  We have found that surface superstructures made of "monolayer alloys" of Tl
and Pb on Si(111), having giant Rashba effect, produce non-reciprocal
spin-polarized photocurrent via circular photogalvanic effect (CPGE) by
obliquely shining circularly polarized near-infrared (IR) light. CPGE is here
caused by injection of in-plane spin into spin-split surface-state bands, which
is observed only on Tl-Pb alloy layers, but not on single-element Tl nor Pb
layers. In the Tl-Pb monolayer alloys, despite their monatomic thickness, the
magnitude of CPGE is comparable to or even larger than the cases of many other
spin-split thin-film materials. The data analysis has provided the relative
permittivity $\epsilon^{\ast}$ of the monolayer alloys to be $\sim$ 1.0, which
is because the monolayer exists at a transition region between the vacuum and
the substrate. The present result opens the possibility that we can optically
manipulate spins of electrons even on monolayer materials.
