Graphene has been heralded as a revolutionary material ever since its isolation in 2004, for which the scientists involved won the Nobel Prize in Physics. Greg Blackman finds out that its unique properties are also beneficial in laser technology
Harder than diamond yet flexible, graphene has been touted as a material that will affect everyone’s life in some respect, at some point. Electronics, computing, medical diagnostics, aerospace, energy storage, solar cells that can be painted onto a surface, environmental monitoring, desalination technology – the list of potential uses for graphene goes on. The material, made up of carbon atoms arranged in a honeycomb structure, is only one atom thick and possesses remarkable mechanical, electronic, optical, thermal, and chemical properties. The scientists who first isolated it in 2004, Andre Geim and Kostya Novoselov at the University of Manchester in the UK, won the Nobel Prize in Physics in 2010 for their work on graphene.
A lot of the research has gone into its electronic properties and how electrons flow through the sheet of carbon atoms. Graphene conducts far better than copper, with electrons travelling through the lattice at a rate of one hundredth that of the speed of light. However, it also has unique optical properties and, although it is the thinnest material ever made, it can still be seen by the human eye, absorbing 2.3 per cent of white light. It also absorbs over a broadband spectrum and its material properties make it an excellent saturable absorber for producing ultrashort laser pulses, which is where most of the research regarding graphene and laser technology has been focused.
In 2009 the group led by Professor Andrea Ferrari, director of Cambridge Graphene Centre at the University of Cambridge, was the first to create a mode-locked laser incorporating graphene. Graphene absorbs light and releases it back in very short bursts – Professor Ferrari’s group has achieved pulse durations of around 100fs, although he said 20fs or even 10fs should theoretically be possible.
Now, according to Prof Ferrari, the technology is close to commercialisation – a spin-off from Cambridge University, called CamLase, is one of the first companies to offer ultrafast laser modules based on graphene nanotube saturable absorbers.
Traditional saturable absorbers are semiconductors. Semiconductors contain an energy band-gap, so there is a minimum energy that has to be overcome before light is absorbed from the valence band into the conduction band. This means that only certain wavelengths are absorbed – therefore, each laser wavelength has to have a specific saturable absorber, which, according to Professor Roy Taylor, a physicist at Imperial College London, is expensive to manufacture. ‘The advantage of graphene is that it is much more easily made and, essentially, one-size-fits-all – once you’ve made the graphene it will act as a saturable absorber for any laser, so it is universal. That means your costs are cut quite dramatically,’ Prof Taylor said. There’s no band-gap in graphene; it has a point band-gap structure, so any wavelength, within limits, can be absorbed, from around 400nm up to the mid-infrared at a couple of microns.
‘To design a semiconductor structure with a band-gap at a certain wavelength means a lot of wavelength engineering,’ Prof Taylor continued. ‘There are certain wavelength ranges where it is actually exceedingly difficult to do because it puts too big a strain on the semiconductor structure. This is where graphene, which is essentially one material acting as a single absorber, has an advantage.’
Prof Taylor’s group at Imperial College is working on a universal laser, a short-pulse fibre laser system. Instead of doping silica fibres with rare earth metals like neodymium, ytterbium or erbium, the group is using the vibrations of the glass structure itself to achieve gain around the Raman spectral region.
Silica glass will have a broad structure in that it will have a totally different structure from one point to the next in the fibre, so the Raman signature is broadband. The team can generate any wavelength they want inside the silica fibre by pumping it with continuous wave light to get a Raman shift of about 400 wave numbers. The light that is shifted will be able to support short pulses by mode-locking with graphene.
This ‘universal laser’ could have uses in environmental sensing for probing pollutants in the air, or in screening for explosives at airports, for example, by interrogating a sample’s Raman spectra.
Prof Ferrari said graphene is easy to handle; mode-locked lasers incorporating graphene can be made with wet chemistry or simple nano-integration to produce a tuneable ultrafast laser. ‘We are close to the point where the laser manufacturers may consider commercialisation of graphene,’ he said, adding that for some kinds of fibre laser the material is mature enough. ‘It needs somebody to do the investment in order to produce the laser.
‘The problem is the laser 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 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.