  We performed high-resolution spin- and angle-resolved photoemission
spectroscopy studies of the electronic structure and the spin texture on the
surface of Bi$_2$Se$_3$, a model topological insulator. By tuning the photon
energy, we found that the topological surface state is well separated from the
bulk states in the vicinity of $k_z=Z$ plane of the bulk Brillouin zone. The
spin-resolved measurements in that region indicate a very high degree of spin
polarization of the surface state, $\sim 0.75$, much higher than previously
reported. Our results demonstrate that the topological surface state on
Bi$_2$Se$_3$ is highly spin polarized and that the dominant factors limiting
the polarization are mainly extrinsic.
