Multiphoton microwave photoresistance in a high-mobility two-dimensional   electron gas
  We report on experimental and theoretical studies of microwave-induced resistance oscillations in a two-dimensional electron gas over a wide range of microwave intensities. We observe a distinct crossover from linear to sublinear power dependence of the oscillation amplitude and a concomitant narrowing of the oscillation extrema. To explain our observations we propose a theory based on the quantum kinetic equation at arbitrary microwave power. Taken together, these findings demonstrate a crucial role of multiphoton processes at elevated microwave intensities. 
A. T. Hatke, M. Khodas, M. A. Zudov, L. N. Pfeiffer, and K. W. West
cond-mat.mes-hall