  Our studies of the cyclotron resonance (CR) photoresistance in GaAs-based
two-dimensional electron systems (2DES) reveal an anomalously low sensitivity
to the helicity of the incoming circularly polarized terahertz radiation. We
find that this anomaly is strongly intensity dependent, and the ratio of the
low-temperature photoresistance signals for the CR-active (CRA) and CR-inactive
(CRI) polarities of magnetic field increases with lowering power, but,
nevertheless, remains substantially lower than expected from conventional
theory assuming interaction of the plane electromagnetic wave with the uniform
2DES. Our analysis shows that all data can be well described by the nonlinear
CR-enhanced electron gas heating in both CRA and CRI regimes. This description,
however, requires a source of anomalous absorption of radiation in the CRI
regime. It can stem from evanescent electromagnetic fields originating from the
near-field diffraction within or in the vicinity of the quantum well hosting
the 2DES.
