Flat lenses thinner than a wavelength can be constructed from optical metamaterials or gratings; however, their focusing efficiency and other performance qualities don’t approach those of conventional curved refractive lenses. Researchers at the California Institute of Technology (Caltech; Pasadena, CA) have created a type of metamaterial flat lens that has a focusing efficiency of 82% for light at a 1550 nm wavelength. The so-called “transmitarray” lens can be fabricated using industry-standard techniques for making computer chips.
The subwavelength-thickness, polarization-insensitive transmitarray lens consists of amorphous silicon nanoposts on fused silica. The varying phase delays that are needed to create a lens are achieved by varying the diameter of the posts as a function of their lateral distance from the optical axis. Lenses with diameters of 50 and 175 μm were fabricated, producing focal spots as small as 0.57 wavelengths full-width half-maximum (FWHM). As with most metamaterial or Fresnel-zone-plate lenses, the transmitarray lenses are monochromatic. Although the prototypes are currently expensive to manufacture, the use of nanoimprint lithography or industry-standard photolithography techniques would allow thousands to be made at once at low cost. Such lenses would benefit on-chip optical systems created by cascading multiple diffractive elements. Reference: A. Arbabi et al., Nature Commun., 6 (2015); doi:10.1038/ncomms8069.
