The first observation of the optical Stark effect of electron-hole continuum states in addition to the bound-exciton states is reported under femtosecond excitation conditions chosen to minimize the generation of real carriers. The experimental results agree well with calculations using the generalized semiconductor Bloch equations. The theory shows that the commonly used adiabatic approximation is correct only for pulses which are longer than the coherence decay time. For shorter pulses coherent dynamic effects strongly influence the spectral changes.

• Received 3 November 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.1185