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John O | July 2018

Graphene photodetector from UCLA enhances thermal imaging performance

By Josh Perry, Editor


Researchers at the University of California Los Angeles (UCLA) have taken advantage of the unique properties of graphene to create and enhanced photodetector that works with more types of light than standard devices and has enhanced sensing and imaging properties.


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.
(Jarrahi Research Group/UCLA)


According to an article from UCLA, the versatility of photodetectors is based on operating speed, sensitivity to lower levels of light, and the amount of the light spectrum that they can sense. Engineers generally improve one of the three properties while impacting at least one of the others, but this photodetector improves all three at once.


“The new photodetector takes advantage of the unique properties of graphene,” the article explained. “Graphene is an excellent material for detecting photons because it can absorb energy from a broad swath of the electromagnetic spectrum — from ultraviolet light to visible light to the infrared and microwave bands. Graphene is also a very good conductor of electrical current — electrons can flow through it unimpeded.”


To create the photodetector, researchers put graphene layers on top of a layer of silicon dioxide, which lies on a base of silicon. “Then, they created a series of comb-like nanoscale patterns, made of gold, with ‘teeth’ about 100 nanometers wide,” the article added.


The graphene catches incoming photons and converts them to an electrical signal, which the gold nanopatterns transfer to a processor to produce the high-quality image, even in low light. The researchers designed the graphene strips and the metal patches so that the incoming light is tightly confined to ensure the electrical signal can follow the variations in the light’s intensity across the entire spectrum.


The research was recently published in Light: Science and Applications. The abstract read:


“Graphene is a very attractive material for broadband photodetection in hyperspectral imaging and sensing systems. However, its potential use has been hindered by tradeoffs between the responsivity, bandwidth, and operation speed of existing graphene photodetectors.


“Here, we present engineered photoconductive nanostructures based on gold-patched graphene nano-stripes, which enable simultaneous broadband and ultrafast photodetection with high responsivity. These nanostructures merge the advantages of broadband optical absorption, ultrafast photocarrier transport, and carrier multiplication within graphene nano-stripes with the ultrafast transport of photocarriers to gold patches before recombination.


“Through this approach, high-responsivity operation is realized without the use of bandwidth-limiting and speed-limiting quantum dots, defect states, or tunneling barriers. We demonstrate high-responsivity photodetection from the visible to infrared regime (0.6 A/W at 0.8 μm and 11.5 A/W at 20 μm), with operation speeds exceeding 50 GHz.


“Our results demonstrate improvement of the response times by more than seven orders of magnitude and an increase in bandwidths of one order of magnitude compared to those of higher-responsivity graphene photodetectors based on quantum dots and tunneling barriers.”

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