Tiny light detectors work like gecko ears

The gecko ears contain a mechanism similar to that of Stanford researchers to detect the angle of incoming light.Credit: Vitaliy HalenovGeckos and many other animals have their heads too small to triangulate the location of noises as we do, with widely spaced ears. Instead, they have a tiny tunnel in their head that measures how incident sound waves bounce to determine the direction from which they come.

Researchers at Stanford University have come up with a similar system to detect the incoming light angle, facing a problem of tiny size and triangulation. Such a system could allow tiny cameras to detect the origin of light, but without the bulk of a large lens.

"Making a small pixel on your camera indicating that the light comes from one direction or another is difficult, because ideally, the pixels are very small, nowadays about a hundredth of a hair," said Mark Brongersma, professor of materials. and engineer who is the main author of an article on this system, published Oct. 29 in Nature Nanotechnology. "So, it's like having two very close eyes and trying to cross them to see where the light came from."

These researchers are working on tiny detectors that can record many characteristics of light, including color, polarity, and now the angle of light. To their knowledge, the system described in this document is the first to demonstrate that it is possible to determine the angle of light with such a small configuration.

"The typical way to determine the direction of light is to use a lens, but they are big and there are no comparable mechanisms when you shrink a device, so it's smaller than most bacteria, "said Shanhui Fan, professor of electrical engineering, co-author on the paper.

More detailed light detection could support advances in lensless cameras, augmented reality and robotic vision, which is important for autonomous cars.

From atoms to geckos

If a sound does not come directly over the gecko, a tympanum essentially steals some of the energy of the sound waves that would otherwise pbad from one to the other. This deduction helps the gecko – and about 15,000 other animal species with a similar tunnel – to understand where a sound comes from.

The researchers mimic this structure in their photodetector by having two silicon nanowires, each of about 100 nanometers in diameter or about 1/1000th of the width of one hair, aligned one beside the other, to the way of the eardrums of the gecko. They are positioned so close that when a light wave enters obliquely, the wire closest to the light source interferes with the waves striking its neighbor, projecting a shadow. The first wire to detect the light would then send the strongest current. By comparing the current in the two wires, researchers can map the angle of incoming light waves.

The geckos were not the inspiration for the initial construction of this system. Soongyu Yi, a graduate student in Electrical and Computer Engineering from the University of Wisconsin-Madison, who is the lead author of the article, discovered the similarity between their design and the ears of geckos after the start of work. They were all surprised by the deep level of similarity. It turns out that the same calculation that explains both the gecko ears and this photodetector also describes a phenomenon of interference between closely arranged atoms.

"At the theoretical level, it is actually very interesting to see many basic concepts of interference, which go up to quantum mechanics, in a device that can be used in practice," Fan said.

A long-term commitment

This project started when one of the co-authors of the newspaper, Zongfu Yu, was a student at the Fan Lab and had taken the initiative to combine his work with the research of Brongersma and his lab. They progressed but had to suspend work while Yu was applying for teaching positions and, subsequently, set up his laboratory at the University of Wisconsin-Madison, where he is now Assistant Professor in Electrical Engineering. and computer and in whose laboratory Soongyu Yi works.

Many years later, and after publishing the current proof of concept, the researchers said that they were eager to take advantage of their results. The next steps are to decide what they want to measure from the light and put several nanowires side by side to see if they can build a complete imaging system that simultaneously captures all the details that interest them.

"We have been working on this for a long time – Zongfu has had a long history between the beginning and the end of this project, which shows that we have not compromised quality," said Brongersma. "And it's amusing to think that we may be here for 20 years to determine the full potential of this system."

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