Tunable graphene device puts terahertz to work

Ashley N. Paddock, ashley.paddock@photonics.com

A microscale graphene device that responds strongly to terahertz light and that can be precisely tuned is the first tool in a kit for putting terahertz light to work.

At the heart of the device, developed by scientists at Lawrence Berkeley National Laboratory and the University of California, is an array composed of graphene ribbons millionths of a meter wide. The varying width of the ribbons and the concentrations of their charge carriers have enabled the scientists to control the collective oscillations, or plasmons, of electrons within the microribbons, said Feng Wang, assistant professor at UC Berkeley.

He explained that, in high-frequency visible light, plasmons happen in three-dimensional metal nanostructures, while graphene – which is only one atom thick – and its electrons move in only two dimensions. In 2-D systems, plasmons occur at much lower frequencies.

“Plasmon resonance in graphene is very interesting scientifically due to the unique two-dimensional Dirac electron characteristics,” Wang said. “We started the project to investigate how one can probe the graphene plasmon using photons. Then we realized that graphene plasmonics can be useful for terahertz metamaterials.”

The graphene microribbon array can be tuned in three ways: Varying the width of the ribbons changes plasmon resonant frequency and absorbs corresponding frequencies of terahertz light. Plasmon response is much stronger when there is a dense concentration of charge carriers (electrons or holes), controlled by varying the top gate voltage. Finally, light polarized perpendicularly to the ribbons is strongly absorbed at the plasmon resonant frequency, while parallel polarization shows no such response. Courtesy of Lawrence Berkeley National Laboratory.

In 2-D graphene, electrons have a tiny rest mass and respond quickly to electric fields. Plasmons’ frequencies depend on how rapidly waves in this electron sea slosh back and forth between the edges of a graphene microribbon. When light of the same frequency is applied, the result is resonant excitation – a marked increase in the strength of the oscillation – and simultaneous strong absorption of the light at that frequency. Because the frequency of the oscillations is determined by the width of the ribbons, varying their width can tune the system to absorb different light frequencies.

The scientists discovered that the strength of the light-plasmon coupling can also be affected by the concentration of charge carriers – electrons and their positively charged counterparts, holes. One remarkable characteristic of graphene is that the concentration of its charge carriers can easily be increased or decreased simply by applying a strong electric field, a process called electrostatic doping.

The new device incorporates both of these methods for tuning the response to terahertz light. Microribbon arrays were made by depositing an atom-thick layer of carbon on a sheet of copper, then transferring the graphene layer to a silicon-oxide substrate and etching ribbon patterns into it. An ion gel with contact points for varying the voltage was placed on top of the graphene.

The gated graphene microarray was then illuminated with terahertz radiation at beamline 1.4 of Berkeley Lab’s Advanced Light Source, and transmission measurements were made with the beamline’s infrared spectrometer. In this way, the research team demonstrated coupling between light and plasmons that was stronger by an order of magnitude than in other 2-D systems.

A final method of controlling plasmon strength and terahertz absorption depends on polarization. Light shining in the same direction as the graphene ribbons shows no variations in absorption according to frequency. But light shining at right angles to the ribbons – the same orientation as the oscillating electron sea – yields sharp absorption peaks. In addition, light absorption in conventional 2-D semiconductor systems, such as quantum wells, can be measured only at temperatures near absolute zero. The team measured prominent absorption peaks at room temperature.

At a constant carrier density, varying the width of the graphene ribbons – from 1 to 4 µm – changes the plasmon resonant frequency from 6 to 3 THz. The spectra of light transmitted through the device (right) show corresponding absorption peaks.

A precursor of devices to come, the experimental setup could control the polarization and modify the intensity of terahertz light and could enable other optical and electronic components in applications from medical imaging to astronomy – all in two dimensions.

Next, the team plans to explore various metamaterial designs to further control the graphene plasmon resonances as well as explore different-length scales to extend the available spectral range that can be reached, Wang said. In addition, they will examine what fundamental limitations on quality factors there are for the plasmon resonances.