Metasurfaces are artificially constructed sub-wavelength patterned structures containing arrays of tiny elements such as rods and rings that interact with light and other electromagnetic waves in unusual ways. For example, a metasurface can be designed to have a negative refractive index so that it bends light in the opposite direction to normal materials. Such a unique property means that these structures have already been used to make "super-lenses" that are able to focus light to a point smaller than its wavelength, allowing optical microscopes to view much smaller objects than is possible today. They have also been used to make holograms and gratings.

Portable and wearable optics are one of the areas in which metasurfaces might come in most useful, but such technologies will require ever more sophisticated spectroscopy techniques that can measure the optical properties of such devices. Conventional spectrometers cannot be used to make such measurements since a relatively long propagation distance is needed to separate out different wavelengths of light at sufficiently high spectral resolution. Although miniaturized spectrometers do exist, their resolution is limited to nanometres in the telecommunications optical wavelength (1550 nm).

Super-dispersive meta-lenses

Now, a team led by Federico Capasso may have overcome this problem with their super-dispersive meta-lenses that can resolve wavelength differences down to just 200 picometres in this part of the electromagnetic spectrum.

The researchers made their lenses from silicon nanofins. The devices comprise unit cells that measure 0.5 µm × 0.5 µm. “Each unit cell contains a nanofin that is rotated to impart the required phase profile for off-axis focusing,” explains lead author of the study Mohammedreza Khorasaninejad. “The nanofins are arranged in a square lattice with a centre-to-centre distance of 500 nm.”

Meta-lenses can also be "stitched" together

To focus light at an extremely large angle (up to 80°), secondary light waves emerging from each nanofin must arrive at the focal point in-phase. “To simultaneously focus and disperse light, we need to control the phase of incoming light very precisely,” says Khorasaninejad. "We achieve this using a so-called geometric phase approach via the sub-wavelength spaced high-aspect ratio nanofins. The resulting meta-lenses can also be stitched together to extend the bandwidth without sacrificing the spectral resolution – something that can not be done using conventional methods.”

The lenses could be integrated into compact or portable devices, he tells, or be used as add-on components for mobile phone cameras in applications as advanced as biosensing and even food safety analyses.

The team, reporting its work in Nano Letters DOI: 10.1021/acs.nanolett.6b01097, says that it is now busy developing off-axis meta-lenses that work in the visible region too. “As well as high spectral resolution applications, we believe that a more advanced version of our device might be able to perform polarization-resolved spectroscopy, which would significantly extend its applications.”

Other members of the team include Wei Ting Chen and Jaewon Oh of SEAS. The research was supported in part by a MURI grant from the Air Force Office of Scientific Research.