  We propose a novel topological defect called Janus bound states in the
continuum (BICs), featuring asymmetric topological charges in upward and
downward radiation channels. Our approach involves a photonic crystal slab
(PCS) that initially exhibits both out-of-plane and in-plane mirror symmetry,
and this PCS possesses one BIC at the $\Gamma$ point and two BICs off the
$\Gamma$ point. By introducing perturbations that break the out-of-plane mirror
symmetry, the two off-$\Gamma$ BICs decompose into four circularly polarized
states (C points) with identical topological charges. Then, we selectively
manipulate the four C points associated with downward radiation channel to
converge at the at-$\Gamma$ BIC, forming a Janus BIC with Janus topological
charges. By further introducing in-plane mirror symmetry perturbation, we can
bring two of the C points with the same handedness and identical topological
charges for upward radiation to merge into the Janus BIC. This process results
in a Janus chiral BIC which exhibits large intrinsic chirality and an infinite
Q factor. Janus BICs can induce distinct Pancharatnam-Berry phase singularities
in momentum space for different incident channels, providing a new approach to
control optical angular momentum. Janus chiral BICs hold promise in enhancing
direction-dependent and spin-dependent asymmetric light-matter interaction,
opening new pathways for improving chirality-dependent operation for on-chip
devices.
