  We predict that a metal nanofilm subjected to an ultrashort (single
oscillation) optical pulse of a high field amplitude $\sim 3 \mathrm{V/\AA}$ at
normal incidence undergoes an ultrafast (at subcycle times $\lesssim 1
\mathrm{fs}$) transition to a state resembling semimetal. Its reflectivity is
greatly reduced, while the transmissivity and the optical field inside the
metal are greatly increased. The temporal profiles of the optical fields are
predicted to exhibit pronounced subcycle oscillations, which are attributed to
the Bloch oscillations and formation of the Wannier-Stark ladder of electronic
states. The reflected, transmitted, and inside-the-metal pulses have non-zero
areas approaching half-cycle pulses. The effects predicted are promising for
applications to nanoplasmonic modulators and field-effect transistors with
petahertz bandwidth.
