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Metalens focuses entire visible light spectrum onto single point

A single metalens capable of focusing the entire visible spectrum of light to a single point and in high resolution has been developed by a team of researchers at the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS).

The achievement, published in Nature Nanotechnology, has only ever been accomplished before by stacking multiple lenses together, and could open new possibilities in virtual and augmented reality.

Different wavelengths move through materials at different speeds – red wavelengths move through glass faster than the blue – resulting in varying foci that make focusing the entire visible spectrum at once particularly challenging. This creates image distortions known as chromatic aberrations.

Cameras and optical instruments currently use multiple conventional curved lenses of different thicknesses and materials to correct these aberrations. The researchers’ new single metalens, however, instead features a simple, flat surface that uses nanostructures to focus light, allowing it to be used to replace these multiple lenses and reduce the bulk of optical devices.

‘Metalenses have advantages over traditional lenses,’ said Federico Capasso, senior author of the research and a professor of applied physics 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 newly developed metalens corrects chromatic aberrations using arrays of titanium dioxide nanofins, the dimensions and shape of which can be optimised to focus different wavelengths at varying distances. The lens design uses mutliple pairs of these nanofins, each tuned to create different time delays in the light passing through them, allowing them to determine the refractive index of the metasurface and ensure that all wavelengths reach the same 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 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,’ added Alexander Zhu, 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 are now looking to scale up the lens to about 1cm in diameter, which could lead to a range of new possibilities, such as applications in virtual and augmented reality. In the meantime, the Harvard Office of Technology Development has licensed the intellectual property relating to this project to a startup, in order to begin commercialisation of the new lens.

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