Collective excitations in a quasi-3-dimensional electron system

The far-infrared magneto-optical properties of remotely doped wide-parabolically graded AlGaAs quantum wells is reviewed. The optical response of the system is the same as that of a single-component three-dimensional plasma. Extensive studies were made of the plasma-shifted cyclotron resonance in the Voigt geometry. It is shown that the resonance frequency is independent of the areal density which demonstrates the generalized Kohn theorem. The internal modes of the system are excited by using wells with periodic and aperiodic perturbations superimposed on the parabolic potential. Dimensional modes and grating modes are identified which permit the wave-vector assignment of the internal magneto-plasma oscillations. From results on different samples and magnetic fields the dispersion relation for magneto-plasmons is measured. A roton excitation is observed at q{reversed tilde equals}2/l₀ which is good agreement with the predictions of calculations based on the single-mode approximation.