Dark Lines and Polarization Singularities in Bright Metasurface-Enhanced Fluorescence

2D scattering lattices can turn incoherent emitter ensembles into bright, spatially coherent sources, a well-established concept in thermal emission control and solid-state lighting communities. Here, a surprising phenomenon is reported: networks of sharp dark lines and polarization singularities in momentum space, observed alongside the usual bright light extraction features of plasmon antenna lattices optimized for extracting light from fluorophores in a 2D waveguide. Using Fourier microscopy with Stokes polarimetry, supported by reciprocity-based T-matrix calculations, these unexpected features are traced to destructive far-field interference between transverse electric (TE) and transverse magnetic (TM) waveguide modes, diffractively coupled through the meta-atoms. Remarkably, these modes generate strong elliptical far-field polarization signatures, with cancellations leading to polarization singularities such as L-lines, V-points, and C-points, analogous to those observed in the incoherent sunlight field in the sky.