Direct observation of an ac Stark splitting in semiconductor microcavitiesexcited above the continuum onset

We study the coherent optical response of a very high-finesse microcavity w ith a single quantum well by means of degenerate subpicosecond pump-probe s pectroscopy. The dynamics of the ac Stark effect versus Pump intensity is i nvestigated upon tuning the empty-cavity;mode-below and above the exciton t ransition energy, at the exciton continuum onset. The evolution from a simp le dynamical excitonic shift to a dynamical splitting is clearly observed i n both eases. The qualitative, agreement with numerical calculations based on the semiconductor Maxwell-Bloch equations is good.