Fall 2012 Magazine

The Data Age

Modeled on the Human Eye

Hooman Mohseni seeks the perfect single-photon detector


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Hooman Mohseni, associate professor of electrical engineering and computer scienceFrom the outside, Hooman Mohseni’s camera doesn’t look like much. Measuring nearly a foot long, its boxy tan body is more reminiscent of a ’70s slide projector or dental x-ray machine than one of the world’s best light-sensing devices.

But inside that unassuming exterior is an imager that rivals the human eye in its ability to sense the tiniest traces of light, but in a wavelength completely invisible to human eye. The result of six years of work, the one-inch flat square is made of tens of thousands of photon detectors, a type of light sensor not unlike those in cameras and cellphones.  

Part of the photon detector

Single-photon detectors are so powerful they can detect and respond to just one photon, the basic unit of light, and capture images in virtually pitch-black conditions. They hold promise for much more than imaging, though; once perfected, they might find application in everything from medicine to homeland security to quantum computing. So far, researchers have been unable to design a single-photon detector accurate and sensitive enough for most applications, but people are taking notice of Mohseni’s camera. Taking cues from the human eye, Mohseni (PhD ’01), associate professor of electrical engineering and computer science, has developed a single-photon detector that could lead to significant advances.

Single-photon detectors, previously a niche topic within the photonics field, became increasingly prevalent within the scientific community during the three years Mohseni worked in the photonics group at Sarnoff Corporation after receiving his PhD from Northwestern. Mohseni grew intrigued by the significant gap between the performance of man-made single-photon detectors and those found in nature. When he returned to McCormick as an assistant professor in 2004, he immediately developed novel ideas to replicate nature’s approach for photon detection in a new semiconductor device.

Hooman Mohseni and his research teamWithin a year of his arrival as a faculty member he had been awarded close to $4 million in grants from the Defense Advanced Research Projects Agency (DARPA) and National Science Foundation to develop his single-photon detector. With the funding and researchers from his aptly named Bio-Inspired Sensors and Optoelectronics Laboratory, he set out to develop a light detector that mimics the eye.

“The human eye is amazing in terms of sensitivity—many thousands of times better than the standard detectors we are using today,” Mohseni says. “I wanted to know how this, nature’s marvel, manages to reach that level of detection.”

To sense light, the eye uses rhodopsin, a collection of light-detecting molecules that float inside the retina’s rod cells, forming a sort of web that is highly efficient at capturing photons. When rhodopsin interacts with light, a cascade of chemical reactions begins, at the end of which a single molecule is triggered to open molecular valves in the rod cell wall, beginning the process of transmitting signals to the brain. That combination—the net of rhodopsin funneling energy into a single molecule—makes the eye both extremely reliable and incredibly sensitive; after 30 minutes of darkness, every rod cell in the human eye is sensitive enough to detect a single photon.

Mohseni’s design uses similar principles: a very large absorbing area that focuses all its energy into a nanoscale switch that controls the flow of electrons through the device. Not only is the resulting device far more sensitive than other single-photon detectors—currently 100 times more sensitive than other cameras at its wavelength—it also produces less “noise,” blips of errant signals that skew results.

“Just like the rod cells in the eye, we have a single-photon detector that has a very high probability of detection and a very low rate of false alarms,” Mohseni says. “And we are still far from the predicted performance limit of our device.”

Hooman Mohseni and his research teamAlthough he knew he’d created something significant, Mohseni has been surprised by the amount of interest in his device, often from unexpected sources such as the medical community.

Because the wavelength at which Mohseni’s camera operates—shortwave infrared—is the wavelength at which human tissue is most transparent, medical applications are likely. Mohseni envisions that his camera could eventually replace CT scans for some applications, achieving better images in a few seconds and eliminating radiation risks. Ultrasensitive and fast infrared imagers are also useful in astronomy because they can significantly increase the probability of finding Earth-like planets. And with another grant from DARPA, Mohseni’s team just started working to perfect his single-photon detector for quantum computing.

“Our research could have a significant impact in a wide range of fields— enabling discoveries in astronomy and quantum physics, even new medical applications that could save lives,” Mohseni says.  “It’s very exciting for my team.”

By: Sarah Ostman