  Similar to light polarization that is selected by a superposition of optical
basis, electron spin direction can be controlled through a superposition of
spin basis. We investigate such a spin interference occurring in photoemission
of the spin-orbit coupled surface state in Bi2Se3 by using spin- and
angle-resolved photoemission spectroscopy combined with laser light source
(laser-SARPES). Our laser-SARPES with three-dimensional spin detection and
tunable laser polarization including elliptical and circular polarization
enables us to directly visualize how the direction of the fully-polarized
photoelectron spin changes according to the optical phase and orientation of
the incident laser polarization. By this advantage of our laser-SARPES, we
demonstrate that such optical information can be projected to the
three-dimensional spin vector of the photoelectrons. Our results, therefore,
present a novel spin-polarized electron source permitting us to optically
control the pure spin state pointing to the arbitrary direction.
