Researchers from three Australian universities have collaborated to develop a light-absorbing device using a new graphene-based film that can absorb unpolarized incident light striking it over a wide range of angles up to 60 degrees.
The 90-nm ultra-thin metamaterial can rapidly heat up to as high as 1600C under sunlight in an open environment. The researchers believe the characteristics of this new class of optical material make it suitable for a wide variety of uses, including desalination of seawater, color displays, photodetectors, and optical components for communication devices.
"Graphene has unique properties," says Han Lin, a senior research fellow at the Centre for Micro-Photonics, Swinburne University of Technology in Melbourne, Australia. "It can absorb any wavelength of light from UV to microwaves." Other absorbents, including various engineered metamaterials all have drawbacks by comparison, he says. "Carbon nanotubes, for instance, are tens to hundreds of micrometers thicker, which impedes device integration."
Lin, first author of a paper on the prototype device published this month in Nature Photonics, notes that attempts to produce practical graphene absorbent devices have been made using multilayer structures, but these approaches have been limited to single polarization, while fabricating them has proved difficult.
"On the other hand, the device we've fabricated with the new graphene based film is polarization insensitive," he says. "That's because it's constructed to lower the effective permittivity to achieve absorption of both transverse electric [TE] and transverse magnetic [TM] polarizations. It is also simple to fabricate."
Eight researchers from Swinburne University of Technology, the University of Sydney, and Australian National University worked together to develop an experimental 2.5 cm x 5 cm prototype device employing the graphene-based film that has approximately 85 percent absorptivity of unpolarized light. The high absorption rate was achieved in part by fabricating a series of layers of the metamaterial.
A multilayer structure is necessary given that a single graphene layer has a light absorption rate of only 2.3 percent. However, creating such a structure and preventing the graphene from turning into graphite and losing important properties proved to be a challenge. This was overcome by separating the graphene layers with layers of polymer dialectric.
A flexible substrate composed of silver mirror (chosen to reflect light back into the device) of 200 nm and an 80-nm thick layer of silicon oxide spacer was laid down using physical vapor deposition. The graphene-based metamaterial is then attached using the wet chemical self-assembly technique in which positively charged polymer dielectric layers and negatively charged graphene-oxide (GO) layers are alternately deposited using static electric force.
"We attach a polymer layer to the substrate first, then a GO layer," says Professor Baohua Jia, research leader, Light Nanomaterial Interaction Research Group, Swinburne University of Technology. "Then another layer of polymer and another layer of graphic oxide and so on. Positive-negative, positive-negative charged films alternatively. They attract each other, so the material can be self-assembled."
About 30 GO layers are stacked using this technique to maximize light absorption. "All we do is simply dip the substrate into the graphene oxide solution and the coating takes place automatically," Jia explains. "Using this method, we not only control the thickness of the film with nanometer accuracy but also its uniformity. It's simple and inexpensive because no special equipment is necessary."
The metamaterial is then sectioned by treating it with a femtosecond laser beam that removes some of the metamaterial, creating a grating structure of air grooves around the resulting sections. The beam also converts the GO layers into graphene-like material from which the oxygen is almost completely removed, and which reduces a GO layer's thickness to approximately 1 nanometer.
The grating structure sends the incident light striking the device sideways into waveguide TE and TM modes that propagate along the surface in both directions. "This structure increases the amount of light absorbed and so is the key to our achieving 85 percent aborptivity," says Lin.
Thermal image of graphene-based ultra-thin light absorbent film. Image: Swinburne University of Technology
In addition, the ultrathin design reduces the amount of material used, which enables it to heat up rapidly. "This is why the device can heat from 300C to 1500C in 30 seconds, he says." Furthermore, the ultrathin property allows easy heat transfer from the metamaterial to the material needed to be heated, such as water, and so could be used to desalinate seawater, for instance.
Lin notes that while some additional optimization will continue to be done, the researchers are quite happy with the results achieved. "We now have a good balance, so we are seeking to collaborate with industry to create applications," he explains. The group is already working with one company to create a machine to scale up the graphene multilayer coating process.
As for commercialization, "Two to three years would be a reasonable time," says Jia. "But we think it could happen even faster than that."