Independently tunable dual quasi-bound states with decoupled eigenfrequencies and Q-factors in an active metasurface

Metasurfaces have become versatile platforms for manipulating the fundamental properties of resonance modes. In particular, some well-designed metasurfaces have enabled independent control of either eigenfrequencies or Q-factors across different resonance modes. However, simultaneously realizing both of these independent tuning parameters in an active metasurface remains a key challenge. Here, we demonstrate an active metasurface that can support independently tunable dual resonances with simultaneously decoupled eigenfrequencies and Q-factors, which consists of a glass plate with two periodic lithium niobate (LN) grating arrays placed on opposite sides. Such dual-resonances are driven by symmetry-protected bound states in the continuum (BICs), whose eigenfrequencies and Q-factors are unaffected by the elements located in zero-eigenfield region. By carefully designing, each grating array is placed in the zero-eigenfield region of one BIC mode. Then, both the eigenfrequencies and the Q-factor of the dual-resonances can be actively and independently tuned through the electro-optic effect of LN. Combining active tuning and guided mode resonance engineering, the Q-factor of the designed metasurface has been increased from 10² to 10⁶. This work provides a powerful framework for expanding the degrees of freedom in metasurface-based resonance control, with the potential to improve the adaptability and performance of applications in multimode lasing, optical filtering, dual-channel sensing, and nonlinear optics.