Resonance coupling between chiral quasi-BICs and achiral molecular excitons in dielectric metasurface J-aggregate heterostructures

A general model for designing the chirality of exciton-polaritons

Chirality of exciton-polaritons can be tuned by the chirality of photons, excitons, and their coupling strength. In this work, we propose a general analytical model based on coupled harmonic oscillators to describe the chirality of exciton-polaritons. Our model predicts the degree of circular polarization (DCP) of exciton-polaritons, which is determined by the DCPs and weight fractions of the constituent excitons and photons. At the anticrossing point, the DCP of exciton-polaritons is equally contributed from both constituents. Away from the anticrossing point, the DCP of exciton-polaritons relaxes toward the DCP of the dominant constituent, with the relaxation rate decreasing as the coupling strength increases. We validate our model through simulations of strongly coupled topological edge states and excitons, showing good agreement with model predictions. Our model provides a valuable tool for designing the chirality of strong coupling systems and offers a framework for the inverse design of exciton-polaritons with tailored chirality.

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.

Near-unity circular dichroism enabled by toroidal dipole empowered bound states in the continuum for electrical switching and sensing

Near-unity circular dichroism (CD) with high quality (Q)-factors has wide applications in chiral lasers, modulators, detectors, etc. In this work, we firstly suggest a feasible approach to realize near-unity CD (∼0.94) with a high Q-factor (>2 × 104) supported by a toroidal dipole (TD) empowered superchiral quasi-bound states in the continuum (BICs) metasurface. Based on intensity, excellent electrical switching is achieved by adjusting the Fermi energy of the graphene on the metasurface. High refractive index sensitivity (136.2 nm/RIU) and figure of merit (1135 RIU-1) demonstrate its superior chiral sensing detection performance. Moreover, the near-unity CD displays a large robustness to the asymmetry offset. Our work paves a feasible avenue for well-designed superchiral quasi-BIC metasurfaces with high Q-factor near-unity CD for chiral applications in electrically tunable modulators, switches, sensors, etc.