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Researchers Confirm Strong Magneto-Optical Resonance in Graphene

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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 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).   

 


Published: July 2019
Glossary
graphene
Graphene is a two-dimensional allotrope of carbon consisting of a single layer of carbon atoms arranged in a hexagonal lattice pattern. It is the basic building block of other carbon-based materials such as graphite, carbon nanotubes, and fullerenes (e.g., buckyballs). Graphene has garnered significant attention due to its remarkable properties, making it one of the most studied materials in the field of nanotechnology. Key properties of graphene include: Two-dimensional structure:...
terahertz
Terahertz (THz) refers to a unit of frequency in the electromagnetic spectrum, denoting waves with frequencies between 0.1 and 10 terahertz. One terahertz is equivalent to one trillion hertz, or cycles per second. The terahertz frequency range falls between the microwave and infrared regions of the electromagnetic spectrum. Key points about terahertz include: Frequency range: The terahertz range spans from approximately 0.1 terahertz (100 gigahertz) to 10 terahertz. This corresponds to...
infrared
Infrared (IR) refers to the region of the electromagnetic spectrum with wavelengths longer than those of visible light, but shorter than those of microwaves. The infrared spectrum spans wavelengths roughly between 700 nanometers (nm) and 1 millimeter (mm). It is divided into three main subcategories: Near-infrared (NIR): Wavelengths from approximately 700 nm to 1.4 micrometers (µm). Near-infrared light is often used in telecommunications, as well as in various imaging and sensing...
nano
An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
optoelectronics
Optoelectronics is a branch of electronics that focuses on the study and application of devices and systems that use light and its interactions with different materials. The term "optoelectronics" is a combination of "optics" and "electronics," reflecting the interdisciplinary nature of this field. Optoelectronic devices convert electrical signals into optical signals or vice versa, making them crucial in various technologies. Some key components and applications of optoelectronics include: ...
magneto-optics
Magneto-optics refers to the study and manipulation of the interaction between magnetic fields and light (electromagnetic radiation). This field of physics explores how the properties of light, such as its polarization and propagation, are affected by the presence of magnetic materials or external magnetic fields. Key aspects of magneto-optics include: Faraday effect: The Faraday effect is a fundamental phenomenon in magneto-optics. It describes the rotation of the plane of polarization of...
Research & TechnologyeducationEuropeUniversity of GenevaUniversity of ManchesterLight SourcesMaterialsgrapheneterahertzinfrarednanooptoelectronicsmagneto-opticspharmaceuticaldirac materials2D materialsAndré GeimTech Pulse

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