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We have determined for the first time the inter-Landau level lifetimes in GaAsJGaAIAs heterostructures in the spin polarized region (filling factor 1 1 ) . Intensity-dependent cyclotron resonance absorption has been measured using a high-intensity optica!ly pumped far-infrared (FIR! laser, A detailed analysis of t h e lineshape of the cyclotron resonance absorption within a Drude model shows saturation at intensities of 0.1 Wcm-'. The electronic lifetimes deduced using a three-level model depend inversely on the electron concentration in the excited Landau level, indicating electron-electron scattering to be the dominant relaxation mechanism.

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The cyclotron-resonance linewidth was studied in GaAs/GaAlAs heterostructures as a function of filling factor, magnetic field, and frequency. Maxima in the cyclotron-resonance linewidth are found for integer and for fractional Landau-level fillings of 3 3 and 3, giving evidence for a reduced screening due to density-of-states gaps. For partly filled levels a strong reduction of the linewidth due to resonant screening is found. The oscillations in the linewidth are highest for high-mobility and low-density samples.