Imaging Spectroscopy of Exciton Spin Diffusion and Relaxation in Quantum Structure Semiconductors

近年来，人们对自旋晶体管等“自旋电子学”产生了浓厚的兴趣，在半导体中，电子自旋比其电荷更重要。因此，自旋弛豫和自旋扩散的知识是成功的关键之一。然而，量子结构中的这些物理机制并不为人所知。在此背景下，我们研究了量子阱结构中激子自旋的扩散和弛豫。为此，我们利用了四波混频光谱与自旋光栅。两个泵浦脉冲在样品表面附近产生共线偏振的激子密度光栅。对于交叉线性偏振，泵浦脉冲不能干涉，因此产生均匀的激子密度。但激子的上自旋和下自旋的混合随空间位置的变化而变化，从而可以产生自旋光栅。密度光栅和自旋光栅的间距可以通过泵浦脉冲之间的交叉角来调节。从这些光栅衍射的时间延迟的探测脉冲给出了光栅的衰减率，该衰减率包括根据所产生的光栅的特性的关于自旋和密度的弛豫和扩散的信息。因此，我们可以在同一个系统中测量自旋弛豫、自旋扩散常数、复合和密度扩散常数。结果表明，自旋扩散速率是密度扩散速率的3.5倍。这个结果是我第一次知道。

In recent years, people have much interest on "spin electronics" such as spin transistors, where electronic spin plays more important role than its charge in semiconductors. Then knowledge of the spin relaxation and spin diffusion is one of keys for success. Nevertheless those physical mechanisms in quantum structures are not well known. Under this background, We studied the diffusion and relaxation of exciton spin in quantum well structures. For this purpose, we utilized the four-wave-mixing spectroscopy with spin grating. Two pump pulses generated the exciton density grating near the sample surface in the co-linear polarizations. For the cross-linear polarizations, the pump pulses cannot interfere and therefore uniform exciton density is created. But the mixture of up-spin and down-spin of exciton changes according to the spatial position and thus the spin grating can be created. The pitch of both density and spin gratings can adjust by the crossing angle between the pump pulses. The time-delayed probe pulse diffracted from those gratings gives the decay rate of the gratings that includes the information about the relaxation and diffusion of the spin and density according to the character of the created grating. So, we could measure the spin relaxation ; spin diffusion constant, recombination and density diffusion constant in the same system. In the result, we obtained that spin diffusion rate is 3.5-times larger than density diffusion. This result is for the first time under my knowledge.