The Harvard-produced lens could usher in a new age of cameras and augmented reality.
From the gargantuan telescopes built to study the universe to the ever smaller cameras inside your smartphones, lenses have come a long way. They've reached incredibly high performance at lower and lower costs, but researchers want to take them to the next level. A team from Harvard has developed a metalens -- a flat surface that uses nanostructures to focus light -- capable of concentrating the entire visible spectrum onto a single spot.
Metalenses aren't exactly a new thing. They've been around for quite a while, but until now, they've struggled to focus a broad spectrum of light, addressing only some of the light wavelengths. This is the first time researchers managed to focus the entire spectrum -- and in high resolution. This raises exciting possibilities.
“Metalenses have advantages over traditional lenses,” says Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the research. “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.”
In a way, creating such a lens is like building a maze for light. Different wavelengths travel at different speeds; red moves the fastest, with violet being the slowest. At macroscopic scales (say, if you were using a prism for light diffraction), this difference is not noticeable. But if you go down to the nanoscale, it becomes evident, leading to so-called chromatic aberrations. Conventional lenses bypass this by having a curved surface, but metalenses need to take a different approach. This is where the innovation takes place. The team from the School of Engineering and Applied Sciences (SEAS) at Harvard used tiny arrays of titanium dioxide nano-sized fins to fix chromatic aberrations.
Previous research had shown that this is possible in theory, but this is the first time a practical solution was designed, and it was no easy feat.
“One of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time,” said Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper.
“By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses.”
Through this approach, they were able to focus all the colors of the rainbow onto a single point -- in other words, they were able to image "normal" white light, using a lens thousands of times thinner than what we're used to.
“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.
The potential for practical applications is practically limitless, not only in photography but also in emerging technologies such as virtual or augmented reality. But while this does bring researchers one step closer to developing smaller, better lenses for your camera or smartphone, there's still a long way to go before the technology will reach consumers. The first step is achieving the same results in macro-scale lenses. Chen and Zhu say that they plan on scaling up the lens to about 1 cm (0.39 in) in diameter, which would make it suitable for real-world applications. It will undoubtedly take them at least a few years to reach that goal, but if they can do it, we're in for quite a treat.
Journal Reference: Wei Ting Chen et al. A broadband achromatic metalens for focusing and imaging in the visible. doi:10.1038/s41565-017-0034-6.