Optical injection of spin current in direct bandgap group IV semiconductors

Ge_1-x Sn_x is an emerging direct bandgap semiconductor that can be monolithically grown on silicon wafers, thus enabling a variety of monolithic photonic and optoelectronic devices. Here we exploit recent progress in group IV bandgap engineering to investigate the optical processes of spin current injection in a silicon-compatible platform. Understanding these processes is a critical step towards coherent optical photon-to-spin conversion, which is a long-sought-after strategy for surmounting current fundamental limits in optical schemes that hinder the long-distance distribution of entanglement. To this end, charge carrier, spin, current, and spin current injection in relaxed and strained Ge_1-x Sn_x alloy are calculated. A full Brillouin zone, 30 band kp model is used to extract the band structure necessary for the evaluation of these injection rates. In the independent particle approximation, carrier injection and optical orientation for one- and two-photon absorption are determined, and for the two-photon absorption, properties such as the anisotropy and linear-circular dichroism are presented. Interference terms related to the coherent control of carrier, spin populations, and charge, spin currents are evaluated for a bichromatic field of frequencies ω and 2ω.