Photodetector Mimics Directional Hearing in Small Animals

Researchers from Stanford University have drawn from nature — specifically, from the ear structure of the gecko — to devise a subwavelength photodetector that can measure both the intensity and incident angle of light.

Gecko ears contain a mechanism similar to the Stanford researchers’ system for detecting the angle of incoming light. Courtesy of Vitaliy Halenov.

Geckos and many other animals have heads that are too small to triangulate the location of noises the way humans do. Instead, they have a tiny tunnel through their heads that measures the way incoming sound waves bounce around. If a sound isn’t coming from directly over the top of the gecko, one eardrum essentially steals some of the sound wave energy that would otherwise tunnel through to the other. This inference helps the gecko (and about 15,000 other animal species with a similar tunnel) understand where a sound is coming from.

The researchers mimicked this structure in their photodetector by lining up two silicon nanowires, each about 100 nm in diameter, next to each other, like the gecko’s eardrums. The nanowires were positioned so closely that when a light wave came in at an angle, the wire closest to the light source interfered with the waves hitting its neighbor, basically casting a shadow. Consequently, the first wire to detect the light sent the strongest current. By comparing the current in both wires, the researchers could map the angle of incoming light waves.

Such a system could be used by tiny cameras to detect where light is coming from, without the bulk of a large lens.

“Making a little pixel on your photo camera that says light is coming from this or that direction is hard because, ideally, the pixels are very small — these days about 1/100th of a hair,” said professor Mark Brongersma. “So it’s like having two eyes very close together and trying to cross them to see where the light is coming from.”

The tiny detectors that the team is developing can record many characteristics of light, including color, polarity, and now angle of light. The researchers believe their system is the first to demonstrate that it’s possible to determine angle of light with a nanoscale setup.

“The typical way to determine the direction of light is by using a lens. But those are big and there’s no comparable mechanisms when you shrink a device so it’s smaller than most bacteria,” said professor Shanhui Fan. More detailed light detection could support advancements in lensless cameras, augmented reality, and robotic vision, the researchers said.

Geckos were not the inspiration for the construction of the system, at least not initially. One of the researchers discovered the similarities between the team’s design and the gecko’s ear structure after work on the project had begun. The researchers further discovered that the same math that explains both the gecko ears and their tiny photodetector also describes an interference phenomenon between closely arranged atoms.

“On the theory side, it’s actually very interesting to see many of the basic interference concepts that go all the way to quantum mechanics show up in a device that can be practically used,” Fan said.

The researchers look forward to building on the results of their current proof of concept. Next steps include deciding what else they might want to measure from light and putting several nanowires side by side to see if they can build a complete imaging system that records all the details they’re interested in at once.

The research was published in Nature Nanotechnology (https://doi.org/10.1038/s41565-018-0278-9).