Novel metalens focuses all colours of rainbow at one point

New York, Jan 2 :A team of Harvard researchers have developed the first single lens that can focus the entire visible spectrum of light -- including white light -- in the same spot and in high resolution.

Known as metalens, the new lens opens new possibilities in virtual and augmented reality, the researchers said.

Metalenses -- flat surfaces that use nanostructures to focus light -- promise to revolutionise optics by replacing the bulky, curved lenses currently used in optical devices with a simple, flat surface.

But, these metalenses have remained limited in the spectrum of light they can focus well.

"Metalenses have advantages over traditional lenses," said Federico Capasso, Professor at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS)

"Metalenses are thin, easy to fabricate and cost effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step," Capasso added.

Focussing the entire visible spectrum and white light -- combination of all the colours of the spectrum -- is so challenging because each wavelength moves through materials at different speeds.

Red wavelengths, for example, will move through glass faster than the blue, so the two colours will reach the same location at different times resulting in different foci. This creates image distortions known as chromatic aberrations.

Cameras and optical instruments use multiple curved lenses of different thicknesses and materials to correct these aberrations.

The metalenses, detailed in the journal Nature Nanotechnology, uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration.

The researchers created units of paired nanofins that control the speed of different wavelengths of light simultaneously.

The 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, ensuring that all wavelengths reach the focal spot at the same time, the researchers said.