  We present an experimental study of the microwave power and the linear
polarization angle dependence of the microwave-induced magnetoresistance
oscillations in the high-mobility GaAs/AlGaAs two-dimensional electron system.
Experimental results show the sinusoidal dependence of the oscillatory
magnetoresistance extrema as a function of the polarization angle. Yet, as the
microwave power increases, the angular dependence includes additional harmonic
content, and it begins to resemble the absolute value of the cosine function.
We present a theory to explain such peculiar behavior.
