Tag Archives: visible spectrum

New, revolutionary metalens focuses entire visible spectrum into a single point

The Harvard-produced lens could usher in a new age of cameras and augmented reality.

The next generation of cameras might be powered by nanotechnology.

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.

An artist’s conception of incoming light being focused on a single point by a metalens. Image credits: Jared Sisler/Harvard SEAS.

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.

Tomorrow’s camera is flash free, regardless of light conditions

As any amateur photographer can tell you, in order to take a clear picture, you require a good light source; so in poor light conditions, the solution was the intense flash. However, there are some obvious disadvantages.

Still, computer scientist Rob Fergus started thinking if we actually need such an intense light source, or if we could actually develop some sort of invisible flash that would solve the inconvenient that come with the traditional camera flash.

F is a multi spectral flash, A is using ambiental lighting, which is way lower than it should be, R is a combined version of the two, and L is a reference long exposure shot

So one year later, the end result was a camera that emits and records light outside the visible spectrum. Practically, the prototype emits a flash, but you just don’t see it, and the photographs are as good as old-school flash ones. How does it work ? Well, usually, cameras have a filter that prevents any type of light from the infrared spectrum.  For this innovative camera, Fergus replaced the filter; the UV however, was a little trickier. His camera could already detect UV, but sending it out, that was a real challenge. So he employed the help of some hobbyists that use UV photography to reveal hidden patterns on flowers: landing strips for insects, polinators, etc.

So the camera is done, but is it any good ? Well, it most definitely is. as you can see for yourself.

“Most pictures you take with a flash look quite crappy,” says Ankit Mohan, an expert in camera technology at the Massachusetts Institute of Technology says. “They look kind of flat, you get the red-eye effect, and one part of the scene is always much brighter than another part. But the problem of capturing a picture with no flash is that you don’t get detail. By combining the two you get the best of both worlds.”

Despite the comfort advantages it provides, this development is also quite useful in some fields.

Cramer Gallimore, a professional photographer based in North Carolina, believes dark-flash photography has great potential. “You might be able to take high-quality photographs of wildlife without disturbing them,” Gallimore says, “and for forensic photography, it would be very useful to have technology like this that could switch between infrared technology and visible light photography to record certain traces of human activity at a crime scene.”

Source: Popular Mechanics