Open Access. Powered by Scholars. Published by Universities.®

Physics Commons

Open Access. Powered by Scholars. Published by Universities.®

Articles 1 - 9 of 9

Full-Text Articles in Physics

Portable, High-Bandwidth Frequency-Domain Photon Migration Instrument For Tissue Spectroscopy, Steen J. Madsen, Eric R. Anderson, Richard C. Haskell, Bruce J. Tromberg Dec 1994

Portable, High-Bandwidth Frequency-Domain Photon Migration Instrument For Tissue Spectroscopy, Steen J. Madsen, Eric R. Anderson, Richard C. Haskell, Bruce J. Tromberg

All HMC Faculty Publications and Research

We describe a novel frequency-domain photon migration instrument employing direct diode laser modulation and avalanche photodiode detection, which is capable of noninvasively determinating the optical properties of biological tissues in near real time. An infinite medium diffusion model was used to extract absorption and transport scattering coefficients from 300-kHz to 800-MHz photon-density wave phase data. Optical properties measured in tissue-simulating solutions at 670 nm agreed to within 10% of those expected.


Low-Frequency Line Shapes In Guided Acoustic-Wave Brillouin Scattering, Benjamin I. Greene, Peter N. Saeta Oct 1994

Low-Frequency Line Shapes In Guided Acoustic-Wave Brillouin Scattering, Benjamin I. Greene, Peter N. Saeta

All HMC Faculty Publications and Research

Guided acoustic‐wave Brillouin scattering (GAWBS) measurements were performed on 20‐cm lengths of optical fibers with particular attention focused on the lowest lying resonance. In 125‐μm‐diam silica fibers, this resonance was observed to occur at ~22 MHz and have a line shape which varied erratically from sample to sample. Significant line shape fluctuations were evident even between sequential samples from the same fiber spool. We speculate that the observed effects are attributable to 0.01-0.1 μm distributed geometric deviations from a perfect cylinder.


Boundary Conditions For The Diffusion Equation In Radiative Transfer, Richard C. Haskell, Lars O. Svaasand, Tsong-Tseh Tsay, Ti-Chen Feng, Matthew S. Mcadams, Bruce J. Tromberg Oct 1994

Boundary Conditions For The Diffusion Equation In Radiative Transfer, Richard C. Haskell, Lars O. Svaasand, Tsong-Tseh Tsay, Ti-Chen Feng, Matthew S. Mcadams, Bruce J. Tromberg

All HMC Faculty Publications and Research

Using the method of images, we examine the three boundary conditions commonly applied to the surface of a semi-infinite turbid medium. We find that the image-charge configurations of the partial-current and extrapolated-boundary conditions have the same dipole and quadrupole moments and that the two corresponding solutions to the diffusion equation are approximately equal. In the application of diffusion theory to frequency-domain photon-migration (FDPM) data, these two approaches yield values for the scattering and absorption coefficients that are equal to within 3%. Moreover, the two boundary conditions can be combined to yield a remarkably simple, accurate, and computationally fast method for ...


Subpicosecond Thomson Scattering Measurements Of Optically Ionized Helium Plasmas, T. E. Glover, Thomas D. Donnelly, E. A. Lipman, A. Sullivan, R. W. Falcone Jul 1994

Subpicosecond Thomson Scattering Measurements Of Optically Ionized Helium Plasmas, T. E. Glover, Thomas D. Donnelly, E. A. Lipman, A. Sullivan, R. W. Falcone

All HMC Faculty Publications and Research

We present the first subpicosecond time-resolved temperature measurements of plasmas produced by high-intensity optical ionization. Thomson scattering is used to measure electron and ion temperatures of helium plasmas created by 125 fs, 800 nm laser pulses focused to an intensity of 2 × 1017 W/cm2. We find that the electron temperature is accurately predicted by a tunneling ionization model. The measured ion temperature is consistent with direct heating by the laser pulse.


Tabletop X-Ray Lasers, D. C. Eder, P. Amendt, L. B. Dasilva, R. A. London, B. J. Macgowan, D. L. Matthews, B. M. Penetrante, M. D. Rosen, S. C. Silks, Thomas D. Donnelly, R. W. Falcone, G. L. Strobel May 1994

Tabletop X-Ray Lasers, D. C. Eder, P. Amendt, L. B. Dasilva, R. A. London, B. J. Macgowan, D. L. Matthews, B. M. Penetrante, M. D. Rosen, S. C. Silks, Thomas D. Donnelly, R. W. Falcone, G. L. Strobel

All HMC Faculty Publications and Research

Details of schemes for two tabletop size x‐ray lasers that require a high‐intensity short‐pulse driving laser are discussed. The first is based on rapid recombination following optical‐field ionization. Analytical and numerical calculations of the output properties are presented. Propagation in the confocal geometry is discussed and a solution for x‐ray lasing in Li‐like N at 247 Å is described. Since the calculated gain coefficient depends strongly on the electron temperature, the methods of calculating electron heating following field ionization are discussed. Recent experiments aimed at demonstrating lasing in H‐like Li at 135 Å ...


X-Rays From Microstructured Targets Heated By Femtosecond Lasers, S. P. Gordon, Thomas D. Donnelly, A. Sullivan, H. Hamster, R. W. Falcone Apr 1994

X-Rays From Microstructured Targets Heated By Femtosecond Lasers, S. P. Gordon, Thomas D. Donnelly, A. Sullivan, H. Hamster, R. W. Falcone

All HMC Faculty Publications and Research

We have demonstrated efficient conversion of ultrashort-pulse laser energy to x rays with energies above 1 keV, using laser-produced plasmas generated on a variety of microstructured surfaces. Lithographically produced grating targets generated 0.1 mJ of kilo-electron-volt x rays, and porous gold and aluminum targets emitted 1 mJ. This represents an improvement of a factor of 100 over flat targets. The K-shell emission spectrum of porous aluminum was composed primarily of heliumlike spectral lines.


Advection Of A Passive Scalar By A Vortex Couple In The Small-Diffusion Limit, Joseph F. Lingevitch, Andrew J. Bernoff Jan 1994

Advection Of A Passive Scalar By A Vortex Couple In The Small-Diffusion Limit, Joseph F. Lingevitch, Andrew J. Bernoff

All HMC Faculty Publications and Research

We study the advection of a passive scalar by a vortex couple in the small-diffusion (i.e. large Péclet number, Pe) limit. The presence of weak diffusion enhances mixing within the couple and allows the gradual escape of the scalar from the couple into the surrounding flow. An averaging technique is applied to obtain an averaged diffusion equation for the concentration inside the dipole which agrees with earlier results of Rhines & Young for large times. At the outer edge of the dipole, a diffusive boundary layer of width O(Pe−½) forms; asymptotic matching to the interior of the dipole yields effective boundary conditions for the averaged diffusion equation. The analysis predicts that first the scalar is homogenized along the streamlines ...


Optical Properties Of Human Uterus At 630 Nm, Steen J. Madsen, Bruce J. Tromberg, Yona Tadir, Pius Wyss, Lars O. Svaasand, Richard C. Haskell Jan 1994

Optical Properties Of Human Uterus At 630 Nm, Steen J. Madsen, Bruce J. Tromberg, Yona Tadir, Pius Wyss, Lars O. Svaasand, Richard C. Haskell

All HMC Faculty Publications and Research

The optical properties of normal and fibriotic human uteri were determined using frequency-domain and steady-state techniques .


Frequency-Domain Photon Migration In Turbid Media, Bruce J. Tromberg, Steen J. Madsen, Curtis Chapman, Lars O. Svaasand, Richard C. Haskell Jan 1994

Frequency-Domain Photon Migration In Turbid Media, Bruce J. Tromberg, Steen J. Madsen, Curtis Chapman, Lars O. Svaasand, Richard C. Haskell

All HMC Faculty Publications and Research

An analytical model is presented for the propagation of diffuse photon density waves in turbid media. The frequency- and wavelength-dependence of photon density waves are measured using Frequency-domain Photon Migration (FDPM). Media optical properties, including absorption, transport, and fluorescence relaxation times are calculated from experimental results.