Tailoring directional chiral emission from molecules coupled to extrinsic chiral quasi-bound states in the continuum

We investigate the chiral emission from non-chiral molecules coupled to metasurfaces with a unit cell formed by dimers of detuned and displaced Si nanodisks. The detuning and displacement lead to the formation of narrow modes, known as quasi-bound states in the continuum (Q-BICs), with different electric and magnetic characteristics. The dispersion and character of the modes are explained by using the guided-mode expansion method and finite-element simulations. The coupling between these modes leads to an extrinsic chiral response with large circular dichroism for defined energies and wavevectors. When the lattice constant of the metasurface is changed, the dispersion of the extrinsic chiral Q-BICs can be tuned and the emission properties of a thin film of dye molecules on top of the metasurface are modified. In particular, we observe strongly directional and circularly polarized emission from the achiral dye molecules with a degree of circular polarization reaching 0.8 at the wavelengths defined by the dispersion of the Q-BICs. These results could enable the realization of compact light sources with a large degree of circular polarization for applications in displays, optical recording, or optical communication.