The photoresponse of a two-dimensional electron gas at the second   harmonic of the cyclotron resonance
  Terahertz spectroscopy experiments at magnetic fields and low temperatures were carried out on samples of different gate shapes processed on a high electron mobility GaAs/AlGaAs heterostructure. For a given radiation frequency, multiple magnetoplasmon resonances were observed with a dispersion relation described within a local approximation of the magnetoconductivity tensor. The second harmonic of the cyclotron resonance was observed and its appearance was interpreted as resulting from a high frequency, inhomogeneous electromagnetic field on the border of a two-dimensional electron gas with a metallic gate and/or an ohmic contact. 
M. Bia{\l}ek, M. Czapkiewicz, J. Wr\'obel, V. Umansky, J.   {\L}usakowski
cond-mat.mes-hall