Graphene – a two-dimensional sheet of carbon that is just one atom thick – was first made three years ago by Andre Geim and colleagues of the University of Manchester and colleagues at the Institute for Microelectronics Technology in Chernogolovka, Russia. The material is made from graphite, the form of carbon that is found in pencils.

Graphene has some unusual physical properties. These include the fact that electrons in the material behave like relativistic particles that have no rest mass and travel at about 106 m/s. Although this is a factor of 300 slower than the speed of light in vacuum, it is still much faster than the speed of electrons in an ordinary conductor.

Now, Sergey Mikhailov of the University of Augsburg has predicted that when graphene is irradiated by electromagnetic waves, it emits radiation at higher frequency harmonics and can thus work as a frequency multiplier. In other words, if you irradiate the sample with a certain frequency of light it reflects light back of a higher frequency.

Such "frequency transformation" could be used to produce radiation at frequencies where suitable sources do not exist. For example, it is difficult to produce frequencies higher than 100 GHz and up to 1–10 THz (1012 Hz, the so-called terahertz gap). In contrast, there are many powerful sources of microwave radiation for frequencies below 100 GHz. Scientists could therefore "multiply" microwave frequencies using a non-linear device like graphene to produce radiation in the terahertz range, says Mikhailov.

"I calculated that graphene should be able to work as a frequency multiplier in the terahertz gap," he told "This could be a very important application of the material."

Such sources of terahertz radiation could find use in many areas, including security and defence, medicine, astronomy and biological research. Terahertz radiation penetrates many materials (except metals) and so can be used to "see" through packages at airports, for example. "In medicine, it could be used to image cancer tumours for early disease diagnosis," said Mikhailov. Astronomers are also interested in terahertz radiation because the cosmic microwave background, which originated in the Big Bang includes a terahertz component.

The work was published in Europhysics Letters.