ISFO, Eurekanet: Nature Physics' paper authored by ICFO researchers, in collaboration with researchers from MIT, Max Planck Institute for Polymer Research, and Graphenea S.L. demonstrates that graphene is able to convert a single photon that it absorbs into multiple hot electrons, and that the higher photons energy, the larger the number of hot electrons created:
"Photoexcitation cascade and multiple hot-carrier generation in graphene"
K. J. Tielrooij, J. C. W. Song, S. A. Jensen, A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L. S. Levitov & F. H. L. Koppens
Apparently, this is possible due to a very small bandgap in graphene, so that photoelectrons can use their excessive energy to multiply themselves - somewhat similar to X-ray absorption in silicon.
Not everything is good about graphene photodetectors. An earlier article in Economist from the same group says that graphene only captures 2.7% of the photons falling on its surface. There are some ways to improve it though.
Thanks to CDM for the link!