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

Chiral light-matter interactions underpin many emerging technologies in sensing, quantum optics, and polarization control. Artificial chiral metasurfaces, which are patterned at nanoscale, can generate very strong circular dichroism and convert linearly polarized light into elliptically or circularly polarized light. However, conventional optical measurements only reveal the far-field response. They do not show how the local geometry of a single meta-atom shapes the near fields, or how these fields evolve on ultrafast time scales.

 

In a new paper published in Light: Science & Applications, a team of scientists led by Professor Xuewen Fu from Nankai University, together with collaborators from Peking University, Shanghai Tech University and other institutes, have now directly visualized how light is transformed inside a chiral metasurface in both real space and real time. Using a home-built ultrafast electron microscopy and photon-induced near-field electron microscopy (PINEM), they achieved nanometer spatial resolution and femtosecond temporal resolution in imaging the near fields of a single Γ-shaped chiral meta-atom inside the chiral metasurface.

 

The researchers combined PINEM near-field imaging and far-field ellipsometry measurements on the same chiral metasurface. Under horizontally polarized excitation from visible to near-infrared range, they observed that the chiral structure supports two distinct near-field components: a symmetric component aligned with the incident polarization and an asymmetric component that is tilted away from it. In contrast, the non-chiral rectangular metasurface shows only symmetric near fields. This comparison proves that the asymmetric near fields are a genuine signature of the chiral geometry.

 

To quantitatively connect the near-field images with the far-field response, the team introduced a “near-field ellipticity” parameter. By decomposing near-field distribution in the high-resolution PINEM image into local symmetric and asymmetric contributions and integrating their intensities, they defined an effective ellipticity that captures the strength and handedness of the near field. Remarkably, the wavelength dependence of this near-field ellipticity closely follows the measured far-field ellipticity spectrum, establishing a direct link between the nanoscale near-field patterns and the macroscopic polarization control.

 

Finite-element simulations further revealed the geometric origin of this behavior. At longer wavelengths, a localized electric dipole that forms at the top-right corner of the Γ-shaped meta-atom grows in strength and couples to the main mode of the structure. This “corner dipole” breaks the symmetry of the near field and drives the enhancement of the ellipticity. In other words, the small geometric feature at the up-right corner of the Γ-shaped meta-atom plays a dominant role in shaping the overall chiral response.

 

Going beyond static imaging, the team also used time-resolved PINEM imaging to probe the ultrafast dynamics of the chiral near fields. They found that the asymmetric near fields decay significantly faster, by tens to hundreds of femtoseconds, than the symmetric near fields. This reveals distinct energy dissipation pathways that are controlled by the chiral geometry and directly influence how free electrons interact with the optical near fields.

 

The authors believe that their approach opens a new window on chiral nanophotonics. By combining real-space and real-time near-field imaging, far-field polarization measurements, and numerical simulations, their work provides a microscopic picture of how chiral metasurfaces transform light. This framework could guide the design of next-generation chiral photonic devices, including compact polarization converters, enhanced chiral sensors, and platforms for controlling quantum states of light and free electrons at the nanoscale.

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