  Manipulating radiation asymmetry of photonic structures is of particular
interest in many photonic applications such as directional optical antenna,
high efficiency on-chip lasers, and coherent light control. Here, we proposed a
term of pseudo-polarization to reveal topological nature of radiation asymmetry
in bilayer metagratings. Robust pseudo-polarization vortex with an integer
topological charge exists in P-symmetry metagrating, allowing for tunable
directionality ranging from -1 to 1 in synthetic parameter space. When
P-symmetry-breaking, such vortex becomes pairs of C points due to the
conservation law of charge, leading to the phase difference of radiation
asymmetry from {\pi}/2 to 3{\pi}/2. Furthermore, topologically enabled coherent
perfect absorption is robust with customized phase difference at will between
two counter-propagating external light sources. This work can not only enrich
the understanding of two particular topological photonic behavriors, i.e.,
bound state in the continuum and unidirectional guided resonance, but also
provide a topological view on radiation asymmetry, opening an unexplored avenue
for asymmetric light manipulation in on-chip laser, light-light switch and
quantum emitters.
