  We observed an ultra-strong photovoltaic effect induced by resonant
intersubband absorption of microwaves in a two-dimensional electrons system on
the surface of liquid helium. The effect emerges in the regime of
microwave-induced vanishing of dissipative conductance reported previously [D.
Konstantinov and K. Kono: Phys. Rev. Lett. 105 (2010) 226801)] and is
characterized by a nonequilibrium spatial distribution of electrons in the
confining electrostatic potential. The electrostatic energy acquired by an
electron exceeds other relevant energies by several orders of magnitude.
