Multiple beneficial optical device properties scale with the thickness of nanostructures. For example, taller nanostructures afford a greater degree of control over multiple colors of light, expanding the range of chromatic behaviors and possibly leading to large-area achromatic metalenses. However, in conventional meta-optics, which employ free-standing nanopillars or nanofins, increasing the nanostructure height while maintaining subwavelength nanostructure spacing requires high aspect ratio structures (aspect ratio = height to width ratio), which are difficult to fabricate and work with. In this study, we employ ultra-deep via-holes as high aspect ratio nanostructures to focus incident infrared light. These holes are drilled in a 5 µm thick free-standing silicon membrane to produce a 2 mm diameter metalens that focuses 1550 nm light. The holes have an aspect ratio approaching 30:1, which represent the highest aspect ratio nanoholes employed in meta-optics to date, and were fabricated with using conventional semiconductor industry processes, allowing such devices to be manufactured at scale in the future. Holey metasurfaces may also be filled in with active or nonlinear materials, such as liquid crystals, which will open up new vistas of dynamic light generation, modulation, and transformation. 

Read the paper in Nano Letters here and the press release from Harvard SEAS here.