Coherent control of one-photon forbidden transitions in semiconductors

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The coherent control technique, where a relative phase of two femtosecond optical pulses is controlled precisely, gives us a new method controlling freely optical transitions in the semiconductors. Combining this coherent control technique with the resonant hyper-Rayleigh scattering, we developed a new spectroscopic method to manipulate the wavefunctions of one-photon forbidden states in semiconductors. This method was applied to the coherent control of one-photon forbidden 2p exciton states in ZnSe thin film on GaAs substrate. As a result, it was found that the 2p excitonic polarization can be manipulated freely by controlling the relative phase between incidence two pulses, and that the phase relaxation time of the 2p exciton state is -2ps. In addition, the quantum beat signal was superimposed to the obtained signal. This means that the 2p exciton state is split to a heavy hole and light hole states due to the strain caused by the lattice mismatch between ZnSe and GaAs. It is found from the period of the quantum beat that the energy separation between the heavy hole and light hole 2p extions is about 2meV.Next, we investigated the symmetry of the 2p exciton state in ZnSe by changing the relative direction of the polarizations of two pulses. It is found that the heavy hole and light hole 2p excitons belong to different symmetry. The spectrum information shows that the wavefunction of the heavy hole 2p exciton is s-like. Whereas, as for the light hole 2p exciton, the wavefunction has d-like symmetry. Moreover, the selection rule of resonant hyper-Rayleigh scattering process involving the 2p exciton states is found to be different from that of the SHG process.