  Although several efforts have been taken to enhance the photonic spin Hall
shift in deep-subwavelength region, according to effective medium theory, the
fundamental confliction between near-zero reflection coefficient and near-zero
incident angle still hinders the further application. Here, we reveal a
fundamental breakdown of effective medium theory due to the existing of
topological singularity in deep-subwavelength region in one-dimensional
photonic crystals. We find that near the topological singularity, huge photonic
spin Hall shift can be achieved for s-polarization and p-polarization. At the
topological singularity, the reflected filed is split as dipole-like
distribution with zero photonic spin Hall shift for both-polarizations, which
is resulted from the interfere of the spin-maintained normal light and
spin-flipped abnormal light. Based on the theoretical research,
dual-polarizations thickness and dielectric constant sensing devices can be
designed in deep-subwavelength region. Further more, by applying more
complicated layered structure, multi-channels dual-polarizations detection and
broadband dual-polarizations huge spin Hall shift platform can be designed.
This work paves the way to exploring the topological properties and
polarization control of photonic crystals and provides a prospective method for
the design of multi-channels sensitive detection spin optical devices.
