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.
The 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. In addition, the lens is capable of dynamically correcting for aberrations such as astigmatism and image shift, which the human eye cannot naturally do.
The research 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. It also shows the possibility of future optical microscopes that operate fully electronically and can correct many aberrations simultaneously.
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 — 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. When trying to scale-up the lens, the researchers found that the file size of the design alone would balloon up to gigabytes or even terabytes. To solve this problem, they developed a new algorithm to shrink the file size.
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. They looked to the engineering field of dielectric elastomer actuators, also known as artificial muscles.
The researchers chose a thin, transparent dielectric elastomer with low loss, meaning light travels through the material with little scattering, to attach to the lens. To do so, they needed to develop a platform to transfer and adhere the lens to the soft surface.
The elastomer is controlled by applying voltage. As it stretches, it shifts the position of nanopillars on the surface of the lens. 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.
Together, the lens and muscle are only 30 microns thick.
All optical systems with multiple components 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. These could be corrected by an adaptive optical element. Because the adaptive metalens is flat, you can correct those aberrations and integrate different optical capabilities onto a single plane of control.
The next step for the researchers is to further improve the functionality of the lens and decrease the voltage required to control it.