Coherent control of one-photon forbidden transitions in semiconductors

相干控制技术，即精确控制两个飞秒光脉冲的相对相位，为我们提供了一种自由控制半导体中光学跃迁的新方法。将这种相干控制技术与共振超瑞利散射相结合，我们发展了一种新的光谱方法来操纵半导体中单光子禁态的波函数。将该方法应用于GaAs基ZnSe薄膜中单光子禁带2p激子态的相干控制。结果发现，通过控制入射脉冲之间的相对相位，可以自由地操纵2p激子的极化，2p激子态的相位驰豫时间为-2ps。此外，将量子拍频信号叠加到所获得的信号上。这意味着由于晶格失配引起的应变，2p激子态分裂为重空穴和轻空穴。从量子拍频的周期中发现，重空穴和轻空穴的2p能级之间的能量间隔约为2 meV。其次，我们通过改变两个脉冲的偏振方向，研究了ZnSe中2p激子态的对称性。结果表明，重空穴激子和轻空穴激子具有不同的对称性。光谱信息表明，重空穴2p激子的波函数是类S的。而对于轻空穴2p激子，波函数具有类d对称性。此外，还发现涉及2p激子态的共振超瑞利散射过程的选择规则与倍频过程不同。

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.