  As is well known the absorption of circularly polarized light in
semiconductors results in optical orientation of electron spins and
helicity-dependent electric photocurrent, and the absorption of linearly
polarized light is accompanied by optical alignment of electron momenta. Here
we show that the absorption of unpolarized light leads to generation of a pure
spin current, although both the average electron spin and electric current
vanish. We demonstrate this for direct interband and intersubband as well as
indirect intraband (Drude-like) optical transitions in semiconductor quantum
wells (QWs).
