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

Abstract

The interaction of femtosecond ${\mathrm{KrF}}^{\mathrm{*}}$-laser pulses with plasmas of various solid target materials has been studied up to intensities exceeding ${10}^{18}$ W/${\mathrm{cm}}^{2}$. Absorption measurements were performed for p- ands-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/${\mathrm{cm}}^{2}$. 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\ifmmode^\circ\else\textdegree\fi{}--80\ifmmode^\circ\else\textdegree\fi{}) 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 \AA{}) and in the soft-x-ray region (i.e., between 25 and 400 \AA{}). 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-\ensuremath{\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 \ensuremath{\sim}8 keV is considerably lower than that deduced from experiments using lasers of longer wavelength and comparable intensities. Scaling indicates that 0.25-\ensuremath{\mu}m lasers can simultaneously fulfill the requirements for both intensity and hot electron temperature for the ``fast ignitor.'' \textcopyright{} 1996 The American Physical Society.