Optically resolving an object in air to as high a resolution as possible (without resorting to superresolution tricks, of which there are many) using light at a given wavelength means pushing the numerical aperture (NA) of the imaging lens to as close to 1 as possible. With high-NA bulk optics, each incremental boost to NA complicates and enlarges the optical design of the imaging lens. (One way to further boost NA in bulk optics is to use a fluid rather than air as the imaging medium and design the lens accordingly; this approach is used in photolithographic optics to create NAs of 1.3 or slightly higher). Flat metamaterial lenses have the advantage of not getting bulkier as their NA goes up—instead, the metamaterial surface pattern gets more complicated. The highest previously reported NA for a metamaterial lens was <0.9.
Now, researchers at the Data Storage Institute (Agency for Science, Technology and Research, A*STAR) and the Photonics Institute, School of Electrical and Electronic Engineering, Nanyang Technological University (both in Singapore) have created a flat metamaterial lens with a thickness of a third of a wavelength and an NA of 0.99 at a 715 nm wavelength. Such an NA corresponds to a collection angle of 82°. According to the researchers, this collection angle is higher than any previously reported value for flat and bulk optics (in air). Made of silicon nanodisks placed on a fused silica substrate and surrounded by air, and fabricated via standard electron-beam lithographic techniques, the flat lens uses the disks as dielectric nanoantennas with asymmetric scattering patterns to redistribute diffracted energy. The lens focused only about 10% of the incident power, but the researchers believe that the total efficiency could be boosted to about 37% by optimizing the fabrication process.
The researchers used the lens in an experimental setup in which a lone subwavelength scatterer was placed on a prism and excited by the evanescent field from total internal reflection. Showing a filled NA, this experiment more than anything confirmed the lens NA of 0.99 (as the diamond was a true subdiffractive emitter). The lens was also tried out in a confocal configuration that mapped the emission from color centers in a subdiffractive diamond nanocrystal. Reference: R. Paniagua-Domínguez et al., Nano Lett. (2018); doi:10.1021/acs.nanolett.8b00368.