  Metasurfaces and photonic crystals have revolutionized classical and quantum
manipulation of light, and opened the door to studying various optical
singularities related to phases and polarization states. However, traditional
nanophotonic devices lack reconfigurability, hindering the dynamic switching
and optimization of optical singularities. This paper delves into the
underexplored concept of tunable bilayer photonic crystals (BPhCs), which offer
rich interlayer coupling effects. Utilizing silicon nitride-based BPhCs, we
demonstrate tunable bidirectional and unidirectional polarization
singularities, along with spatiotemporal phase singularities. Leveraging these
tunable singularities, we achieve dynamic modulation of
bound-state-in-continuum states, unidirectional guided resonances, and both
longitudinal and transverse orbital angular momentum. Our work paves the way
for multidimensional control over polarization and phase, inspiring new
directions in ultrafast optics, optoelectronics, and quantum optics.
