Graphene is a sheet of carbon just one atom thick and has a host of unique mechanical and electronic properties. Although research has mainly focused on graphene's electronic properties (and rightly so, since the material has even been touted to replace silicon as the material of choice in future electronics), it also has a number of other special characteristics. For example, it is impermeable to gases, is very elastic – it can be stretched by up to 20% – and is transparent to light. Its elasticity means that bubbles of various shapes can be "blown" from the material and all three of these last properties could make graphene ideal for making optical lenses.

Novoselov (who together with Andre Geim at Manchester first fabricated graphene in 2004) is now saying that circular-shaped graphene bubbles could be used to make lenses whose curvature can be controlled by applying an external voltage. This means that their focal length can be varied – just like in conventional adaptive focus lenses, which try to mimic how the human eye works.

Such lenses are employed in many modern optical systems, such as mobile phone cameras, webcams and auto-focusing eye glasses. Current adaptive focus lens are usually made of transparent liquid crystals (LCs) or fluids. The LC ones work thanks to the fact that the refractive index of liquid-crystal materials changes in response to an external electric field. The fluid-filled lenses work slightly differently in that they change shape when compressed, which also results in a change in the focal length of the lens.

Easier fabrication
Although such devices work well, they are relatively difficult to make because they generally involve placing two or more liquids or liquid-crystal layers between transparent electrodes – for example, those made of indium tin oxide (ITO). ITO has the added disadvantage of being expensive as indium is becoming increasingly rare. Graphene-based optics could be fabricated using much simpler methods, say the Manchester researchers, and would be cheaper too as methods to scale up graphene become easier.

The team began by preparing large graphene flakes on flat silicon oxide substrates. When the air underneath the graphene cannot escape, a bubble of the material naturally forms. The researchers identified such structures, which are remarkably stable and range in size from a few tens of nanometres to tens of microns, using an optical microscope.

To show that the bubbles could work as adaptive focus lenses, Novoselov and co-workers fabricated devices that contained titanium/gold electrodes contacted to the bubbles in a transistor-like arrangement. In this way, they could apply a gate voltage to the set-up. The researchers then took optical microphotographs of the structures while tuning the gate voltage from –35 to +35 V. As expected, they saw the shape of the bubbles go from being highly curved to more flat as the voltage increased.

Real, working lenses would simply be made by filling the graphene bubbles with a high-refractive index liquid or by covering the bubbles with a flat layer of this liquid, says the team. Indeed, the new graphene bubble lenses are a cross between conventional liquid-crystal and fluid-filled adaptive lenses, it adds.

So what is next? "We have shown that controlling the curvature of these bubbles is an easy task," Novoselov told nanotechweb.org. "We are now looking at performing other experiments where more complicated deformations in graphene would be created and controlled."

Novoselov and Geim won the 2010 Nobel Prize in Physics for their work on graphene.

The current results are published in Applied Physics Letters.