Observation of plasma waves with anomalously weak damping in a two-dimensional electron system

The microwave absorption spectra of a stripe of two-dimensional electrons in a GaAs/AlGaAs heterostructure are investigated using the optical detection of microwave absorption. A previously unknown low-frequency microwave-absorption resonance corresponding to the excitation of a weakly damped plasma wave in the two-dimensional electron system is observed. The new plasma mode is anomalously narrow, its width being considerably smaller than the inverse relaxation time of two-dimensional electrons. The measured dependences of the frequency of this mode on the density of two-dimensional electrons and the magnetic field give evidence of its plasmon-polariton nature.