Similar to light polarization that is selected by a superposition of the optical basis, the electron spin direction can be controlled through a superposition of the spin basis. We investigate such a spin interference occurring in the photoemission of a spin-orbit coupled surface state in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by using spin- and angle-resolved photoemission spectroscopy combined with a 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 spin-polarized electron source permitting us to optically control the pure spin state pointing to an arbitrary direction.

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• Electronic structure
• Photoemission
• Spin-orbit coupling
• Surface states
• Spin-resolved photoemission spectroscopy