  Studying the cyclotron resonance (CR)-induced photoconductivity in GaAs and
HgTe two-dimensional electron structures, we observed an anomalous
photoresponse for the CR-inactive geometry being of almost the same magnitude
as the CR-active one. This observation conflicts with simultaneous transmission
measurements and contradicts the conventional theory of CR which predicts no
resonant response for the CR-inactive geometry. We provide a possible route to
explain this fundamental failure of the conventional description of
light-matter interaction and discuss a modified electron dynamics near strong
impurities that may provide a local near-field coupling of the two helicity
modes of the terahertz field at low temperatures. This should result in a
CR-enhanced local absorption and, thus, CR photoconductivity for both magnetic
field polarities.
