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Graphene Photodetector Demonstrates Speed, High Responsivity

Photonics Spectra
Sep 2018
A new photodetector, made with graphene, can operate across a broader range of wavelengths and process images faster than existing photodetectors. The detector is also more sensitive to low levels of light than current technology.

Graphene photodetector, UCLA.
The photodetector operates across a broad range of light, processes images more quickly, and is more sensitive to low levels of light than current technology. Courtesy of UCLA Engineering.

Researchers from University of California, Los Angeles (UCLA) fabricated the photodetector using photoconductive nanostructures based on gold-patched graphene nanostripes. They laid stripes of graphene over a silicon dioxide layer; then they created a series of comb-shaped nanoscale patterns made from gold, with “teeth” about 100 nm wide. The graphene nanostripes act as a net to catch incoming photons. The graphene then converts the photons into an electrical signal. The gold comb-shaped nanopatterns quickly transfer that information into a processor, which produces a corresponding high-quality image, even under low-light conditions.

Through this approach, the team achieved high responsivity without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers.

The graphene photodetector demonstrated high-responsivity (ampere per watt; A/W) photodetection from the visible to the IR regime of 0.6 A/W at 0.8 μm, and 11.5 A/W at 20 μm, with operation speeds exceeding 50 GHz. The results indicate an improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude, compared to higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.

“We specifically designed the dimensions of the graphene nanostripes and their metal patches such that incoming visible and infrared light is tightly confined inside them," said researcher Semih Cakmakyapan. "This design efficiently produces an electrical signal that follows ultrafast and subtle variations in the light’s intensity over the entire spectral range, from visible to infrared.”

The team believes that the combination of broadband and ultrafast photodetection with high responsivity could have an impact on future hyperspectral imaging and sensing systems. Researchers said that to further enhance performance, the symmetric gold patches could be replaced with asymmetric metal patches; this could lead to symmetry breaking and enable bias-free, low-dark-current device operation.

“Our photodetector could extend the scope and potential uses of photodetectors in imaging and sensing systems," said professor Mona Jarrahi. "It could dramatically improve thermal imaging in night vision or in medical diagnosis applications where subtle differences in temperatures can give doctors a lot of information on their patients. It could also be used in environmental sensing technologies to more accurately identify the concentration of pollutants.”

The research was published in Light: Science & Applications (doi:10.1038/s41377-018-0020-2).

GLOSSARY
hyperspectral imaging
Methods for identifying and mapping materials through spectroscopic remote sensing. Also called imaging spectroscopy; ultraspectral imaging.
photodetector
A device used to sense incident radiation.
Research & TechnologyAmericaseducationimaginglight sourcesNanopositioningopticsSensors & Detectorshyperspectral imagingthermal sensingmedical imagingphotodetectorgraphenegraphene photodetectormaterialsnanoenvironmentUCLA Samueli School of EngineeringTech Pulse

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