Giant AC Stark shift in germanium

We observe a strong ultrafast AC Stark shift of the direct band transition in strained germanium quantum wells grown on silicon. At 150 meV, the maximum measured blue shift of the band edge is one order of magnitude larger than typically found in III-V materials. The power dependence shows a linear behavior between the electrical field intensity and the magnitude of the shift which is in good agreement with a dressed-exciton model. The coherent excitation dynamics in germanium are thus mainly governed by the direct transitions.