ABSORPTION OF SUBPICOSECOND ULTRAVIOLET-LASER PULSES IN HIGH-DENSITY PLASMA

The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 1012-1014 W cm-2 and as a function of polarization and angle of incidence for the intensity range of 1014-2.5×1015 W cm-2. As the intensity increases from 1012 W cm-2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5-0.61 at 1014 W cm-2. For this intensity maximum absorption of 63-80% has been observed for p-polarized radiation at angles of incidence in the range of 54°-57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/ω, in the range of 0.13-0.15 at critical density and normalized density gradient scale lengths, L/λ0, in the range of 0.018-0.053 for a laser intensity of 1014 W cm-2. At 2.5×1015 W cm-2 a small amount of preplasma is present and maximum absorption of 64-76% has been observed for p-polarized radiation at angles of incidence in the range of 45°-50°. © 1990 Springer-Verlag.