On-demand control of terahertz and infrared waves

The ability to control infrared and terahertz waves with the help of magnetic or electric fields is one of the great challenges of physics that could revolutionize optoelectronics, telecommunications and medical diagnosis. A theory of 2006 predicts that it should be possible to use graphene – a monoatomic layer of carbon atoms – in a magnetic field not only to absorb terahertz light and infrared on demand, but also to control the direction of circular polarization. Researchers from the University of Geneva (UNIGE) in Switzerland and the University of Manchester have successfully tested this theory and achieved the expected results. The study, to appear in the journal Nature Nanotechnology, shows that scientists have found an effective way to control infrared and terahertz waves. It also shows that graphene holds its initial promise and presents itself as the material of the future, whether on earth or in space.

"There is a class of Dirac materials, where the electrons behave as they do not have mass, like photons," says Alexey Kuzmenko, a researcher at the Department of Quantum Physics at the University of California. UNIGE. Faculty of Science, who conducted this research with Ievgeniia Nedoliuk. One of these Dirac materials is graphene, a monolayer of carbon atoms arranged in a honeycomb structure, akin to the graphite used to make pencils.

The interaction between graphene and light suggests that this material could be used to control infrared and terahertz waves. "It would be a huge step forward for optoelectronics, security, telecommunications and medical diagnosis," says the Geneva-based researcher.

Saving an old theory via experimentation

A theoretical prediction of 2006 postulated 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 travels in a circular orbit and absorbs electromagnetic energy at the orbiting frequency, or cyclotron, as in the large CERN hadron collider, by example, "explains Alexey Kuzmenko. "And when the particles are charged but not mass, as electrons in graphene, the absorption of light is at its maximum!"

To demonstrate this maximum absorption, physicists needed a very pure graphene so that electrons traveling long distances did not disperse on impurities or crystalline defects. But this level of purity and this network order are very difficult to obtain and are achieved only when the graphene is encapsulated in another two-dimensional material, boron nitride.

UNIGE researchers teamed up with the University of Manchester group led by André Geim, the Nobel Prize winner in physics for the discovery of graphene in 2010, to develop extremely pure graphene samples. These samples, exceptionally large for this type of graphene, were nevertheless too small to quantify the cyclotron resonance with well established techniques. That is why Genevan researchers have come up with a special experimental setup to focus infrared and terahertz radiation on small samples of pure graphene in the magnetic field. "And the result of the experiment confirmed the theory of 2006!" Alexey Kuzmenko adds.

Custom controlled polarization

The results demonstrated for the first time that a gigantic magneto-optical effect occurred if we used a pure graphene layer. "The maximum possible magneto-absorption of infrared light is now achieved in a monatomic layer," said Kuzmenko.

In addition, physicists discovered that it was possible to choose which circular polarization – left or right – had to be absorbed. "Natural or intrinsic graphene is electrically neutral and absorbs all light, whatever its polarization, but if we introduce electrically charged carriers, whether they are positive or negative, we can choose the polarization to absorb, which works as well in the infrared as in the terahertz, "continues the scientist. This ability plays a crucial role, especially in pharmacy, where some key molecules of the drug interact with light according to the direction of polarization. Interestingly, this control is considered promising for life research on exoplanets, as it is possible to observe the signatures of the molecular chirality inherent in biological matter.

Finally, physicists have found that to observe a powerful effect in the terahertz range, it was sufficient to apply magnetic fields, which could already be generated by inexpensive permanent magnets. Now that the theory has been confirmed, researchers will continue to work on sources and detectors with magnetic adjustment of terahertz and infrared light. Graphene continues to surprise them.