The interaction of ultrashort (~50 fs), high-intensity XUV (l ~ 85 nm) pulses with solids has been investigated experimentally at the TTF-FEL Facility, DESY, Hamburg. Experiments were carried out at various intensities ranging from 1011 W/cm2 to 1014 W/cm2. Ions and atomic clusters ejected from the irradiated surfaces have been registered and analyzed with respect to their masses, energies, and charge states. Desorption, ablation, and plasma formation have been studied for a large variety of materials (insulators, semiconductors, and metals). Damaged surfaces have been investigated using light, electron, and atomic force microscopy. Results obtained for different materials, e. g. metals (Au), semiconductors (Si), inorganic insulators (Ce:YAG, BaF2, SiO2), and organic polymers (polymethylmethacrylate) ) show very interesting differences in the way a surface is damaged by XUV laser beam, as compared to the conventional one. Short-wavelength ablation is very efficient and clean when proper irradiation conditions (i.e. low fluency level) are chosen. The observed edges of craters are sharp, and the area around the craters is clean. There is a distinct difference in the behavior of conducting materials and insulators. In the case of all investigated insulators the morphology of the irradiated surface and the crater depth hardly depend on the beam intensity. In contrast, the irradiated silicon surface becomes very rough when the intensity exceeds the damage threshold. At certain illumination conditions the laser-induced periodic surface structures (LIPSS) were observed with a period of 87 nm. At high intensities a plasma was generated and multiple-ionized ions (charge states up to 5) were registered. Kinetic energy of the ions increases with charge state and reaches keV range for highly-charged ions. Again, a clear difference between insulators and conducting material is observed. High charge states and energetic ions are typical for conductors and semiconductors. Only single ions states and low energetic ions (~50 eV) are detected for insulators for all irradiation conditions. Such behavior was not anticipated because the photon energy was higher than the band energy gap, and one could expect a strong ionization. The results will be discussed in the frame of the modified inverse bremsstrahlung model.
Institute of Physics of Polish Academy of Sciences Institute of Physics of Polish Academy of Sciences