Reflectivity and Optical Brightness of Laser-Induced Shocks in Silicon

Abstract

We report the first simultaneous measurement of the reflectivity and optical emission of a strong (4-8 Mbar) shock front emerging at a free surface of a solid. Planar shock waves were driven by thermal x rays from a laser-heated cavity. The inferred model-independent brightness temperature of the shock front in silicon turns out to be significantly below the expected Hugoniot temperature. We find that our data cannot be explained within the two-temperature model which assumes instantaneous metallization of silicon in the density jump. [S0031-9007(98)06017-7]

Outstanding questions in electron–ion energy relaxation, lattice stability, and dielectric function of warm dense matter

“…In fact, it was found in the quantum kinetic calculations that for a two-temperature hydrogen plasma, the coupled-mode energy relaxation rate increases substantially with time during the relaxation process [10]. A more appropriate comparison between experiment and theory should focus instead on temperature of the shock front that was measured in the second experiment on shocked silicon [5]. As explained above, temperature obtained from optical pyrometry would be localized to a layer behind the shock front with a thickness comparable with optical mean free path, which is typically 5-10 nm for a metal.…”

“…Accordingly, pyrometric measurements would yield emission intensity or electron temperature significantly lower than that of an equilibrated shocked state as illustrated in Figure 1; their exact values would depend on the electron-ion coupling constant. This was the basis of our experimental investigation in shocked silicon [4,5].…”

“…Our interpretation of an electron temperature gradient behind the shock front was based on the comparison of observed shock emission intensity to results of calculation that (i) assumed a constant value for electron-ion coupling coefficient and (ii) relied on the use of a dense plasma conductivity model [6]. Such limitations were eliminated in a second experiment conducted at the Max-Planck Institute for Quantum Optics based on simultaneous measurements of emission intensity and reflectivity of a shock front in flight in silicon [5]. The use of these two measured quantities in Kirchhoff's law allowed direct determination of the brightness temperature of the shock front free from any opacity or transport models.…”

The Investigation of s‐ and p‐Polarized Reflectivities of Nonideal Plasma

“…Reflectivity measurements under shock wave compression have been performed for different materials [1][2][3][4][5][6][7]. Of particular interest are optical reflectance measurements on materials in which a transition from a dielectric to a metal-like state occurs with increasing density due to pressure ionization [2][3][4][5][6][7].…”

Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility

“…Active probing techniques can provide a wealth of information on transport properties, primarily through quantitative measurement of optical reflectance. Measurements of target reflectance with active probes have revealed detailed information on electrical conductivities in release profiles [12] and in the shock front [13,14].…”