Multiphoton processes at cyclotron resonance subharmonics in a 2D   electron system under DC and microwave excitation
  We investigate a two-dimensional electron system (2DES) under microwave illumination at cyclotron resonance subharmonics. The 2DES carries sufficient direct current, $I$, that the differential resistivity oscillates as $I$ is swept. At magnetic fields sufficient to resolve individual Landau levels, we find the number of oscillations within an $I$ range systematically changes with increasing microwave power. Microwave absorption and emission of $N$ photons, where $N$ is controlled by the microwave power, describes our observations in the framework of the displacement mechanism of impurity scattering between Hall-field tilted Landau levels. 
S. Chakraborty and A. T. Hatke and L. W. Engel and J. D. Watson and M.   J. Manfra
cond-mat.mes-hall