Coherence control of electron spin currents in semiconductors

Abstract

AbstractWe provide an overview of some of our recent work on the use of one color and two color optical techniques to generate and control electronic spin currents in semiconductors for which a spin–orbit interaction exists. The generation process relies on the quantum interference between different absorption pathways, such as that between single and two photon absorption or those involving different polarization states of a monochromatic beam. For different crystal orientations and/or beam polarizations it is possible to generate a spin current with or without an electric current, and an electrical current with or without a spin current. In our experiments, which are conducted either at 80 K or 295 K, we typically employ nominally 100 fs pulses centered near 1500 and 750 nm. The currents generated are quasi‐ballistic and the carriers typically move distances of ∼1–10 nm, determined by the momentum relaxation time, which is of the order of 100 fs. The transient characteristics of spin‐polarized electrical currents generated in strained GaAs at room temperature by ∼100 fs pulses is detected by the emitted THz radiation. Pure spin currents can be detected by taking advantage of the accumulation of up and down spins on opposite sides of tightly focused pump beams. The spin states are detected through differential transmission measurements of tightly focused right and left circularly polarized, near‐band‐edge probe pulses, delayed by several picoseconds from the pump pulses to allow carrier thermalization to occur. By spatial scanning across the differential spin profiles and determining the amplitude of the response we are able to translate this into nm spatial resolution of spin displacement. Finally, the ability to generate ballistic currents using purely optical techniques allows us to generate transverse Hall‐like currents, with transverse charge currents generated from pure spin currents and transverse spin currents generated from pure charge currents. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)