Study of close to solid density plasmas generated by ultra-short laser pulses

Complete test of publication follows. The interaction of high contrast sub 10 fs laser pulses with solid targets has been studied using XUV spectroscopy and laser absorption measurements. Laser pulses with intensities up to 1016 W/cm2 have been used on different targets including carbon, boron nitride and lithium fluoride. The experimental results indicate that close to solid density plasmas with very steep density gradients are produced. Time integrated K shell emission spectroscopy has been used to record the series limits for H-like and He-like resonance lines of carbon, boron and lithium. The limits are explained by pressure ionization, a 'signature' of high density plasmas. From the emission spectra, the plasma density and electron temperature were inferred. A simple computer code was used to model the expansion of the plasma and its time dependence for parameters similar to ones used in the experiment. The calculations predict a peak electron temperature for a lithium plasma of about 150 eV at a plasma density of 75% times solid. The rise time of the electron temperature is about 15 fs with plasma expansion of about 5%. Time dependent synthetic spectra have been calculated using the FLY code to analyze the temporal evolution of the resonance emission lines starting from close to solid density. In order to understand the energy transfer mechanisms from the sub 10 fs laser pulse to solid matter, absorption measurements have been performed. The absorption fraction of the laser energy A was determined from the relation A = 1 - R, where R represents the fraction of the laser energy reflected by the solid target which was measured using an integrating Ulbricht's sphere. The dependence of the absorption fraction versus the incident angle for p-polarized laser beam was investigated. The experimental data were compared with various absorption mechanisms including resonance absorption, vacuum heating and skin depth absorption. The model developed by Gibbon and Bell for a steep but finite density gradient which describes the transition between resonance absorption and vacuum heating fits best the data. For our experimental conditions, a plasma density scale length L/λ < 0.1 is inferred.