  Relativistic surface high harmonics have been considered a unique source for
the generation of intense isolated attosecond pulses in the extreme
ultra-violet (XUV) and X-ray spectral range. However, its experimental
realization is still a challenging task requiring identification of the optimum
conditions for the generation of isolated attosecond pulses as well as their
temporal characterization. Here, we demonstrate measurements in both
directions. Particularly, we have made a first step towards the temporal
characterization of the emitted XUV radiation by adapting the attosecond streak
camera concept to identify the time domain characteristics of relativistic
surface high harmonics. The results, supported by PIC simulations, set the
upper limit for the averaged (over many shots) XUV duration to <6 fs, even when
driven by not CEP controlled relativistic few-cycle optical pulses. Moreover,
by measuring the dependence of the spectrum of the relativistic surface high
harmonics on the carrier envelope phase (CEP) of the driving infrared laser
field, we experimentally determined the optimum conditions for the generation
of intense isolated attosecond pulses.
