Physics World – A tiny bubble of graphene could be used to make an optical lens with an adjustable focal length. That is the claim of physicists in the UK, who have shown that the curvature of such bubbles can be controlled by applying an external voltage. Devices based on the discovery could find use in adaptive-focus systems that try to mimic how the human eye works.
Graphene is a sheet of carbon just one atom thick and has a host of unique mechanical and electronic properties. It is extremely elastic and can be stretched by up to 20%, which means that bubbles of various shapes can be “blown” from the material. This, combined with the fact that graphene is transparent to light yet impermeable to most liquids and gases, could make the material ideal for creating adaptive-focus optical lenses.
An atomic-force-microscope image of a graphene bubble. The bubble is about 3 µm in diameter. (Courtesy: Applied Physics Letters)
Raised above the substrate and elastically deformed areas of graphene in the form of bubbles are found on different substrates. They come in a variety of shapes, including those which allow strong modification of the electronic properties of graphene. We show that the shape of the bubble can be controlled by an external electric field. This effect can be used to make graphene-based adaptive focus lenses.
Such lenses are employed in mobile-phone cameras, webcams and auto-focusing eye glasses, and are usually made of transparent liquid crystals or fluids. Although such devices work well, they are relatively difficult and expensive to make. In principle, graphene-based adaptive optics could be fabricated using much simpler methods than those used for existing devices. They could also become cheaper to produce if industrial-scale processes to manufacture graphene devices become available.
Now Andre Geim and Konstantin Novoselov – who shared the 2010 Nobel Prize for Physics for discovery of graphene – have built tiny devices that show how graphene could be used in adaptive optical systems. Working with colleagues at the University of Manchester, the physicists 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 bubbles are extremely stable and range in size from a few tens of nanometres to tens of micrometres in diameter.
To show that the bubbles could work as adaptive-focus lenses, the team made devices that contained titanium/gold electrodes contacted to the bubbles in a transistor-like arrangement. In this way, the researchers were able to apply a gate voltage to the set-up. They then obtained optical-microscope images 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 changed.
Real, working lenses could 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, say the researchers.
So, what is next? “We have shown that controlling the curvature of these bubbles is an easy task,” says Novoselov. “We are now looking at performing other experiments where more complicated deformations in graphene would be created and controlled.”