  We have observed long-lived (~30 ps) coherent oscillations of charge carriers
due to cyclotron resonance (CR) in high-mobility two-dimensional electrons in
GaAs in perpendicular magnetic fields using time-domain terahertz spectroscopy.
The observed coherent oscillations were fitted well by sinusoids with
exponentially-decaying amplitudes, through which we were able to provide direct
and precise measures for the decay times and oscillation frequencies
simultaneously. This method thus overcomes the CR saturation effect, which is
known to prevent determination of true CR linewidths in high-mobility electron
systems using Fourier-transform infrared spectroscopy.
