Together with their colleagues from Germany and the Netherlands, scientists at the Moscow Institute of Physics and Technology (MIPT) have found a way to significantly improve computer performance. In their paper published in Nature Photonics, they propose the use of the so-called T-waves, or terahertz radiation as a means of resetting computer memory cells. This process is several thousand times faster than magnetic-field-induced switching.
“We have demonstrated an entirely new way of controlling magnetization, which relies on short electromagnetic pulses at terahertz frequencies. This is an important step towards terahertz electronics. As far as we know, our study is the first to make use of this mechanism to trigger the oscillations of magnetic subsystems,” says Anatoly Zvezdin of Prokhorov General Physics Institute and MIPT, a coauthor of the paper and a USSR State Prize-winning scientist heading MIPT’s Laboratory of Physics of Magnetic Heterostructures and Spintronics for Energy-Saving Information Technologies.
The rapidly increasing amounts of digital data that have to be manipulated, along with the growing complexity of the computation tasks at hand, compel hardware designers to achieve ever higher computational speeds. Many experts believe that classical computation is currently approaching a limit, beyond which no further increase in data processing speed will be practicable. This is motivating scientists all over the world to investigate possibilities of entirely different computer technologies. One of the weak spots in modern computers holding back their evolution is memory: it takes time to complete every set/reset operation for a magnetic memory cell, and reducing the duration of this cycle is a very challenging task.
A group of scientists including Sebastian Baierl of the University of Regensburg, Anatoly Zvezdin, and Alexey Kimel of Radboud University Nijmegen (the Netherlands) and Moscow Technological University (MIREA) proposed that electromagnetic pulses at terahertz frequencies (with wavelengths of about 0.1 millimeters, i.e., between those of microwaves and infrared light) could be used in memory switching instead of external magnetic fields. A more familiar device that makes use of terahertz radiation is the airport body scanner. T-rays can expose weapons or explosives concealed under a person’s clothing, without causing any harm to live tissues.
To find out whether T-rays could be used for convenient memory states switching (storing “magnetic bits” of information), the researchers performed an experiment with thulium orthoferrite (TmFeO?). As a weak ferromagnet, it generates a magnetic field by virtue of the ordered alignment of the magnetic moments, or spins of atoms in the microcrystals (magnetic domains). In order to induce a reorientation of spins, an external magnetic field is necessary.
However, the experiment has shown that it is also possible to control magnetization directly by using terahertz radiation, which excites electronic transitions in thulium ions and alters the magnetic properties of both iron and thulium ions. Furthermore, the effect of T-rays proved to be almost ten times greater than that of the external magnetic field. In other words, the researchers have devised a fast and highly efficient remagnetization technique—a solid foundation for developing ultrafast memory.
The scientists expect their “T-ray switching” to work with other materials as well. Thulium orthoferrite, which was used in the experiment, happens to be convenient for the purposes of demonstration, but the proposed magnetization control scheme itself is applicable to many other magnetic materials.
“There was a Soviet research group that used orthoferrites in their studies, so this was always kind of a priority field for us. This research can be seen as a follow-up on their studies,” says Anatoly Zvezdin.
Tunable radiation source that reaches coveted THz region of spectrum could be used for medical imaging or security applications
Terahertz radiation, the no-man’s land of the electromagnetic spectrum, has long stymied researchers. Optical technologies can finagle light in the shorter-wavelength visible and infrared range, while electromagnetic techniques can manipulate longer-wavelength radiation like microwaves and radio waves. Terahertz radiation, on the other hand, lies in the gap between microwaves and infrared, whether neither traditional way to manipulate waves works effectively. As a result, creating coherent artificial sources of terahertz radiation in order to harness it for human use requires some ingenuity.
Difficulties of generating it aside, terahertz radiation has a wide variety of potential applications, particularly in medical and security fields. Because it’s a non-ionizing form of radiation, it is generally considered safe to use on the human body. For instance, it can distinguish between tissues of different water content or density, making it a potentially valuable tool for identifying tumors. It could also be used to detect explosives or hidden weapons, or to wirelessly transmit data.
In a step towards more widespread use of terahertz radiation, researchers have designed a new device that can convert a DC electric field into a tunable source of terahertz radiation. Their results are published this week in the Journal of Applied Physics, from AIP Publishing.
This device exploits the instabilities in the oscillation of conducting electrons at the device’s surface, a phenomenon known as surface plasmon resonance. To address the terahertz gap, the team created a hybrid semiconductor: a layer of thick conducting material paired with two thin, two-dimensional crystalline layers made from graphene, silicene (a graphene-like material made from silicon instead of carbon), or a two-dimensional electron gas. When a direct current is passed through the hybrid semiconductor, it creates a plasmon instability at a particular wavenumber. This instability induces the emission of terahertz radiation, which can be harnessed with the help of a surface grating that splits the radiation.
By adjusting various parameters — such as the density of conduction electrons in the material or the strength of the DC electric field — it is possible to tune the cutoff wavenumber and, consequently, the frequency of the resulting terahertz radiation.
“[Our work] demonstrates a new approach for efficient energy conversation from a dc electric field to coherent, high-power and electrically tunable terahertz emission by using hybrid semiconductors,” said Andrii Iurov, a researcher with a dual appointment at the University of New Mexico‘s Center for High Technology Materials and the City University of New York. “Additionally, our proposed approach based on hybrid semiconductors can be generalized to include other novel two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide and tungsten diselenide.”
Other labs have created artificial sources of terahertz radiation, but this design could enable better imaging capabilities than other sources can provide. “Our proposed devices can retain the terahertz frequency like other terahertz sources but with a much shorter wavelength for an improved spatial resolution in imaging application as well as a very wide frequency tuning range from a microwave to a terahertz wave,” said Iurov.