  Spintronic devices usually rely on long spin relaxation times and/or lengths
for optimum performance. Therefore, the ability to modulate these quantities
with an external agent offers unique possibilities. The dominant spin
relaxation mechanism in most technologically important semiconductors is the
D'yakonov-Perel' (DP) mechanism which vanishes if the spin carriers (electrons)
are confined to a single conduction subband in a quantum wire grown in certain
crystallographic directions, or polycrystalline quantum wires. Here, we report
modulating the DP spin relaxation rate (and hence the spin relaxation length)
in self assembled 50-nm diameter InSb nanowires with infrared light at room
temperature. In the dark, almost all the electrons in the nanowires are in the
lowest conduction subband at room temperature, resulting in near-complete
absence of DP relaxation. This allows observation of spin-sensitive effects in
the magnetoresistance. Under infrared illumination, electrons are photoexcited
to higher subbands and the DP spin relaxation mechanism is revived, leading to
a three-fold decrease in the spin relaxation length. Consequently, the spin
sensitive effects are no longer observable under illumination. This phenomenon
may have applications in spintronic room-temperature infrared photodetection.
