Either 1 or 0. Either current flows or it does not. In electronics, everything is controlled via the binary system. Electrons already generate information quite fast and well, pass them on and take over various switching functions. But it can be done even faster. Paul Herrmann and Sebastian Klimmer from the Friedrich Schiller University of Jena have proven that. To this end, the two doctoral students at the Institute of Solid State Physics have experimented with monocrystalline 2D materials and laser light. They combined the well-known physical method of frequency doubling of light with a special material property, valleypolarization, and achieved astonishing results.
Asked about their favorite toy from childhood, Paul Herrmann and Sebastian Klimmer don’t have to think twice. They answer unanimously: the Lego construction set. The best thing about it was that there were so many possible combinations, they both explain in unison. The young physicists have retained their enthusiasm for construction kits to this day - but for some time now, they have been working on a construction kit of a completely different format for their doctorate: so-called 2D materials, which they break down into atomic layers in order to manipulate them with "Valleytronik".
Paul Herrmann and Sebastian Klimmer are conducting research on this topic at the Institute of Solid State Physics at Jena University in the "Ultrafast Optical Spectroscopy" group led by Junior Professor Dr. Giancarlo Soavi. Their goal is to find new materials and technical possibilities that help to make information processing and transmission with modern electronics orders of magnitude faster.
To do this, they use light as a tool - a major topic not only in physics at Jena University - and the high-tech construction kit of 2D materials.These materials, which consist of only one layer of atoms, have outstanding optical properties that make them so interesting for research," explains Paul Herrmann, who has been a member of Giancarlo Soavi’s research group for a year. ,,In 2004, Nobel laureates Geim and Novoselov succeeded for the first time in producing two-dimensional layers of carbon atoms, or graphene. Since then, many other 2D materials have been discovered by scientists around the world," adds Sebastian Klimmer. "Theoretical models also predict that there should be about 1,800 of them. This is practically our modern Lego box, whose building blocks offer us endless combination possibilities."
Using light to alternately manipulate local extrema
The two Jena physicists chose tungsten diselenide, which belongs to the group of transition metal dichalcogenides, from this construction kit.This special semiconductor material has local extrema in its electronic band structure, so-called valleys, which we can manipulate with light," Paul Herrmann explains the choice.
The young researchers are working successfully with these materials in the laboratory.We bombard the material with a circularly polarized laser. This can be done in two different directions, so we can use it to determine in which valley we excite electrons," Herrmann explains.This phenomenon of valley polarization - the state in which one valley is more excited than the other - can be exploited to encode, manipulate and read out information," Klimmer continues.
At the same time, the researchers are exploiting the effect of "second-harmonic generation," i.e., the frequency doubling of light, which has been known since the 1960s.We use an infrared laser at a wavelength of 1,500 nanometers. This allows us to resonantly drive the frequency doubling in tungsten diselenide with two photons and thus further enhance the induced valleypolarization," Herrmann describes the complicated process.Furthermore, the use of frequency doubling allows us a much simpler separation of the excitation light and the signal of interest to us, which is correspondingly at 750 nanometers, half the wavelength and twice the frequency, respectively," adds Klimmer.
Light makes electronics 1,000 times faster
Until now, the binary system has been used in electronics; electricity is switched on or off to transmit information. A transistor can perform about one billion calculations per second. By switching electronics with light instead of electricity, this can be increased to one trillion calculations per second. This means that our system is 1,000 times faster than conventional electronics," says Paul Herrmann, summarizing the Jena research results. This makes the solution interesting for many areas of optoelectronics and technology.
Herrmann and Klimmer report on their research on ,,Nonlinear Optical Coherent Generation and Readout of Valleys in Atomically Thin Semiconductors" in a new paper in the journal ,,Small", which is already online. What makes this Jena work original is the combination of the method of resonant two-photon frequency doubling and valley polarization.
Research field of 2D materials booming at the University of Jena.
Sebastian Klimmer and Paul Herrmann assume that it will be several years before the new findings on 2D materials and technical solutions from Jena can be used on a larger scale. After all, they are busy with basic research. But the two twenty-somethings are not the only ones who are convinced of the opportunities offered by the new high-tech materials. At the Institute of Solid State Physics at the University of Jena, around 15 physicists in Giancarlo Soavi’s research group are currently working on 2D materials. They are also involved in the work of the Collaborative Research Center SFB 1375 ,,NOA - Nonlinear Optics down to the Atomic Range" en , which was established at Friedrich Schiller University in July 2019 and has just been extended. The Institute of Solid State Theory and Optics has been a cooperation partner, he said, as has the Research Training Group ,,Tailored Meta Surfaces - Generation, Programming and Detection of Light," in which Klimmer is a member. In addition, 2D materials also play an important role in several other institutes at Friedrich Schiller University Jena.