For excitation of the model semiconductor GaAs with optical pulses which are both extremely short ( $5\mathrm{fs}$) and extremely intense ( $\approx {10}^{12}\mathrm{W}{\mathrm{cm}}^{-2\mathrm{}}$), we can meet the condition that the Rabi frequency becomes comparable to the band gap frequency—a highly unusual and previously inaccessible situation. Specifically, in this regime, we observe carrier-wave Rabi flopping, a novel effect of nonlinear optics which has been predicted theoretically and which is related to the failure of the area theorem.

• Received 13 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.057401