Ultrathin, high speed photodetector coating developed

Researchers, including one of Indian-origin, have developed 3-atom-thick, high speed photodetecting material that may someday enable technologies which use light as a source of communication such as LiFi.
Photodetection is used in high-speed optoelectronic applications, including optical fibre networks.
Graduate student Haining Wang from Cornell University came up with an inventive way of measuring the near-instantaneous electrical current generated using a light detector made using an atomically thin material.
The team, headed by Farhan Rana, associate professor at Cornell, measured the ultrafast response of their 2D photodetector using a strobe-like process called two-pulse photovoltage correlation.
Researchers came up with the idea of 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 said.

Researchers, including Sandip Tiwari, used a 3-atoms-thick sheet of molybdenum disulfide (MoS2), a material Rana and others have tested previously in photodetection studies.
The MoS2 photodetector had intrinsic response times as short as 3 picoseconds, the researchers found.
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," Wang said.
"We find that this single device can work up to 300 GHz, which is an amazing speed," he said.
Despite being just 3-atoms thick, MoS2 is "easy to make" and relatively inexpensive, adding to its appeal, Wang said.
As with all photodetectors, the downside is the low quantum efficiency, which is a measure of the number of charges generated by the detector in the external circuit per incident photon, the researchers said.

Only a small percentage of the light-generated charges - 1 to 2 per cent - were able to escape the photodetector and make it into the external circuit; most recombined inside the device, producing heat.
Rana said the photodetection technology will play a major role in emerging fields, such as LiFi - using light as a source of wireless communication.
Windows and walls could be coated with atomically thin layers of material that would interact with light and carry Internet signals, he said.
The study was published in the journal Nature Communications.