Single Metalens Focuses White Light

This flat metalens is the first single lens that can focus the entire visible spectrum of light in the same spot and in high resolution. It uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. (Source: J. Sisler, Harvard SEAS)

Metalenses with flat surfaces promise to revo­lutionize 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. Now a team of researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS has 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. This has only ever been achieved in conven­tional lenses by stacking multiple lenses.

Focusing the entire visible spectrum and white light 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 colors 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, which, of course, adds to the bulk of the device.

“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. “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 Harvard Office of Tech­nology Development has protected the intellectual property relating to this project and is exploring commercia­lization oppor­tunities.

The meta­lenses developed by Capasso and his team use arrays of titanium dioxide nanofins to equally focus wave­lengths of light and eliminate chromatic aber­ration. Previous research 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 this latest design, 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,” said Wei Ting Chen, a post­doctoral fellow at SEAS. “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. This brings us one step closer to the goal of incorporating them into common optical devices such as cameras,” said Alexander Zhu. Next, the researchers aim to scale up the lens, to about 1 cm in diameter. This would open a whole host of new possi­bilities, such as appli­cations in virtual and aug­mented reality. (Source: SEAS)

Reference: W. T. Chen et al.: A broadband achromatic metalens for focusing and imaging in the visible, Nat. Nano., online 1 January 2018; DOI: 10.1038/s41565-017-0034-6

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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