Adaptive metalens maintains focus with artificial muscle

Artificial muscle and metalens technology have been combined to create a tuneable metalens that can change its focus in real time.

Silicon metalens mounted on a transparent, stretchy polymer film (Harvard SEAS)

Developed by researchers at the Harvard John A Paulson School of Engineering and Applied Sciences (SEAS), the adaptive metalens is said to simultaneously control three of the major contributors to blurry images, namely focus, astigmatism, and image shift. The research is published in Science Advances.

“This research combines breakthroughs in artificial muscle technology with metalens technology to create a tuneable metalens that can change its focus in real time, just like the human eye,” said Alan She, a graduate student at SEAS and first author of the paper. “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 and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper. “It also shows the possibility of future optical microscopes, which operate fully electronically and can correct many aberrations simultaneously.”

To build the artificial eye the team had to scale-up metalenses, which are currently very small and 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 centimetre 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.”

The team overcame this by developing a new algorithm to shrink the file size and, in a paper published in Optics Express, the researchers demonstrated the design and fabrication of metalenses up to centimetres or more in diameter.

Illustration of the adaptive metalens (Second Bay Studios/Harvard SEAS)

The team then had to join 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 colleagues 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 (artificial muscles).

The researchers chose a thin, transparent dielectric elastomer with low loss to attach to the lens. The elastomer is controlled by applying voltage and as it stretches, the position of nanopillars on the surface of the lens shift. Controlling the position of the pillars and the total displacement of the structures helps tune the metalens. 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 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.”

The researchers now plan to improve the functionality of the lens and decrease the voltage required to control it. The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialisation opportunities.