  The microscopic dynamics of laser-driven coherent synchrotron emission
transmitted through thin foils are investigated using particle-in-cell
simulations. For normal incidence interactions, we identify the formation of
two distinct electron nanobunches from which emission takes place each
half-cycle of the driving laser pulse. These emissions are separated temporally
by 130 attoseconds and are dominant in different frequency ranges, which is a
direct consequence of the distinct characteristics of each electron nanobunch.
This may be exploited through spectral filtering to isolate these emissions,
generating electromagnetic pulses of duration ~70 as.
