New material system to convert and generate terahertz waves developed

 

https://www.sciencedaily.com/releases/2020/12/201218112511.htm

On the electromagnetic spectrum, terahertz light is located between infrared radiation and microwaves. It holds enormous potential for tomorrow's technologies: Among other things, it might succeed 5G by enabling extremely fast mobile communications connections and wireless networks. The bottleneck in the transition from gigahertz to terahertz frequencies has been caused by insufficiently efficient sources and converters. A German-Spanish research team with the participation of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now developed a material system to generate terahertz pulses much more effectively than before. It is based on graphene, i.e., a super-thin carbon sheet, coated with a metallic lamellar structure. The research group presented its results in the journal ACS Nano.

Some time ago, a team of experts working on the HZDR accelerator ELBE were able to show that graphene can act as a frequency multiplier: When the two-dimensional carbon is irradiated with light pulses in the low terahertz frequency range, these are converted to higher frequencies. Until now, the problem has been that extremely strong input signals, which in turn could only be produced by a full-scale particle accelerator, were required to generate such terahertz pulses efficiently."This is obviously impractical for future technical applications," explains the study's primary author Jan-Christoph Deinert of the Institute of Radiation Physics at HZDR. "So, we looked for a material system that also works with a much less violent input, i.e., with lower field strengths."

For this purpose, HZDR scientists, together with colleagues from the Catalan Institute of Nanoscience and Nanotechnology (ICN2), the Institute of Photonic Sciences (ICFO), the University of Bielefeld, TU Berlin and the Mainz-based Max Planck Institute for Polymer Research, came up with a new idea: the frequency conversion could be enhanced enormously by coating the graphene with tiny gold lamellae, which possess a fascinating property: "They act like antennas that significantly amplify the incoming terahertz radiation in graphene," explains project coordinator Klaas-Jan Tielrooij from ICN2. "As a result, we get very strong fields where the graphene is exposed between the lamellae. This allows us to generate terahertz pulses very efficiently."

Surprisingly effective frequency multiplication

To test the idea, team members from ICN2 in Barcelona produced samples: First, they applied a single graphene layer to a glass carrier. On top, they vapor-deposited an ultra-thin insulating layer of aluminum oxide, followed by a lattice of gold strips. The samples were then taken to the TELBE terahertz facility in Dresden-Rossendorf, where they were hit with light pulses in the low terahertz range (0.3 to 0.7 THz). During this process, the experts used special detectors to analyze how effectively the graphene coated with gold lamellae can multiply the frequency of the incident radiation.

"It worked very well," Sergey Kovalev is happy to report. He is responsible for the TELBE facility at HZDR. "Compared to untreated graphene, much weaker input signals sufficed to produce a frequency-multiplied signal." Expressed in numbers, just one-tenth of the originally required field strength was enough to observe the frequency multiplication. And at technologically relevant low field strengths, the power of the converted terahertz pulses is more than a thousand times stronger thanks to the new material system. The wider the individual lamellae and the smaller the areas of graphene that are left exposed, the more pronounced the phenomenon. Initially, the experts were able to triple the incoming frequencies. Later, they attained even larger effects -- fivefold, sevenfold, and even ninefold increases in the input frequency.

Compatible with chip technology

This offers a very interesting prospect, because until now, scientists have needed large, complex devices such as accelerators or large lasers to generate terahertz waves. Thanks to the new material, it might also be possible to achieve the leap from gigahertz to terahertz purely with electrical input signals, i.e., with much less effort. "Our graphene-based metamaterial would be quite compatible with current semiconductor technology," Deinert emphasizes. "In principle, it could be integrated into ordinary chips." He and his team have proven the feasibility of the new process -- now implementation in specific assemblies may become possible.

The potential applications could be vast: Since terahertz waves have higher frequencies than the gigahertz mobile communications frequencies used today, they could be used to transmit significantly more wireless data -- 5G would become 6G. But the terahertz range is also of interest to other fields -- from quality control in industry and security scanners at airports to a wide variety of scientific applications in materials research, for example.

---

Story Source:

Materials provided by Helmholtz-Zentrum Dresden-Rossendorf. Note: Content may be edited for style and length.

---

Journal Reference:

1. Jan-Christoph Deinert, David Alcaraz Iranzo, Raúl Pérez, Xiaoyu Jia, Hassan A. Hafez, Igor Ilyakov, Nilesh Awari, Min Chen, Mohammed Bawatna, Alexey N. Ponomaryov, Semyon Germanskiy, Mischa Bonn, Frank H.L. Koppens, Dmitry Turchinovich, Michael Gensch, Sergey Kovalev, Klaas-Jan Tielrooij. Grating-Graphene Metamaterial as a Platform for Terahertz Nonlinear Photonics. ACS Nano, 2020; DOI: 10.1021/acsnano.0c08106

Research team presents novel transmitter for terahertz waves

If a gallium-arsenide crystal is irradiated with short laser pulses, charge carriers are formed. These charges are accelerated by applying a voltage which enforces the generation of a terahertz wave. Credit: HZDR/Juniks

by Helmholtz Association of German Research Centres

https://phys.org/news/2020-03-team-transmitter-terahertz.html

Terahertz waves are becoming ever more important in science and technology. They enable us to unravel the properties of future materials, test the quality of automotive paint and screen envelopes. But generating these waves is still a challenge. A team at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Dresden and the University of Konstanz has now made significant progress. The researchers have developed a germanium component that generates short terahertz pulses with an advantageous property: the pulses have an extreme broadband spectrum and thus deliver many different terahertz frequencies at the same time. As it has been possible to manufacture the component employing methods already used in the semiconductor industry, the development promises a broad range of applications in research and technology, as the team reports in the journal Light: Science & Applications.

Just like light, terahertz waves are categorized as electromagnetic radiation. In the spectrum, they fall right between microwaves and infrared radiation. But while microwaves and infrared radiation have long since entered our everyday lives, terahertz waves are only just beginning to be used. The reason is that experts have only been able to construct reasonably acceptable sources for terahertz waves since the beginning of the 2000s. But these transmitters are still not perfect—they are relatively large and expensive, and the radiation they emit does not always have the desired properties.

One of the established generation methods is based on a gallium-arsenide crystal. If this semiconductor crystal is irradiated with short laser pulses, gallium arsenide charge carriers are formed. These charges are accelerated by applying voltage which enforces the generation of a terahertz wave—basically the same mechanism as in a VHF transmitter mast where moving charges produce radio waves.

However, this method has a number of drawbacks: "It can only be operated with relatively expensive special lasers," explains HZDR physicist Dr. Harald Schneider. "With standard lasers of the type we use for fiber-optic communications, it doesn't work." Another shortcoming is that gallium-arsenide crystals only deliver relatively narrowband terahertz pulses and thus a restricted frequency range—which significantly limits the application area.

Precious metal implants

That is why Schneider and his team are placing their bets on another material—the semiconductor germanium. "With germanium we can use less expensive lasers known as fiber lasers," says Schneider. "Besides, germanium crystals are very transparent and thus facilitate the emission of very broadband pulses." But, so far, they have had a problem: If you irradiate pure germanium with a short laser pulse, it takes several microseconds before the electrical charge in the semiconductor disappears. Only then can the crystal absorb the next laser pulse. Today's lasers, however, can fire off their pulses at intervals of a few dozen nanoseconds—a sequence of shots far too fast for germanium.

In order to overcome this difficulty, experts searched for a way of making the electrical charges in the germanium vanish more quickly. And they found the answer in a prominent precious metal—gold. "We used an ion accelerator to shoot gold atoms into a germanium crystal," explains Schneider's colleague, Dr. Abhishek Singh. "The gold penetrated the crystal to a depth of 100 nanometers." The scientists then heated the crystal for several hours at 900 degrees Celsius. The heat treatment ensured the gold atoms were evenly distributed in the germanium crystal.

Success kicked in when the team illuminated the peppered germanium with ultrashort laser pulses: instead of hanging around in the crystal for several microseconds, the electrical charge carriers disappeared again in under two nanoseconds—about thousand times faster than before. Figuratively speaking, the gold works like a trap, helping to catch and neutralize the charges. "Now the germanium crystal can be bombarded with laser pulses at a high repetition rate and still function," Singh is pleased to report.

Inexpensive manufacture possible

The new method facilitates terahertz pulses with an extremely broad bandwidth: instead of 7 terahertz using the established gallium-arsenide technique, it is now ten times greater—70 terahertz. "We get a broad, continuous, gapless spectrum in one fell swoop", Harald Schneider enthuses. "This means we have a really versatile source at hand that can be used for the most diverse applications." Another benefit is that, effectively, germanium components can be processed with the same technology that is used for microchips. "Unlike gallium arsenide, germanium is silicon compatible," Schneider notes. "And as the new components can be operated together with standard fiber-optic lasers, you could make the technology fairly compact and inexpensive."

This should turn gold-doped germanium into an interesting option not just for scientific applications, such as the detailed analysis of innovative two-dimensional materials such as graphene, but also for applications in medicine and environmental technology. One could imagine sensors, for instance, that trace certain gases in the atmosphere by means of their terahertz spectrum. Today's terahertz sources are still too expensive for the purpose. The new methods, developed in Dresden-Rossendorf, could help to make environmental sensors like this much cheaper in the future.

A laser for penetrating waves

An international research team has been able to show that it is relatively easy to generate terahertz waves with an alloy of mercury, cadmium and tellurium. To examine the behavior of the electrons in the material, the physicists use the free-electron laser FELBE at HZDR. Circularly polarized terahertz pulses (orange spiral) excite the electrons (red) from the lowest to the next higher energy level (parabolic shell). The energy gap of these so-called Landau levels can be adjusted with the help of a magnetic field.

Image source: HZDR

https://www.biophotonics.world/magazine/article/878/a-laser-for-penetrating-waves

The "Landau-level laser" is an exciting concept for an unusual radiation source. It has the potential to efficiently generate so-called terahertz waves, which can be used to penetrate materials as well as for future data transmission. So far, however, nearly all attempts to make such a laser reality have failed. An international team of researchers has now taken an important step in the right direction: In the journal Nature Photonics (DOI: 10.1038/s41566-019-0496-1), they describe a material that generates terahertz waves by simply applying an electric current. Physicists from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) played a significant role in this project.

Like light, terahertz waves are electromagnetic radiation, in a frequency range between microwaves and infrared radiation. Their properties are of great technological and scientific interest, as they allow fundamental researchers to study the oscillations of crystal lattices or the propagation of spin waves. Simultaneously "terahertz waves are of interest for technical applications because they can penetrate numerous substances that are otherwise opaque, such as clothing, plastics and paper," HZDR researcher Stephan Winnerl explains. Terahertz scanners are already used today for airport security checks, detecting whether passengers are concealing dangerous objects under their clothing - without having to resort to harmful X-rays.

Because terahertz waves have a higher frequency than the radio waves we use today, they could also be harnessed for data transmission one day. Current WLAN technology, for instance, operates at frequencies of two to five gigahertz. Since terahertz frequencies are about a thousand times higher, they could transmit images, video, and music much faster, albeit across shorter distances. However, the technology is not yet fully developed. "There has been a lot of progress in recent years," Winnerl reports. "But generating the waves is still a challenge - experts speak of a veritable terahertz gap." A particular issue is the lack of a terahertz laser that is compact, powerful, and tunable at the same time.

Flexible frequencies

Laser light is generated by the electrons in the laser material. According to the quantum effect, energized electrons emit light, but they cannot absorb just any random amount of energy, only certain portions. Accordingly, light is also emitted in portions, in a specific color and as a focused beam. For some time now, experts have set their sights on a specific concept for a terahertz laser: the "Landau-level laser". It is special because it can use a magnetic field to flexibly adjust the electrons' energy levels. These levels, in turn, determine the frequencies that are emitted by the electrons, which makes the laser tunable - a huge advantage for many scientific and technical applications.

There is just one issue: Such a laser does not exist yet. "So far, the problem has been that the electrons pass their energy on to other electrons instead of emitting them as the desired light waves," Winnerl explains. Experts call this physical process the "Auger effect". To their chagrin, this phenomenon also occurs in graphene, a material that they deemed particularly promising for a "Landau-level laser". This two-dimensional form of carbon showed strong Auger scattering in HZDR experiments.

A question of material

The research team therefore tried another material: a heavy metal alloy of mercury, cadmium, and tellurium (HgCdTe) that is used for highly sensitive thermal imaging cameras, among other things. The special feature of this material is that its mercury, cadmium, and tellurium contents can be very precisely chosen, which makes it possible to fine-tune a certain property that experts call the "band gap".

As a result, the material showed properties similar to graphene, but without the issue of strong Auger scattering. "There are subtle differences to graphene that avoid this scattering effect," says Stephan Winnerl. "Put simply, the electrons can't find any other electrons that could absorb the right amount of energy." Therefore, they have no choice but to get rid of their energy in the form that the scientists want: terahertz radiation.

The project was an international team effort: Russian partners had prepared the HgCdTe samples, which the project's lead group in Grenoble then analyzed. One of the pivotal investigations took place in Dresden-Rossendorf: Using the free-electron laser FELBE, experts fired strong terahertz pulses at the sample and were able to observe the electrons' behavior in temporal resolution. The result: "We noticed that the Auger effect that we had observed in graphene had actually disappeared," Winnerl is happy to report.

LED for Terahertz

Lastly, a work group in Montpellier observed that the HgCdTe compound actually emits terahertz waves when electric current is applied. By varying an additional magnetic field of only about 200 millitesla, the experts were able to vary the frequency of the emitted waves in a range of one to two terahertz - a tunable radiation source. "It's not quite a laser yet, but rather like a terahertz LED," Winnerl describes. "But we should be able to extend the concept to a laser, even though it will take some effort." And that's exactly what the French partners want to tackle next.

There is one limiting factor, however: Up to now, the principle has only worked when cooled to very low temperatures, just above absolute zero. "This is certainly a hindrance for everyday applications," Winnerl summarizes. "But for use in research and in certain high-tech systems, we should be able to make it work with this kind of cooling."

Related journal article: http://dx.doi.org/10.1038/s41566-019-0496-1

Source: Helmholtz Center Dresden Rossendorf 

A laser for penetrating waves

https://www.intelligent-aerospace.com/resources/article/14038472/a-laser-for-penetrating-waves-terahertz-data-transmission

DRESDEN, Germany - The Landau-level laser is an exciting concept for an unusual radiation source. It could efficiently generate so-called terahertz waves, which can be used to penetrate materials, with possible applications in data transmission. So far, however, nearly all attempts to make such a laser have failed. An international team of researchers has now taken an important step in the right direction: In the journal Nature Photonics, they describe a material that generates terahertz waves by simply applying an electric current. Physicists from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) played a significant role in this project, says Phys.org. Continue reading original article

The Intelligent Aerospace take:

August 19,2019- The Helmholtz Association of German Research Centres describe a material that generates terahertz waves by applying an electric current. Currently, teraherz waves are used in detecting objects under clothing in airports, but the high-frequency waves could drive innovation in data transmission. Fiber optic cables use light to transfer data at much faster rates compared to traditional copper connectors, but a tunable laser capable of transmitting the high frequency has not yet been developed. However, with terahertz waves transmitting at frequencies at 1000 times higher than current WLAN technologies, the technology could prove useful in moving large amounts of data across short distances should some very difficult tech hurdles be cleared.

A laser for penetrating waves

An international research team has been able to show that it is relatively easy to generate terahertz waves with an alloy of mercury, cadmium and tellurium. To examine the behavior of the electrons in the material, the physicists use the free-electron laser FELBE at HZDR. Circularly polarized terahertz pulses (orange spiral) excite the electrons (red) from the lowest to the next higher energy level (parabolic shell). The energy gap of these so-called Landau levels can be adjusted with the help of a magnetic field. CREDIT HZDR / Juniks

https://www.eurekalert.org/multimedia/pub/208908.php

Research team develops a new principle to generate terahertz radiation

The "Landau-level laser" is an exciting concept for an unusual radiation source. It has the potential to efficiently generate so-called terahertz waves, which can be used to penetrate materials as well as for future data transmission. So far, however, nearly all attempts to make such a laser reality have failed. An international team of researchers has now taken an important step in the right direction: In the journal Nature Photonics (DOI: 10.1038/s41566-019-0496-1), they describe a material that generates terahertz waves by simply applying an electric current. Physicists from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) played a significant role in this project.

Like light, terahertz waves are electromagnetic radiation, in a frequency range between microwaves and infrared radiation. Their properties are of great technological and scientific interest, as they allow fundamental researchers to study the oscillations of crystal lattices or the propagation of spin waves. Simultaneously "terahertz waves are of interest for technical applications because they can penetrate numerous substances that are otherwise opaque, such as clothing, plastics and paper," HZDR researcher Stephan Winnerl explains. Terahertz scanners are already used today for airport security checks, detecting whether passengers are concealing dangerous objects under their clothing - without having to resort to harmful X-rays.

Because terahertz waves have a higher frequency than the radio waves we use today, they could also be harnessed for data transmission one day. Current WLAN technology, for instance, operates at frequencies of two to five gigahertz. Since terahertz frequencies are about a thousand times higher, they could transmit images, video, and music much faster, albeit across shorter distances. However, the technology is not yet fully developed. "There has been a lot of progress in recent years," Winnerl reports. "But generating the waves is still a challenge - experts speak of a veritable terahertz gap." A particular issue is the lack of a terahertz laser that is compact, powerful, and tunable at the same time.

Flexible frequencies

Laser light is generated by the electrons in the laser material. According to the quantum effect, energized electrons emit light, but they cannot absorb just any random amount of energy, only certain portions. Accordingly, light is also emitted in portions, in a specific color and as a focused beam. For some time now, experts have set their sights on a specific concept for a terahertz laser: the "Landau-level laser". It is special because it can use a magnetic field to flexibly adjust the electrons' energy levels. These levels, in turn, determine the frequencies that are emitted by the electrons, which makes the laser tunable - a huge advantage for many scientific and technical applications.

There is just one issue: Such a laser does not exist yet. "So far, the problem has been that the electrons pass their energy on to other electrons instead of emitting them as the desired light waves," Winnerl explains. Experts call this physical process the "Auger effect". To their chagrin, this phenomenon also occurs in graphene, a material that they deemed particularly promising for a "Landau-level laser". This two-dimensional form of carbon showed strong Auger scattering in HZDR experiments.

A question of material

The research team therefore tried another material: a heavy metal alloy of mercury, cadmium, and tellurium (HgCdTe) that is used for highly sensitive thermal imaging cameras, among other things. The special feature of this material is that its mercury, cadmium, and tellurium contents can be very precisely chosen, which makes it possible to fine-tune a certain property that experts call the "band gap".

As a result, the material showed properties similar to graphene, but without the issue of strong Auger scattering. "There are subtle differences to graphene that avoid this scattering effect," says Stephan Winnerl. "Put simply, the electrons can't find any other electrons that could absorb the right amount of energy." Therefore, they have no choice but to get rid of their energy in the form that the scientists want: terahertz radiation.

The project was an international team effort: Russian partners had prepared the HgCdTe samples, which the project's lead group in Grenoble then analyzed. One of the pivotal investigations took place in Dresden-Rossendorf: Using the free-electron laser FELBE, experts fired strong terahertz pulses at the sample and were able to observe the electrons' behavior in temporal resolution. The result: "We noticed that the Auger effect that we had observed in graphene had actually disappeared," Winnerl is happy to report.

LED for Terahertz

Lastly, a work group in Montpellier observed that the HgCdTe compound actually emits terahertz waves when electric current is applied. By varying an additional magnetic field of only about 200 millitesla, the experts were able to vary the frequency of the emitted waves in a range of one to two terahertz - a tunable radiation source. "It's not quite a laser yet, but rather like a terahertz LED," Winnerl describes. "But we should be able to extend the concept to a laser, even though it will take some effort." And that's exactly what the French partners want to tackle next.

There is one limiting factor, however: Up to now, the principle has only worked when cooled to very low temperatures, just above absolute zero. "This is certainly a hindrance for everyday applications," Winnerl summarizes. "But for use in research and in certain high-tech systems, we should be able to make it work with this kind of cooling."

###

Publication:

D.B. But, M. Mittendorff, C. Consejo, F. Teppe, N.N. Mikhailov, S.A. Dvoretskii, C. Faugeras, S. Winnerl, M. Helm, W. Knap, M. Potemski, M. Orlita: Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons, in Nature Photonics, 2019 (DOI: 10.1038/s41566-019-0496-1)

Energy-efficient, tunable laser source

http://www.electronics-eetimes.com/news/energy-efficient-tunable-laser-source
By Christoph Hammerschmidt

A joint team of researchers from the Helmholtz research center (HZDR) in Dresden and from the Trinity College in Dublin has created a terahertz source that is both tunable and efficient. The achievement could be utilized to implement wireless local area networks with very high data rates.

The scientist group around Karsten Rode at the Trinity College grew multiple layers of different manganese gallium compounds with a thickness of 45 to 65 nanometers. The researchers irradiated these layers with strong laser pulses, which in turn caused a synchronous, oscillating movement of the magnetic moments within these layers and thus emitting electromagnetic radiation in the terahertz frequency range. What makes Rode’s achievement so interesting is the fact that he was able to control the frequency of the emitted radiation through the composition of the compounds. Plus, the emission exhibited a surprisingly high energetic efficiency. “This terahertz wave generating technology is very unique in that it is possible to adjust the frequency of the radiation as desired,” said Rode. The capability of varying the frequency is said to be an important requirement of next-gen communications devices and networks.

Another group of researchers led by Michael Gensch from the HZDR surveyed these layers by means of lasers and short, intensive terahertz pulses. “We think that this approach is extremely interesting”, Gensch said. According to Gensch, so far only a small number of techniques to generate “monochromatic” terahertz radiation have been known. These methods are rather complex and costly. In contrast, Rode’s thin layers are cost-effective and suit well for mass production. It even could be possible to integrate these layered terahertz sources into semiconductors, explained Helmholtz researcher Alina Deac.

Against the background of the promising results in the research lab, the HZDR researchers plan to go one step further towards production-ready super-fast WiFi transmitter modules with bandwidths up to 100 Gbps. In the subsequent project, they intend to excite the layered stacks electrically instead of by laser pulses. If this project will be successful, it could pave the way for a first prototype of a terahertz wireless data network module.

Further information: www.hzdr.de

A new league of compact Terahertz lightsources

International collaboration of users, accelerator and laser physicists demonstrates novel concept for compact high-field high-repetition-rate accelerator-based Terahertz user facilities

http://www.hzdr.de/db/Cms?pOid=46636&pNid=0

Press release of March 7, 2016

View of the two THz sources at the ELBE accelerator: The pulses from the diffraction radiator source (right) and from the undulator source (orange part) are transported into the laboratory on the roof by two optical beamlines

Foto: HZDR/F. Bierstedt

Download

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) succeeded in constructing a prototype facility for research with high Terahertz (THz) fields. The performance of the TELBE facility was successfully tested by conducting a time-resolved pilot experiment. The achieved THz fields exceed those of existing THz sources at similar repetition rates by orders of magnitude, although TELBE has not yet reached its design parameters. The results have been published in the journal “Scientific Reports”.

The resonances of many important fundamental excitations are in the THz frequency range. Intense, specifically shaped THz pulses can be utilized to manipulate material properties on ultra-fast timescales selectively as several groundbreaking experiments in the past years could show. Scientists hope to understand complex processes such as high-temperature superconductivity better when analyzing these short-lived exotic THz driven states of matter. Existing THz sources have so far failed to provide optimal parameters for many of the envisioned investigations, e.g. by providing only moderate repetition rates.

The collaboration involving experts from several accelerator laboratories like Deutsches Elektronen-Synchrotron (DESY),Karslruhe Institute of Technology (KIT), SLAC National Accelerator Laboratory and the European XFEL GmbH, now demonstrated that intense THz pulses can be generated at unprecedented repetition rates utilizing a very compact, quasi-cw linear electron accelerator. The concept combines superconducting radio frequency accelerator technology with the superradiant THz emission principle. Two of the results are of particular importance. Firstly, it could be shown that multiple THz sources can be operated in parallel by one accelerator. Thereby future user facilities, like TELBE itself, can provide multiple user groups with THz pulses of individually adjustable parameters. Secondly, during the pilot experiments a timing accuracy between the THz pulses and external laser systems in the 10 femtosecond regime was demonstrated routinely. These measurements originate from an interdisciplinary collaboration between accelerator physicists, laser physicists, material scientists and life scientists which aimed to make the prototype facility at the center for high power radiation sources in Dresden suitable for experiments in diverse research areas right from the start.

Pilot experiment: Coherent spin waves, excited by THz pulses

The chosen pilot experiment investigated the coherent, selective THz excitation of a spin wave in nickel oxide. The high transient magnetic field of the multicycle THz pulses from the TELBE facility couples in this case directly to the electron spins and induces a coherent spin motion. A synchronized femtosecond laser pulse is utilized to measure the spin deflection as a function of time via the Faraday effect. Due to the orders of magnitude higher repetition rate, experiments like this can be performed with much higher accuracy or speed. „It took us all by surprise that we could take such nice data at such an early stage during the commissioning. Once the target parameters of the TELBE facility have been reached we can work with by a factor of 100 stronger THz pulses, which in the case of the spin deflection would yield values close to what would be necessary for inducing an actual spin flip. This would have an enormous technological importance,” says Michael Gensch, corresponding author of the publication.

More pulses per seconds – opportunity for unique experiments

“The high repetition rate enables to employ a number of techniques in combination with the THz pump pulses of which scientists so far could only be dreaming of. We are discussing with our pilot users which additional probe techniques should be established at the TELBE facility. One of the currently investigated options is to implement time resolved UV photoelectron spectroscopy,” says Dr. Michael Gensch. 

„Friendly user“ operation starting in summer 2016

The facility is beginning to start “friendly user” operation for a limited number of proposals in summer 2016. “Particularly in the initial phase of such a new facility we cannot always guarantee to provide optimal parameters for individual experiments over typical beamtime durations,” says Dr. Sergey Kovalev, the future beamline scientist at TELBE. “For that purpose we will provide special support during the beamtimes and continue to work on the optimization of the facility.” Deadline for proposals for the first run of “friendly user” operation is April 15, 2016.

---

Publication: Green, B. et al., „High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter“, Sci. Rep. 6, 2016 (DOI: 10.1038/srep22256)

---

Further information:

Dr. Michael Gensch
Institute of Radiation Physics at HZDR
Phone +49 351 260-2464 | Email: m.gensch@hzdr.de 

Graphene flakes as an ultra-fast stopwatch

The external antenna on the detector captures long-wave infrared and terahertz radiation and funnels it to a graphene flake which is located in the center of the structure. Credit: M. Mittendorff
 
http://phys.org/news/2015-10-graphene-flakes-ultra-fast-stopwatch.html#jCp

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), working with colleagues from the US and Germany, have developed a new optical detector from graphene which reacts very rapidly to incident light of all different wavelengths and even works at room temperature. It is the first time that a single detector has been able to monitor the spectral range from visible light to infrared radiation and right through to terahertz radiation. The HZDR scientists are already using the new graphene detector for the exact synchronization of laser systems.

A tiny flake of graphene on silicon carbide and a futuristic-looking antenna, and there it is - the new graphene detector. Like no other single detector system which has gone before, this comparatively simple and inexpensive construct can cover the enormous spectral range from visible light all the way to terahertz radiation. "In contrast to other semiconductors like silicon or gallium arsenide, graphene can pick up light with a very large range of photon energies and convert it into electric signals. We only needed a broadband antenna and the right substrate to create the ideal conditions," explained Dr. Stephan Winnerl, physicist at the Institute of Ion Beam Physics and Materials Research at the HZDR.

Back in 2013 Martin Mittendorff, who was a PhD student at the HZDR at that time, had developed the precursor to the graphene detector. In his present position as a postdoc at the University of Maryland, he has now perfected it with his Dresden colleagues and with scientists from Marburg, Regensburg and Darmstadt. How it works: the graphene flake and antenna assembly absorbs the rays, thereby transferring the energy of the photons to the electrons in the graphene. These "hot electrons" increase the electrical resistance of the detector and generate rapid electric signals. The detector can register incident light in just 40 picoseconds - these are billionths of a second.

Wide spectral range achieved through silicon carbide substrate

The choice of substrate has now proved a pivotal step in improving the little light trap. "Semiconductor substrates used in the past have always absorbed some wavelengths but silicon carbide remains passive in the spectral range," explained Stephan Winnerl. Then there is also an antenna which acts like a funnel and captures long-wave infrared and terahertz radiation. The scientists have therefore been able to increase the spectral range by a factor of 90 in comparison with the previous model, making the shortest detectable wavelength 1000 times smaller than the longest. By way of comparison, red light, which has the longest wavelength visible to the human eye, is only twice as long as violet light which has the shortest wavelength on the visible spectrum.

This optical universal detector is already being used at the HZDR for the exact synchronization of the two free-electron lasers at the ELBE Center for High-Power Radiation Sources with other lasers. This alignment is particularly important for "pump probe" experiments, as they are called, where researcher take one laser for the excitation of a material ("pump") and then use a second laser with a different wavelength for the measurement ("probe"). The laser pulses must be exactly synchronized for such experiments. So the scientists are using the graphene detector like a stopwatch. It tells them when the laser pulses reach their goal, and the large bandwidth helps to prevent a change of detector from being a potential source of error. Another advantage is that all the measurements can take place at room temperature, obviating the need for the expensive and time-consuming nitrogen or helium cooling processes with other detectors.

 Explore further: How long do electrons live in graphene?

More information: Martin Mittendorff et al. Universal ultrafast detector for short optical pulses based on graphene, Optics Express (2015). DOI: 10.1364/OE.23.028728

All-purpose optical method for observing physical, chemical or biological processes at the nanoscale

Studying a known thin-layer sample using the novel nanoscope. Laser pulses excite the electrons in the bright stripes, whereby the otherwise transparent sample at these locations becomes reflexive. Credit: TU Dresden

 http://phys.org/news/2015-08-all-purpose-optical-method-physical-chemical.html#jCp

To gain even deeper insights into the smallest of worlds, the thresholds of microscopy must be expanded further. Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the TU Dresden, in cooperation with the Freie Universität Berlin, have succeeded in combining two established measurement techniques for the first time: near-field optical microscopy and ultra-fast spectroscopy. Computer-assisted technology developed especially for this purpose combines the advantages of both methods and suppresses unwanted noise. This makes highly precise filming of dynamic processes at the nanometer scale possible. The results were recently published in the research journal Scientific Reports.

Many important but complex processes in the natural and life sciences, for example, photosynthesis or high-temperature superconductivity, have yet to be understood. On the one hand, this is due to the fact that such processes take place on a scale of a millionth of a millimeter (nanometer) and therefore cannot be observed by conventional optical microscopic imaging. On the other hand, researchers must be able to precisely observe very rapid changes in individual stages to better understand the highly complex dynamics. The development of high-resolution temporal and spatial technologies has therefore been promoted for decades.

The new camera from Dresden combines the advantages of two worlds: microscopy and ultra-fast spectroscopy. It enables unaltered optical measurements of extremely small, dynamic changes in biological, chemical or physical processes. The instrument is compact in size and can be used for spectroscopic studies in a large area of the electromagnetic spectrum. Time increments from a few quadrillionths of a second (femtoseconds) up to the second range can be selected for individual images. "This makes our nanoscope suitable for viewing ultra-fast physical processes as well as for biological process, which are often very slow," says the HZDR's Dr. Michael Gensch.

Combining two methods guarantees high spatial and temporal Resolution

The nanoscope is based on the further development of near-field microscopy, in which laser light is irradiated on a ultra-thin metal point. This creates highly bundled light - a hundred times smaller than the wavelength of light, which otherwise represents the limit of "normal" optics with lenses and mirrors. "In principle, we can use the entire wavelength spectrum of near-field microscopy, from ultraviolet to the terahertz range," says Dr. Susanne Kehr from the TU Dresden. "The focused light delivers energy to the sample, creating a special interaction between the point and the sample in what is known as the near-field. By observing the back-scattered portion of the laser light, one can achieve a spatial resolution in the order of the near-field magnitude, that is, in the nanometer range." This technology, known as SNOM (Scanning Near-Field Optical Microscopy), is typically only utilized for imaging static conditions.

Using ultra-fast spectroscopy is the crucial tool, on the other hand, enabling scientists to study dynamic processes on short timescales and with extreme sensitivity. The spatial resolution has, until now, been limited to the micrometer range however. The principle in such pump-probe experiments that function, for example, with light, pressure or electric field pulses is as follows: while a first pulse excites the sample under study, a second pulse monitors the change in the sample. If the time between them is varied, snapshots can be taken at different times, and a movie can be assembled. A clever correction of the measurement errors leads to the high sensitivity of the spectroscopic procedure. Activation by an excitation pulse means a type of disturbance for the entire sample system, which needs to be filtered out so that noise or the "background" is eliminated. This is achieved by probing the unperturbed sample with a second reference pulse directly before the excitation. This particular technology could not be combined with near-field optical microscopy until now. For the first time, the teams led by the two Dresden physicists have managed to combine all the advantages of both methods in their nanoscope.

"We have developed software with a special demodulation technology with which—in addition to the outstanding resolution of near-field optical microscopy that is at least three orders of magnitude better than the resolution of common ultra-fast spectroscopy—we can now also measure dynamic changes in the sample with high sensitivity," explains Kehr. The clever electronic method enables the nanoscope to exclusively record only the changes actually occurring in the sample's properties due to the excitation. Although other research groups have only recently reported good temporal resolution with their nanoscopes, they could not, however, obtain this important correction mode. An additional advantage to the Dresden solution is that it can easily be integrated into existing near-field microscopes.

Universal in every respect

"With our nanoscope's considerable wavelength coverage, dynamic processes can be studied with the best suited wavelengths for the specific process under study. This is an important step in understanding these processes. Our colleagues at the Freie Universität Berlin have, for example, the ambitious dream of tracking structural changes during the photocycle of an individual membrane protein at specific wavelengthes in the infrared spectrum," Gensch says. Together with his TU colleague, Susanne Kehr, he demonstrated the new method on a known sample system, a semi-conducting layer made of silicon and germanium. "Had we used an unknown sample for the demonstration, we would not have been in the position to correctly interpret the functionality of our approach," Kehr stresses.

The Dresden nanoscope is universally adaptable to respective scientific questions. The probe pulse wavelengths can, in principle, reach from the low terahertz range to the ultraviolet range. The sample can be stimulated with laser, pressure, electric field or magnetic field pulses. The principle was tested at the HZDR on a typical laboratory laser as well as on the free-electron laser FELBE. First tests on the new terahertz source TELBE, which provides extremely short electric and magnetic field pulses for excitation, are in preparation. "In the future, we will not only see how quickly a process occurs, but we can also better localize where exactly it takes place in the sample. This is especially important for our TELBE facility, which will be in operation next year," explains Michael Gensch, head of the TELBE project at the HZDR.

 Explore further: Physicists confine electrons inside nano-pyramids

More information: Optical nanoscopy of transient states in condensed matter, in: Scientific Reports 5, 12582, DOI: 10.1038/srep12582

.