Absorption and hot electron production by high intensity femtosecond UV-laser pulses in solid targets

The interaction of femtosecond KrF*-laser pulses with plasmas of various solid target materials has been studied up to intensities exceeding 10(18) W/cm(2). Absorption measurements were performed for p- and s-polarized laser light and as a function of the laser intensity and the angle of incidence. They reveal substantial absorption up to 70% even at intensities in excess of 10(18) W/cm(2). The results have also been compared to simulations of the absorption at high intensities and, in particular, the peaking of the absorption for large angles of incidence (70 degrees-80 degrees) appears to be consistent with the anomalous skin effect as an important contribution to the total laser pulse absorption. X-ray spectra were measured in the keV range (i.e., between 6.5 and 8.4 Angstrom) and in the soft-x-ray region (i.e., between 25 and 400 Angstrom). The electron density and temperature of the plasma has been estimated by comparison of the experimental spectra with spectral simulations. A systematic study of the hot electrons produced by 248-nm light is presented. Targets consisting of an Al layer on a Si substrate have been used to determine the hot electron yield and the corresponding energy. The K-alpha line emission produced by the hot electrons has been observed as a function of the Al-layer thickness. The measurements have been compared to simulations. The estimated hot electron temperature similar to 8 keV is considerably lower than that deduced from experiments using lasers of longer wavelength and comparable intensities. Scaling indicates that 0.25-mu m lasers can simultaneously fulfill the requirements for both intensity and hot electron temperature for the ''fast ignitor.''