The superstructure surface provides a new idea for the design of diffractive optical elements. In 2016, the Harvard University Capasso team developed a flat lens in visible light based on hyper surface. The results were included in one of the top ten breakthroughs in science magazine. However, the lens can only focus on one color of light at the same time.

In January 2017, Nano Letters online reported that the team further research achievements. The report shows an achromatic super lens (AML) that can be used in the visible wavelength range, which is simple to manufacture, requires only one step of photolithography, na=0.2. There is a constant focal length in a continuous band of 490 nm~550 nm. The flat lens is composed of titanium dioxide nano columns with square cross section arranged on a metal mirror. And a silicon dioxide layer is sandwiched between the nano column and the metal mirror surface. In order to prove the validity and the customization of this method, the researchers also designed a super lens with reverse dispersion. The focal length of the lens increases with the increase of the wavelength.

The method can be used to develop devices with specific requirements for dispersion. Thus, it will play an unprecedented role in the applications of LED lighting, fluorescence and photoluminescence spectrum imaging.

Nano Lett, 2017, 17(3): 1819-1824.

DOI: 10.1021/acs.nanolett.6b05137.