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Theoretical framework illuminates graphene’s optomechanical properties

By Rachel Berkowitz January 11, 2018
Theoretical framework
Charge distributions in a two-layer stacking of graphene sheets which result in (a) attraction or (b) repulsion between the two sheets. Excitation wavelength λ = 10 μm and separation distance d = 100 nm. Re{Ex} is the real-part corresponding to the x-component of the electric field.

Graphene is an important candidate for optomechanical devices, for applications in which optical forces manipulate a material’s mechanical properties. But while scientists know how to deform thin film structures under the pressure of laser light, enhancing these weak optical forces without increasing the incident light power has been a challenge. Now, a research team has determined how to amplify optical forces and control the resulting mechanical deformation in stacks of graphene sheets.

In addition to desirable mechanical properties such as low mass density, high elastic strength, and flexibility, graphene has unique optical properties: incident light couples with charge carriers on the two-dimensional sheets, causing the charges to oscillate together and create a self-sustaining, propagating wave called a surface plasmon polariton—like a standing wave on the sheet’s surface. When excited, the mode of this standing wave exerts pressure that deforms the sheet itself. Hossein Mosallaei, professor of physics in the Department of Electrical and Computer Engineering at Northeastern University in Boston, has developed a theoretical framework that shows how these optical modes lead to different mechanical effects in stacks of graphene.

“The novel idea is that we can manage optical forces in graphene sheets without any contact,” Mosallaei says. His student Mohammad Salary, lead author on the study published in a recent issue of the Journal of Materials Research, adds that “we can create tiny blisters on the graphene, useful for flattening wrinkles during fabrication or for deforming the sheets for a wide range of applications.”

In their calculations, they first determined the electromagnetic fields that arise when light of 1 mW/µm intensity and 10 µm wavelength strikes a layered structure of three to five graphene sheets, spaced 100 nm apart. They also calculated the optical pressures exerted on each graphene sheet. “You need to link each layer to the next layer,” Mosallaei says.  “As we increase the number of layers, we can have access to more numbers of optical modes, and therefore more mechanical states.” Finally, they determined the resulting deformation profiles for the stacked layers.

They found that coupling between excited optical modes can enhance the optical forces and lead to small regions of deflection in the sheets, several nanometers in height. Stacking the sheets provides access to a spectrum of modes which can be selected to tailor the optical forces and the resulting mechanical deformation. The sign of these optical forces—whether the charges distributed across the graphene sheets attract or repel one another, and therefore which direction the sheet “bends”—depends on the coupling of the optical modes. “Both attractive and repulsive forces can be achieved, giving rise to concave or convex graphene blisters,” Salary says. 

“[The researchers] suggest mechanical deformation of atomically thin graphene membrane[s] utilizing optical force in the guided modes in the layered systems. Experimental realization of this modelling work can be an exciting new route to create optomechanical devices based on stacks of graphene,” says Philip Kim, professor of physics and applied physics at Harvard University who was not involved in this work.

One immediate application is using light to smooth the wrinkles that appear when fabricating graphene on a substrate. “The light is like a pulse, with which you can push the graphene up or down at different places,” Mosallaei says. Another possibility is increasing the number of layers to access different mechanical situations, and switching between these mechanical states by applying a voltage. This has promise for optically controlled nanoactuators. “The layered structures are already being fabricated,” Salary says. “We expect these optical forces will be verified experimentally in the near future.”

Read the abstract in the Journal of Materials Research.