Carrier-wave Rabi flopping on radiatively coupled shallow donor transitions in n-type GaAs

Ultrafast phase-resolved terahertz propagation experiments reveal the nonlinear optical response of shallow donor transitions in n-type GaAs. At a lattice temperature of 100 K, carrier-wave Rabi oscillations between bound impurity levels govern the nonlinear terahertz response for terahertz driving fields up to 5 kV/cm. The radiative coupling between impurity transitions results in a coherently excited macroscopic 1S-2P polarization that oscillates for several picoseconds. A comparison with the Bloch equations for radiatively coupled two-level systems demonstrates a breakdown of the two-level approach for field amplitudes above 5 kV/cm.