Meta-Atom

A meta-atom is the fundamental unit cell of a metasurface or metalens. It is a nanoscale structure placed on a surface with dimensions smaller than the wavelength of light, so the structure can influence the electromagnetic field without producing strong diffraction into higher orders. In a metalens, the meta-atom is the element that converts a wavefront from the incident form into the desired outgoing form.

The key concept is that the light-matter interaction is determined by the geometry of the meta-atom: its height, width, shape, spacing, and material. By changing those parameters, one changes the local scattering response. This gives each meta-atom the ability to impose a specific phase delay, amplitude transmission, and sometimes polarization conversion. A metalens is therefore a spatially varying arrangement of these local optical responses.

Typical meta-atoms include cylindrical dielectric nanopillars, rectangular blocks, rings, or more complex resonant structures. In dielectric designs, the nanostructure often acts as a waveguide-like element that supports a tuned propagation phase. In resonant designs, the meta-atom may leverage Mie or Fabry–Pérot resonances to achieve a rapid phase shift over a small geometric range.

The design of a metalens can therefore be seen as a two-step process:

Choose a target phase profile for the desired optical function.
Encode that phase profile using a set of meta-atoms with distinct optical responses.

This makes meta-atoms the physical basis for flat optics: they replace the curved surface of a lens with a discrete set of nanostructures that collectively produce the same wavefront shaping.

Key parameters

Subwavelength dimensions to avoid diffraction orders.
Geometry-dependent phase response.
Tunable amplitude and polarization behavior.
High-index dielectric or metallic materials for strong field control.

A meta-atom is the microscopic optical building block that turns a flat surface into a lens.

References

[1] Khorasaninejad, M., Chen, W. T., Devlin, R. C., Oh, J., Zhu, A. Y., & Capasso, F. (2016). Metalenses at visible wavelengths: diffraction-limited focusing and subwavelength resolution imaging. Science, 352(6290), 1190–1194. https://www.science.org/doi/10.1126/science.aaf1110

[2] Hsiao, H. H., Chu, C. H., & Tsai, D. P. (2017). Fundamentals and applications of metasurfaces. Reports on Progress in Physics, 80(6), 066401. https://iopscience.iop.org/article/10.1088/1361-6633/aa5f8f

[3] Wikipedia contributors. “Metasurface.” https://en.wikipedia.org/wiki/Metasurface