Metalens With Artificial Muscles

The adaptive metalens focuses light rays onto an image sensor. An electrical signal controls the shape of the metalens to produce the desired optical wavefronts resulting in better images. (Source: Harvard SEAS / Second Bay Studios)

Inspired by the human eye, researchers at the Harvard John A. Paulson School of Engi­neering and Applied Sciences SEAS have developed an adap­tive metalens, that is essen­tially a flat, elec­tronically controlled arti­ficial eye. The adaptive metalens simul­taneously controls for three of the major contri­butors to blurry images: focus, astig­matism, and image shift. “This research combines break­throughs in arti­ficial muscle technology with metalens tech­nology to create a tunable metalens that can change its focus in real time, just like the human eye,” said Alan She, a graduate student at SEAS. “We go one step further to build the capability of dynami­cally correc­ting for aber­rations such as astig­matism and image shift, which the human eye cannot naturally do.”

“This demon­strates the feasi­bility of embedded optical zoom and autofocus for a wide range of appli­cations including cell phone cameras, eye­glasses and virtual and aug­mented reality hardware,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics at SEAS. “It also shows the possi­bility of future optical micro­scopes, which operate fully elec­tronically and can correct many aber­rations simul­taneously.” The Harvard Office of Tech­nology Develop­ment has protected the intel­lectual property relating to this project and is exploring commercia­lization oppor­tunities.

To build the arti­ficial eye, the researchers first needed to scale-up the metalens. Prior meta­lenses were about the size of a single piece of glitter. They focus light and eli­minate spherical aber­rations through a dense pattern of nano­structures, each smaller than a wave­length of light. “Because the nano­structures are so small, the density of infor­mation in each lens is incredibly high,” said She. “If you go from a 100 micron-sized lens to a centi­meter sized lens, you will have increased the infor­mation required to describe the lens by ten thousand. Whenever we tried to scale-up the lens, the file size of the design alone would balloon up to giga­bytes or even tera­bytes.”

To solve this problem, the researchers developed a new algo­rithm to shrink the file size to make the metalens compatible with the tech­nology currently used to fabri­cate inte­grated circuits. Recently, the researchers demon­strated the design and fabri­cation of meta­lenses up to centi­meters or more in diameter. “This research provides the possi­bility of unifying two industries: semi­conductor manu­facturing and lens-making, whereby the same tech­nology used to make computer chips will be used to make meta­surface-based optical components, such as lenses,” said Capasso.

Next, the researchers needed to adhere the large metalens to an arti­ficial muscle without compro­mising its ability to focus light. In the human eye, the lens is sur­rounded by ciliary muscle, which stretches or compresses the lens, changing its shape to adjust its focal length. Capasso and his team colla­borated with David Clarke, Extended Tarr Family Professor of Materials at SEAS and a pioneer in the field of engi­neering appli­cations of dielec­tric elastomer actuators, also known as arti­ficial muscles. The researchers chose a thin, transparent dielec­tric elastomer with low loss to attach to the lens. To do so, they needed to developed a platform to transfer and adhere the lens to the soft surface.

“Elasto­mers are so different in almost every way from semi­conductors that the challenge has been how to marry their attri­butes to create a novel multi-func­tional device and, especially how to devise a manu­facturing route,” said Clarke. “As someone who worked on one of the first scanning electron micro­scopes (SEMs) in the mid 1960’s, it is exhi­larating to be a part of creating an optical micro­scope with the capa­bilities of an SEM, such as real-time aber­ration control.” The elastomer is controlled by applying voltage. As it stretches, the position of nano­pillars on the surface of the lens shift. The metalens can be tuned by control­ling both the position of the pillars in relation to their neighbors and the total dis­placement of the structures. The researchers also demon­strated that the lens can simul­taneously focus, control aber­rations caused by astig­matisms, as well as perform image shift. Together, the lens and muscle are only 30 microns thick.

“All optical systems with multiple components from cameras to micro­scopes and telescopes have slight misalign­ments or mechanical stresses on their compo­nents, depending on the way they were built and their current environ­ment, that will always cause small amounts of astig­matism and other aber­rations, which could be corrected by an adaptive optical element,” said She. “Because the adaptive metalens is flat, you can correct those aber­rations and inte­grate different optical capabi­lities onto a single plane of control.” Next, the researchers aim to further improve the func­tionality of the lens and decrease the voltage required to control it. (Source: Harvard SEAS)

Reference: A. She et al.: Adaptive metalenses with simultaneous electrical control of focal length, astigmatism, and shift, Sci. Adv. 4, eaap9957 (2018); DOI: 10.1126/sciadv.aap9957

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

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