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Making an incredibly fast photodetector is one thing, but actually measuring its speed is another.
Graduate student Haining Wang came up with an inventive way of measuring the near-instantaneous electrical current generated using a light detector that he and a team of Cornell engineers made using an atomically thin material.
The team, headed by Farhan Rana, associate professor in the School of Electrical and Computer Engineering, measured the ultrafast response of their two-dimensional photodetector using a strobe-like process called two-pulse photovoltage correlation.
The team’s paper, “Ultrafast response of monolayer molybdenum disulfide photodetectors,” was published in Nature Communications, Nov. 17.
“It was very clever,” Rana said of Wang’s idea. “He came up with this idea of essentially hitting the device with an optical pulse [to initiate an electrical charge] and after a small delay, hitting it with the pulse again. By varying the time between the first and second pulse, and looking at the response of the device as a result, you can sort of see what the intrinsic speed of the device is.”
Rana’s team used a 3-atoms-thick sheet of molybdenum disulfide (MoS2), a material Rana and others have tested previously in photodetection studies. Photodetection is used in various high-speed optoelectronic applications, including optical fiber networks.
According to Wang’s experimentation, the MoS2 photodetector had intrinsic response times as short as 3 picoseconds; a picosecond is one-trillionth of a second. Co-author Wang said the speed at which the MoS2 detector responds is vastly superior to current technology, and is partly due to the extremely short distance the charges generated by light must travel before making it out of the device and into the external electrical circuit.
“State-of-the-art optical communication links work at around 10 GHz per channel, so if you make 10 channels in parallel, you have a 100 GHz optical communication link,” he said. “We find that this single device can work up to 300 GHz, which is an amazing speed.”
Wang also said that, despite being just 3-atoms thick, MoS2 is “extremely easy to make” and relatively inexpensive, adding to its appeal.
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