  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.
