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FEATURE GRAPHENE


manufacturers do not sell billions of pounds of these ultrafast lasers. They sell a few every year,’ he continued. ‘The market is not huge like consumer electronics, or computers, or mobile phones. That is the major problem. It might take a few years to get people to change their minds and use this kind of laser.’ There has been some research into using graphene as a laser gain material, although Prof Ferrari explained that to engineer graphene as a gain material would be very difficult and it might only work in the terahertz range. Work led by Jigang Wang at Iowa State University found a photo-excited state of graphene that could mean using graphene as a gain medium for light amplification. However, it remains unlikely,


according to Prof Taylor, purely because of graphene’s material properties. ‘It’s probably not really going to happen,’ he said. ‘The emission process is rather weak. The recovery time of the absorption is ultrafast, in the hundred- femtoseconds regime, so in terms of it ever being an efficient laser material – well, who can say never – but I would doubt if it would be looked as


GRAPHENE PROPERTIES


At one atom thick, graphene is the thinnest known material.


More conductive than copper; electrons travel through the lattice at one hundredth that of the speed of light.


Excellent thermal conductivity; heat dissipation in electronics will be more important as electrical devices shrink and circuit density increases.


Harder than diamond and about 300 times harder than steel – yet also flexible.


Isolated in 2004 by Andre Geim and Kostya Novoselov at the University of Manchester; they were awarded the Nobel Prize in Physics in 2010 for their work on graphene.


www.electrooptics.com | @electrooptics


a serious laser material. ‘In 10 years time, I could be wrong,’ he added. ‘People could possibly rearrange the recovery time to be longer so there was storage in the material. But essentially there’s no gain storage because the recombination is ultrafast and femtosecond pulse excitation is presently needed to obtain emission.’ The main use for graphene in laser technology, though, is as a saturable absorber. ‘There’s nowhere where a semiconductor saturable absorber


works where graphene can’t. That, of course, is a huge advantage,’ said Prof Taylor. He went on to say that, at the moment, the biggest consideration is the damage threshold of the graphene host, which is typically a polymer. ‘Once people can find new hosts or other unique ways of putting the graphene at an interface, then the damage problem will be negated.’ Prof Taylor feels that both graphene and semiconductor saturable absorbers are operating at roughly


the same power densities – and, if improvements can be made to the polymer host, then graphene will win the damage threshold competition. It might be a few more years before graphene mode-locked ultrafast lasers are available to buy, but the technology is mature enough. With large amounts of funding being pumped into graphene research, other interesting uses for the material in photonics are bound to develop. l


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