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.

Terahertz Waves Could Push 5G to 6G

At the Brooklyn 5G summit, experts said terahertz waves could fix some of the problems that may arise with millimeter-wave networks

By Michael Koziol

https://spectrum.ieee.org/tech-talk/telecom/wireless/at-the-6th-annual-brooklyn-5g-summit-some-eyes-are-on-6g

It may be the sixth year for the Brooklyn 5G Summit, but in the minds of several speakers, 2019 is also Year Zero for 6G. The annual summit, hosted by Nokia and NYU Wireless, is a four-day event that covers all things 5G, including deployments, lessons learned, and what comes next.

This year, that meant preliminary research into terahertz waves, the frequencies that some researcher believe will make up a key component of the next next generation of wireless. In back-to-back talks, Gerhard Fettweis, a professor at TU Dresden, and Ted Rappaport, the founder and director of NYU Wireless, talked up the potential of terahertz waves.

As a quick primer on the electromagnetic spectrum, terahertz waves (despite what the name implies) occupy the 300 gigahertz to 3 terahertz band of spectrum. This means the frequencies are higher than the highest frequencies that will be used by 5G, which are known as millimeter waves, and fall between 30 and 300 GHz.

In his talk, Fettweis discussed the potential of terahertz waves and 6G to solve some of the problems of 5G. He pointed to the trend established by previous generations of wireless: While 1G provided us with mobile telephony, 2G expanded on that and addressed some of its predecessor’s shortcomings. 3G 

and 4G did the same with mobile data. Now that we’re moving on to 5G, which is expected to support many new applications like the Internet of Things and AR/VR, Fettweis said it was only natural that 6G will function similarly to 2G and 4G to correct the flaws of the previous generation.

As to what, exactly, terahertz waves will correct—that’s still largely unknown. Service providers around the world are only now rolling out their mobile 5G networks, and it will take time to identify the shortcomings. Even so, the physical properties of terahertz waves point to some general ways in which they could help.

Terahertz waves, as mentioned, have shorter wavelengths and higher frequencies than millimeter waves. That suggests terahertz waves should be able to carry more data more quickly, though they will not be able to propagate as far. In general, that means that the introduction of terahertz waves into mobile networks could address any areas in which 5G isn’t able to deliver high enough data throughput or low enough latency. During his talk, Fettweis revealed the results of tests in which terahertz waves were able to transmit 1 terabit per second of data for a grand total of 20 meters (yeah, not very far at all).

But if you think those results are less than impressive, they don’t dissuade Rappaport, who gave a very earnest talk on the future of terahertz waves as they relate to 6G and, dare I say it, 7G. Rappaport, who was one of the pioneering researchers into millimeter waves and played a large role in proving they would be viable for 5G networks, suggested that with these frequencies, as well as additional improvements in cellular technology, we’ll someday see thousand-dollar smartphones that have the computational power of the human brain.

Of course, it’s all highly speculative at this point, but if past trends continue, we can expect to see service providers harnessing terahertz waves for communications in areas with many devices or large amounts of data a decade from now. And that will all be thanks to the fundamental research that’s just getting underway today.

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

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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