Terahertz dynamics of a topologically protected state: quantum Hall   effect plateaus near cyclotron resonance in a GaAs/AlGaAs heterojunction
  We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron resonance frequency by employing a highly sensitive Faraday rotation method coupled with electrical gating of the sample to change the electron density. We observe clear plateau-and step-like features in the Faraday rotation angle vs. electron density and magnetic field (Landau-level filling factor), which are the high frequency manifestation of quantum Hall plateaus - a signature of topologically protected edge states. The results are compared to a recent dynamical scaling theory. 
A.V. Stier, C.T. Ellis, H. Zhang, D. Eason, G. Strasser, B.D. McCombe   T. Morimoto, H. Aoki, J. Cerne
cond-mat.mes-hall