Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session L5: Spectrosopy & Optical Properties I |
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Chair: Marcus Knudson, Sandia National Laboratories Room: Hyatt Regency Constellation F |
Tuesday, August 2, 2005 3:30PM - 3:45PM |
L5.00001: Calculation of phase-dependent optical absorption in materials Lorin Benedict, John Klepeis, Frederick Streitz We calculate the optical absorption spectra of different phases of various materials (Al, Bi, Fe) in an effort to support work in which optical constants are measured during the course of a shock compression experiment. In addition to showing that the optical properties of various solid phases of a given material are quite different, we compute the optical properties of Al across its solid-to-liquid transition and show that a prominent peak in the spectrum of the solid smoothly disappears as the temperature is raised above Tmelt. This is in agreement with earlier measurements on solid and liquid Al, but is in disagreement with a more recent measurement on the liquid. Our results suggest that in situ optical constants measurements may be used as a diagnostic for the shock melting of Al. [Preview Abstract] |
Tuesday, August 2, 2005 3:45PM - 4:00PM |
L5.00002: Anharmonic vibrational properties of PETN, HMX, and TATB from temperature dependent Raman Shawn McGrane, Jeffrey Barber, Jason Quenneville Raman spectra from 50-3500 cm$^{-1}$ and 4-296 K are analyzed for PETN, HMX, and TATB. Shock initiation of detonation is postulated to occur by vibrational up-pumping, which implies the presence of diagnostic features in the vibrational spectra of molecules with vastly different shock initiation properties. Temperature dependent Raman is utilized for its sensitivity to anharmonic couplings between thermally populated phonons and higher frequency vibrations. The data are analyzed with anharmonic perturbation theory (APT), which is shown to have significant fundamental limitations. Fitting to APT revealed no significant difference in averaged anharmonicities among the three explosives. Calculations of the multiphonon densities of states also failed to correlate clearly with shock sensitivity. However, striking differences in temperature dependent lifetimes were obvious: PETN has long lived phonons and vibrons, HMX has long lived phonons but short lived vibrons, while TATB has short lived phonons and vibrons. These data suggest the further hypothesis that hindered vibrational energy transfer in the molecular crystals is a factor in shock sensitivity. [Preview Abstract] |
Tuesday, August 2, 2005 4:00PM - 4:15PM |
L5.00003: Optical Properties of Shock-compressed Materials Reed Patterson, Jeff Nguyen, John Klepeis, Lorin Benedict, Neil Holmes Real-time {\em in-situ} measurements of complex optical constants during dynamic compression experiments provide a wealth of information beyond standard Hugoniot-EoS or sound speed measurements. At LLNL we have developed an ellipsometer for use on our two-stage light-gas gun, giving us the ability to measure the indices of refraction of shocked materials. Optical constants not only provide information about the existence of phase transformations, but also can be coupled with calculations to propose high pressure - high temperature crystal structures, yielding phase diagram information. Time-resolved ellipsometric measurements can potentially be used to extract information about the dynamic behavior of materials such as the kinetics of phase transitions, or elastic/plastic deformation/relaxation rates. We have observed solid-solid or solid-liquid phase transitions in Fe,Sn, and Bi as well as stress-induced birefringence in LiF. We will also discuss the implications of time-dependent phenomenon such as lattice relaxation in LiF. \\ \\ \noindent This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. [Preview Abstract] |
Tuesday, August 2, 2005 4:15PM - 4:30PM |
L5.00004: Coherent optical photons from shock waves in polarizable crystals Evan Reed, Marin Soljacic, John Joannopoulos, Richard Gee We predict that coherent electromagnetic radiation can be generated in polarizable crystalline materials when subject to a shock wave or shock-like propagating excitation. To our knowledge, this phenomenon represents a fundamentally new source of coherent optical radiation source in the 1-100 THz frequency range that is distinct from lasers and free-electron lasers. Analytical theory, finite-difference time-domain simulations of Maxwell's equations, and molecular dynamics simulations demonstrate coherence lengths on the order of mm (at 16 THz) and potentially greater. [Preview Abstract] |
Tuesday, August 2, 2005 4:30PM - 5:00PM |
L5.00005: Shock compression spectroscopy with high time and space resolution Invited Speaker: Shock compression of molecules with high time and space resolution is obtained using a combination of femtosecond laser-driven shock waves and coherent nonlinear vibrational spectroscopy. In a molecular monolayer, vibrational spectroscopy is used to look at a single atomic group, CH$_{3}$, as the $<$ 3 ps rise time shock front passes. Through comparison with molecular simulations, the detailed nature of shock-induced molecular deformations is determined. [Preview Abstract] |
Tuesday, August 2, 2005 5:00PM - 5:15PM |
L5.00006: Optical properties of silica under shock loading R.S. McWilliams, R. Jeanloz, D.G. Hicks, P.M. Celliers, J.H. Eggert, G.W. Collins The high-pressure behavior of Silica (SiO2) has been explored extensively with shock-compression and is of great importance in condensed matter physics and planetary science. A number of interesting phenomena, such as shock-induced shear banding and superheating, were first observed in silica and subsequently observed in a variety of dielectrics including planetary silicates and alkali halides. Thus silica can serve as an excellent system in which to understand the high-pressure and -temperature behavior of dielectrics. We have investigated the optical opacity, reflectivity, and index of refraction of alpha-quartz and fused silica to shock compressions of $\sim $2 Mbar and temperatures of $\sim $1 eV with laser driven shock waves. With increasing pressure, we observe a sharp increase in the opacity of these initially transparent solids, followed by a more gradual rise of reflectivity at higher pressure. These results may be crucial for interpreting thermal emission measurements from shocks in these materials, as well as providing new detail on the nature of silica and other dielectrics at extreme conditions. This work was performed under the auspices of the US DOE by LLNL under Contract No.$\backslash $W-7405-ENG-48. [Preview Abstract] |
Tuesday, August 2, 2005 5:15PM - 5:30PM |
L5.00007: Determination of the deformation potentials of GaN using shock wave compression M.D. McCluskey, H.Y. Peng, Y.M. Gupta Gallium nitride (GaN) is a wide-band-gap semiconductor used widely in blue lasers and light-emitting diodes. Since the layers in these devices are under considerable strain, an accurate determination of the optical deformation potentials is important for modeling and design. Toward that end, in the present work, the band-gap shift of GaN has been studied as a function of uniaxial compression along the $c$-axis using time-resolved, optical absorption measurements in shock wave experiments. Single-pass, optical transmission measurements were obtained in thin GaN samples grown heteroepitaxially on sapphire substrates. Previous estimates of the band-gap deformation potentials, which were based on samples under hydrostatic pressure or biaxial stress, varied over a wide range. Shock compression provided a novel approach for determining the deformation potentials accurately. Based on the experimental results, a set of deformation potentials was obtained. The values indicate that the deformation potentials in wurtzite GaN are anisotropic, contrary to common assumptions. [Preview Abstract] |
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