Observation of the Resonant Optical Stark Effect in a Semiconductor

This Letter presents for the first time the observation of the resonant optical Stark effect in a semiconductor. The excitonic system of ${\mathrm{Cu}}_2$O is well suited to the investigation of this effect because of its rather large binding energy and well-resolved resonances at low temperatures. The mixing between the $1S$ and $2P$ excitons by the electric field of a tunable high-power C${\mathrm{O}}_2$ laser leads to drastic changes in the one-photon spectrum around the $2P$ absorption line. Even details of the line shape are consistently described in terms of the frequency-dependent nonlinear susceptibility.