Effective interaction of intense ultra-short laser pulse with nano-structured targets

Summary form only given. A main problem of laser target interaction physics is an efficient transformation of laser pulse energy into particle or radiation energy. Recently for size limited thin foils an enhancement of laser to ion energy conversion was demonstrated at a moderate laser absorption level. [1].In this work a significant increase of target absorption is found by an optimized target structure enabling maximization of energy to accelerated particles transfer, as well as reflected (transmitted) light pulses radiate attosecond pulses. The optimum target characteristics are evaluated by an analytical and a multi-dimensional numerical modeling. We investigated metall and plastic foils with a size of few hundred nanometers, where different elements are topped as a relief on the surface. Experimentally periodic nanostructures were generated at the surface of extremely thin metal foils, by a Femtosecond-laser moving substrate method [2] in air. Few micron thick Cu and Ti specimen were translated through a linear or point-like focus of a linearly polarized and frequency-converted femtosecond laser (pulse duration 120 fs, center wavelength 400 nm). It was found that two different types of ripple structures with high and low spatial frequencies corresponding to structure periods in the range of 70 nm and 300 nm, respectively, can be formed. The ripple type can be adjusted by confining the number and energy of the pulses and the scanning velocity. Between the distinct parameter fields of stable formation, a transition range with partially changing ripple size and orientation appears. The laser pulse with relativisitc intensity irradiated the structured foils. It was found that the optimal relief is capable to increase absorption of a target to extremely high values. The shape of profile does not change significantly the absorption but affect other object properties. For effective acceleration of ions, the volume of the relief should be less than the volume of the substrate foil. To redirect the additional absorbed laser energy to protons rather than ions, it is necessary to use a heterogeneous target. We have studied the stability of the target relief structures and found that for this scheme to work, one needs a very high-contrast laser-pulse and ns pre-pulse duration [3]. An optimal target relief rise laser pulse absorption to maximal possible value and significantly improves X-ray yield. The most effective generation of short dense electron bunches appears at a big angle of incidence of a laser pulse with a thin semi-limited foil. Streams of electron bunches can be used for generation the short pulse EUV radiation when the fast electrons hit a second target. It is shown, that effective atto-pulse can be generated during the interaction of a laser pulse with a double foil target.