  We propose a new mechanism for generating coherent subcycle optical pulse by
directing a relativistic electron beam (REB) into a plasma with a density
up-ramp. The subcycle pulse is coherently emitted by bubble-sheath electrons in
REB-induced superluminal plasma wake. Using three-dimensional particle-in-cell
and far-field time-domain radiation simulations as well as analytical modeling,
we show that an isolated subcycle optical shock can be produced at the
Cherenkov angle. This radiation has ultra-short attosecond-scale duration and
high intensity and exhibits excellent directionality with ultra-low angular
divergence and stable carrier envelope phase. Its central frequency can be
easily tuned over a wide range, from the far-infrared to the ultra-violet, by
adjusting the plasma and driver-beam density.
