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 

Magnetic fields and lasers elicit graphene secret

                                                                  Auger scattering in graphene

http://www.sciencedaily.com/releases/2014/11/141124125343.htm

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have studied the dynamics of electrons from the "wonder material" graphene in a magnetic field for the first time. This led to the discovery of a seemingly paradoxical phenomenon in the material. Its understanding could make a new type of laser possible in the future. Together with researchers from Berlin, France, the Czech Republic and the United States, the scientists precisely described their observations in a model and have now published their findings in the scientific journal Nature Physics.

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Graphene is considered a "wonder material": its breaking strength is higher than steel and it conducts electricity and heat more effectively than copper. As a two-dimensional structure consisting of only a single layer of carbon atoms, it is also flexible, nearly transparent and approximately one million times thinner than a sheet of paper. Furthermore, shortly after its discovery ten years ago, scientists recognized that the energy states of graphene in a magnetic field -- known as Landau levels -- behave differently than those of semiconductors. "Many fascinating effects have been discovered with graphene in magnetic fields, but the dynamics of electrons have never been studied in such a system until now," explains physicist Dr. Stephan Winnerl from HZDR.

The HZDR researchers exposed the graphene to a four-Tesla magnetic field -- forty times stronger than a horseshoe magnet. As a result, the electrons in graphene occupy only certain energy states. The negatively charged particles were virtually forced on tracks. These energy levels were then examined with free-electron laser light pulses at the HZDR. "The laser pulse excites the electrons into a certain Landau level. A temporally delayed pulse then probes how the system evolves," explains Martin Mittendorff, doctoral candidate at the HZDR and first author of the paper.

Electron redistribution surprises scientists

The result of the experiments has astonished the researchers. This particular energy level, into which new electrons were pumped using the laser, gradually emptied. Winnerl illustrates this paradoxical effect using an everyday example: "Imagine a librarian sorting books on a bookshelf with three shelves. She places one book at a time from the lower shelf onto the middle shelf. Her son is simultaneously 'helping' by taking two books from the middle shelf, placing one of them on the top shelf, the other on the bottom. The son is very eager and now the number of books on the middle shelf decreases even though this is precisely the shelf his mother wishes to fill."

Because there were neither experiments nor theories regarding such dynamics before, the Dresden physicists initially had difficulty interpreting the signals correctly. After a number of attempts, however, they found an explanation: collisions between electrons cause this unusual rearrangement. "This effect has long been known as Auger scattering, but no one expected it would be so strong and would cause an energy level to become depleted," explains Winnerl.

This new discovery could be used in the future for developing a laser that can produce light with arbitrarily adjustable wavelengths in the infrared and terahertz ranges. "Such a Landau-level laser was long considered impossible, but now with graphene this semiconductor physicists' dream could become a reality," says Winnerl enthusiastically.

Berlin researchers calculate complex model for Dresden experiments

After the fundamental model used in the experiments had worked satisfactorily, the precise theoretical work followed, which was carried out at the Technical University Berlin. Berlin scientists Ermin Malic and Andreas Knorr confirmed, using complex calculations, the Dresden group's assumptions and provided detailed insights into the underlying mechanisms. The HZDR researchers additionally cooperated with the French High Magnetic Field Laboratory in Grenoble (Laboratoire National des Champs Magnétiques Intenses -- LNCMI), the Charles University Prague and the Georgia Institute of Technology in Atlanta (USA).

The research has been funded by the German research association DFG (Deutsche Forschungsgemeinschaft) within the program "Graphene."

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Story Source:

The above story is based on materials provided by Helmholtz-Zentrum Dresden-Rossendorf. Note: Materials may be edited for content and length.

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Journal Reference:

1. Martin Mittendorff, Florian Wendler, Ermin Malic, Andreas Knorr, Milan Orlita, Marek Potemski, Claire Berger, Walter A. de Heer, Harald Schneider, Manfred Helm, Stephan Winnerl. Carrier dynamics in Landau-quantized graphene featuring strong Auger scattering. Nature Physics, 2014; DOI:10.1038/NPHYS3164

Intersublevel Spectroscopy on Quantum Dots by terahertz Near-Field Microscopy

                                   Near-field microscopy using the free electron laser at HZDR: An adjusting laser is employed to align the measuring tip of the microscope that comes from above. Below the movable sample stage is to be seen. (Credit: Image courtesy of Helmholtz Association of German Research Centres)

http://www.sciencedaily.com/releases/2012/09/120928103756.htm

ScienceDaily — Quantum dots are nanostructures of semiconducting materials that behave a lot like single atoms and are very easy to produce. Given their special properties, researchers see huge potential for quantum dots in technological applications. Before this can happen, however, we need a better understanding of how the electrons "trapped" inside them behave. Dresden physicists have recently observed how electrons in individual quantum dots absorb energy and emit it again as light.

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Their results were recently published in the journal Nano Letters.

Quantum dots look like miniscule pyramids. Inside each of these nano-pyramids are always only one or two electrons that essentially "feel" the constricting walls around them and are therefore tightly constrained in their mobility. Scientists from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), TU Dresden. TU Dresden and the Leibniz Institute for solid State and Materials Research Dresden (IFW) have now studied the special energy states of the electrons trapped inside individual quantum dots.

Sharp energy levels

The behaviour of electrons in a material essentially determines its properties. Being spatially constrained in all three spatial dimensions, electrons inside a nano-pyramid can only occupy very specific energy levels -- which is why quantum dots are also called "artificial atoms." Where these energy levels lie depends on the chemical composition of the semiconductor material as well as the size of the nano-pyramid. "These sharply defined energy levels are exploited, for example, in highly energy-efficient lasers based on quantum dots. The light is produced when an electron drops from a higher energy level into a lower one. The energy difference between the two levels determines the colour of the light," Dr. Stephan Winnerl of HZDR explains.

Seeing electrons inside individual quantum dots

The researchers in Dresden working with Dr. Winnerl were recently the first to succeed in scanning transitions between energy levels in single quantum dots using infrared light. Although, they could only do this after overcoming a certain hurdle: While the pyramids of indium arsenide or indium gallium arsenide form spontaneously during a specific mode of crystal growth, their size varies within a certain range. Studying them with infrared light, for example, one obtains blurred signals because electrons in different sized pyramids respond to different infrared energies. This is why it is so important to obtain a detailed view of the electrons trapped inside a single quantum dot.

The scientists approached this task with the special method of scanning near-field microscopy. Laser light is shone onto a metallic tip less than 100 nanometers thick, which strongly collimates the light to a hundred times smaller than the wavelength of light, which is the spatial resolution limit for "conventional" optics using lenses and mirrors. By focusing this collimated light precisely onto one pyramid, energy is donated to the electrons, thereby exciting them to a higher energy level. This energy transfer can be measured by watching the infrared light scattered from the tip in this process. While near-field microscopy involves major signal losses, the light beam is still strong enough to excite the electrons inside a nano-pyramid. The method is also so sensitive that it can create a nanoscale image in which the one or two electrons inside a quantum dot stand out in clear contrast. In this fashion, Stephan Winnerl and his colleagues from HZDR, plus physicists from TU and IFW Dresden, studied the behaviour of electrons inside a quantum dot in great detail, thereby contributing towards our understanding of them.

Infrared light from the free electron laser

The infrared light used in the experiments came from the free electron laser at HZDR. This special laser is an ideal infrared radiation source for such experiments because the energy of its light can be adjusted to precisely match the energy level inside the quantum dots. The laser also delivers such intense radiation that it more than makes up for the unavoidable losses inherent to the method.

"Next, we intend to reveal the behaviour of electrons inside quantum dots at lower temperatures," Dr. Winnerl says. "From these experiments, we hope to gain even more precise insights into the confined behavior of these electrons. In particular, we want to gain a much better understanding of how the electrons interact with one another as well as with the vibrations of the crystal lattice." Thanks to its intense laser flashes in a broad, freely selectable spectral range, the free electron laser offers ideal conditions for the method of near-field microscopy in Dresden, which benefits particularly from the close collaboration with Prof. Lukas Eng of TU Dresden in the scope of DRESDEN-concept.

Journal Reference:

1. Rainer Jacob, Stephan Winnerl, Markus Fehrenbacher, Jayeeta Bhattacharyya, Harald Schneider, Marc Tobias Wenzel, Hans-Georg von Ribbeck, Lukas M. Eng, Paola Atkinson, Oliver G. Schmidt, Manfred Helm. Intersublevel Spectroscopy on Single InAs-Quantum Dots by Terahertz Near-Field Microscopy. Nano Letters, 2012; 12 (8): 4336 DOI: 10.1021/nl302078w