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Full-Text Articles in Physics

How Much Can Guided Modes Enhance Absorption In Thin Solar Cells?, Peter N. Saeta, Vivian E. Ferry, Domenico Pacifici, Jeremy N. Munday, Harry A. Atwater Nov 2009

How Much Can Guided Modes Enhance Absorption In Thin Solar Cells?, Peter N. Saeta, Vivian E. Ferry, Domenico Pacifici, Jeremy N. Munday, Harry A. Atwater

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Absorption enhancement in thin metal-backed solar cells caused by dipole scatterers embedded in the absorbing layer is studied using a semi-analytical approach. The method accounts for changes in the radiation rate produced by layers above and below the dipole, and treats incoherently the subsequent scattering of light in guided modes from other dipoles. We find large absorption enhancements for strongly coupled dipoles, exceeding the ergodic limit in some configurations involving lossless dipoles. An antireflection-coated 100-nm layer of a-Si:H on Ag absorbs up to 87% of incident above-gap light. Thin layers of both strong and weak absorbers show similar strongly ...


Designs And Optical Tests Of Thermal Links For An Optical Refrigerator, John Parker, David Mar, Steven Von Der Porten, John Hankinson, Kevin Byram, Chris Lee, Kai Mayeda, Richard C. Haskell, Qimin Yang, Scott R. Greenfield, Richard I. Epstein Feb 2008

Designs And Optical Tests Of Thermal Links For An Optical Refrigerator, John Parker, David Mar, Steven Von Der Porten, John Hankinson, Kevin Byram, Chris Lee, Kai Mayeda, Richard C. Haskell, Qimin Yang, Scott R. Greenfield, Richard I. Epstein

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Dielectric mirror leakage at large angles of incidence limits the effectiveness of solid state optical refrigerators due to reheating caused by photon absorption in an attached load. In this paper, we present several thermally conductive link solutions to greatly reduce the net photon absorption. The Los Alamos Solid State Optical Refrigerator (LASSOR) has demonstrated cooling of a Yb/sup 3+/ doped ZBLANP glass to 208 K. We have designed optically isolating thermal link geometries capable of extending cooling to a typical heat load with minimal absorptive reheating, and we have tested the optical performance of these designs. A surrogate source ...


Role Of Beat Noise In Limiting The Sensitivity Of Optical Coherence Tomography, Richard C. Haskell, David Liao, Adam E. Pivonka, Tera L. Bell, Brendan R. Haberle, Barbara M. Hoeling, Daniel C. Petersen Jun 2006

Role Of Beat Noise In Limiting The Sensitivity Of Optical Coherence Tomography, Richard C. Haskell, David Liao, Adam E. Pivonka, Tera L. Bell, Brendan R. Haberle, Barbara M. Hoeling, Daniel C. Petersen

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The sensitivity and dynamic range of optical coherence tomography (OCT) are calculated for instruments utilizing two common interferometer configurations and detection schemes. Previous researchers recognized that the performance of dual-balanced OCT instruments is severely limited by beat noise, which is generated by incoherent light backscattered from the sample. However, beat noise has been ignored in previous calculations of Michelson OCT performance. Our measurements of instrument noise confirm the presence of beat noise even in a simple Michelson interferometer configuration with a single photodetector. Including this noise, we calculate the dynamic range as a function of OCT light source power, and ...


Harmonic Generation In Thin Films And Multilayers, William S. Kolthammer '04, Dustin Barnard '03, Nicole Carson, Aaron D. Edens '00, Nathan A. Miller '01, Peter N. Saeta Jul 2005

Harmonic Generation In Thin Films And Multilayers, William S. Kolthammer '04, Dustin Barnard '03, Nicole Carson, Aaron D. Edens '00, Nathan A. Miller '01, Peter N. Saeta

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A general method for computing harmonic generation in reflection and transmission from planar nonmagnetic multilayer structures is described. The method assumes plane waves and treats harmonic generation in the parametric approximation. The method is applied in studying the second- and third-harmonic generation properties of thin crystal silicon layers surrounded by thermal oxide. Most independent components of the nonlinear susceptibility tensor have unique signatures with silicon layer thickness d, allowing their strength to be determined in principle by measuring harmonic generation as a function of d. Surface and bulk contributions to third-harmonic generation are cleanly distinguished, with the bulk signal dominating ...


X-Ray Generation From Metal Targets Coated With Wavelength-Scale Spheres, D. R. Symes, H. A. Sumeruk, I. V. Churina, Thomas D. Donnelly, J. Landry, T. Ditmire May 2005

X-Ray Generation From Metal Targets Coated With Wavelength-Scale Spheres, D. R. Symes, H. A. Sumeruk, I. V. Churina, Thomas D. Donnelly, J. Landry, T. Ditmire

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X-ray yield measurements from targets coated with wavelength-scale spheres are compared with measurements from polished targets. Evidence for a hotter resonant electron temperature due to field enhancements from Mie resonances in the spheres is investigated.


Femtosecond Spectrotemporal Magneto-Optics, J.-Y. Bigot, L. Guidoni, E. Beaurepaire, Peter N. Saeta Aug 2004

Femtosecond Spectrotemporal Magneto-Optics, J.-Y. Bigot, L. Guidoni, E. Beaurepaire, Peter N. Saeta

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A new method to measure and analyze the time and spectrally resolved polarimetric response of magnetic materials is presented. It allows us to study the ultrafast magnetization dynamics of a CoPt3 ferromagnetic film. The analysis of the pump-induced rotation and ellipticity detected by a broad spectrum probe beam shows that magneto-optical signals predominantly reflect the spin dynamics in ferromagnets.


Breakdown Of The Slowly Varying Amplitude Approximation: Generation Of Backward Traveling Second Harmonic Light, J. Z. Sanborn '01, C. Hellings '02, Thomas D. Donnelly Jan 2003

Breakdown Of The Slowly Varying Amplitude Approximation: Generation Of Backward Traveling Second Harmonic Light, J. Z. Sanborn '01, C. Hellings '02, Thomas D. Donnelly

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By numerically solving the nonlinear field equations, we simulate second-harmonic generation by laser pulses within a nonlinear medium without making the usual slowly-varying-amplitude approximation, an approximation which may fail when laser pulses of moderate intensity or ultrashort duration are used to drive a nonlinear process. Under these conditions we show that a backward-traveling, second-harmonic wave is created, and that the magnitude of this wave is indicative of the breakdown of the slowly-varying-amplitude approximation. Conditions necessary for experimental detection of this wave are discussed.


Distinguishing Surface And Bulk Contributions To Third-Harmonic Generation In Silicon, Peter N. Saeta, Nathan A. Miller '01 Oct 2001

Distinguishing Surface And Bulk Contributions To Third-Harmonic Generation In Silicon, Peter N. Saeta, Nathan A. Miller '01

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We report measurements of third-harmonic generation from ultrathin crystalline silicon layers of gradually varying thickness. Both the angular and thickness dependence of the third-harmonic light generated in transmission at normal incidence are consistent with negligible surface contribution to third-harmonic generation in silicon, even under tight focusing. This work illustrates a method for distinguishing surface and bulk contributions to harmonic generation.


An Optical Coherence Microscope For 3-Dimensional Imaging In Developmental Biology, Barbara M. Hoeling, Andrew D. Fernandez, Richard C. Haskell, Eric Huang, Whittier R. Myers, Daniel C. Petersen, Sharon E. Ungersma, Ruye Wang, Mary E. Williams, Scott E. Fraser Mar 2000

An Optical Coherence Microscope For 3-Dimensional Imaging In Developmental Biology, Barbara M. Hoeling, Andrew D. Fernandez, Richard C. Haskell, Eric Huang, Whittier R. Myers, Daniel C. Petersen, Sharon E. Ungersma, Ruye Wang, Mary E. Williams, Scott E. Fraser

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An optical coherence microscope (OCM) has been designed and constructed to acquire 3-dimensional images of highly scattering biological tissue. Volume-rendering software is used to enhance 3-D visualization of the data sets. Lateral resolution of the OCM is 5 mm (FWHM), and the depth resolution is 10 mm (FWHM) in tissue. The design trade-offs for a 3-D OCM are discussed, and the fundamental photon noise limitation is measured and compared with theory. A rotating 3-D image of a frog embryo is presented to illustrate the capabilities of the instrument.


Probing Nonequilibrium Electron Distributions In Gold By Use Of Second Harmonic Generation, K. L. Moore '99, Thomas D. Donnelly Jul 1999

Probing Nonequilibrium Electron Distributions In Gold By Use Of Second Harmonic Generation, K. L. Moore '99, Thomas D. Donnelly

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Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 109 to 1011W/cm2 . At laser intensities below 1010W/cm2 , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron–electron or electron–phonon collisions can ...


Ultrafast Phenomena: A Laboratory Experiment For Undergraduates, Thomas D. Donnelly, Carl Grossman Aug 1998

Ultrafast Phenomena: A Laboratory Experiment For Undergraduates, Thomas D. Donnelly, Carl Grossman

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We present a set of experiments that introduce students to ultrafast science. We discuss the relationship between the description of an ultrashort laser pulse in the frequency domain and the time domain. Using experimental results we demonstrate that this relationship is constrained by the lower limit of the Heisenberg uncertainty principle. Students carrying out the experiments will become familiar with ultrafast techniques, such as autocorrelation and laser cavity design, as well as various other concepts such as dispersion, Fourier transformation, interference, and nonlinear optics.


Experimental And Theoretical Investigation Of Recombination Pumped X-Ray Lasers Driven By High-Intensity, Short Pulse Lasers, Thomas D. Donnelly, L. Da Silva, R. W. Lee, S. Mrowka, M. Hofer, R. W. Falcone Jan 1996

Experimental And Theoretical Investigation Of Recombination Pumped X-Ray Lasers Driven By High-Intensity, Short Pulse Lasers, Thomas D. Donnelly, L. Da Silva, R. W. Lee, S. Mrowka, M. Hofer, R. W. Falcone

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We have experimentally investigated a recombination-pumped soft-x-ray laser on a Lyman-α transition (135 Å) of hydrogenlike lithium. Furthermore, we have modeled the dynamics of this system, including the effects of the multipeaked electron distribution function that is obtained from the sequential, optical-field ionization of an atom. We compare the predictions of our model and our experimental results.


Optical Measurements Of The Core Radius Of High-Δ Fibers With 1-Nm Resolution, Peter N. Saeta Jan 1995

Optical Measurements Of The Core Radius Of High-Δ Fibers With 1-Nm Resolution, Peter N. Saeta

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An optical technique for measuring the core radius of high-Δ optical fibers is described. Variations in the core radius of step-index fibers can be measured down to a scale of 1 nm.


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

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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

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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

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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 ...


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

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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.


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

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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

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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.


Short Terahertz Pulses From Semiconductor Surfaces: The Importance Of Bulk Difference‐Frequency Mixing, Peter N. Saeta, Benjamin I. Greene, Shun Lien Chuang Dec 1993

Short Terahertz Pulses From Semiconductor Surfaces: The Importance Of Bulk Difference‐Frequency Mixing, Peter N. Saeta, Benjamin I. Greene, Shun Lien Chuang

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The crystallographic orientation dependence of the far‐infrared (FIR) light generated at the (001) surface of a zincblende semiconductor is shown to derive principally from bulk difference‐frequency mixing. A strong modulation is observed for 1‐GW/cm2 pulses on InP, which demonstrates that the radiated FIR wave produced by bulk optical rectification is comparable to that generated by the transport of photoinjected carriers. Using the bulk rectification light as a clock, we show that more than 95% of the light produced from an InP (111) crystal by 100‐fs, 100‐μJ pulses is generated in a time shorter ...


Properties Of Photon Density Waves In Multiple-Scattering Media, Bruce J. Tromberg, Lars O. Svaasand, Tsong-Tseh Tsay, Richard C. Haskell Feb 1993

Properties Of Photon Density Waves In Multiple-Scattering Media, Bruce J. Tromberg, Lars O. Svaasand, Tsong-Tseh Tsay, Richard C. Haskell

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Amplitude-modulated light launched into multiple-scattering media, e.g., tissue, results in the propagation of density waves of diffuse photons. Photon density wave characteristics in turn depend on modulation frequency (ω) and media optical properties. The damped spherical wave solutions to the homogeneous form of the diffusion equation suggest two distinct regimes of behavior: (1) a highfrequency dispersion regime where density wave phase velocity Vp has a ω dependence and (2) a low-frequency domain where Vp is frequency independent. Optical properties are determined for various tissue phantoms by fitting the recorded phase (Φ) and modulation (m) response to simple ...


Optical Rectification At Semiconductor Surfaces, Shun Lien Chuang, Stefan Schmitt-Rink, Benjamin I. Greene, Peter N. Saeta, Anthony F. J. Levi Jan 1992

Optical Rectification At Semiconductor Surfaces, Shun Lien Chuang, Stefan Schmitt-Rink, Benjamin I. Greene, Peter N. Saeta, Anthony F. J. Levi

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We show that far-infrared radiation can be generated in the depletion field near semiconductor surfaces via the inverse Franz-Keldysh effect or electric-field-induced optical rectification. This mechanism is conceptually different from those previously proposed and accounts for many recent experimental observations.


Problems Of Channel Correlation And Statistical Bias In Photon-Correlation Spectroscopy, Richard C. Haskell, Gary L. Pisciotta Apr 1985

Problems Of Channel Correlation And Statistical Bias In Photon-Correlation Spectroscopy, Richard C. Haskell, Gary L. Pisciotta

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Correlation between channels of the normalized photocount-rate correlation function g(2)(τ) becomes significant at high count rates and leads to a number of data-analysis problems. We derive an expression for channel correlation that is valid for a detector area of arbitrary extent and compare the theoretical predictions with measured values. A data-analysis procedure is demonstrated that employs the theoretical expression for channel correlation and provides a rigorous test of an assumed fitting function. The procedure facilitates the use of the cumulant method in determining the polydispersity of scatterers. An expression for the statistical bias of g(2)(τ) is ...