Strong coupling of double excitons with guided mode resonances and quasi-bound states in the continuum in heterogeneous metamaterials

Heterogeneous metamaterials containing excitonic materials provide an ideal platform for strong exciton-photon coupling. In this Letter, we theoretically demonstrate four strong couplings in a heterogeneous metamaterial consisting of a TiO₂ grating standing on a perovskite-WS₂-perovskite waveguide layer by tuning the structural sizes. The quasi-bound state in the continuum (qBIC) and the guided mode resonance (GMR) both strongly coupled with the excitons of both perovskite and WS₂ under oblique incident illumination, resulting in four large Rabi splittings of 177.32, 187.53, 406.25, and 435.09 meV via a reasonable combination of oscillator strengths of perovskite and WS₂. Double strong coupling behaviors are also achieved when the grating period equals 222 nm with an incident light angle of 19.3°. Moreover, double ultrastrong coupling can even be realized by the GMR and qBIC respectively interacting with the exciton of WS₂ when its oscillator strength reaches a certain value. Our work paves an effective avenue to realizing strong coupling and even ultrastrong coupling between multiple excitons and multiple optical modes.