  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.
