All-Optical Switching of Photon Helicity at Direct-gap Transition in Germanium

Germanium is a prominent candidate for the development of semiconductor spintronics. It is compatible with Silicon microelectronic processing and it has quasi-direct behavior. These latter properties result in direct-gap transitions wich leads to efficient angular momentum transfer from circularly polarized light to the carriers. Germanium has thus the potential of merging spintronics and photonics in the so-called spin-optoelectronics field. We investigate the state of the polarization of the direct-gap photoluminescence (PL) emission in bulk Ge and we provide also an overview of the processes governing the recombination of carriers in Γ valley. It will then be shown how the polarization of excited carriers can be controlled simply by tuning the excitation power density. In this study we achieve the control over the helicity of emitted photons across the direct-gap without the need of any external magnetic field or optical retarder. Our result provides a step forward the implementation of novel polarized light sources, such as Ge-based spin LEDs and spin lasers monolithically integrated on Silicon.