Graphene makes for a nearly ideal photodetector: It absorbs light over a broad range of wavelengths—from the UV to the far-IR—and then ballistically conducts the photoexcited electrons, even at room temperature. But as a single monolayer of atoms, graphene can only absorb 2.3% of incident light.

Ever since IBM’s Phaedon Avouris and colleagues demonstrated an ultrafast graphene photodetector three years ago,1 his group and others have been developing strategies to ameliorate the limited absorption. One can, for instance, place graphene in an optical cavity to repeatedly pass light through it or position plasmonic nanostructures near electrodes on opposite sides of the graphene to dramatically amplify the local electric field.

Researchers led by Gerasimos Konstantatos and Frank Koppens at the Institute of Photonic Sciences in Barcelona, Spain, have now developed a fundamentally different approach. They used graphene as an ultrafast charge-transport channel but relegated the light absorption to a roughly...

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