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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 2000年第4期18卷 2322-2327页

作者： Yifei Zhang Yulia Smorchkova C. Elsass Stacia Keller James Ibbetson Steven DenBaars Umesh Mishra Jasprit Singh Applied Physics Program University of Michigan Ann Arbor Michigan 48109 Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor Michigan 48109-2122 Department of Electrical and Computer Engineering University of California Santa Barbara California 93106 Department of Electrical Engineering and Computer Engineering University of Michigan Ann Arbor Michigan 48109

The AlGaN/GaN heterostructure is important for both electronic and optoelectronic devices. In addition to having a large band gap, the heterostructure has a strong piezoelectric effect and a large spontaneous polarization. This allows one to incorporate a large electric field (>106 V/cm) and high sheet charge (>1013 cm−2) without doping. Theoretical studies are done to examine how polarization effects controls the sheet charge density. The studies also focus on how interface roughness, aluminum mole fraction in the barrier, impurity scattering, sheet charge density, and phonon scattering influence mobility. Results are compared with experimental studies on samples grown by both molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). We find that interface roughness is a dominant source of scattering in the samples reported. Due to the variation in growth techniques we find that the MBE samples have a smoother interface quality compared to the MOCVD samples. By carefully fitting the experimental data we present results on interface roughness parameters for MBE and MOCVD samples.

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An effective hybrid fractal-wavelet image coder using quadtree partitioning and pruning

An effective hybrid fractal-wavelet image coder using quadtr...

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Canadian Conference on electrical and Computer engineering (CCECE)

作者： M. Ghazel E.R. Vrscay Department of Electrical & Computer Engineering Queen''s University Kingston ONT Canada Department of Applied Mathematics University of Waterloo Waterloo ONT Canada

We describe a simple hybrid fractal-wavelet coder that demonstrates extremely competitive compression characteristics in terms of rate-distortion curves with minimal computational cost. An energy-threshold criterion is used to perform the quadtree partitioning. The compression is enhanced in two ways: (1) pruning of negligible subtrees and (2) variable length encoding of domain pools that is implicitly built into the quadtree partitioning code. To the best of our knowledge, the rate-distortion characteristics of this coder are surpassed only by 2 highly adaptive partitioning schemes that employ intensive searching of domain pools, hence requiring much higher computational times.

关键词： Fractals Image coding Wavelet transforms Wavelet coefficients Image storage Power engineering and energy mathematics Computational efficiency Rate-distortion Data compression

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A room temperature unipolar quantum dot intersubband long wavelength (13.3 /spl mu/m) laser

A room temperature unipolar quantum dot intersubband long wa...

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IEEE Photonics Conference (IPC)

作者： S. Krishna P. Bhattacharya J. Singh J. Urayama T.B. Norris P.J. McCann K. Namjou Applied Physics Program Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor MI USA School of Electrical and Computer Engineering University of Oklahama Norman OK USA

Summary form only given. We report the room temperature operation of a novel unipolar self-organized quantum dot FIR laser (13.3 /spl mu/m) based on transitions between discrete bound electron states in self-organized... 详细信息

关键词： Quantum dot lasers US department of Transportation Electrons Infrared detectors Stationary state Finite impulse response filter Land surface temperature Infrared spectra Spectroscopy Laser radar

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Gaussian analysis of one-dimensional unsaturated flow in randomly heterogeneous soils

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Special Paper of the Geological Society of America 2000年 348卷 105-117页

作者： Amir, Orna Neuman, Shlomo P. Faculty of Engineering Tel Aviv University Tel Aviv Israel Program in Applied Mathematics Department of Hydrology and Water Resources University of Arizona Tucson AZ 85721 United States

We propose a new method for the solution of stochastic unsaturated flow problems in randomly heterogeneous soils that applies to both steady-state and transient regimes, avoids linearizing the soil constitutive relations, and places no theoretical limit on the variance of constitutive parameters. The proposed method applies to a broad class of soils with flow properties that scale according to the linearly separable model of L. Vogel and coworkers, provided that the dimensionless pressure head ψ has a near-Gaussian distribution. Upon treating ψ as a multivariate Gaussian function, we obtain a closed system of coupled nonlinear differential equations for the first and second moments of pressure head. We apply this Gaussian closure to one-dimensional steady-state unsaturated flow through a randomly stratified soil with hydraulic conductivity that varies exponentially with αψ where ψ=1/α ψ is dimensional pressure head and α is a random field with given statistical properties. Upon disregarding correlation between αand ψ, we obtain good agreement between Gaussian closure and Monte Carlo results for the mean of ψover a wide range of parameters;the agreement between variances is good far from the soil bottom where pressure head is prescribed, but not as good near the bottom boundary. We expect that the latter problem will be remedied once we account for correlation between α and ψ. Our solution provides considerable insight into the analytical behavior of the stochastic flow problem.

关键词： Soils

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An efficient wavelet-based algorithm for image superresolution

An efficient wavelet-based algorithm for image superresoluti...

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IEEE International Conference on Image Processing

作者： N. Nguyen P. Milanfar Scientific Computing and Computational Mathematics Program Stanford CA USA Department of Electrical Engineering University of California Santa Cruz Santa Cruz CA USA

Superresolution produces high quality, high resolution images from a set of degraded, low resolution frames. We present a new and efficient wavelet-based algorithm for image superresolution. The algorithm is a combination of interpolation and restoration processes. Unlike previous work, our method exploits the interlaced sampling structure in the low resolution data. Numerical experiments and analysis demonstrate the effectiveness of our approach and illustrate why the computational complexity only doubles for 2-D superresolution versus 1-D case.

关键词： Image resolution Interpolation Image sampling Degradation Spatial resolution Scientific computing mathematics Image restoration Pixel Additive noise

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Parrondo’s paradoxical games and the discrete Brownian ratchet

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AIP Conference Proceedings 2000年第1期511卷 189-200页

作者： Gregory P. Harmer Derek Abbott Peter G. Taylor Juan M. R. Parrondo 1Centre for Biomedical Engineering (CBME) and Department of Electrical and Electronic Engineering University of Adelaide Australia 5005 2Department of Applied Mathematics University of Adelaide Australia 5005 3Department Fisca Atomica Nuclear y Mulecular Universidad Complutense Madrid Spain

We introduce Parrondo’s paradox that involves games of chance. We consider two fair games. A and B, both of which can be made to lose by changing a biasing parameter. The apparently paradoxical situation arises when the two games are played in any alternating order. A winning expectation is produced, even though both games A and B are losing when we play them individually. We develop an explanation of the phenomenon in terms of a Brownian ratchet model, and also develop a mathematical analysis using discrete-time Markov chains. From the analysis we investigate the regions of parameter space in which Parrondo’s paradox can occur. We also consider some open questions that arise from this paradox and possible areas it could be applied.

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On the Perception of Transients: Applying Psychophysical Constraints to Improve Audio Analysis and Synthesis 26

On the Perception of Transients: Applying Psychophysical Con...

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26th International Computer Music Conference, ICMC 2000

作者： Wakefield, Gregory H. Heller, Laurie M. Carney, Laurel H. Mellody, Maureen Department of Electrical Engineering and Computer Science University of Michigan Ann ArborMI48109 United States Naval Submarine Medical Research Laboratory GrotonCT06359 United States Hearing Research Center Dept. of Biomedical Engineering Boston University BostonMA02215 United States Applied Physics Program University of Michigan United States

In a psychophysical experiment, a wideband, 4-ms noise is compared with spectrally smoothed versions of the noise. To isolate on the magnitude spectrum, the phase spectrum is controlled by assigning the same random phase spectrum to both the original and smoothed signals. Depending on the choice of phase spectrum, discrimination thresholds for spectral shape vary from a maximum spectral deviation of 2-3 dB to as much as 25 dB. The exact source of this effect remains an open question. However, it suggests that not all transients require the same signal fidelity. Given a proper characterization of the perceptual response, it may be possible to reduce the size of the codebook required to maintain acceptable perceptual results. © 2000 ICMC. All Rights Reserved.

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Biophysical and spectral characteristics of cool- and warm-season grasslands under three land management practices in Eastern Kansas

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Natural Resources Research 2000年第4期9卷 321-331页

作者： Guo, Xulin Price, Kevin P. Stiles, James M. Kansas Applied Remote Sensing Program (KARS) Department of Geography University of Kansas Lawrence KS 66045 2335 Irving Hill Road United States Radar Systems and Remote Sensing Laboratory (RSL) Department of Electrical Engineering and Computer Science University of Kansas Lawrence KS 66045 2335 Irving Hill Road United States Lawrence KS 66045 2335 Irving Hill Road United States

Grasslands are the largest of the Earth's four major vegetation types and are among the most agriculturally productive lands. Grassland management practices alter biophysical factors, such as plant species composition, soil structure and chemistry, and primary productivity. Large-scale changes in land management practices can have significant impacts on available natural resources, therefore impacting a regional socioeconomics. For these reasons, an improved understanding of the geographic distribution of changing land management practices and how these changes affect grassland biophysical characteristics of an area is critical to improve land use planning and practices. This study investigated the spectral characteristics, as measured using satellite remotely sensed data, of three land management practices [grazing, haying, and management under Conservation Reserve program (CRP)] on areas dominated by cool- and warm-season grasslands. The satellite imagery was acquired in May, July, and September in 1997 over Douglas County, Kansas. Biomass, vegetation cover, and species richness data were collected around the satellite overflight periods. Our results indicate that the biophysical characteristics and spectral reflectances are significantly different among the three management practices over cool- and warm-season grasslands. Interestingly, sites grazed by livestock had the highest species richness and the highest forb cover, although the total cover is almost the same as the other practices, and they have the highest near-infrared reflectance throughout the growing season. © 2000 International Association for Mathematical Geology.

关键词： Landsat Tallgrass prairie Thematic Mapper (TM)

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Information entropy and parrondo’s discrete-time ratchet

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AIP Conference Proceedings 2000年第1期502卷 544-549页

作者： Gregory P. Harmer Derek Abbott Peter G. Taylor Charles E. M. Pearce Juan M. R. Parrondo 1Centre for Biomedical Engineering (CBME) and Department of Electrical and Electronic Engineering University of Adelaide Adelaide SA 5005 Australia 2Department of Applied Mathematics University of Adelaide Adelaide SA 5005 Australia 3Department Fisica Atomica Nuclear y Molecular Universidad Computense Madrid Spain

Parrondo’s paradox involves two losing games of chance. The paradox is that a winning situation is produced when the two games are played in a random sequence in accordance with fixed probabilities. In this paper we ...

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Random focusing of sound into spatially coherent regions

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Waves in Random and Complex Media 2000年第2期10卷 199-216页

作者： J. R. Potter[a][b] B. J. Uscinski[b][c] T. Akal[d] [a] Acoustic Research Laboratory Tropical Marine Science Institute National University of Singapore Singapore [b] Department of Applied Mathematics and Theoretical Physics Cambridge University Cambridge UK [c] Applied Physics Laboratory and Department of Electrical Engineering University of Washington Seattle WA USA [d] Saclantcen Undersea Research Centre La Spezia Italy

Oceanographic variability creates a weak random propagation medium for acoustic waves. The impact on acoustic transmission is becoming increasingly appreciated as the deterministic modelling of sound propagation in the ocean has become tractable and better understood. Beyond the near field (where phase fluctuations are weak) and the far field (where the scintillation index becomes saturated) multiple-scattering theory predicts that random focusing will greatly enhance the acoustic energy density over small volumetric regions, which we call ‘ribbons’. In 1986 an experiment was carried out in the eastern Mediterranean to test this prediction using acoustic propagation along distinct, resolvable ray paths. This experiment is one of the few to map the spatial structure of acoustic intensity with such a large vertical aperture, and as far as the authors are aware it remains the only experiment to attempt to detect the two-dimensional structure of the predicted focused ribbons for individual energy paths. Renewed impetus to publish the results has been provided by the recent focus on moderate- to high-frequency acoustics in near-shore and shallow-water environments. The experiment is described and high-intensity regions consistent with the theoretical predictions are reported. A 3.5 kHz pulsed signal was transmitted over ranges of 11–23 km and sampled over a vertical aperture of 250–350 m and horizontal apertures of 4–4.5 km. The acoustic signals travelling along individual ray paths developed randomly focused regions of 6–18 dB over regions with a vertical dimension of about 20 m and whose horizontal length could possibly be up to 1 km. The understanding of these features allows system limitations to be estimated quantitatively and opens up the way to their constructive tactical use. The results are applicable to many systems from towed array sonars to high-frequency bathymetric sidescan and minehunting.

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