Optical measurements of electron and nuclear spin polarization in semiconductors

In this talk, I will give an overview of spin-based, or spintronic, devices and describe optical methods to probe spin dynamics and spin-orbit effects. Current-induced spin polarization is a phenomenon in which carrier spins are oriented when subjected to current flow.  This effect is of interest as an all-electrical method of generating electron spin polarization that does not require magnetic materials or applied magnetic field.  However, the mechanism that produces this spin polarization remains an open question.  Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend has been observed experimentally.  We perform experiments on samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and measure the electrical spin generation efficiency and spin-orbit splitting [1].  Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be larger for momentum directions corresponding to smaller spin-orbit splitting.  In addition, angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation.  Furthermore, we show that this electrically-generated electron spin polarization can be used to produce a nuclear spin hyperpolarization through dynamic nuclear polarization [2].

[1] "Current-Induced Spin Polarization in Anisotropic Spin-Orbit Fields," B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, Phys. Rev. Lett. 112, 056601 (2014).

[2] "Dynamic nuclear polarization from current-induced electron spin polarization," C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, Phys. Rev. B 90, 085122 (2014).

Professor Sih is investigating electron spins and photons in semiconductor nanostructures for potential applications in spin-based devices that combine logic and storage and for quantum and optical information processing in solid-state systems. Ultrafast, spatially-resolved magneto-optical spectroscopy explores electron spin coherence and electrically generating spin polarization in non-magnetic materials through spin-orbit coupling and the spin hall effect. The study of fabrication and optical characterization of prototype photonic devices, such as waveguides, ring resonators and photonic crystals, focuses on developing novel methods for controlling light.