Coherent control of two-photon transitions

Dipole-forbidden transitions have been investigated by means of coherent co ntrol techniques. The 2p heavy-hole excitonic transition of a GaAs/AlGaAs m ultiple quantum well system was excited by a pair of phase-locked pulses wi th their energy at half the transition energy. The temporal evolution of th e amplitude of coherent control, monitored in the secondary emission at the 1s heavy-hole exciton energy, showed two different contributions: one at t he frequency of the optical field, and a second one at twice that frequency , from the interference of the excitonic polarisation. The latter component contains, besides the energy of the dark transition, information about the dynamics of the state.