  The reflection of intense laser radiation from solids appears as a result of
relativistic dynamics of the electrons driven by both incoming and
self-generated electromagnetic fields at the periphery of the emerging dense
plasma. In the case of highly-relativistic motion, electrons tend to form a
thin oscillating layer, which makes it possible to model the interaction and
obtain the temporal structure of the reflected radiation. The modelling reveals
the possibility and conditions for producing singularly intense and short XUV
bursts of radiation, which are interesting for many applications. However, the
intensity and duration of the XUV bursts, as well as the high-energy end of the
harmonic spectrum, depends on the thickness of the layer and its internal
structure which are not assessed by such macroscopic modelling. Here we analyse
the microscopic physics of this layer and clarify how its parameters are bound
and how this controls outlined properties of XUV bursts.
