Spin Electronics Novel Optical Probe of Carrier Spin Coherence in Semiconductors

该提案是根据21世纪自旋电子学倡议（项目征求NSF 02-036）收到的。该提案的重点是测量半导体载流子自旋相干性的新光学技术。与传统的微电子器件相比，基于自旋的电子器件有望提供更高的处理速度、更高的集成密度和更低的功耗。此外，半导体中的电子自旋对于量子信息处理的实现具有吸引力。这些进步需要对半导体中的电子自旋相干性有坚定的理解，并开发探测自旋相干性的技术。先前的研究表明，n掺杂的GaAs具有非常长的自旋相干时间，这促使了进一步的研究。半磁半导体具有较短的自旋相干，但其较强的载流子-离子相互作用使其成为制造自旋滤波器和相关器件的较好候选材料。本项目开发的技术将补充法拉第旋转的主要技术，并提供法拉第旋转无法获得的信息。特别令人感兴趣的是为什么法拉第旋转信号在高光激发密度下饱和。该技术基于瞬态四波混频。具体地说，利用三脉冲激励方案，可以探测自旋相干性并在背景自由方向上产生信号。使用五阶非线性光学响应的修改可以产生拉曼自旋回波，这将消除非均匀自旋进动率的影响，例如，由于g因子与k向量的变化。实验将在n掺杂的GaAs和半磁半导体（如CdMnTe和ZnMnTe）上进行。前一种样品是市售晶圆，而后者将由德国多特蒙德大学的合作者提供。在后一种情况下，将进行初步实验来探测光学相干性（与自旋相干性相反），因为它不是很好地理解，并提供了对载流子离子自旋散射的重要见解，这对电子控制铁磁性至关重要。初步的法拉第旋转实验将在GaAs和半磁半导体样品上进行，以提供与其他小组实验结果的联系。在此资助期间，将在n掺杂GaAs上进行自旋相干和拉曼自旋回波实验。对于半磁性半导体，将进行光相干和自旋相干实验。

This proposal was received in response to the Spin Electronics for the 21st century initiative, Program Solicitation NSF 02-036. The proposal focuses on novel optical techniques for measuring carrier spin coherence on semiconductors. Spin based electronics promise to provide increased processing speed, increased integration density and decreased power consumption as compared to traditional microelectronic devices. In addition, electronic spins in semiconductors are attractive for implementation of quantum information processing. These advances require a firm understanding of electronic spin coherence in semiconductors and the development of techniques to probe spin coherence. Previous work has shown that n-doped GaAs has a remarkably long spin coherence time, which has prompted further investigation. Semimagnetic semiconductors display much shorter spin coherences, however the stronger carrier-ion interaction makes them better candidate materials for fabrication of spin filters and related devices.The techniques developed in this project will complement the predominant technique of Faraday rotation and provide information that is not available using Faraday rotation. Of specific interest is why the Faraday rotation signal saturates for high optical excitation density. The techniques are based on transient four-wave-mixing. Specifically, by using a three pulse excitation scheme, it is possible to probe the spin coherence and generate a signal in a background free direction. A modification of this that uses the fifth order nonlinear optical response can generate a Raman-spin echo, which will remove the effects of inhomogeneous spin precession rates due, for example, to a variation in the g-factor with k-vector. The experiments will be performed on n-doped GaAs and semimagnetic semiconductors such as CdMnTe and ZnMnTe. The former samples are commercially available wafers, while the latter will be provided by collaborators at the University of Dortmund, Germany. In the latter case, initial experiments to probe the optical coherence (as opposed to spin coherence) will be performed because it is not well understood and provides important insight into carrier-ion spin scattering, which is crucial for electronically controlled ferromagnetism. Preliminary Faraday-rotation experiments will be performed on both the GaAs and semimagnetic semiconductor samples to provide a connection to experimental results from other groups. During this grant, both spin-coherence and Raman-spin echo experiments will be performed on n-doped GaAs. For the semimagnetic semiconductors, optical coherence and spin coherence experiments will be performed.