Perfect narrowband circular dichroism based on intrinsic chiral dual quasi-bound states in the continuum in terahertz metasurfaces

•The degeneracy of the two quasi-BICs provides selectivity for different bandwidth transmission of circularly polarized waves.•Both BICs of the structure exhibit CD at the same time.•The free choice of mode eigenstate is realized.•In the study of narrowband chirality, the Q-factor reaches 6822 and the CD reaches 0.99. Circular dichroism (CD) of chiral structures has wide applications in ultra-sensitive biosensing, wavefront manipulation, and other fields. However, 3D chiral structures require a complex fabrication process, and 2D non-intrinsic chiral structures are difficult to achieve narrow-band perfect CD. Here we proposed a tilted cylindrical structure of a terahertz intrinsic chiral metasurface. Through energy band calculation, topological analysis, multipole decomposition and other research methods, we have realized the separation and merging of topological charges, the degeneracy of modes with opposite polarizations, the free selection of mode eigenstates, and the perfect CD with ultra-high quality (Q) factor. In the study of narrowband chirality, the Q-factor reaches 6822 and the CD reaches 0.99. In addition, both bound states in the continuum (BICs) of the structure exhibit CD at the same time, with Q-factors of 862 and 2673 and CD of –0.98 and 0.98 for the two BICs, respectively. Our work provides new solutions for the study of terahertz narrowband chirality and spin-selective asymmetric transport.