  Ultrashort electron bunches are useful for applications like ultrafast
imaging and coherent radiation production. Currently, however, the shortest
achievable bunches, at attosecond time scales, have only been realized in the
single or very few electron regime, limited by Coulomb repulsion and electron
energy spread. Using ab initio simulations and theoretical analysis, we show
that highly-charged bunches are achievable by subjecting relativistic (few
MeV-scale) electrons to a superposition of terahertz and optical pulses. Using
realistic electron bunches and laser pulse parameters which are within the
reach of current compact setups, we provide two detailed examples: one with
final bunches of ~1 fC contained within sub-400 as durations, and one with
bunches of >25 electrons contained within 20 as durations. Our results reveal a
route to achieve such extreme combinations of high charge and attosecond pulse
durations with existing technology.
