  The photovoltaic effect induced by terahertz radiation in a gated
two-dimensional electron gas in magnetic field is considered theoretically. It
is assumed that the incoming radiation creates an ac voltage between the source
and gate and that the gate length is long compared to the damping length of
plasma waves. In the presence of pronounced Shubnikov-de Haas oscillations, an
important source of non-linearity is the oscillating dependence of the mobility
on the ac gate voltage. This results in a photoresponse oscillating as a
function of magnetic field, which is enhanced in the vicinity of the cyclotron
resonance, in accordance with recent experiments. Another, smooth component of
the photovoltage, unrelated to SdH oscillations, has a maximum at cyclotron
resonance.
