Chiral Bound States in the Continuum in Plasmonic Metasurfaces

Bound states in the continuum (BICs) offer novel mechanisms to boost the quality factor (Q‐factor) of resonances. Unfortunately, current studies on chiral BICs metasurfaces suffer from a fundamental trade‐off between Q‐factor and circular dichroism (CD), presenting a significant hurdle that severely limits the independent control between CD and Q‐factors. Here, 3D plasmonic metasurfaces are numerically demonstrated that overcome the trade‐off and offer high‐Q quasi‐BIC resonances (Q ≈ 938) with strong CD (≈0.67) in the mid‐infrared. These metasurfaces are made of integrated‐resonance units consisting of a twisted vertical split‐ring resonator (VSRR) and a wall. Importantly, this dissimilar dimer configuration unlocks a new degree of freedom to decouple the Q‐factor and CD, that is, the Q‐factor and CD can be relatively independently manipulated by the height of the wall and the twisted angle of the VSRR, respectively. These results provide novel paradigms to manipulate advanced chiroptical responses, with various applications that require strong CD with enhanced light–matter interaction. Bound states in the continuum (BICs) enable sharp resonances. However, the independent control of circular dichroism (CD) and quality‐factor (Q‐factor) of quasi‐BIC resonances is challenging due to their strong correlation. Here, by exploiting plasmonic metasurfaces with asymmetric integrated‐resonance units, this challenge was addressed and novel mechanisms were revealed for at‐will manipulation of CD in high Q‐factor chiral quasi‐BIC resonances.