Cyclotron resonance inAlxGa1−xAs-GaAs heterostructures with tunable charge density via front gates

${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$-GaAs heterostructures have been prepared in which the charge density $N_S$ can be varied over a wide regime: $N_S=1\mathrm{}\times {10}^{10}{\mathrm{cm}}^{-2\mathrm{}} \mathrm{to} 5\times {10}^{11}{\mathrm{cm}}^{-2\mathrm{}}$. This gave us the possibility to study in detail filling-factor-($\nu$) dependent effects on the cyclotron-resonance mass $m^{*}$, linewidth, and amplitude. We found abrupt jumps for $m^{*}$ near $\nu =1\mathrm{} \mathrm{and} 2$ which are explained by energy renormalization effects and combined interacting cyclotron resonances from different Landau or spin levels.