Strong coupling regime of a quasi-bound state in a continuum in a plasmonic nanohole array with broken symmetry

The strong-coupling regime of light-matter interaction is a peculiar situation in which a photonic-like and a matter-like excitation are coupled to form mixed quasiparticles, which share the properties of their constituents. In this work, we report the prediction of chiral optical response in the strong coupling regime, produced by a plasmonic bound state in the continuum (BIC) coupled with an active medium. We consider a gold metasurface with oval nanoholes, which supports a quasi-BIC upon symmetry breaking. We introduce an active medium characterized by the Lorentz model, which allows tuning of the oscillator strength to achieve strong coupling between the quasi-BIC mode and the active medium's resonances. These strongly coupled modes - which we name plasmonic polariton BICs - become chiral at a finite angle of incidence, with nearly maximum circular dichroism (CD) in absorption and transmission. This new type of chiral plasmonic polariton BICs opens up a new approach for studying chiral phenomena in the strong coupling regime of light-matter interaction. This can be achieved by properly designed metasurfaces infused with colloidal quantum dots tuned to resonate with the quasi-BIC.