Cyclotron Resonance in a High Mobility 2DEG*

We have systematically studied the cyclotron resonance lifetimes of a high mobility ($\mu =3.4\times 10^{6}\mathrm{cm}^{2}\,\mathrm{V}^{-1}\,\mathrm{s}^{-1}$) two-dimensional electron gas as a function of temperature (0.38 K-80 K) using ultrafast terahertz spectroscopy. The cyclotron lifetime increases by $\sim$2x from 1.2 to 0.38 K, which results in a concomitant decrease in transmission amplitude due to the saturation effect. The differential amplitude of the time-delayed secondary pulse field is larger than the primary pulse and is not consistent with a Drude free carrier model.