  Microwave-induced resistance oscillations (MIRO) and "zero-resistance" states
(ZRS) were discovered in ultraclean two-dimensional electron systems in
2001-2003 and have attracted great interest from researchers. A comprehensive
theory of these phenomena was developed in 2011: It was shown that all
experimentally observed dependencies can be naturally explained by the
influence of the ponderomotive forces which arise in the near-contact regions
of the two-dimensional electron gas under the action of microwaves. Now we show
that the same near-contact physical processes should lead to another nonlinear
electrodynamic phenomenon - the second-harmonic generation. We calculate the
frequency, magnetic field, mobility, and power dependencies of the
second-harmonic intensity and show that it can be as large as ~ 0.5 mW/cm2
under realistic experimental conditions. A part of this paper is devoted to a
further discussion of the MIRO/ZRS phenomena: we explain how the
ponderomotive-force theory explains different experimental details, including
those which were not known in 2011, and critically discuss alternative
theories.
