Harvard University scientists may have achieved a breakthrough in optical technology. They developed an artificial eye lens that focuses in the same way as a real eye.
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 three of the main 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 SEAS student at the Graduate School of Arts and Sciences, and first author of the paper. "We are going one step further and building the ability to dynamically correct for aberrations such as astigmatism and image shift, something the human eye obviously cannot do."
"This demonstrates the feasibility of embedded optical zoom and autofocus for a wide range of applications, including mobile phone cameras, eyewear and virtual and augmented reality hardware," said Federico Capasso, professor of engineering physics and senior author of the paper. "It also shows the possibility of future optical microscopes, which are fully electronic and can correct many aberrations simultaneously."
Scaling up
To build the artificial eye, the researchers first had to scale up the metalens. Metalens focus light and eliminate spherical aberrations through a dense pattern of nanostructures, each smaller than a wavelength of light. Previous metallenses were about the size of a single piece of glitter.
"Because the nanostructures are so small, the density of information in each lens is incredibly high," states She. "If you go from a 100-micron lens to a centimetre-sized lens, you have increased the information needed to describe the lens by 10,000. Every time we tried to scale up the lens, the file size of the design alone would add up to gigabytes or even terabytes. "
To solve this problem, the researchers developed a new algorithm to reduce the file size to make the metalens compatible with technology currently used to fabricate integrated circuits. In a paper recently published in Optics Express, the researchers showed the design and fabrication of metalenses of up to centimetres or more in diameter. "This research offers the opportunity to unify two industries, semiconductor manufacturing and lens making, where the same technology used to make computer chips will be used to make metasurface-based optical components, such as lenses."
Related
This flat metal lens is the first single lens that can focus the entire visible spectrum of light - including white light - at the same spot and in high resolution. It uses arrays of titanium dioxide nanofins to focus even wavelengths of light and eliminate chromatic aberration. A single metalens focuses all colours of the rainbow at a single point.
The researchers also had to attach the large metal lens to an artificial muscle without compromising its ability to focus light. In the human eye, the lens is surrounded by a ciliary muscle, which stretches or compresses the lens, or changes its shape to adjust the focal length. Capasso and his team collaborated with David Clarke, a pioneer in technical applications of dielectric actuators made of elastomers, also called artificial muscles.
The researchers chose a thin, transparent low-loss dielectric elastomer - meaning light travels through the material with little scattering - to adhere to the lens. To do this, they had to develop a platform to transfer and attach the lens to the soft surface. "Elastomers are so different in almost every way from semiconductors that the challenge was to bring their attributes together to create a new multifunctional device and, most importantly, to devise a manufacturing route," Clarke says. "As someone who worked on one of the first scanning electron microscopes (SEMs) in the mid-1960s, it's special to be part of creating an optical microscope with the capabilities of an SEM, such as real-time aberration control."
The elastomer is controlled by adding tension. As it stretches, the position of nanopillars on the surface of the lens shifts. The metallic lenses can be tuned by controlling both the position of the pillars relative to their neighbours and the overall displacement of the structures. The researchers also showed that the lens can simultaneously focus, control aberrations caused by astigmatisms and perform image shift. Together, the lens and muscles are only 30 microns thick. "All multi-component optical systems - from cameras to microscopes and telescopes - have small misalignments or mechanical stresses on their components, which will always cause small amounts of astigmatism and other aberrations. This can be corrected by an adaptive optical element. Because the adaptive metalens is flat, you can correct those aberrations and integrate different optical capabilities on one control surface." The researchers are now working on improving the functionality of the lens and using less stress.
By: Kelly Bakker
Source: Harvard
