  We calculate the photocurrent in a clean graphene sample normally irradiated
by a monochromatic electromagnetic field and subject to a step-like
electrostatic potential. We consider the photon energies $\hbar\Omega$ that
significantly exceed the height of the potential barrier, as is the case in the
recent experiments with graphene-based photodetectors. The photocurrent comes
from the resonant absorption of photons by electrons and decreases with
increasing ratio $\hbar\Omega/U_0$. It is weakly affected by the background
gate voltage and depends on the light polarization as $\propto\sin^2\gamma$,
$\gamma$ being the angle between the potential and the polarization plane.
