  The electronic properties of graphene are unique and are attracting increased
attention to this novel 2-dimensional system. Its photonic properties are not
less impressive. For example, this single atomic layer absorbs through direct
interband transitions a considerable fraction of the light (~2.3%) over a very
a broad wavelength range. However, while applications in electronics are
vigorously being pursued, photonic applications have not attracted as much
attention. Here, we report on ultrafast photocurrent response measurements in
graphene (single and few-layers) field-effect-transistors (FETs) up to 40 GHz
light intensity modulation frequencies, using a 1.55 micron excitation laser.
No photoresponse degradation is observable up to the highest measured
frequency, demonstrating the feasibility and unique benefits of using graphene
in photonics. Further analysis suggests that the intrinsic bandwidth of such
graphene FET based photodetectors may exceed 500 GHz. Most notably, the
generation and transport of the photo-carriers in such graphene photodetectors
are fundamentally different from those in currently known semiconductor
photodetectors, leading to a remarkably high bandwidth, zero source-drain bias
(hence zero dark current) operation, and good internal quantum efficiency.
