  We observe the inverse spin Hall effect in a two-dimensional electron gas
confined in AlGaAs/InGaAs quantum wells. Specifically, we find that an
inhomogeneous spin density induced by the optical injection gives rise an
electric current transverse to both the spin polarization and its gradient. The
spin Hall conductivity can be inferred from such a measurement through the
Einstein relation and the Onsager relation, and is found to have the order of
magnitude of $0.5(e^{2}/h)$. The observation is made at the room temperature
and in samples with macroscopic sizes, suggesting that the inverse spin Hall
effect is a robust macroscopic transport phenomenon.
