  Optical spin orientation and depolarization phenomena in semiconductors are
of overwhelming importance for the development of spin-optoelectronics. In this
paper we employ Ge-based spin-photodiodes to investigate the room temperature
spectral dependence of optical spin orientation in Germanium, in the range
400-1550 nm, and the photo-carrier spin relaxation phenomena. Apart from the
maximum in the spin polarization of photo-carriers for photon energy resonant
with the direct gap (1550 nm), we experimentally demonstrate the presence of a
second sizable peak at 530 nm due to photo-generation far away from the center
of the Brillouin zone, within the L valleys. Furthermore, our data indicate
that the equivalent the spin lifetime of holes in Ge is in the order of 5-10
ps, meaning that the spin diffusion length for holes at room temperature is
larger than expected, in the order of 150-220 nm.
