The discovery of topological insulators has opened a new chapter of condensed matter physics. Recently, several experiments have confirmed that the magnetoelectric susceptibility of a topological insulator with time-reversal symmetry is quantized in terms of the fine-structure constant. To realize such measurements, a number of challenges have been overcome; particularly, high-precision time-domain terahertz polarimetry is devised. Here an experiment is proposed to observe the topological magnetoelectric effect. Based on the generalized multisphere Mie theory, it is shown that the optical activity of a cluster of topological insulator spheres elevates as metal spheres are deliberately brought near to them: The destructive interference of fields of the induced electric dipoles allows fields of the induced magnetic dipoles, which originate from the topological magnetoelectric effect, to play a prominent role. For example, for clusters of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and $\mathrm{InSb}$ spheres, the elevated azimuth and ellipticity angles are about $\pm {20}^{\circ }$. Such a giant optical activity serves as an optical bridge to the determination of the fine-structure constant.

• Received 12 May 2021
• Revised 24 January 2022
• Accepted 7 April 2022

DOI:https://doi.org/10.1103/PhysRevB.105.165139