  Momentum and energy balance equations are developed for steady-state electron
transport and optical absorption under the influence of a dc electric field, an
intense ac electric field of terahertz (THz) frequency in a two-dimensional
(2D) semiconductor in the presence of a strong magnetic field perpendicular to
the 2D plane. These equations are applied to study the intensity-dependent
cyclotron resonance (CR) in far-infrared transmission and THz-radiation-induced
photoconductivity of GaAs heterostructures in Faraday geometry. We find that
the CR peaks and line shapes of the transmittance exhibit different intensity
dependence when the intensity of THz field increases in the range above or
below a certain critical value. The CR in photoresistivity, however, always
enhances with increasing the intensity of the THz field. These results
qualitatively agree with the experimental observations. We have clarified that
the CR in photoconductivity is not only the result of the electron heating, but
also comes from photon-assisted scattering enhancement, especially at high
temperatures. The effects of an intense THz field on Faraday angle and
ellipticity of magnetically-biased 2D semiconductors have also been
demonstrated.
