Nicole Wu

Major: Electrical Engineering 

Mentors: Roark Chao, Jon Schuller

A Transmissive Metalens That Uses Dielectric Nanopillars Forms Adequate Focal Spots But Suffers From Transmission Limitations

A metasurface is a flat optical device that manipulates light. A metalens is a type of metasurface that focuses light. The surface of a metalens is covered with nanostructures called meta-atoms. Meta-atoms can be any shape, but they are traditionally cylindrical. While cylindrical meta atoms are easy to fabricate, it remains unclear if this simple geometry can simultaneously achieve high transmission, 2π phase coverage, and effectively focus light to a sharp spot. We used the Ansys Lumerical finite-difference time-domain (FDTD) simulation software to design a cylindrical meta-atom and spherical metalens. For a fixed simulation period, the cylindrical pillar successfully reached 2π phase coverage across a range of varied radii. Yet, transmission varied from a maximum of 80% to dropping to nearly zero. With a lens radius of 20 micrometers, the metalens formed a slightly elongated, bright focal spot at the target focal length of 100 micrometers. The resulting low, inconsistent transmission suggests most of the light wave is not passing through the meta-atom. By achieving 2π phase coverage, the meta atoms can be arranged on the metalens so that parts of the incident plane wave are shifted by the amount needed to converge to a focal point. However, the focal point the metalens produced could have been tighter and more circular. For future work, one possible strategy to overcome the limitations of cylindrical meta-atoms is to break symmetry in the meta-atom design to get higher, uniform transmission and improve overall metalens performance.