Research on optical properties of warm dense matter using ultra-short-pulse lasers

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The purpose of this research is to explore properties of matter at novel conditions. Using ultra-short pulse lasers, we produce and study matter at high energy density, corresponding to temperatures of 1,000〜100,000 Kelvin. Although very high energy densities can be created by various laboratory technologies, the resulting pressure is too high to confine and the heated material expands rapidly. For this reason it is important to develop high-speed (dynamic) diagnostic techniques.Short-pulse laser pump-probe experiments can now be performed with high repetition rate and give a continuous time scan of target reflectivity changes during and after the heat pulse. The probe pulse duration provides a time resolution 100fsec. A single reflected probe pulse yields four signals, which characterize the elliptic polarization of the reflected light, a technique called "ellipsometry". We also have developed new computer tools to analyze or interpret the results.In the analysis of experiments on low … More -temperature gold plasmas, we found evidence for a strong reduction of free-electron electrical conductivity consistent with localization of electrons. Solutions of the Saha equation predict an anomalous plasma composition in which positive ions Au+ and negative ions Au- are balanced with very few free electrons. Evidence for anomalous low conductivity in low-temperature plasmas has emerged from other experimental techniques including direct measurement of DC conductivities in pulse-heated systems such as wire arrays, capillaries, etc. Under certain circumstances, quantum molecular-dynamic simulations also predict low conductivity.This work is not yet ready to discuss the details in the warm dense matter. Rather we are making a first quantitative exploration with the goal of identifying the main physical phenomena, especially as regards metal-insulator transitions, applicability of basic ideas of plasma EOS, Drude conductivity, etc.In near future, we will have a database of many atomic polarizabilities and dielectric functions for real materials, data qualified by comparison to experiments. Less