Multi-mode optical chirality extremizations on the incident momentum sphere.

We study the momentum-space evolutions for chiral optical responses of multi-mode resonators scattering plane waves of varying incident directions. It was revealed, in our previous study [Phys. Rev. Lett.126, 253901 (2021)10.1103/PhysRevLett.126.253901], that for single-mode resonators the scattering optical chiralities characterized by circular dichroism (CD) are solely decided by the third Stokes parameter distributions of the quasi-normal mode (QNM) radiations: CD = S3. Here we extend the investigations to multi-mode resonators, and explore numerically the dependence of optical chiralities on incident directions from the perspectives of QNM radiations and their circular polarization singularities. In contrast to the single-mode regime, for multi-mode resonators it is discovered that CDs defined in terms of extinction, scattering and absorption generally are different and cannot reach the ideal values of ±1 throughout the momentum sphere. Though the exact correspondence between CD and S3 does not hold anymore in the multi-mode regime, we demonstrate that the positions of the polarization singularities still serve as an efficient guide for identifying those incident directions where the optical chiralities can be extremized.