Researchers from Helmholtz-Zentrum Dresden-Rossendorf, Universität Duisburg-Essen, CENTERA Laboratories, Indian Institute of Technology, University of Maryland and the U.S. Naval Research Laboratory have used graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency.
The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, the team estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm−2.
The team fired short terahertz pulses at thousands of micrometer-sized discs of graphene, which briefly turned these minuscule objects into surprisingly strong magnets. This discovery may prove useful for developing future magnetic switches and storage devices. To achieve the best possible conditions, the research group used a particular light source for the experiment: The FELBE free-electron laser at the HZDR can generate extremely intense terahertz pulses.
The result: "The tiny graphene disks briefly turned into electromagnets," reports HZDR physicist Dr. Stephan Winnerl.
"We were able to generate magnetic fields in the range of 0.5 Tesla, which is roughly ten thousand times the Earth's magnetic field." These were short magnetic pulses, only about ten picoseconds or one-hundredth of a billionth of a second long.
To achieve their results, the researchers had to polarize the terahertz flashes in a specific way. Specialized optics changed the direction of oscillation of the radiation so that it moved, figuratively speaking, helically through space. When these circularly polarized flashes hit the micrometer-sized graphene discs, the decisive effect occurred: Stimulated by the radiation, the free electrons in the discs began to circle.
As the basic laws of physics dictate, a circulating current always generates a magnetic field, so the graphene disks mutated into tiny electromagnets.
"The idea is actually quite simple," says Martin Mittendorff, professor at the University of Duisburg-Essen. "In hindsight, we are surprised nobody had done it before." The team said the process was very efficient: compared to experiments irradiating nanoparticles of gold with light, the experiment at the HZDR was a million times more efficient.
The new phenomenon could initially be used for scientific experiments in which material samples are exposed to short but strong magnetic pulses to investigate certain material properties in more detail. "With our method, the magnetic field does not reverse polarity, as is the case with many other methods," explains Winnerl. "It, therefore, remains unipolar." In other words, during the ten picoseconds that the magnetic pulse from the graphene disks lasts, the north pole remains a north pole and the south pole a south pole - a potential advantage for certain series of experiments.
In the long run, those minuscule magnets might even be useful for certain future technologies: as ultra-short radiation flashes generate them, the graphene discs could carry out extremely fast and precise magnetic switching operations.
This would be interesting for magnetic storage technology, for example, but also for spintronics - a form of magnetic electronics. Here, instead of electrical charges flowing in a processor, weak magnetic fields in the form of electron spins are passed on like tiny batons. This could significantly speed up the switching processes once again. Graphene disks could conceivably be used as switchable electromagnets to control future spintronic chips.
However, experts would have to invent very small, highly miniaturized terahertz sources for this purpose - certainly still a long way to go. "You cannot use a full-blown free-electron laser for this, like the one we used in our experiment," comments Stephan Winnerl. "Nevertheless, radiation sources fitting on a laboratory table should be sufficient for future scientific experiments." Such significantly more compact terahertz sources can already be found in some research facilities.
