  We report on a tunable - by magnetic field and gate voltage - conversion of
terahertz radiation into a dc current in spatially modulated bilayer graphene.
We experimentally demonstrate that the underlying physics is related to the
so-called ratchet effect. Our key findings are the direct observation of a
sharp cyclotron resonance in the photocurrent and the demonstration of two
effects caused by electron-electron interaction: the plasmonic splitting of the
resonance due to long-range Coulomb coupling and the partial suppression of its
second harmonic due to fast interparticle collisions. We develop a theory which
perfectly fits our data. We argue that the ratchet current is generated in the
hydrodynamic regime of non-ideal electron liquid.
