  Friedrich-Wintgen (FW) interference is an atypical coupling mechanism that
grants loss exchange between leaky resonances in non-Hermitian classical and
quantum systems. Intriguingly, such an mechanism makes it possible for
destructive interference scenario in which a radiating wave becomes a bound
state in the continuum (BIC) by giving away all of its losses. Here we propose
and demonstrate experimentally an original concept to tailor FW-BICs as
polarization singularity at on-demand wavevectors in optical metasurface. As a
proof-of-concept, using hybrid organic-inorganic halide perovskite as active
material, we empower this novel polarization singularity to obtain lasing
emission exhibiting both highly directional emission at oblique angles and
polarization vortex in momentum space. Our results pave the way to steerable
coherent emission with tailored polarization pattern for applications in
optical communication/manipulation in free-space, high-resolution imaging
/focusing and data storage.
