  Terahertz (THz) emissions from fast electron and ion currents driven in
relativistic, femtosecond laser-foil interactions are examined theoretically.
We first consider the radiation from the energetic electrons exiting the
backside of the target. Our kinetic model takes account of the coherent
transition radiation due to these electrons crossing the plasma-vacuum
interface as well as of the synchrotron radiation due to their deflection and
deceleration in the sheath field they set up in vacuum. After showing that both
mechanisms tend to largely compensate each other when all the electrons are
pulled back into the target, we investigate the scaling of the net radiation
with the sheath field strength. We then demonstrate the sensitivity of this
radiation to a percent-level fraction of escaping electrons. We also study the
influence of the target thickness and laser focusing. The same sheath field
that confines most of the fast electrons around the target rapidly sets into
motion the surface ions. We describe the THz emission from these accelerated
ions and their accompanying hot electrons by means of a plasma expansion model
that allows for finite foil size and multidimensional effects. Again, we
explore the dependencies of this radiation mechanism on the laser-target
parameters. Under conditions typical of current ultrashort laser-solid
experiments, we find that the THz radiation from the expanding plasma is much
less energetic -- by one to three orders of magnitude -- than that due to the
early-time motion of the fast electrons.

Plasma Physics