Deciphering light transformation in chiral metasurface in real space and time by ultrafast electron microscopy.

Optical activity in chiral structures, i.e., circular dichroism (CD), has led to significant advances in nanoscale optical manipulation, including chiral metasurfaces, helicoid crystals, and chiral macromolecules. Although the local geometric design of chiral structures fundamentally governs their optical responses, the microscopic origin of CD remains unresolved due to the inability to probe optical chirality generation and local geometry effects with sufficient spatiotemporal resolution. Here, we unveil the light transformation process in a Γ-shaped chiral metasurface by combining far-field ellipticity measurements with direct near-field imaging at nanometer-femtosecond scale using photon-induced near-field electron microscopy (PINEM). By decomposing the near-field distributions into local symmetric and asymmetric components, we define a near-field ellipticity that quantitatively follows the wavelength-dependent far-field ellipticity. Finite-element simulations reveal that an electric dipole at the top-right corner of the Γ-shaped meta-atom dominates the ellipticity, which increases as the dipole contribution grows with wavelength. Crucially, time-resolved PINEM reveals that asymmetric near-fields dissipate faster than the symmetric counterparts by tens to hundreds of femtoseconds, indicating chiral-geometry-dependent energy dissipation pathways. This work provides microscopic insight into light transformation in chiral structures and lays the foundation for advanced chiral photonic device design.