  The dispersion of the volume relativistic magnetoplasma mode in a gated
GaAs/AlGaAs quantum well is measured using a coupled resonators detection
technique. The weakly damped relativistic mode exhibits an unusual
zigzag-shaped magnetodispersion dependence dictated by the diagonal component
of the resistivity tensor $\rho_{xx}$. The plasma excitation easily hybridizes
with photon modes due to a large spatial delocalization of its electromagnetic
field. The effects of electron density and structure geometry on the excitation
spectrum have been investigated.
