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Tunable Metalens Can Change Its Focus in Real Time, Like a Human Eye

Photonics Spectra
May 2018
CAMBRIDGE, Mass. — Researchers have demonstrated electrically tunable large-area metalenses controlled by artificial muscle technology. The adaptive metalens simultaneously controls for three of the major contributors to blurry images: focus, astigmatism and image shift. The device thickness is only 30 μm.

To create the tunable metalens, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) first needed to scale up existing metalens structures. Current metalenses focus light through a dense pattern of nanostructures, each smaller than a wavelength.

Metalens (made of silicon) mounted on a transparent, stretchy polymer film, without any electrodes. Harvard University/SEAS.
The metalens (made of silicon) mounted on a transparent, stretchy polymer film, without any electrodes. The colorful iridescence is produced by the large number of nanostructures within the metalens. Courtesy of Harvard SEAS.

“Because the nanostructures are so small, the density of information in each lens is incredibly high,” said researcher Alan She. “If you go from a 100-micron-size lens to a centimeter-size 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 research team developed an algorithm to shrink the file size.

The next step was to adhere the scaled-up metalens to an artificial muscle without compromising its ability to focus light. Professor Federico Capasso and his team collaborated with professor David Clarke to develop dielectric elastomer actuators for this task.

The researchers chose a thin, transparent dielectic elastomer with low loss to attach to the lens and 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 multifunctional device and especially how to devise a manufacturing route. . . . It is exhilarating to be a part of creating an optical microscope with the capabilities of an SEM [scanning electron microscope], such as real-time aberration control,” said Clarke.

Voltage was applied to the elastomer to control it. As the elastomer stretched, the position of nanopillars on the surface of the lens shifted. Researchers found that the metalens could be tuned by controlling both the position of the nanopillars in relation to their neighbors and the displacement of the structures. The team showed that the lens could simultaneously perform focal-length tuning, control aberrations caused by astigmatisms, and perform image-shift corrections.

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 (shown in red), resulting in better images. Harvard University/SEAS.
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 (shown in red), resulting in better images. In the future, adaptive metalenses could be built into imaging systems, such as cell phone cameras and microscopes, enabling flat, compact autofocus as well as the capability for simultaneously correcting optical aberrations and performing optical image stabilization, all in a single plane of control. Courtesy of Second Bay Studios/Harvard SEAS.

“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 plan to further improve the functionality of the lens and decrease the voltage required to control it.

“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 Capasso. “It also shows the possibility of future optical microscopes, which operate fully electronically and can correct many aberrations simultaneously.”

The research was published in Science Advances (doi:10.1126/sciadv.aap9957).

GLOSSARY
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
elastomer
Any material of a macromolecular nature that can stretch at room temperature to more than twice its length and return to approximately its original shape when stress is released.
Research & TechnologyeducationAmericasimagingmaterialsNanopositioningopticslensesmetalensadaptive opticscamerasConsumerimage shiftelastomerartificial muscleadaptive metalensoptical microscopeTech Pulse

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