Researchers Confirm Strong Magneto-Optical Resonance in Graphene

The interaction between graphene and light suggests that graphene could be used to control infrared (IR) and terahertz (THz) waves. Researchers from the University of Geneva (UNIGE) and the University of Manchester have demonstrated an efficient way to control IR and THz waves using graphene, in a study that confirms a 2006 theory predicting that graphene could be used in a magnetic field to absorb THz and IR light on demand and control the direction of the circular polarization.

“There exist a class of the so-called Dirac materials, where the electrons behave as if they do not have a mass, similar to the light particles, the photons,” researcher Alexey Kuzmenko said. One such material is graphene, which is composed of a monolayer of carbon atoms arranged in honeycomb structure.

The theoretical prediction from 2006 posited that if a Dirac material was placed in a magnetic field, it would produce a very strong cyclotron resonance. “When a charged particle is in the magnetic field, it moves on a circular orbit and absorbs the electromagnetic energy at the orbiting, or cyclotron, frequency,” Kuzmenko said. “And when the particles have charge but no mass, as electrons in graphene, the absorption of light is at its maximum.”

The experimental device that focused infrared and terahertz radiation on small samples of pure graphene in the magnetic field, built by the UNIGE team. Courtesy of UNIGE, Ievgeniia Nedoliuk.

The researchers needed a very pure graphene to demonstrate maximum absorption, so that the electrons traveling long distances would not scatter when met with impurities or crystal defects. To achieve purity and lattice order, the UNIGE researchers teamed up with a University of Manchester group led by André Geim, a winner of the Nobel Prize in 2010 for his work on graphene.

The researchers developed exceptionally large samples of pure graphene that were nevertheless too small to quantify the cyclotron resonance with well-established techniques. So the Geneva researchers built a special experimental setup to concentrate the IR and THz radiation on small samples of pure graphene in the magnetic field.

Using a custom-designed setup for magneto-IR microspectroscopy, the researchers measured magneto-transmission and Faraday rotation in high-mobility monolayer graphene encapsulated in boron nitride. Results showed strongly enhanced magneto-optical activity in the IR and THz ranges, characterized by absorption of light near the 50% maximum allowed, 100% magnetic circular dichroism, and high Faraday rotation.

The results demonstrated for the first time that a large magneto-optical effect occurs if a layer of pure graphene is used, the researchers said. “The maximum possible magneto-absorption of the infrared light is now achieved in a monoatomic layer,” Kuzmenko said.

The researchers found that they could choose in which direction (to the left or to the right) circular polarization would be absorbed. “Natural or intrinsic graphene is electrically neutral and absorbs all the light, regardless of its polarization,” Kuzmenko said. “But if we introduce electrically charged carriers, either positive or negative, we can choose which polarization is absorbed, and this works both in the infrared and terahertz ranges.” This finding could have an impact on pharmaceuticals development, where certain drug molecules interact with light depending on polarization direction. The researchers also found that magnetic fields generated by an inexpensive permanent magnet were sufficient to observe a strong effect in the THz range.

Now that they have confirmed the 2006 theory, the researchers will continue to work on magnetically adjustable sources and detectors of THz and IR light. Their findings demonstrate the potential of magnetic tuning in 2D Dirac materials for long-wavelength optoelectronics and plasmonics.

The research was published in Nature Nanotechnology (https://doi.org/10.1038/s41565-019-0489-8).