Broadband metalens opens new possibilities in virtual, augmented reality

Technology News | January 5, 2018

By Rich Pell

This is said to be the first single lens to achieve this – a feat only previously made possible in conventional lenses by stacking multiple lenses. The new single metalens — a flat surface that uses nanostructures to focus light – opens up new possibilities in virtual and augmented reality, say the researchers.

While holding promise for revolutionizing optics by replacing traditional bulky, curved lenses with a simple, flat surface, metalenses until now have been limited in the spectrum of light they can focus well – a result of the different speed at which various light wavelengths move through materials. These chromatic aberrations have traditionally been corrected in cameras and optical instruments through the use of multiple curved lenses of different thicknesses and materials.

“Metalenses have advantages over traditional lenses,” says Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of a paper on the research. “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.”

The researchers used arrays of titanium dioxide nanofins in their metalens to equally focus wavelengths of light and eliminate chromatic aberration. Their previous research had demonstrated that different wavelengths of light could be focused – but at different distances – by optimizing the shape, width, distance, and height of the nanofins.

In their latest design, which comprises a single layer of nanostructures whose thickness is on the order of the wavelength, 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.

“One of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time,” says Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper. “By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses.”

“Using our achromatic lens, we are able to perform high quality, white light imaging,” says Alexander Zhu, a co-author of the study. “This brings us one step closer to the goal of incorporating them into common optical devices such as cameras.”

The researchers next plan to scale up the lens, to about 1 cm in diameter, which they say could open a range of new possible applications, including in virtual and augmented reality. For more, see “A broadband achromatic metalens for focusing and imaging in the visible.”