equations in which the dielectric permittivity tensor is perturbed by the applied stress (or strain) field through the piezo‐optic effect. The dielectric resonator, which is surrounded by infinite free space, can be isotropic or anisotropic and of arbitrary shape. By using previously obtained two‐dimensional closed‐form solutions as the approximate unperturbed solutions, stress effect on the electromagnetic resonances in a dielectric circular disk is studied. Frequency changes of both the transverse electric and transverse magnetic modes are computed for disks of gallium arsenide and under three cases of loading: (1) a pair of diametral forces, (2) steady vertical acceleration, and (3) steady horizontal acceleration. In the latter two cases, the bottom face of the disk is supported by a rigid base. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361015

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Strain‐compensated InAsP/InGaAsP multi‐quantum‐wells (MQWs) grown by metalorganic molecular beam epitaxy (MOMBE) are characterized by conventional photoluminescence (PL), micro‐PL, transmission electron microscopy, and x‐ray diffraction measurements and applied to fabricate 1.3 &mgr;m wavelength laser diodes. These methods reveal that there is no deterioration in the optical properties or structure of strain‐compensated MQWs having up to 25 wells, which means that the critical thickness of InAsP grown by MOMBE exceeds 1000 A˚. The critical conditions of strain and thickness over which misfit dislocations are generated are determined for the MQWs. The MQW lasers with ten wells (Lz=55 A˚) have no misfit dislocations and have uniform threshold current densities of 0.9±0.1 kA/cm2. The maximum operating temperatureTmaxof the lasers increases with increasing well number, the highestTmaxis 155 °C, which is obtained for MQW lasers with 15 wells. The lasers have no problems in terms of long‐term reliability. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361016

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The gain switching of an erbium‐doped waveguide laser is investigated both theoretically and experimentally. We analyze the lasing characteristics of gain‐switched laser based on coupled rate equations, taking ground state absorption and the saturation effect into account. The analyzed theory indicates that the repetition rate, pulse width, and peak power vary when the pump conditions are changed. Gain‐switched optical pulses are experimentally generated in the erbium‐doped waveguide laser with intensity modulated optical pumping at 651 nm using a dye laser and an acousto‐optic modulator. The pulses are controlled with widths of 660–2600 ns and peak powers of 1.0–6.5 mW at repetition rates of 18–60 kHz by changing the average pump rate between 2.0 and 7.0. These experimental results agree with theoretical calculations of the pulse width, peak power, and repetition rate against the pump rate. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361017

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Third‐order nonlinear optical properties of Au/SiO2composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility &khgr;(3)exhibits a peak at the wavelength of the absorption peak and the maximum value of &khgr;(3)is obtained to be 2.0×10−7esu. The size‐dependent enhancement of the &khgr;(3)in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of &khgr;(3)/&agr; (&agr;: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constant of the metal particles. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361018

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The initial expansion of a copper plasma generated at reduced pressure (20 mTorr air) by XeCl excimer laser irradiation of 308 nm has been studied by fast intensified charge‐coupled device photography of the overall visible plume emission with a time resolution of 3 ns. The spatial distribution and its temporal development along the target normal as well as in a plane parallel to the sample surface were measured in a single laser shot (28 ns). We report the firstinsitumeasurement and observation of a one‐dimensional plasma expansion with very small divergence during the initial stage (&dgr;t<150 ns) at reduced pressure for low laser fluence (5.4 J cm−2). The related angular distribution of ablated particles has a cosn &THgr; form wheren≳36 for &dgr;t≤150 ns and &THgr;<20°. The observed effect corresponds to a highly collisional plasma, where strongest forward peaking beyond the Knudsen‐layer edge takes place. For times greater than 150 ns, the plasma switches into a three‐dimensional expansion corresponding to an effusion model with recondensation. A nearly constant center‐of‐mass velocity in the range of 6×103m s−1has been observed. A particle velocity at the surface ofuk=2×103m s−1, in good agreement with the theory of unsteady adiabatic expansion, has been confirmed through a measured excitation temperature ofT≊104K revealing an image of the kinetic energy. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361019

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

When pumped with a 1.48 &mgr;m laser diode, Er‐implanted Al2O3ridge waveguides emit a broad spectrum consisting of several distinct peaks having wavelengths ranging from the midinfrared (1.53 &mgr;m) to the visible (520 nm). In order to explain these observations, three different upconversion mechanisms are considered: cooperative upconversion, excited state absorption, and pair‐induced quenching. It is found that for samples with a high Er concentration (1.4 at. %), cooperative upconversion completely dominates the deexcitation of the Er3+ions. For a much lower concentration (0.12 at. %), the influence of cooperative upconversion is strongly reduced, and another upconversion effect becomes apparent: excited state absorption. These conclusions are based on measurements of the luminescence emission versus pump intensity, and also on measured luminescence decay curves. The upconversion coefficient is found to be (4±1)×10−18cm3/s; the excited state absorption cross section is (0.9±0.3)×10−21cm2. It is shown that in spite of these upconversion effects, a high fraction of the Er3+can be excited at low pump powers. For pump powers between 2 and 10 mW, the optimum Er concentration is calculated. The results show that for an Er concentration of 0.5 at. %, more than 2 dB/cm net optical gain is achievable at a pump power less than 10 mW. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361020

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Quadratic electro‐absorption (QEA) and quadratic electro‐optic measurements were performed with a guest/host system consisting of 4(4’‐N,N‐Dimethylaminophenyl)‐1,1‐bis(trifluoromethyl sulfonyl)‐1,3‐butadiene doped in poly(ethyl)methacrylate host polymer. The imaginary part of the QEA response was measured over a wavelength range of 450–600 nm. The real and imaginary parts of the electric field induced optical Kerr coefficient were measured at wavelengths near and far from resonance. Under different polarizations of the optical field in the QEA measurement, the two contributing coefficients of the third order susceptibility (&khgr;(3)) were deduced. A distinctive temperature variation of the third order response was observed. Temperature dependence measurements from ambient to well above the glass transition of the guest‐host system were carried out. The same chromophore in a different polymer host, poly(vinyl)chloride, was also investigated. Correct expressions for the real and imaginary parts of the Kerr coefficient are provided. The present results confirm the importance of the electronic contribution to &khgr;(3)at room temperature where molecular reorientation is insignificant. At high temperatures (above the glass transition temperature of the polymer) the ratio of &khgr;(3)3333/&khgr;(3)1133deviates from −2, which is predicted by the elastic model assuming molecular reorientation to be the dominant mechanism. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361021

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Under certain conditions of flow rate, height, and feedback, periodic or chaotic fluttering oscillations can be observed as the sheet of water from a dam or waterfall fountain descends through the air. Numerical and analytical interpretations of this phenomenon have recently been reported. The extension of these results to other fountain geometries is discussed here together with experimental observations on an annular waterfall fountain. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361022

出版商:AIP    

年代:1996

数据来源: AIP