  Using high-resolution, two-dimensional particle-in-cell simulations, we
investigate numerically the mechanisms of terahertz (THz) emissions in
submicron-thick carbon solid foils driven by ultraintense ($\sim 10^{20}\,\rm
W\,cm^{-2}$), ultrashort ($30\,\rm fs$) laser pulses at normal incidence. The
considered range of target thicknesses extends down to the relativistic
transparency regime that is known to optimize ion acceleration by femtosecond
laser pulses. By disentangling the fields emitted by longitudinal and
transverse currents, our analysis reveals that, within the first picosecond
after the interaction, THz emission occurs in bursts as a result of coherent
transition radiation by the recirculating hot electrons and antenna-type
emission by the shielding electron currents traveling along the fast-expanding
target surfaces.
