New method to track ultrafast change of magnetic state

In the new study together with their international colleagues, Professor Dr Dmitry Turchinovich (left) and Wentao Zhang demonstrate how the ultrafast change of magnetic states can be measured. Credit: Bielefeld University/M.-D. Müller

https://phys.org/news/2020-08-method-track-ultrafast-magnetic-state.html

An international team of physicists from Bielefeld University, Uppsala University, the University of Strasbourg, University of Shanghai for Science and Technology, Max Planck Institute for Polymer Research, ETH Zurich, and the Free University Berlin have developed a precise method to measure the ultrafast change of a magnetic state in materials. They do this by observing the emission of terahertz radiation that necessarily accompanies such a magnetization change. Their study, titled "Ultrafast terahertz magnetometry," is being published today in Nature Communications.

Magnetic memories are not just acquiring higher and higher capacity by shrinking the size of magnetic bits, they are also getting faster. In principle, the magnetic bit can be flipped—that is, it can change its state from one to zero or vice versa—on an extremely fast timescale of shorter than one picosecond. One picosecond (1 ps = 10-12 s) is one millionth of one millionth of a second. This could allow the operation of magnetic memories at terahertz (1 THz = 1 x 1012 hertz) switching frequencies, corresponding to extremely high terabit per second (Tbit/s) data rates.

"The actual challenge is to be able to detect such a magnetization change quickly and sensitively enough," explains Dr. Dmitry Turchinovich, professor of physics at Bielefeld University and the leader of this study. "The existing methods of ultrafast magnetometry all suffer from certain significant drawbacks such as, for example, operation only under ultrahigh vacuum conditions, the inability to measure on encapsulated materials, and so on. Our idea was to use the basic principle of electrodynamics. This states that a change in the magnetization of a material must result in the emission of electromagnetic radiation containing the full information on this magnetization change. If the magnetization in a material changes on a picosecond timescale, then the emitted radiation will belong to the terahertz frequency range. The problem is, that this radiation, known as 'magnetic dipole emission,' is very weak, and can be easily obscured by light emission of other origins."

As this illustration shows, the researchers were able to measure the magnetization dynamics in the iron nanofilm caused by ultrafast electronic and acoustic processes. Credit: Bielefeld University/W. Zhang

Wentao Zhang, a Ph.D. student in the lab of Professor Dmitry Turchinovich, and the first author of the published paper says: "It took us time, but finally we succeeded in isolating precisely this magnetic dipole terahertz emission that allowed us to reliably reconstruct the ultrafast magnetization dynamics in our samples: encapsulated iron nanofilms."

In their experiments, the researchers sent very short pulses of laser light onto the iron nanofilms, causing them to demagnetize very quickly. At the same time, they were collecting the terahertz light emitted during such a demagnetization process. The analysis of this terahertz emission yielded the precise temporal evolution of a magnetic state in the iron film.

"Once our analysis was finished, we realized that we actually saw far more than what we had expected," continues Dmitry Turchinovich. "It has already been known for some time that iron can demagnetize very quickly when illuminated by laser light. But what we also saw was a reasonably small, but a very clear additional signal in magnetization dynamics. This got us all very excited. This signal came from the demagnetization in iron—actually driven by the propagation of a very fast pulse of sound through our sample. Where did this sound come from? Very easy: when the iron film absorbed the laser light, it not only demagnetized, it also became hot. As we know, most materials expand when they get hot—and this expansion of the iron nanofilm launched a pulse of terahertz ultrasound within our sample structure. This sound pulse was bouncing back and forth between the sample boundaries, internal and external, like the echo between the walls of a big hall. And each time this echo passed through the iron nanofilm, the pressure of sound moved the iron atoms a little bit, and this further weakened the magnetism in the material." This effect has never been observed before on such an ultrafast timescale.

"We are very happy that we could see this acoustically-driven ultrafast magnetization signal so clearly, and that it was so relatively strong. It was amazing that detecting it with THz radiation, which has a sub-mm wavelength, worked so well, because the expansion in the iron film is only tens of femtometres (1 fm = 10-15 m) which is ten orders of magnitude smaller," says Dr. Peter M. Oppeneer, a professor of physics at Uppsala University, who led the theoretical part of this study. Dr. Pablo Maldonado, a colleague of Peter M. Oppeneer who performed the numerical calculations that were crucial for explaining the observations in this work, adds: "What I find extremely exciting is an almost perfect match between the experimental data and our first-principles theoretical calculations. This confirms that our experimental method of ultrafast terahertz magnetometry is indeed very accurate and also sensitive enough, because we were able to distinguish clearly between the ultrafast magnetic signals of different origins: electronic and acoustic."

The remaining co-authors of this publication have dedicated it to the memory of their colleague and a pioneer in the field of ultrafast magnetism, Dr. Eric Beaurepaire from the University of Strasbourg. He was one of the originators of this study, but passed away during its final stages.

Extreme conditions in semiconductors

This is a close-up of the experimental setup in the University of Konstanz's high-field Terahertz lab. Under the extreme conditions of the experiment, a bright red glow can be seen to emanate from the gallium arsenide crystal used as a semiconductor. This is due to the system's extremely high optical nonlinearity, which occurs when Wannier-Stark localization sets in.CREDIT Leitenstorfer research team

Physicists from the Universities of Konstanz, Paderborn and ETH Zurich have succeeded in experimentally demonstrating Wannier-Stark localization

UNIVERSITY OF KONSTANZ

https://www.eurekalert.org/pub_releases/2018-07/uok-eci073018.php

Scientists from the University of Konstanz and Paderborn University have succeeded in producing and demonstrating what is known as Wannier-Stark localization for the first time. In doing so, the physicists managed to overcome obstacles that had so far been considered insurmountable in the field of optoelectronics and photonics. Wannier-Stark localization causes extreme imbalance within the electric system of crystalline solids. "This fundamental effect was predicted more than 80 years ago. But it has remained unclear ever since whether this state can be realized in a bulk crystal, that is, on the level of chemical bonds between atoms", says Professor Alfred Leitenstorfer, Professor of Experimental Physics at the University of Konstanz. Analogues of the effect have so far been demonstrated only in artificial systems like semiconductor superlattices or ultracold atomic gases. In a bulk solid, Wannier-Stark localization can only be maintained for an extremely short period of time, shorter than a single oscillation of infrared light. Using the ultrafast laser systems at the University of Konstanz, Wannier-Stark localization has now been demonstrated for the first time. The experiment was conducted in a high-purity gallium arsenide crystal grown at ETH Zurich using epitaxial growth. The research results were published in the scientific journal Nature Communications on 23 July 2018.

What is Wannier-Stark localization?

If we tried to picture the atoms of a crystal, it would have to be as a three-dimensional grid composed of small beads that repel each other and are only kept together by rubber bands. The system remains stable as long as the rubber band is as strong as the repulsion is. If this is the case, the beads neither move closer to each other, nor do they move away from each other - the distance between them remains about the same. Wannier-Stark localization occurs when the rubber bands are removed abruptly. It is the electronic state that happens at the precise moment in time when the rubber bands have already gone but the beads still remain in place: The chemical bonds that hold the crystal together have been suspended.

If this state is maintained for too long, the beads will break apart and the crystal dissolves. To analyze Wannier-Stark localization, the physicists had to remove the stabilizing structures, capture the system within a fraction of a light oscillation using light pulses, and finally to stabilize it again to prevent the atoms from breaking apart. The experiment was made possible through the highly intense electric field of an ultrashort infrared light pulse, which is present in the crystal for a few femtoseconds only. "This is what we specialize in: studying phenomena that only exist on very short time scales", explains Alfred Leitenstorfer.

"In perfect insulators and semiconductors, electronic states expand throughout the entire crystal. According to an 80-year-old prediction, this changes as soon as electrical voltage is applied", says Professor Torsten Meier from Paderborn University. "If the electric field inside the crystal is strong enough, the electronic states can be localized to a few atoms. This state is called the Wannier-Stark ladder", explains the physicist, who is also Vice-President for International Relations at Paderborn University.

New electronic characteristics

"A system that deviates so extremely from its equilibrium has completely new characteristics", says Alfred Leitenstorfer about why this state is so interesting from a scientific perspective. The short-lived Wannier-Stark localization correlates with drastic changes to the electronic structure of the crystal and results, for example, in extremely high optical nonlinearity. The scientists also assume that this state is chemically particularly reactive.

The first-ever experimental realization of Wannier-Stark localization in a gallium arsenide crystal was made possible through highly intense Terahertz radiation with field intensities of more than ten million volts per centimetre. The application of more ultrashort optical light pulses resulted in changes to the crystal's optical characteristics, which was instrumental to proving this state. "If we use suitably intense light pulses consisting of a few oscillations lasting some ten femtoseconds only, we can realize the Wannier-Stark localization for a short period of time", says Alfred Leitenstorfer. "Our readings match the theoretical considerations and simulations carried out both by my own research team and by that of my colleague, Professor Wolf Gero Schmidt", adds Torsten Meier. The researchers are planning to study the extreme state of Wannier-Stark localization on the atomic scale in more detail in the future and intend to make its particular characteristics usable.

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Facts:

• Scientists from the Universities of Konstanz, Paderborn and ETH Zurich succeed in experimentally demonstrating Wannier-Stark localization in a high-purity gallium arsenide crystal grown at ETH Zurich.
• Original publication: C. Schmidt, J. Bühler, A.-C. Heinrich, J. Allerbeck, R. Podzimski, D. Berghoff, T. Meier, W. G. Schmidt, C. Reichl, W. Wegscheider, D. Brida, A. Leitenstorfer: Signatures of transient Wannier-Stark localization in bulk gallium arsenide, Nature Communications 9, 2890 (2018), direct link: https://rdcu.be/3ntP
• Wannier-Stark localization occurs in extremely high electric fields in solid-state crystals and results in new characteristics such as optical nonlinearity. The scientists also assume that this state exhibits extremely high chemical reactivity.
• Implementation of Wannier-Stark localization through highly intense Terahertz radiation with field intensities of more than ten million volts per centimetre. Demonstration through ultrashort light pulses in the femtosecond range.
• Research funded through: ERC Advanced Grant 290876 "UltraPhase" of the European Research Council (ERC, A. Leitenstorfer, Konstanz), Emmy Noether Programme of the German Research Foundation (DFG, D. Brida, Konstanz), Collaborative Research Centre SFB-TRR 142 (DFG, T. Meier and W. G. Schmidt, Paderborn), Carl Zeiss Foundation (J. Bühler, Konstanz), Swiss National Science Foundation (C. Reichl and W. Wegscheider, Zurich).

Note to editors:

You can download photos here:

https://cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Extreme_Zustaende.jpg

Caption: Close-up of the experimental setup in the University of Konstanz's high-field Terahertz lab. Under the extreme conditions of the experiment, a bright red glow can be seen to emanate from the gallium arsenide crystal used as a semiconductor. This is due to the system's extremely high optical nonlinearity, which occurs when Wannier-Stark localization sets in.

Photo: Leitenstorfer research team

https://cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Extreme_Zustaende_Leitenstorfer.jpg

Caption: Professor Alfred Leitenstorfer, Professor of Experimental Physics at the University of Konstanz

Photo: University of Konstanz

QCL-based THz sources generate 'record' ultrashort pulses

TUW and ETHZ researchers add lateral absorber to laser resonator, creating emission bandwidth across a full octave.

http://optics.org/news/8/2/16

Broadband terahertz amplifier based on a quantum cascade laser.

Researchers at TU Wien (Vienna, Austria) and ETH Zurich (Switzerland) have succeeded in generating ultrashort terahertz waves. With lengths of just a few picoseconds, these pulses are suited to spectroscopic applications and can enable extremely precise frequency measurements to be taken, says the team.

A working group led by Prof. Karl Unterrainer at the Photonics Institute at TUW has been developing quantum cascade lasers (QCLs) as an efficient means of generating terahertz waves. QCLs consist of a precisely defined sequence of several hundred semiconductor layers a few nanometers thick.

This construction means there is the possibility for the operator to select the exact energy state at which the electrons remain within the semiconductor structure. This in turn allows the frequency of the laser light emitted to be tuned to suit various inspection and spectroscopic applications.

‘Laser sandwich’

With the capability of being able to determine the laser wavelengths themselves, several quantum cascade structures with different output frequencies can be stacked on top of one another, with the aim of generating broadband terahertz radiation.

“Heterogeneous active zones of this kind are ideally suited for implementing broadband terahertz amplifiers and generating ultrashort terahertz pulses,” said Dominic Bachmann from the Photonics Institute.

Furthermore, if the discrete laser lines are linked together to establish a fixed phase relationship between the laser modes, a “frequency comb” is formed. Frequency combs make it possible to take extremely precise measurements of the absolute frequency of the light being used, which is essential for a huge number of applications.

The discovery of the frequency comb more or less revolutionized optical metrology and was honoured with the Nobel Prize for Physics in 2005. Over the past four years, researchers worldwide have been working to generate a terahertz frequency comb using a quantum cascade laser as part of the EU project TERACOMB, completed in 2015. Led by Dr Juraj Darmo from the Vienna Photonics Institute, the team of international research groups has succeeded in generating the first broadband terahertz frequency comb based on semiconductor technology.

Karl Unterrainer, Dominic Bachmann and Juraj Darmo at the Vienna Photonics Institute.

Vienna breakthough

A method developed by the group led by Prof. Unterrainer makes it possible to analyze internal quantum cascade laser parameters during laser operation. This technique is based on time-resolved spectroscopy, with broadband terahertz pulses penetrating the sample to be measured.

Based on femtosecond lasers, this technology can be used to collect the full information content relating to the time and frequency range with just one single measurement. As a result, the scientists at the Photonics Institute have managed to quantify the optical gain coefficients as well as the optical dispersion in broadband terahertz quantum cascade lasers, improving their understanding of the complex dynamics at play. “These findings allow us to increase the laser bandwidth even further and to improve the efficiency of frequency combs,” explains Juraj Darmo.

One unresolved issue with terahertz quantum cascade lasers had been the existence of laser lines with different propagation speeds. If there are laser modes with a higher lateral order, the intensity is distributed very unevenly between the laser lines, thereby reducing the usable bandwidth and preventing the generation of a frequency comb.

To prevent these modes from oscillating, the losses must be increased to such an extent that they do not reach the laser threshold. By adding a tailored lateral absorber to the edges of the laser resonator, the TUW/ETHZ researchers suppressed the higher lateral modes entirely, without having any relevant impact on the fundamental modes. The result was an emission bandwidth covering a full octave, even mode distribution in the middle at 700 GHz, and a frequency comb with a bandwidth of 440 GHz.

Furthermore, the lateral absorbers enable the generation of ultrashort terahertz pulses with pulse widths of less than 3 ps, which represents what they say is a new world record for terahertz pulses generated using a quantum cascade laser. “It was truly amazing to see how a relatively minor adjustment to the waveguide could bring about such a dramatic improvement,” said Dominic Bachmann, who has just finished writing his dissertation on broadband quantum cascade lasers.

New record achieved in terahertz pulse generation

http://www.alphagalileo.org/ViewItem.aspx?ItemId=172465&CultureCode=en

A group of scientists from TU Wien and ETH Zurich have succeeded in their attempts to generate ultrashort terahertz light pulses. With lengths of just a few picoseconds, these pulses are ideally suited to spectroscopic applications and enable extremely precise frequency measurements to be taken.

The unique properties of terahertz radiation mean it is of interest for a wide range of potential applications, including non-invasive medical imaging and the detection of hazardous substances. Terahertz waves can penetrate many materials that are opaque to visible light and, unlike X-radiation, do not pose a risk of damage to biological tissue. In addition to this, many substances have a molecular fingerprint in the terahertz range, allowing them to be detected using spectroscopic methods. One efficient way of generating these terahertz waves is using quantum cascade lasers, which a working group led by Prof. Karl Unterrainer at the Photonics Institute at TU Wien has been researching and developing. Quantum cascade lasers consist of a precisely defined sequence of several hundred semiconductor layers that measure just a few nanometres in thickness. This special construction means there is the freedom to select the exact energy state at which the electrons stay within the semiconductor structure. This allows the frequency of the laser light emitted to be adjusted to suit the application in question.

Creating a frequency comb with a broadband ‘laser sandwich’

With this special feature of being able to determine the laser wavelengths themselves, several quantum cascade structures with different emission frequencies can be stacked on top of one another, with the aim of generating broadband terahertz radiation.  “Heterogeneous active zones of this kind are ideally suited for implementing broadband terahertz amplifiers and generating ultrashort terahertz pulses,” explains Dominic Bachmann from the Photonics Institute. Plus, if the discrete laser lines are linked together to establish a fixed phase relationship between the laser modes, something known as a ‘frequency comb’ will be created. Frequency combs make it possible to take extremely precise measurements of the absolute frequency of the light being used, which is essential for a huge number of applications. The discovery of the frequency comb more or less revolutionised optical metrology and was honoured with the Nobel Prize for Physics in 2005. Over the past four years, researchers have been working hard to generate a terahertz frequency comb using a quantum cascade laser as part of the EU project TERACOMB. Headed up by Dr Juraj Darmo from the Photonics Institute, the team of international research groups has succeeded in generating the first broadband terahertz frequency comb based on semiconductor technology.

Watching lasers at work

One method developed by the group led by Prof. Unterrainer makes it possible to analyse internal quantum cascade laser parameters during laser operation. This technique is based on time-resolved spectroscopy, with broadband terahertz pulses penetrating the sample to be measured. Based on femtosecond lasers, this technology can be used to collect the full information content relating to the time and frequency range with just one single measurement. As a result, the scientists at the Photonics Institute have managed to quantify the optical gain coefficients as well as the optical dispersion in broadband terahertz quantum cascade lasers, improving their understanding of the complex dynamics at play. “These findings allow us to increase the laser bandwidth even further and to improve the efficiency of frequency combs,” explains Juraj Darmo.

Targeting losses

One unresolved issue with terahertz quantum cascade lasers had been the existence of laser lines with different propagation speeds. If there are laser modes with a higher lateral order, the intensity is distributed very unevenly between the laser lines, thereby reducing the usable bandwidth and preventing the generation of a frequency comb. In order to stop these modes from oscillating, the losses have to be increased to such an extent that they do not reach the laser threshold. By adding a tailored lateral absorber to the edges of the laser resonator, the researchers managed to suppress the higher lateral modes entirely, without having any relevant impact on the fundamental modes. The result was an emission bandwidth covering a full octave, very even mode distribution in the middle at 700 GHz, and a frequency comb with a bandwidth of 440 GHz. What's more, the lateral absorbers enable the generation of ultrashort terahertz pulses with pulse widths of less than 3 ps, which represents a new world record for terahertz pulses generated using a quantum cascade laser. “It was truly amazing to see how a relatively minor adjustment to the waveguide could bring about such a dramatic improvement,” explains Dominic Bachmann, who has just finished writing his dissertation on broadband quantum cascade lasers.

Switching light with a silver atom

The switch is based on the voltage-induced displacement of one or more silver atoms in the narrow gap between a silver and a platinum plate.

Credit: Alexandros Emboras / ETH Zurich

http://www.sciencedaily.com/releases/2016/02/160201103943.htm

The quantity of data exchanged via communications networks around the globe is growing at a breathtaking rate. The volume of data for wired and mobile communications is currently increasing by 23% and 57% respectively every year. It is impossible to predict when this growth will end. This also means that all network components must constantly be made more efficient.

These components include so-called modulators, which convert the information that is originally available in electrical form into optical signals. Modulators are therefore nothing more than fast electrical switches that turn a laser signal on or off at the frequency of the incoming electrical signals. Modulators are installed in data centres in their thousands. However, they all have the disadvantage of being quite large. Measuring a few centimetres across, they take up a great deal of space when used in large numbers.

From micromodulators to nanomodulators

Six months ago, a working group led by Jürg Leuthold, Professor of Photonics and Communications already succeeded in proving that the technology could be made smaller and more energy-efficient. As part of that work, the researchers presented a micromodulator measuring just 10 micrometres across -- or 10,000 times smaller than modulators in commercial use.

Leuthold and his colleagues have now taken this to the next level by developing the world's smallest optical modulator. And this is probably as small as it can get: the component operates at the level of individual atoms. The footprint has therefore been further reduced by a factor of 1,000 if you include the switch together with the light guides. However, the switch itself is even smaller, with a size measured on the atomic scale. The team's latest development was recently presented in the journal Nano Letters.

In fact, the modulator is significantly smaller than the wavelength of light used in the system. In telecommunications, optical signals are transmitted using laser light with a wavelength of 1.55 micrometres. Normally, an optical device can not be smaller than the wavelength it should process. "Until recently, even I thought it was impossible for us to undercut this limit," stresses Leuthold.

New structure

But his senior scientist Alexandros Emboras proved the laws of optics wrong by successfully reconfiguring the construction of a modulator. This construction made it possible to penetrate the order of magnitude of individual atoms, even though the researchers were using light with a "standard wavelength."

Emboras's modulator consists of two tiny pads, one made of silver and the other of platinum, on top of an optical waveguide made of silicon. The two pads are arranged alongside each other at a distance of just a few nanometres, with a small bulge on the silver pad protruding into the gap and almost touching the platinum pad.

Short circuit thanks to a silver atom

And here's how the modulator works: light entering from an optical fibre is guided to the entrance of the gap by the optical waveguide. Above the metallic surface, the light turns into a surface plasmon. A plasmon occurs when light transfers energy to electrons in the outermost atomic layer of the metal surface, causing the electrons to oscillate at the frequency of the incident light. These electron oscillations have a far smaller diameter than the ray of light itself. This allows them to enter the gap and pass through the bottleneck. On the other side of the gap, the electron oscillations can be converted back into optical signals.

If a voltage is now applied to the silver pad, a single silver atom or, at most, a few silver atoms move towards the tip of the point and position themselves at the end of it. This creates a short circuit between the silver and platinum pads, so that electrical current flows between them. This closes the loophole for the plasmon; the switch flips and the state changes from "on" to "off" or vice versa. As soon as the voltage falls below a certain threshold again, a silver atom moves back. The gap opens, the plasmon flows, and the switch is "on" again. This process can be repeated millions of times.

ETH Professor Mathieu Luisier, who participated in this study, simulated the system using a high-performance computer at the CSCS in Lugano. This allowed him to confirm that the short circuit at the tip of the silver point is brought about by a single atom.

A truly digital signal

As the plasmon has no other options than to pass through the bottleneck either completely or not at all, this produces a truly digital signal -- a one or a zero. "This allows us to create a digital switch, as with a transistor. We have been looking for a solution like this for a long time," summarises Leuthold.

As yet, the modulator is not ready for series production. Although it has the advantage of operating at room temperature, unlike other devices that work using quantum effects at this order of magnitude, it still remains very slow for a modulator: so far, it only works for switching frequencies in the megahertz range or below. The ETH researchers want to fine-tune it for frequencies in the gigahertz to terahertz range.

Improving the lithography process

The researchers also want to further improve the lithography method, which was redeveloped by Emboras from scratch to build the parts, so that components like this can be produced reliably in future. At present, fabrication is only successful in one out of every six attempts. Nevertheless, the researchers consider this a success, as lithography processes on the atomic scale remain uncharted territory.

In order to continue his research into the nanomodulator, Leuthold has strengthened his team. However, he points out that greater resources would be required to develop a commercially available solution. Despite this, the ETH professor is confident that he and his team will be able to present a practicable solution within the next few years.

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Story Source:

The above post is reprinted from materials provided by ETH Zurich. Note: Materials may be edited for content and length.

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Journal Reference:

1. Alexandros Emboras, Jens Niegemann, Ping Ma, Christian Haffner, Andreas Pedersen, Mathieu Luisier, Christian Hafner, Thomas Schimmel, Juerg Leuthold. Atomic Scale Plasmonic Switch. Nano Letters, 2016; 16 (1): 709 DOI: 10.1021/acs.nanolett.5b04537

TERACOMB FET Open project: tuning quantum cascade lasers to the terahertz

http://ec.europa.eu/digital-agenda/en/news/teracomb-fet-open-project-tuning-quantum-cascade-lasers-terahertz

The TERACOMB FET Open project successfully developed quantum cascade lasers emitting stably in the terahertz region of the electromagnetic spectrum. This technology would allow for the creation of compact high resolution spectroscopy devices.

A new paper in Nature Photonics from ETH Zürich partner at the TERACOMB project reports about the first experimental realization of an octave-spanning semiconductor injection laser. This ambitious project is focused on pursuing the technology of quantum cascade lasers (QCL) to generate a frequency comb (FC) in the terahertz frequency region. QCLs are currently useful for spectroscopic applications such as remote sensing of environmental gases and pollutants in the atmosphere.

The devices developed by TERACOMB emit in the THz region of the electromagnetic spectrum. Octave spanning lasers are the building block of frequency combs, which constitute nowadays a powerful platform for high resolution spectroscopy in the visible and Mid-IR as well as in metrology. TERACOMB devices not only show octave spanning output but, thanks to careful bandstructure engineering, also display a remarkably stable output with sub-KHz RF beatnotes indicating comb operation on more than 600 GHz spectral bandwidth.

The team at TERACOMB points that such devices pave the way towards compact, comb-based THz spectrometers, overcoming many of the current technological limits on in identification and quantification of complex heavy molecules such as those in toxic chemicals, explosives, and drugs.

The TERACOMB project consortium brings together partners from Austria, France, Switzerland, Germany and the United Kingdom.

Observed live with X-ray laser: Electricity controls magnetism

Principle of the experiment. The motion of the magnetic moments in TbMnO3 (shown as arrows on the right hand side) is excited by a terahertz pulse (red beam) and probed by a pulse from the x-ray laser LCLS (blue beam). Credit: Teresa Kubacka

by Paul Piwnicki

 http://phys.org/news/2014-03-x-ray-laser-electricity-magnetism.html#jCp

Researchers from ETH Zurich and the Paul Scherrer Institute PSI demonstrate how the magnetic structure can be altered quickly in novel materials. The effect could be used in efficient hard drives of the future.

Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now changed the magnetic arrangement in a material much faster than is possible with today's hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled. Applied in future devices, this kind of strong interaction between magnetic and electric properties can have numerous advantages. For instance, an electrical fieldcan be generated more easily in a device than a magnetic one. In the experiment, the changes in magnetic arrangement took place within a picosecond (a trillionth of a second) and could be observed with x-ray flashes at the American x-ray laser LCLS. The flashes are so short that you can virtually see how the magnetisation changes from one image to the next - similar to how we are able to capture the movement of an athlete with a normal camera in a series of images with a short exposure time. In future, such experiments should also be possible at PSI's new research facility, the x-ray laser SwissFEL.

The results will be published in the journal Science. They appear online in advance of print in Science Express on 6 March.

One common method of data storage uses materials in which different magnetic domains can be oriented in different directions. In other words, the tiny elementary magnets inside the material are aligned along two possible directions, which enables one bit to be saved in the material. A bit is the smallest unit of information, for which there are two possibilities, often referred to as 0 and 1. In the storage device, these correspond to the two different magnetic directions. In a real hard drive, which must store a large amount of information, there are many small areas that correspond to single bits. To change the information on the hard drive, the direction of the magnetism in one domain must be flipped. In modern consumer devices this is achieved using a small magnetic field.

An electric field can be generated in a small space more easily than a magnetic field, which means that, in principle, smaller storage devices can be constructed if magnetism is switched by electric fields. A strong connection between magnetic and electric properties is exhibited by so-called multiferroic materials, which have been one of the hottest topics in materials research for a number of years. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now studied the material TbMnO3 and demonstrated that its magnetic arrangement can be changed by an electric field in a matter of picoseconds (10-12 s = one trillionth of a second), which is considerably shorter than the time it takes for today's hard drives to be switched. "This shows that multiferroic materials can be switched quickly enough electrically for them to be used in magnetic storage devices," explains Urs Staub, a research group leader at PSI and one of the research project supervisors. "Electric switching could have numerous advantages. In order to generate a magnetic field, you need a coil through which a current flows. An electric field can be generated without current.

"The material we studied can't be used in technical devices - you need very low temperatures and strong electrical fields to observe the relevant phenomena. However, the basic result probably also applies for materials that are more suitable for applications and will presumably consist of a combination of thin layers of different materials."

Exposure time: 0.000 000 000 000 1 seconds

The arrangement of magnetic moments in TbMnO3.Neighbouring moments are tilted in respect to each other. There are two possible directions in which the moments can turn that might correspond to the two values of a bit in future storage …more

The experiment is based on the interaction between pulsed light produced by two lasers - terahertz light generated by a laser which can easily fit into a lab, and the radiation from the x-ray laser Linac Coherent Light Source (LCLS), a large-scale research facility located at SLAC National Accelerator Laboratory in Menlo Park, California, that is roughly three kilometres in length. In the experiment, the material was illuminated with short flashes of terahertz-frequency light which were only a few picoseconds long. Light consists of an electric and a magnetic field, which periodically become stronger and weaker. The terahertz flashes were so short that the electric fields in them were only able to perform a few oscillations. With experiments at the LCLS, the researchers were able to demonstrate that the magnetic arrangement was distorted by the flash of light and - with a slight delay - this distortion followed the oscillation of the electrical field within the flash. The magnetic component of the light was too weak to influence the magnetic structure. The x-ray laser generates very short (100 femtoseconds = 0.000 000 000 000 1 seconds) and intense flashes of x-ray light which are so much shorter than the terahertz flash. This allows the x-rays to measure the magnetic distortion along the different stages of its motion, similar to how a camera with a fast shutter speed captures still images of rapid motions. Today, the LCLS is one of two facilities where such experiments are possible. In the future, they will also be possible at the x-ray laser SwissFEL, which is currently under construction at the Paul Scherrer Institute. "An experiment like this can only be conducted at an x-ray laser because only the pulses from the x-ray laser show the magnetic order and are short enough for you to follow the chronological sequences," explains Staub.

Tilted elementary magnets

Magnetic materials which can be used to store data can have different magnetic arrangements. In today's hard drives, the magnetic areas are arranged ferromagnetically, which means that the elementary magnets or, to use the technical term, magnetic moments are all pointing in same direction within the area encoding one bit. In the material studied in the experiment, the moments are arranged in rows but in such a way that two neighbouring moments are slightly rotated with respect to each other as opposed to being parallel. If you move from one moment to the next, the direction of the moments keeps turning and overall the sequence of magnetic moments forms a cycloid. Generally speaking, there are two directions in which the moments can turn, clockwise and anticlockwise - and these could correspond to the two values of a bit. To change between "0" and "1", the magnetic moments would have to change the turning direction within the sequence, which is equivalent to rotating the entire sequence of magnetic moments by 180 degrees.

Positive and negative - offset from each other

The multiferroic material also has another property: electric polarisation, which means that the positive and negative charges are shifted slightly against each other. The interior of the material is constructed from atoms that have fixed positions in a three-dimensional structure. As there are just as many negative charges (electrons) as positive ones (atomic nuclei) in the atoms, the entire material is electrically neutral. Some of the electrons, however, are not bound rigidly to the atomic nuclei. These electrons can be displaced with respect to the atomic nuclei, which means that one side of the material is positively charged, the other negatively. In other words, the material is electrically polarised. In everyday life, electrically polarised materials are primarily known thanks to the piezoelectric effect used to produce sparks in lighters or sound in loudspeakers, for instance.

Electrically and magnetically linked

The deflection of the magnetic moments (black line) follows the electric field of the terahertz pulse (red line) with a short delay. The blue dots show the results of the measurement. Credit: Kubacka et al., Science Express (2014) DOI: 10.1126/science.1242862

In TbMnO3, the electrical polarisation is linked to the magnetic arrangement, which means that if the magnetic moments turn in one direction, this always corresponds to an alignment of the electric polarisation; if you reverse the polarisation, the rotational direction of the magnetic moments also turns around. The researchers studied this coupling in their experiment. Using the alternating electric field of the terahertz pulse, they influenced the electric polarisation and observed the extent to which the magnetic arrangement followed the alternating field. Although the electric field was too weak to actually turn the sequence of magnetic moments by 180 degrees, the scientists were able to observe that it was turned by around four degrees in time with the electrical field. "This procedure is also important for possible applications," explains Teresa Kubacka, a doctoral student in the Ultrafast Dynamics Group at ETH Zurich and first author of the paper. "The terahertz pulse is designed in such a way that it influences the magnetic arrangement only in this particular way. If the magnetic arrangement in a device could be changed so specifically, much less energy would be wasted and the material would not heat up as much."

Precision measuring

It is the first time that it was possible to measure such a rapid change in a multiferroic material so precisely. The angle by which the magnetic moments were turned was determined using the short flashes from the LCLS x-ray laser in a scattering experiment. It involved sending the x-ray beam through the sample studied and observing the directions in which the x-ray light was deflected by the sample. In the case of this material, there are directions in which the light is deflected by the atomic structure and others where the deflection is caused by the magnetic moments. If the magnetic arrangement is changed, the intensity of the deflected x-ray light changes. In the experiment, the researchers measured the intensity of the deflected x-ray beam at different times for a selected direction. Then they calculated how the magnetic moments react to the electric field within the terahertz flash.

Experimental challenges

"One of the challenges of the experiment was to create the terahertz flashes with the correct frequency and guarantee that enough of their intensity reaches the sample. Such pulses were not created directly by a laser, but rather with the aid of special organic crystals hit by laser pulses with another frequency. At ETH Zurich, we are also working on facilities that generate terahertz pulses and working together with the specialists from PSI and LCLS we were able to adapt the lasers available at the LCLS to our experiment's needs," says Kubacka.

 Explore further: 'Electromagnon' effect couples electricity and magnetism in materials

More information: "Large-Amplitude Spin Dynamics Driven by a THz Pulse in Resonance with an Electromagnon." T. Kubacka, et al. Science DOI: 

Observed live with x-ray laser: electricity controls magnetism

http://www.chemeurope.com/en/news/147248/observed-live-with-x-ray-laser-electricity-controls-magnetism.html

0-03-2014: Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now changed the magnetic arrangement in a material much faster than is possible with today’s hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled. Applied in future devices, this kind of strong interaction between magnetic and electric properties can have numerous advantages. For instance, an electrical field can be generated more easily in a device than a magnetic one. In the experiment, the changes in magnetic arrangement took place within a picosecond (a trillionth of a second) and could be observed with x-ray flashes at the American x-ray laser LCLS. The flashes are so short that you can virtually see how the magnetisation changes from one image to the next – similar to how we are able to capture the movement of an athlete with a normal camera in a series of images with a short exposure time. In future, such experiments should also be possible at PSI’s new research facility, the x-ray laser SwissFEL. The results will be published in the journal Science. They appear online in advance of print in Science Express on 6 March.

One common method of data storage uses materials in which different magnetic domains can be oriented in different directions. In other words, the tiny elementary magnets inside the material are aligned along two possible directions, which enables one bit to be saved in the material. A bit is the smallest unit of information, for which there are two possibilities, often referred to as 0 and 1. In the storage device, these correspond to the two different magnetic directions. In a real hard drive, which must store a large amount of information, there are many small areas that correspond to single bits. To change the information on the hard drive, the direction of the magnetism in one domain must be flipped. In modern consumer devices this is achieved using a small magnetic field.

An electric field can be generated in a small space more easily than a magnetic field, which means that, in principle, smaller storage devices can be constructed if magnetism is switched by electric fields. A strong connection between magnetic and electric properties is exhibited by so-called multiferroic materials, which have been one of the hottest topics in materials research for a number of years. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now studied the material TbMnO₃ and demonstrated that its magnetic arrangement can be changed by an electric field in a matter of picoseconds (10^-12 s = one trillionth of a second), which is considerably shorter than the time it takes for today’s hard drives to be switched. “This shows that multiferroic materials can be switched quickly enough electrically for them to be used in magnetic storage devices,” explains Urs Staub, a research group leader at PSI and one of the research project supervisors. “Electric switching could have numerous advantages. In order to generate a magnetic field, you need a coil through which a current flows. An electric field can be generated without current.

“The material we studied can’t be used in technical devices – you need very low temperatures and strong electrical fields to observe the relevant phenomena. However, the basic result probably also applies for materials that are more suitable for applications and will presumably consist of a combination of thin layers of different materials.”

Exposure time: 0.000 000 000 000 1 seconds

The experiment is based on the interaction between pulsed light produced by two lasers – terahertz light generated by a laser which can easily fit into a lab, and the radiation from the x-ray laser LCLS, a large-scale research facility at SLAC National Accelerator Laboratory in Menlo Park, California, that is roughly three kilometres in length. In the experiment, the material was illuminated with short flashes of terahertz-frequency light which were only a few picoseconds long. Light consists of an electric and a magnetic field, which periodically become stronger and weaker. The terahertz flashes were so short that the electric fields in them were only able to perform a few oscillations. With experiments at the LCLS, the researchers were able to demonstrate that the magnetic arrangement was distorted by the flash of light and – with a slight delay – this distortion followed the oscillation of the electrical field within the flash. The magnetic component of the light was too weak to influence the magnetic structure. The x-ray laser generates very short (100 femtoseconds = 0.000 000 000 000 1 seconds) and intense flashes of x-ray light which are so much shorter than the terahertz flash. This allows the x-rays to measure the magnetic distortion along the different stages of its motion, similar to how a camera with a fast shutter speed captures still images of rapid motions. Today, the LCLS is one of two facilities where such experiments are possible. In the future, they will also be possible at the x-ray laser SwissFEL, which is currently under construction at the Paul Scherrer Institute. “An experiment like this can only be conducted at an x-ray laser because only the pulses from the x-ray laser show the magnetic order and are short enough for you to follow the chronological sequences,” explains Staub.

Tilted elementary magnets

Magnetic materials which can be used to store data can have different magnetic arrangements. In today’s hard drives, the magnetic areas are arranged ferromagnetically, which means that the elementary magnets or, to use the technical term, magnetic moments are all pointing in same direction within the area encoding one bit. In the material studied in the experiment, the moments are arranged in rows but in such a way that two neighbouring moments are slightly rotated with respect to each other as opposed to being parallel. If you move from one moment to the next, the direction of the moments keeps turning and overall the sequence of magnetic moments forms a cycloid. Generally speaking, there are two directions in which the moments can turn, clockwise and anticlockwise – and these could correspond to the two values of a bit. To change between “0” and “1”, the magnetic moments would have to change the turning direction within the sequence, which is equivalent to rotating the entire sequence of magnetic moments by 180 degrees.

Positive and negative – offset from each other

The multiferroic material also has another property: electric polarisation, which means that the positive and negative charges are shifted slightly against each other. The interior of the material is constructed from atoms that have fixed positions in a three-dimensional structure. As there are just as many negative charges (electrons) as positive ones (atomic nuclei) in the atoms, the entire material is electrically neutral. Some of the electrons, however, are not bound rigidly to the atomic nuclei. These electrons can be displaced with respect to the atomic nuclei, which means that one side of the material is positively charged, the other negatively. In other words, the material is electrically polarised. In everyday life, electrically polarised materials are primarily known thanks to the piezoelectric effect used to produce sparks in lighters or sound in loudspeakers, for instance.

Electrically and magnetically linked

In TbMnO₃, the electrical polarisation is linked to the magnetic arrangement, which means that if the magnetic moments turn in one direction, this always corresponds to an alignment of the electric polarisation; if you reverse the polarisation, the rotational direction of the magnetic moments also turns around. The researchers studied this coupling in their experiment. Using the alternating electric field of the terahertz pulse, they influenced the electric polarisation and observed the extent to which the magnetic arrangement followed the alternating field. Although the electric field was too weak to actually turn the sequence of magnetic moments by 180 degrees, the scientists were able to observe that it was turned by around four degrees in time with the electrical field. “This procedure is also important for possible applications,” explains Teresa Kubacka, a doctoral student in the Ultrafast Dynamics Group at ETH Zurich and first author of the paper. “The terahertz pulse is designed in such a way that it influences the magnetic arrangement only in this particular way. If the magnetic arrangement in a device could be changed so specifically, much less energy would be wasted and the material would not heat up as much.”

Precision measuring

It is the first time that it was possible to measure such a rapid change in a multiferroic material so precisely. The angle by which the magnetic moments were turned, was determined using the short flashes from the LCLS x-ray laser in a scattering experiment. It involved sending the x-ray beam through the sample studied and observing the directions in which the x-ray light was deflected by the sample. In the case of this material, there are directions in which the light is deflected by the atomic structure and others where the deflection is caused by the magnetic moments. If the magnetic arrangement is changed, the intensity of the deflected x-ray light changes. In the experiment, the researchers measured the intensity of the deflected x-ray beam at different times for a selected direction. Then they calculated how the magnetic moments react to the electric field within the terahertz flash.

Experimental challenges

“One of the challenges of the experiment was to create the terahertz flashes with the correct frequency and guarantee that enough of their intensity reaches the sample. Such pulses were not created directly by a laser, but rather with the aid of special organic crystals hit by laser pulses with another frequency. At ETH Zurich, we are also working on facilities that generate terahertz pulses and working together with the specialists from PSI and LCLS we were able to adapt the lasers available at the LCLS to our experiment’s needs,” says Kubacka.

Original publication:
Large-amplitude spin dynamics driven by a THz pulse in resonance with an electromagnon T. Kubacka et al., Science Express, 6 March 2014