| The extension of the pump-probe technique to the extreme ultra-violet (XUV) and soft x-ray (SXR) regime with attosecond resolution entails the availability of intense attosecond pulses. The advent of such pulses will open up the way to real-time observation of a wide range of fast evolving phenomena in atomic, molecular and plasma physics. Modern laser systems although capable of producing extremely high intensities can not be appropriately exploited for generating intense sub-femtosecond pulses in the conventional approach using gas target. A new medium that exhibits strong nonlinearities, but also can withstand arbitrarily high fields is in need. Using solid targets irradiated at relativistic intensities as harmonic-generation media appears to be a very promising possibility. Based on previous experimental results, recent theoretical findings, and simulations using an 1-D particle-in-cell code, it appears quite feasible using surface harmonics generated at laser intensities of 10**20 W/cm**2 to produce a train or even single attosecond pulses in the 15 - 70 eV spectral range with duration of ~130 as and efficiency of few percent. Moreover, simulations with a 3-D particle-in-cell code have revealed valuable information concerning the divergence, temporal and spatial coherence, and focusability of this new harmonic radiation source. The generation of single attosecond pulses relies on mechanisms akin to those used with atomic harmonics, i.e., either on the availability of intense 5 fs laser pulses with carrier-envelope phase control or on the employment of the polarization gating technique. Equally challenging to the generation physics is the method of characterization of these pulses. Some general characteristics of this new harmonic source will be shortly outlined and the approach of generation and detection along with preliminary experimental results will be discussed. |
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