  Understanding the spin-texture behavior of boundary modes in ultrathin
topological insulator films is critically essential for the design and
fabrication of functional nano-devices. Here by using spin-resolved
photoemission spectroscopy with p-polarized light in topological insulator
Bi2Se3 thin films, we report tunneling-dependent evolution of spin
configuration in topological insulator thin films across the metal-to-insulator
transition. We observe strongly binding energy- and wavevector-dependent spin
polarization for the topological surface electrons in the ultra-thin
gapped-Dirac-cone limit. The polarization decreases significantly with enhanced
tunneling realized systematically in thin insulating films, whereas magnitude
of the polarization saturates to the bulk limit faster at larger wavevectors in
thicker metallic films. We present a theoretical model which captures this
delicate relationship between quantum tunneling and Fermi surface spin
polarization. Our high-resolution spin-based spectroscopic results suggest that
the polarization current can be tuned to zero in thin insulating films forming
the basis for a future spin-switch nano-device.
