Researchers from the University of California, Berkeley built an optical modulator (switches light on and off) using Graphene. This is the basis of network modulators (which use light to transmit data). The graphene based modulator is the world's smallest and fastest - which could help create faster communication devices. In fact Graphene can be used to create modulators that are up to ten times faster than any current technology based modulators.
The researchers found out that tuning graphene electrically (applying voltage) causes it to absorb light in wavelength that are used for data communications (it alters the Graphene's Fermi level). At certain voltages Graphene becomes transparent, and lets light through. If you change the voltage around that level you can change whether the material is transparent or not - and basically it becomes a light switch.
The researchers say that Graphene can also be tuned to modulate mid-infrared light (used in molecular sensing).