Independent tuning of circular dichroism and Q-factor via Quasi-BIC resonances in bilayer plasmonic structures

Bound states in the continuum (BICs) provides an effective mechanism for realizing high-quality-factor (high Q-factor) optical resonances by suppressing radiative losses. However, there is a critical challenge in simultaneously achieving a high Q-factor and strong field enhancement for modulating circular dichroism (CD) in metallic systems. In this work, we propose a bilayer plasmonic structure composed of rectangular metallic nanoapertures and nanorods, which enables strong CD responses while preserving a Q-factor on the order of 10². We demonstrate that the generation of CD arises from distinct localized resonant interactions between the perforated upper metallic layer and the dipolar resonances supported by the lower nanorods. Notably, both the Q-factor and CD can be simultaneously tuned by adjusting the interlayer separation, revealing a clear positive correlation between them. The CD can be effectively tuned by geometric parameters such as rotational angle, nanorod length, and width, while keeping the Q-factor nearly unchanged. By adjusting the length of the upper-layer holes, the Q-factor can be modulated without altering the CD response. This study presents a robust strategy for achieving independent control of CD and Q-factor in plasmonic metasurfaces via resonant coupling between structurally engineered layers.