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ABSTRACT: Experiments on thermal diodes in InSb constructed with a p-type blocking layer in an n*pn structure have shown the existence of an optimum in acceptor concentration and width. Results for various potential barrier shapes show that triangle-shaped barriers outperform barriers with rectangular shape. Enhancements have been demonstrated in PbTe thermal diodes. In InSb and in PbTe diodes, the optimum width is on the order of the scattering length, which is different in the two semiconductors by an order of magnitude.
Thermoelectrics, 2005. ICT 2005. 24th International Conference on; 07/2005
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ABSTRACT: We have developed a nonlocal generalization of the Onsager current relation to study the enhancement of the open-circuit voltage observed in InSb thermal diode experiments. Numerical solutions are obtained by brute force. Nonlocal effects produce an apparent enhancement of the thermopower in regions where the doping concentration changes. Models show voltage drops associated with npn barriers similar to experiment below 350 K, but are not as great as experimental results at high temperature near 600 K. This is thought to be due to thermal drops at the barrier not accounted for so far.
Thermoelectrics, 2005. ICT 2005. 24th International Conference on; 07/2005
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ABSTRACT: A characterization of the electrical and thermal properties of thermoelectric diode structures indicates that the figure of merit for thermal to electrical energy conversion is significantly enhanced in our devices over thermoelectric values. Enhancements are due to current injection and blockage of the ohmic return current within the devices. The resulting device takes advantage of both thermoelectric and thermionic effects, and can be considered to be a hybrid. Experiments indicate an enhancement as high as a factor of 8 in the effective figure of merit. The best results are consistent with a conversion efficiency on the order of 35% of the Carnot limit. Enhancements have been observed in InSb and in HgCdTe, and we believe that the approach applies generally to all thermoelectric semiconductors. © 2002 American Institute of Physics.
Applied Physics Letters 07/2002; 81(3):559-561. · 3.84 Impact Factor
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ABSTRACT: Photonic de Broglie wave interferometers display interesting,
qualitatively nonclassical effects. We present theory and simulations
demonstrating a photonic de Broglie interferometer based on nonlinear
directional couplers. The Kerr nonlinearity in the couplers allows an
N-photon pulse to tunnel between waveguides. Our calculations show the
characteristic λ/N fringes of this device even in the presence of
photon number uncertainty, albeit with reduced contrast.
Physical Review A 09/2001; 64(4):43813. · 2.88 Impact Factor
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ABSTRACT: Summary form only given. Interferometers involving special quantum states can show fascinating and counter-intuitive effects. While the general principles governing these effects have been understood for some time, the recent discussion of photon DeBroglie waves has provided an interesting and useful perspective. The basic idea is that whenever photons display correlated dynamics as they pass through a nonlinear interferometer, a description in terms of multi-photon bound particles can be useful. Multi-photon particles behave quite differently from uncorrelated photons: a bound collection of N photons has total momentum Nhk. Interference of such particles has fringe period λ/N. This runs against our intuition for linear optics, although some implementations are intuitive. For example, if a system includes up-conversion, then λ/2 photons are produced, and the presence of interference at this wavelength is quite natural. Naturally, loss will impose limitations on this kind of coherent manipulation, and must be carefully studied. The Kerr coupler was proposed as a potential implementation of the above interferometer. The Kerr effect can provide a "binding force" holding a packet of photons together as it passes through the coupler. A number of issues must be addressed in any realistic design-including loss and temporal pulse shape. Here we study the basic effect using a simplified two-mode model.
Quantum Electronics and Laser Science Conference, 2001. Technical Digest. Summaries of Papers Presented at the; 02/2001
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ABSTRACT: Experiments demonstrating thermal to electrical energy conversion using thermal diodes have shown an enhancement of the open circuit voltage over the thermoelectric open circuit voltage. Two different physical mechanisms are proposed to be responsible for the effects seen: (1) Thermionic injection from the emitter can occur when a temperature gradient is present, which induces an increased ohmic return current under zero-current conditions. (2) Blockage of the ohmic return current leads to a voltage increase for both thermoelectric and thermionic forward currents. Both effects increase the efficiency of energy conversion. Experiments show enhancements of the figure of merit in the range of 5-8 over the thermoelectric values. The best results are consistent with a single-side conversion efficiency in excess of 30% of the Carnot limit.
MRS Proceedings. 12/2000; 691.
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ABSTRACT: We have developed a semiempirical model to describe the line shape
in the vicinity of the exciton absorption lines in GaAs multiple quantum
well (MQW) structures. This model is based on a conventional
line-broadening analysis similar in spirit to that used in atomic and
plasma physics. The absorption spectrum is determined through the line
positions, oscillator strengths, homogeneous and inhomogeneous
broadening, and continuum shifts. Values for these parameters are
derived from a combination of theoretical models, some existing in the
literature and other developed by the authors, and experimental data
where theory is lacking. Our results are in good agreement with the
absorption measurements and the nonlinear coefficients available in the
literature. This model shows improvements in the nonlinearity at low
temperature and small inhomogeneous linewidth, which is as
expected
IEEE Journal of Quantum Electronics 12/1994; · 1.88 Impact Factor
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ABSTRACT: We identified for the first time the 3d
94d
1
S – 3d
94p
1
P line in Ni-like Nb at 204.2 that was predicted to show gain. When pumped with a train of pulses containing less than 1 J per pulse, significant emission was recorded at 204.2 following the second and the third pulses. We measured the small signal gain coefficient per Joule of incident laser energy to be 1.490.42 cm–1 J–1 for this laser transition, which is higher by several orders of magnitude than that reported for other collisional laser systems in this wavelength range.
Applied Physics B 01/1993; 57(5):303-307. · 2.19 Impact Factor
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ABSTRACT: The authors have examined optical constants and predicted
reflectivities of candidate surface coatings for whisper gallery mirrors
(WGMs) in the extreme ultraviolet. The spectral regime near 100-150
Å has been identified in the literature as particularly promising
due to the high WGM reflectivities of the noble metals in the vicinity
of their Cooper minima in this regime. The authors confirm this basic
result using newer EUV optical data, and they have sought candidate
surface materials which would extend the range over which WGMs may be
used to longer wavelengths. It is found that substantial WGM
reflectivities are predicted for a variety of elements in the EUV, and
that TE peak reflection is larger than TM peak reflection by on the
order of 10%; however, most of the elements which do reflect well have
surfaces which are vulnerable to oxygen contamination
IEEE Journal of Quantum Electronics 06/1992; · 1.88 Impact Factor
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ABSTRACT: For pt.I see ibid., vol.27, p.1069-77 (1991). Although the
whispering-gallery mirror shows great promise for use at short
wavelengths, undesired beam divergence at the mirror exit can be a
significant issue in the design of laser resonators. This problem can be
alleviated by appropriate choice of the mirror geometry: elongated
mirror shapes can give reduced beam divergence without requiring too
large an overall size. It is shown that, for circular geometries, beam
propagation along the mirror surface is conventionally described in
terms of normal modes. Elliptical mirrors are shown to admit a similar
analysis, thereby providing a convenient prototype for the elongated
geometries of interest. They are found to offer a substantial, though
ultimately limited, reduction of beam divergence
IEEE Journal of Quantum Electronics 02/1992; · 1.88 Impact Factor
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ABSTRACT: Whispering-gallery optics for use at soft X-ray and extreme
ultraviolet wavelengths are examined for their potential applications in
laser resonators. An examination is made of beam divergence at the exit
of a whispering gallery resonator. The properties of whispering-gallery
eigenmodes are reviewed, and the features of their Fourier transforms
discussed. The Fraunhofer approximation is then used to describe the
farfield pattern resulting from the diffraction of a whispering-gallery
eigenmode. Explicit formulas are given for the farfield beamwidth and
the Rayleigh distance. For modes of high order the results can be
understood in terms of geometrical optics
IEEE Journal of Quantum Electronics 05/1991; · 1.88 Impact Factor
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ABSTRACT: We are constructing a small-scale XUV laser to operate at near 191 {Angstrom} at a repetition rate of 6 ppm. For this project, we have chosen the Ni-like collision excitation scheme. To date, collisional excitation schemes have been used to generate gain in high Z materials at wavelengths as short as 43.18 {Angstrom}. High Z materials require very high electron temperatures to match the steady state'' production of sufficient number of Ni-like ions and the 3d-4d excitation energy. Based on an earlier idea, we are attempting to demonstrate gain in a low Z material, which is to be pumped with a series of short pulses rather than with one long pulse. In this case, it is predicted that favorable density and temperature conditions will be present after a few initial pulses to generate gain of the order of 5--10 cm{sup {minus}1}. In this paper, we summarize the status of our experiments.
12/1990
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ABSTRACT: Two related schemes for detecting X-rays based on (1) the
scattering and (2) the reflection of a pulsed optical probe beam by
carriers produced by the absorption of a single energetic X-ray photon
are described. The detection method is based on the strong carrier
density dependence of the (complex) index of refraction of the exciton
absorption lines in a GaAs/AlGaAs multiple-quantum-well structure. These
schemes can be used to construct a class of very sensitive gated X-ray
detectors with micron spatial resolution and temporal resolution in the
10-30-ps regime
IEEE Journal of Quantum Electronics 02/1990; · 1.88 Impact Factor
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ABSTRACT: The goal of the MIT Short-Wavelength Laser Project is to develop a small-scale, high repetition rate, EUV laser suitable for practical applications. The present report summarizes recent progress in this effort, both experimental and theoretical. Our experimental work has dealt with development of the pump laser, design of the target chamber and the associated target alignment system, investigation of short-wavelength laser cavities, and design of appropriate EUV diagnostics. Our theoretical work has included a study of the kinetics of transiently pumped laser plasmas, the development of efficient numerical algorithms applicable to laser-plasma rate equations, an investigation of frequency mixing in the EUV, and an analysis of x-ray detection using GaAs quantum-well structures.
12/1989
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ABSTRACT: The stochastic theory presented by Drummond, Gardiner, and Walls [Phys. Rev. A 24, 914 (1981)] is an interesting approach to problems in quantum optics. In this theory, an exact, quantum evolution is written in terms of classical functions (not operators) driven by explicit, quantum noise. We examine the origin of uncertainty in the formalism through the simple example of a single, nonlinear oscillator. We then test the stochastic theory applied to the problem of soliton propagation. We extend the linearized stochastic model by computing analytically quantum uncertainties in the four basic soliton parameters: photon number, momentum, phase, and position. Agreement with second-quantized and configuration-space soliton theories verifies the stochastic formalism.
Phys. Rev. A. 57(6).
