Coherent control of two-photon transitions

Dipole-forbidden transitions have been investigated by means of coherent control techniques. The 2p heavy-hole excitonic transition of a GaAs/AlGaAs multiple quantum well system was excited by a pair of phase-locked pulses with their energy at half the transition energy. The temporal evolution of the amplitude of coherent control, monitored in the secondary emission at the 1s heavy-hole exciton energy, showed two different contributions: one at the 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.