Category: HZDR

Ultrafast and tunable

Ultrafast and tunable

Terahertz-to-visible light conversion for future telecommunications

A study carried out by a research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Catalan Institute of Nanoscience and Nanotechnology (ICN2), University of Exeter Centre for Graphene Science, and TU Eindhoven demonstrates that graphene-based materials can be used to efficiently convert high-frequency signals into visible light, and that this mechanism is ultrafast and tunable, as the team presents its findings in Nano Letters (DOI: 10.1021/acs.nanolett.3c00507). These outcomes open the path to exciting applications in near-future information and communication technologies.

The ability to convert signals from one frequency regime to another is key to various technologies, in particular in telecommunications, where, for example, data processed by electronic devices are often transmitted as optical signals through glass fibers. To enable significantly higher data transmission rates future 6G wireless communication systems will need to extend the carrier frequency above 100 gigahertz up to the terahertz range. Terahertz waves are a part of the electromagnetic spectrum that lies between microwaves and infrared light. However, terahertz waves can only be used to transport data wirelessly over very limited distances. “Therefore, a fast and controllable mechanism to convert terahertz waves into visible or infrared light will be required, which can be transported via optical fibers. Imaging and sensing technologies could also benefit from such a mechanism,” says Dr. Igor Ilyakov of the Institute of Radiation Physics at HZDR.

Read more on the HZDR website

Image: A graphene-based material converts incoming terahertz pulses (from above) into visible light in an ultrafast and controllable manner – optimal for data transport in optical fibers.

Credit: B. Schröder/HZDR

Magnetic sandwich mediating between two worlds

Magnetic sandwich mediating between two worlds

Scientists couple terahertz radiation with spin waves

An international research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a new method for the efficient coupling of terahertz waves with waves of much shorter wavelengths, so-called spin waves. As the experts report in the journal Nature Physics (DOI:  10.1038/s41567-022-01908-1), their experiments, in combination with theoretical models, clarify the fundamental mechanisms of this process previously thought impossible. The results are an important step for the development of novel, energy-saving spin-based technologies for data processing.

“We were able to efficiently excite high-energy spin waves using terahertz light in a sandwich-like material system consisting of two metal films a few nanometers thick, with a ferromagnetic layer sandwiched in between,” says Dr. Sergey Kovalev of the Institute of Radiation Physics at HZDR, where the experiments were conducted. Electrons have an effective spin which behaves like a spinning top. And like a gyroscope, an external perturbation can tilt the spin’s axis of rotation: A gyroscopic motion, called precession, follows suit. In ferromagnetic materials, there is a very strong interaction between the electron spins, and as a result, a precession started locally continues in the form of a spin wave throughout the ferromagnetic material layer. This is interesting because a spin wave – like any wave – can be used as an information carrier.  While each electron spin is in motion, in the ferromagnets considered it remains in its position in the atomic lattice, therefore no current flow is involved. So, unlike in today’s computer chips, there are no heat losses due to currents in spin-based devices.

Read more on the HZDR website

Image: A terahertz light wave (from left) is converted into a spin wave (right) in a sample of thin metallic layers. In a heavy metal layer (left), electrical currents are first excited by the terahertz field. Within an ultrashort time, the spin Hall effect leads to the accumulation of spins with a certain orientation at the interface with a ferromagnetic layer (right). This directed spin current then triggers a coherent, nanomater-wavelength spin wave in the ferromagnetic material.

Credit: HZDR/Juniks

Milestone for laser technology

Milestone for laser technology

Research team demonstrates free-electron laser driven by plasma accelerated electron beams and seeded by additional light pulses.

Extremely intense light pulses generated by free-electron lasers (FELs) are versatile tools in research. Particularly in the X-ray range, they can be deployed to analyze the details of atomic structures of a wide variety of materials and to follow fundamental ultrafast processes with great precision. Until now, FELs such as the European XFEL in Germany are based on conventional electron accelerators, which make them long and expensive. An international team led by Synchrotron SOLEIL, France, and Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany, has now achieved a breakthrough on the way to an affordable alternative solution: they were able to demonstrate seeded FEL lasing in the ultraviolet regime based on a still young technology – laser-plasma acceleration. In the future, this might allow to build more compact systems, which would considerably expand the possible applications of FELs. The research collaboration presents their results in the journal Nature Photonics Nature Photonics (DOI: 10.1038/s41566-022-01104-w).

Read more on the HZDR website

Image: Together with colleagues from Synchrotron SOLEIL, LOA, PhLAM and HZDR, the German-French team succeeded for the first time in generating well-controllable laser light in a free-electron laser via plasma acceleration (Dr. Marie-Emanuelle Couprie, Dr. Arie Irman, Prof. Ulrich Schramm, Dr. Marie Labat, Dr. Amin Ghaiht, Dr. Maxwell LaBerge, Dr. Driss Oumbarek-Espinos, Dr. Alexandre Loulergue, Dr. Jurjen Couperus Cabadağ, Patrick Ufer, Dr. Yen-Yu Chang; from left to right)

Credit: HZDR/Sylvio Dittrich

An X-ray view of carbon

An X-ray view of carbon

New measurement method promises spectacular insights into the interior of planets

At the heart of planets, extreme states are to be found: temperatures of thousands of degrees, pressures a million times greater than atmospheric pressure. They can therefore only be explored directly to a limited extent – which is why the expert community is trying to use sophisticated experiments to recreate equivalent extreme conditions. An international research team including the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has adapted an established measurement method to these extreme conditions and tested it successfully: Using the light flashes of the world’s strongest X-ray laser the team managed to take a closer look at the important element, carbon, along with its chemical properties. As reported in the journal Physics of Plasmas (DOI: 10.1063/5.0048150), the method now has the potential to deliver new insights into the interior of planets both within and outside of our solar system.

The heat is unimaginable, the pressure huge: The conditions in the interior of Jupiter or Saturn ensure that the matter found there exhibits an unusual state: It is as dense as a metal but, at the same time, electrically charged like a plasma. “We refer to this state as warm dense matter,” explains Dominik Kraus, physicist at HZDR and professor at the University of Rostock. “It is a transitional state between solid state and plasma that is found in the interior of planets, although it can occur briefly on Earth, too, for example during meteor impacts.” Examining this state of matter in any detail in the lab is a complicated process involving, for example, firing strong laser flashes at a sample, and, for the blink of an eye, heating and condensing it.

Read more on the HZDR website

Image: High-resolution spectroscopy will enable unique insights into chemistry happening deep inside planets

Credit: HZDR / U. Lehmann

Electrons riding a double wave

Electrons riding a double wave

Since they are far more compact than today’s accelerators, which can be kilometers long, plasma accelerators are considered as a promising technology for the future. An international research group has now made significant progress in the further development of this approach: With two complementary experiments at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and at the Ludwig-Maximilians-Universität Munich (LMU), the team was able to combine two different plasma technologies for the first time and build a novel hybrid accelerator. The concept could advance accelerator development and, in the long term, become the basis of highly brilliant X-ray sources for research and medicine, as the experts describe in the journal Nature Communications (DOI: 10.1038/s41467-021-23000-7).

In conventional particle accelerators, strong radio waves are guided into specially shaped metal tubes called resonators. The particles to be accelerated – which are often electrons – can ride these radio waves like surfers ride an ocean wave. But the potential of the technology is limited: Feeding too much radio wave power into the resonators creates a risk of electrical charges that can damage the component. This means that in order to bring particles to high energy levels, many resonators have to be connected in series, which makes today’s accelerators in many cases kilometers long.

That is why experts are eagerly working on an alternative: plasma acceleration. In principle, short and extremely powerful laser flashes fire into a plasma – an ionized state of matter consisting of negatively charged electrons and positively charged atomic cores. In this plasma, the laser pulse generates a strong alternating electric field, similar to the wake of a ship, which can accelerate electrons enormously over a very short distance. In theory, this means facilities can be built far more compact, shrinking an accelerator that is a hundred meters long today down to just a few meters. “This miniaturization is what makes the concept so attractive,” explains Arie Irman, a researcher at the HZDR Institute of Radiation Physics. “And we hope it will allow even small university laboratories to afford a powerful accelerator in the future.”

Read more on the HZDR website

Image: Numerical rendering of the laser-driven acceleration (left side) and a subsequent electron-driven acceleration (right side), forming together the hybrid plasma accelerator.

Credit: Alberto Martinez de la Ossa, Thomas Heinemann