- The metalens can focus, perform image shift and control aberrations caused by astigmatisms, all at the same time.
- The lens and muscle together have a total thickness of 30 microns.
- The metalens’ shape is controlled by an electric signal to form the necessary optical wavefronts.
Researchers at Harvard University have built an adaptive metalens that controls 3 of the crucial factors of blurry pictures – Focus, Astigmatism and Image shift. This flat, electronically controlled eye combines the advances in metalens technology with artificial muscle technology.
The artificial eye can simultaneously control all of the 3 crucial factors, and can be configured to change its focus in real time. It works like a normal human eye, however, in the future, the technology can be further improved to do things human eye can’t naturally do, like dynamically correct image shift and astigmatism.
The technology also shows the feasibility of inbuilt autofocus and optical zoom for several applications, ranging from eyeglasses and optical microscopes to smartphones and VR/AR devices.
How Did They Do That?
Usually, metalenses focus light and reduce spherical aberrations via a dense nanostructure pattern that is smaller than the light’s wavelength. Since they’re too small, the information density in each lens is extremely high.
To create an artificial eye, the very first task is to scale up the metalens. However, whenever scientists tried to do this, the file size of the design alone would reach up to terabytes.
To deal with this issue, they developed an algorithm that compresses the size of the file to a significant level, making metalens compatible with the techniques used to fabricate integrated circuit. The metalenses were scaled up to centimeters in diameter.
As you can see in the below image, the colorful iridescence within the metalens (made of silicon) is generated by the huge number of nanostructures.
Silicon metalens mounted on transparent polymer sheet | Credit: Harvard SEAS
The next step is to stick the metalens to an artificial muscle in a way that it doesn’t impact the metalens’ light focusing ability. The natural eye lens is surrounded by cilary muscles, a ring of smooth muscle that controls accommodation for viewing objects at varying distances by changing the shape of the lens.
The scientists selected a thin, transparent dielectric elastomer to attach to the lens, through which light can travel with less- scattering. They built an entirely new platform for transferring and adhering the lens to the soft surface. Needless to say, this was the biggest challenge in the whole process of developing an artificial eye.
Metlens focusing light ray onto an image sensor | Credit: Capasso Lab / Harvard SEAS
As you can see in the above image, the metalens’ shape is controlled by an electric signal to form the necessary optical wavefronts (red).
The elastomer is tuned by applying varying voltage. The nanopillars’ position on the surface of the lens shifts when the elastomer stretches. The pillars’ position with respect to their neighbors, and the structures’ total displacement could be used to configure the metalens.
The lens and muscle together have a total thickness of 30 microns. It can focus, perform image shift and control aberrations caused by astigmatisms, all at the same time.
The reliability of the instrument was tested with a sinusoidal voltage ranging from 2 to 100 hertz at 2.5 kV amplitude. It didn’t fail at all, nor was picture quality seemed to degrade after one thousand cycles.
However, dielectric breakdown was observed at nearly 3.5 kV, when the electric current started flowing through it, damaging the instrument. It was a ‘soft’ breakdown associated with local burning. The same instruments were able to resume operation after cycling power.
Almost all optical devices with several modules (including telescopes, microscopes and cameras) contain very little mechanical stresses/misalignments on their modules. This is usually based on how there were created and their surroundings that cause slight aberrations.
These errors could be corrected through adaptive optical component. Since the metalens described here is flat, one can use it to correct those aberrations and integrate numerous capabilities of optics onto a single plane of control.
For now, the next goal is to further enhance the functionality of the metalens while reducing the voltage needed to operate it.