Metalenses are flat surfaces that use nanostructures to focus light. They are thin, easy to fabricate, and cost-effective and of interest for applications in nanoscale optical devices such as cameras, lighting, displays, and wearable optics. However, creating a metalens that focuses the entire visible spectrum is challenging because each wavelength moves through materials at different speeds. This creates image distortions known as chromatic aberrations. To correct these, cameras and optical instruments currently use multiple curved lenses of different thicknesses and materials.

Federico Capasso, Harvard University, Cambridge, MA, USA, and colleagues have developed a metalens that can focus the entire visible spectrum of light in the same spot and in high resolution. Their metalens comprises of a single layer of titanium dioxide nanofins. The layer has a thickness in the order of the wavelength. Units of paired nanofins control the refractive index on the metasurface and are tuned to result in different time delays for the light passing through different fins. This ensures that all wavelengths reach the focal spot at the same time.

The team has demonstrated diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. The metalens has an efficiency of about 20 % at 500 nm. Currently, the researchers are optimizing their approach to meet the demand of future applications. They also want to scale up the lens to about 1 cm in diameter.

• A broadband achromatic metalens for focusing and imaging in the visible,
  Wei Ting Chen, Alexander Y. Zhu, Vyshakh Sanjeev, Mohammadreza Khorasaninejad, Zhujun Shi, Eric Lee, Federico Capasso,
  Nature Nanotechnol. 2018.
  https://doi.org/10.1038/s41565-017-0034-6