The excitonic AC Stark effect in high energy excited semiconductor microcavities

Summary form only given. The recent observation of an "atomic-like" AC Stark splitting in a semiconductor microcavity in which the Fabry-Perot mode is resonant with the fundamental quantum-well excitonic transition has opened the following question: is it possible to observe an AC Stark splitting when exciting in the exciton continuum? To answer this question we have studied the AC Stark effect-by means of fs pump-probe spectroscopy-in a GaAs /spl lambda/ microcavity having an extremely high finesse (quality factor Q>2/spl times/10/sup 4/) and embedding a single, 8-nm-thick In/sub 0.03/Ga/sub 0.97/As quantum well. For zero exciton-photon detuning, our previous results are completely reproduced: when increasing pump intensity, the exciton-polariton doublet evolves continuously to an AC Stark triplet and field-driven Rabi oscillations appear in the time domain. To investigate the role of continuum states, we tuned the cavity mode slightly above (1-2 meV) the quantum-well exciton continuum onset. In the linear optical spectra, this fact provokes an abrupt intensity decrease and broadening of the upper, photon-like polariton, resonantly coupled to the continuum band-edge. Astonishingly, when the pump photon density increases (up to /spl sime/10/sup 14/ photons cm/sup 2/ per pulse), the probe transmission spectrum evolves continuously from an excitonic AC Stark redshift to an AC Stark splitting. Corresponding (field-driven) Rabi oscillations are detected in the time-integrated transmitted probe intensity versus pump-probe delay.