Saturday, September 6, 2014

Yield from graphene much higher than first thought
Generating terahertz light via graphene may soon be an interesting alternative to the current methods used to generate THz light. This is the conclusion reached by researchers in the Optics Research Group and recently published in an article in ACS Nano.
‘We have discovered that after illuminating it with an ultrashort (femtosecond) laser pulse, a single layer of graphene emits a short flash of terahertz light,’ says Prof. Paul Planken. Light with a terahertz frequency (around 1012 Hz) can be used for a wide range of practical applications, making this an interesting development for advancements in imaging and spectroscopy, for example. The main advantage is that light of this frequency is not harmful, unlike X-rays. ‘The light can be used to look inside and through packaging, for example, and to analyse works of art.’

Relatively strong

Terahertz light is normally generated via completely different methods, such as by using antennas fabricated on semi-conductors. The idea of illuminating ultrathin graphene layers has come up before, but it was thought that the THz light yield would be too low. Planken: ‘In absolute terms, the terahertz light is weak, but considering the fact that graphene is just 1 atom thick, it is actually relatively strong. The mechanism responsible for the partial conversion of laser light into THz light occurs when ultrafast electric impulses are directly generated in the graphene as, in a manner of speaking, the light particles in the laser pulse collide with the electrons in the graphene.’


‘Even more important is the discovery that when we lay the graphene on a layer of gold and then induce so-called surface plasmons (optical surface waves) on the surface of the gold using femtosecond laser pulses, the terahertz yield of the graphene rises by up to a factor of 300!’, continues Planken. This makes graphene an interesting possible alternative. ‘We had expected that there would be an enhancement factor if we induced plasmons, but we hadn’t expected it to be this significant.’ Researchers have not yet got to the bottom of the mechanism responsible for this huge increase, but the high pump light intensity in the graphene layer of the surface plasmons plays an important role.

Well before 2030

‘Just two years ago, the Nobel Prize winner Novoselov wrote that he did not expect to see a practical THz generator using graphene before 2030, but I now think that it could be developed, at least an optically pumped one, well before 2030. In any case, we will definitely know whether it’s a possibility within the next few years.’
The article entitled Plasmon enhanced terahertz emission from single-layer graphene published inACS Nano is the result of collaboration between the Optics Research Group and the research group of Prof. Dai-Sik Kim and other researchers at Seoul National University and Ajou University in South Korea.

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