Transport of spin coherence in semiconductors

This thesis describes an optics-based approach to the study of electron spins in semiconductors. Spin transport between two semiconductors of widely different band gaps, doping levels, and g-factors is studied using two-color pump-probe optical spectroscopy. Electron spin polarization can be selectively excited in a GaAs substrate and subsequently measured in an adjacent n-ZnSe epilayer as spins cross the interface.; Even though the band gap of ZnSe is nearly twice that of GaAs and no electrical bias is applied, between 2–10% of the spin polarization in GaAs is spontaneously transferred into ZnSe. Temporal distributions of spin arrival, along with a discontinuity of g-factors produce a new type of inhomogeneous spin dephasing at the interface. The data show the preservation of spin information by transport to regions of low spin decoherence, and demonstrate that regional boundaries can be used to control the resulting spin coherent phase.; Additionally, an applied electric field can be used to transport optically generated coherent electron spins from an n-GaAs substrate to an n-ZnSe epilayer. As the spin lifetimes in n-GaAs are an order of magnitude longer than those in the n-ZnSe epilayers, biasing allows access to a regime where the substrate acts as an electronic spin reservoir. Resonant spin amplification and time-resolved Kerr rotation measurements show that spins can be continuously extracted from the reservoir using electric fields. The sourcing of spin polarization from the reservoir can be maintained for the duration of the reservoir lifetime, and the transfer efficiency can be increased almost 500%, resulting in an almost 85% spin transfer. Furthermore, built-in biases across p-n junctions can be used to increase the interface spin transfer by a factor of 40.; Finally, spin polarization of photoexcited electrons is seen in a GaAs epilayer due to the proximity of a MnAs ferromagnetic layer. The spin polarization follows the hysteretic behavior of MnAs, and can be initialized in any direction by changing the magnetization orientation. This proximity polarization occurs within ∼10–50 ps after which the spins precess coherently about an applied transverse magnetic field.