Kevin J. Webb

Bio: Kevin J. Webb is an academic researcher from Purdue University. The author has contributed to research in topics: Speckle pattern & Scattering. The author has an hindex of 31, co-authored 339 publications receiving 3910 citations. Previous affiliations of Kevin J. Webb include Massachusetts Institute of Technology & University of Maryland, College Park.

Fluorescence optical diffusion tomography

Abstract: A nonlinear, Bayesian optimization scheme is presented for reconstructing fluorescent yield and lifetime, the absorption coefficient, and the diffusion coefficient in turbid media, such as biological tissue. The method utilizes measurements at both the excitation and the emission wavelengths to reconstruct all unknown parameters. The effectiveness of the reconstruction algorithm is demonstrated by simulation and by application to experimental data from a tissue phantom containing the fluorescent agent Indocyanine Green.

Characterization and imaging in optically scattering media by use of laser speckle and a variable-coherence source

Abstract: We demonstrate the application of laser-speckle statistics formed by a variable-coherence source illuminating a scattering medium, for determining the scattering parameter μs′ of a diffusion model for the medium. Furthermore, we apply this technique to visualize laterally localized inhomogeneities embedded within a highly scattering sample.

A temperature-dependent nonlinear analytic model for AlGaN-GaN HEMTs on SiC

Abstract: A temperature-dependent large-signal model for continuous-wave (CW) and pulsed-mode operation is presented and applied to aluminum gallium nitride, gallium nitride (AlGaN-GaN) high electron-mobility transistors (HEMTs) on silicon-carbide (SiC) substrates. The model includes thermal, RF dispersion, and bias-dependent capacitance model elements, and is suitable for application with a harmonic-balance simulator. Temperature- and bias-dependent on-wafer pulsed I-V and S-parameter measurements from 27/spl deg/C to 200/spl deg/C are used to examine trapping and thermal effects, and to determine temperature- and bias-dependent parameterized model coefficients for the nonlinear model. Large-signal measurement and model results are presented for 2 /spl times/ 0.35 /spl mu/m /spl times/ 125 /spl mu/m and 12 /spl times/ 0.35 /spl mu/m /spl times/ 125 /spl mu/m GaN HEMTs fabricated on SiC. The nonlinear model shows good agreement with measured CW power sweep data at an elevated temperature of 150/spl deg/C under more than 5-W power dissipation, and with measured pulsed load-pull data.

Diffusive media characterization with laser speckle

Abstract: The statistical properties of laser speckle with partially coherent light are related to the scattering characteristics of an optically diffuse material. A diffusion equation model is shown to yield a speckle contrast ratio that agrees well with measurements of opaque plastics of varying thicknesses. We show that partially coherent light can be used to determine material parameters for highly scattering media. Measured data for stratified materials with differing scattering properties indicate that this technique may be useful in detecting inhomogeneities.

Imaging in scattering media by use of laser speckle

Abstract: We present a method for the detection and localization of inhomogeneities embedded within highly scattering media that employs speckle statistics with a partially coherent light source. Variations in speckle contrast as a function of position are used to interrogate inhomogeneities deeply embedded within scattering media. A numerical model based on photon diffusion theory is introduced to predict speckle contrast as a function of scan position. This model uses measured speckle data to determine scattering and absorption parameters.

Plasmonics: Fundamentals and Applications

Abstract: Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

The Observation of Gravitational Waves from a Binary Black Hole Merger

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160) Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Light in tiny holes

Abstract: The presence of tiny holes in an opaque metal film, with sizes smaller than the wavelength of incident light, leads to a wide variety of unexpected optical properties such as strongly enhanced transmission of light through the holes and wavelength filtering. These intriguing effects are now known to be due to the interaction of the light with electronic resonances in the surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes. This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics.

Looking and listening to light: the evolution of whole-body photonic imaging.

Abstract: Optical imaging of live animals has grown into an important tool in biomedical research as advances in photonic technology and reporter strategies have led to widespread exploration of biological processes in vivo. Although much attention has been paid to microscopy, macroscopic imaging has allowed small-animal imaging with larger fields of view (from several millimeters to several centimeters depending on implementation). Photographic methods have been the mainstay for fluorescence and bioluminescence macroscopy in whole animals, but emphasis is shifting to photonic methods that use tomographic principles to noninvasively image optical contrast at depths of several millimeters to centimeters with high sensitivity and sub-millimeter to millimeter resolution. Recent theoretical and instrumentation advances allow the use of large data sets and multiple projections and offer practical systems for quantitative, three-dimensional whole-body images. For photonic imaging to fully realize its potential, however, further progress will be needed in refining optical inversion methods and data acquisition techniques.