Vertical chiral emission from an intrinsically achiral metasurface enabled with anisotropic continuum.

Manipulating emission polarization is crucial for advanced applications such as polarized fluorescence and lasing, thermal radiation control, stereochemistry and spintronics. Existing approaches for generating circularly polarized emission primarily rely on nanostructures with true chirality, where the far-field polarization at the Γ point is believed to require out-of-plane symmetry breaking. This raises a question: can an intrinsically achiral metasurface that preserves out-of-plane symmetry generate chiral emission? Here, we introduce the concept of anisotropic continuum, defined as a continuum state exhibiting distinct responses for orthogonal polarizations of incidence, as a new degree of freedom for intrinsically achiral metasurfaces. In such metasurfaces, chiral emission arises from manipulating the anisotropic continuum together with the metasurface's in-plane perturbations. Degrees of circular polarization of 0.83 for upward emission and -0.9 for downward emission are achieved experimentally by integrating the fabricated silicon metasurface with fluorescent organic dyes. Our findings not only address a long-standing gap in the understanding of structural chirality, but also open new opportunities for high-performance applications in polarized fluorescence and thermal emission.