The recently introduced concept of quasi-bound states in the continuum (quasi-BIC) enables the observation of high Q-factor resonances, which is crucial to enhance the intensity of light-matter interaction. However, the resonance wavelength is fixed by the geometrical design, which may be a limitation for certain practical applications. To address this challenge, the combination of symmetry-broken all dielectric metasurfaces, designed to support quasi-BIC states, along with phase change materials, has been proposed.

Bound states in the continuum (BICs) are states that remain localized within the continuum of radiating modes, with infinite radiative Q-factors due to their lack of coupling to incident radiation. They can be classified into symmetry-protected BICs, which arise from incompatible spatial and radiative wave symmetries, and accidental BICs, which result from continuous parameter tuning. However, achieving infinite Q-factors experimentally is hindered by factors such as sample size, parasitic scattering, absorption, or structural disorder, leading to the transformation of BICs into leaky modes known as quasi-BICs in realistic scenarios, characterized by finite, but still high Q-factors. In this study by Wang-Ze Lv et al,. a hybrid dielectric metasurface is presented, comprising unit cells consisting of two Si-VO₂ composite nanobricks. The metasurface demonstrates various types of BICs originating from different multipole interactions with vanadium dioxide in its insulating phase. By breaking the symmetry of the metasurface, BICs are transformed into quasi-BICs. Furthermore, the metasurface also supports two toroidal dipole modes with dipole moments aligned in different directions. By considering the phase transition of VO₂ from insulating to metallic, the quasi-BIC and toroidal dipole modes can be turned off. These results hold promise for applications in optical modulators, tunable harmonic generation, and biosensors.

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