It is shown that absorption of circularly polarized infrared radiation due to intraband (Drude-like) transitions in n-type bulk semiconductors and due to intra-subband or inter-subband transitions in quantum well (QW) structures results in a monopolar spin orientation of free electrons. Spin polarization in zinc-blende-structure based QWs is demonstrated by the observation of the spin-galvanic and the circular photogalvanic effects.
The monopolar spin orientation in n-type materials is shown to be possible if an admixture of valence band states to the conduction band wave function and the spin-orbit splitting of the valence band are taken into account.