Lenses have come a long way from the chunky old bifocals to tiny modern day contact lenses. But, all the lenses we encounter and use in our day to day life are either concave or convex. These curved, transparent materials help us see better and are an integral part of optical technology like cameras.
Now, scientists have created Metalenses — lenses with flat surfaces that have embedded nanostructures that help focus light — which are much smaller and easier to make than conventional lenses.
Though these lenses broke on to the scene a couple of years ago in 2016, there have been some limitations that have kept them from taking over the market. These metalenses remained limited in the spectrum of light they can focus on well. Now a team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed the first single flat lens that can focus the entire visible spectrum of light — including white light — in the same spot and in high resolution.
What is special about this is that, in the past, this was only achieved with several curved lenses stacked one over the other and was unheard of for flat lenses.
The challenge the team overcame to make this breakthrough was to focus the entire visible spectrum of light to make it white light. When the different colours of the light spectrum move through a medium, they have different speeds. This is because of the different wavelengths. The red part of the light spectrum will move through glass faster than the blue, so the two colours will reach the same location at different times resulting in different foci.
This obviously leads to blurry images as the images’ focus is off. The distortions this effect creates is known as chromatic aberrations and it was only corrected by stacking conventional lenses to balance out the speeds.
Even complex optical instruments like cameras and telescopes use many concave and convex lenses of different thickness and materials to correct these aberrations which makes these devices bulky.
“Metalenses have advantages over traditional lenses,” said Federico Capasso, senior researcher in Electrical Engineering at SEAS, Harvard, and senior author of the research in a press release. “Metalenses are thin, easy to fabricate and cost effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step.”
The team overcame the problem of chromatic aberration by using titanium dioxide nanofins arranged onto the flat lens surfaces to make every wavelength of light to focus at the same time.
According to the team, the different wavelengths of light can be focused but at different distances by optimizing the shape, width, distance, and height of the nanofins. As the team furthered this ground-breaking design, they found that a pair of nanofins can control the speed of different wavelengths of light simultaneously.
These nanofins help regulate the refractive index of the material, thereby making tiny alterations to the pathway each wavelength takes through the lens. When different wavelengths are passed through particular nanofins, the team could focus all wavelengths at the focal spot without any aberration.
“Using our achromatic lens, we are able to perform high quality, white light imaging. This brings us one step closer to the goal of incorporating them into common optical devices such as cameras,” said Alexander Zhu, co-author of the study in the release.
The possibilities of this entering mainstream optical device technology is endless. If stacked lenses become a thing of the past, the potential applications of upcoming visual technology like augmented reality headsets grow manifold. As the lenses occupy lesser space in the device, more can be done with the image processing, quality and definition which will make virtual reality (VR) experiences more realistic and engrossing.
Virtual and augmented reality will become an $80 million market by 2025, predicts a new report conducted by the world-recognized investment bank Goldman Sachs. To put this in perspective, this is roughly the size of the desktop PC market today. This potential growth can only infiltrate other fields like offices and space technology.
In fact, NASA is already using VR headsets to train people they send into space, making sure they have some amount of experience in becoming detached from their shuttle and having to use a backpack to navigate their way back, or performing complicated tasks in zero gravity. VR already helps NASA simulate all these situations from Earth.
More advancements in this field will only make it more widespread, sooner.
The study was published in the journal Nature Nanotechnology on Jan. 1.