metasurfaces

Metasurfaces are two-dimensional equivalents of metamaterials composed of arrays of subwavelength scatterers arranged on a planar substrate. They are designed to impose spatially varying amplitude, phase, and polarization on incident electromagnetic waves, enabling control of wavefronts with thicknesses far smaller than the wavelength.

Operation relies on local interactions between the incident field and each constituent element, treated as a nanoantenna. By engineering the resonant response and orientation of elements, designers implement phase shifts across the surface, producing wavefront shaping such as refraction, focusing, or steering. Some metasurfaces exploit the geometric (Pancharatnam-Berry) phase through element rotation to control polarization-dependent phase.

Types include dielectric metasurfaces, which use high-index non-metallic resonators to reduce absorption losses, and plasmonic/metallic metasurfaces that rely on metal resonances. Huygens-type metasurfaces use balanced electric and magnetic responses for high efficiency. Dynamic or tunable metasurfaces incorporate materials such as liquid crystals, phase-change materials, or graphene to vary response post-fabrication.

In design, generalized Snell's law describes apparent angle changes due to phase gradients. Metasurfaces offer compact alternatives to conventional optics, enabling flat lenses (metalenses), beam steering, holography, polarization conversion, and compact optical components. They are also explored for microwave, terahertz, and infrared applications.

Fabrication challenges include nanoscale patterning, material losses, and fabrication throughput. Performance metrics include efficiency, bandwidth, and angular tolerance. Ongoing research addresses broadband operation, active tuning, and large-area manufacturing.

The concept emerged in the early 2010s as an extension of metamaterials. Early optical metasurfaces demonstrated phase control with subwavelength elements; since then, the field has expanded across frequency ranges and applications, with ongoing work to integrate metasurfaces into imaging systems, communications, and sensing.