  Spin orbit interaction and the resulting Spin Hall effect of light are under
recent intensive investigations because of their fundamental nature and
potential applications. Here, we report an extraordinary spin specific beam
shift of light and demonstrate its tunability in an inhomogeneous anisotropic
medium exhibiting spatially varying retardance level. The spin specificity
(shift occurs only for one circular polarization mode, keeping the other
orthogonal mode unaffected) is shown to arise due to the combined spatial
gradients of the geometric phase and the dynamical phase of light. The
constituent two orthogonal circular polarization modes of an input linearly
polarized light evolve in different trajectories, eventually manifesting as a
large and tunable spin separation. The spin specificity of the beam shift and
the demonstrated principle of simultaneously tailoring space-varying geometric
and dynamical phase of light for achieving its tunability (of both magnitude
and direction), may provide an attractive route towards development of
spin-optical devices.
