Optical properties of femtosecond laser-heated ultrathin foils

Summary form only given, as follows. The absorption of femtosecond laser pulses provides an experimental method for the deposition of high energy densities into materials. For very high-contrast 100-femtosecond laser pulses, "moderate" intensities of 10/sup 13/ /sup 14/W/cm/sup 2/ are sufficient to create energy densities in the order of MJ/cm/sup 3/. The target remains at near-solid density due to the rapid heating and reaches, according to the simulations, electron temperatures of a few eV. Modeling and predicting the properties of this partially ionized, strongly coupled state of matter, also known as warm dense matter, is extremely difficult. Significant differences in theoretical predictions of the temperature, the pressure, and the charge state in currently implemented equation-of-state (EOS) models can be found. Moreover, theoretical predictions of the optical properties, such as reflectivity and transmission, using recent conductivity models show large disagreement, too. Measurements were performed at the Livermore UltraShort-Pulse laser facility that probe the optical properties of this high-energy density regime utilizing a pump-probe technique. The use of ultrathin (few 10-nm thin) free-standing targets allowed us to measure the reflectance and transmittance during the first few picoseconds after the heating occurred. The experimental data yield the temporal behavior of the dielectric function. The comparison with simulations give access to both, conductivity models and EOS models. In particular, the comparison of preliminary data with the calculated values favors the Lee-More conductivity model and the activity expansion (ACTEX) EOS model.