Ultrafast Optical Measurements of Spin Polarization in Semiconductors
While both spin dynamics and electron orbital motion have been separately studied for many decades, we are just beginning to probe and understand the intricate phenomena that may be expected in the presence of spin-momentum coupling. For example, it is still a mystery what the microscopic mechanism is that produces a bulk electron spin polarization in semiconductors in response to an applied accelerating electric field and how the electrical spin generation rate depends on material parameters. Existing theories predict that the magnitude of this electrically-generated spin polarization should be proportional to the magnitude of the spin-orbit splitting, but our measurements have shown the opposite trend . Another example exists in monolayer transition metal dichalcogenides, where a lack of spatial inversion symmetry leads to a coupling between the spin and valley degrees of freedom. Our measurements of tungsten diselenide reveal a long valley polarization decay time that is unaffected by transverse applied magnetic fields (up to 0.3 T) . I will describe how optical pump-probe spectroscopy enables us to generate and measure electron spin and valley dynamics in semiconductors with sub-picosecond temporal and sub-micron spatial resolution.
 B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, Phys. Rev. Lett. 112, 056601 (2014)
 X. Song, S. Xie, K. Kang, J. Park, and V. Sih, Nano Letters 16, 5010-5014 (2016)