Multiple cyclotron resonances in GaAs-Al${}_x$Ga${}_{1-x}$As quantum wells detected by resonant inelastic light scattering

Cyclotron resonance excitations in modulation-doped Al${}_x$Ga${}_{1-x}$As-GaAs quantum wells have been investigated by means of resonant inelastic light scattering. Besides the cyclotron resonance excitation $ħ{\omega }_C,$ we observe up to fifteen additional peaks at multiples of $ħ{\omega }_C.$ The constant energy separation between these resonances indicates that they are multiple excitations of $ħ{\omega }_C$ (and not higher harmonics). These multiple resonances all have nearly the same strength. For an explanation, we propose a cascade model, where the photocreated hole in the valence band couples via direct and exchange Coulomb interactions to the Fermi sea. This allows the hole to relax resonantly by creating cyclotron resonance excitations in the conduction band.