Tunable bound states in the continuum with loss compatibility

Dynamic control of bound states in the continuum (BICs) is usually achieved by engineering structural geometries of lossless optical systems, leading to a passive nature for most current BIC devices. Introducing materials with tunable permittivity, i.e., refractive index and loss, may offer a new degree of freedom in designing reconfigurable BIC metadevices with active functionalities. However, achieving loss-accompanied or loss-driven BIC manipulation while preserving its ultrahigh Q factor is extremely challenging. Here, we report a loss-compatible BIC manipulation mechanism based on far-field interference in a mirror-assisted photonic crystal slab, wherein the loss of tunable material not only harmoniously coexists with ultrahigh Q factor, but also serves as a pivotal joystick of BIC dynamics in momentum space. By modulating loss and refractive index of tunable material through the amorphous-crystalline phase transition, simulation results show the active switching of topological charge for BICs, as well as the multidimensional control of chiroptical effect for quasi-BICs, including steerable response/emission direction and chirality continuum with far-field ellipticity ranging from -0.944 to +0.943. Our findings suggest a distinct route to construct BIC metadevices with active functionalities and foster deeper exploration of intrinsic loss applications within the ultrahigh-Q photonic system.