High harmonic generation from solid surfaces with relativistically intense few cycle laser pulses

Complete text of publication follows. Coherent high harmonic radiation from the interaction of intense femtosecond laser pulses with solid targets promises the generation of attosecond pulses orders of magnitude more intense than those generated from gaseous media. Such pulses would, for the first time, enable researchers to explore a whole new class of attosecond time resolution XUV-pump XUV-probe type experiments not accessible with the relatively weak gas-harmonic sources available today. Two mechanisms can contribute to the generation of high harmonics from solid surfaces, namely coherent wake emission (CWE) and the relativistic ally oscillating mirror (ROM). We present experimental results giving deeper insight into the CWE mechanism. Variations of the chirp, the carrier envelope phase and the polarization of the incident laser pulse all show to drastically influence the emitted harmonic spectra. By analyzing these spectra better understanding of the motion of the electrons at the target surface during the interaction can be achieved. At the same time we show the first observation of ROM harmonics generated by a relativistically intense few cycle laser pulse (8 fs, 150 mJ). The broad, almost continuous shape of the recorded harmonic spectra suggests, that the emission of attosecond pulses is limited to only very few cycles around the peak of the envelope of the driving laser pulse what can be explained by strong intensity gating effects. The limitation of the harmonic emission to only very few, down to a single laser cycle is opening the path towards the generation of ultra-intense single attosecond pulses with novel high power, few cycle laser systems and especially interesting in the prospect of ELI.