Terahertz radiation technique opens a new door for studying atomic behavior

A compressor using terahertz radiation to shorten electron bunches is small enough to fit into the palm of a hand. Credit: Dawn Harmer/SLAC National Accelerator Laboratory

https://phys.org/news/2020-03-terahertz-technique-door-atomic-behavior.html

Researchers from the Department of Energy's SLAC National Accelerator Laboratory have made a promising new advance for the lab's high-speed "electron camera" that could allow them to "film" tiny, ultrafast motions of protons and electrons in chemical reactions that have never been seen before. Such "movies" could eventually help scientists design more efficient chemical processes, invent next-generation materials with new properties, develop drugs to fight disease and more.

The new technique takes advantage of a form of light called terahertz radiation, instead of the usual radio-frequency radiation, to manipulate the beams of electrons the instrument uses. This lets researchers control how fast the camera takes snapshots and, at the same time, reduces a pesky effect called timing jitter, which prevents researchers from accurately recording the timeline of how atoms or molecules change.

The method could also lead to smaller particle accelerators: Because the wavelengths of terahertz radiation are about a hundred times smaller than those of radio waves, instruments using terahertz radiation could be more compact.

The researchers published the findings in Physical Review Letters on February 4.

A Speedy Camera

SLAC's "electron camera," or ultrafast electron diffraction (MeV-UED) instrument, uses high-energy beams of electrons traveling near the speed of light to take a series of snapshots—essentially a movie—of action between and within molecules. This has been used, for example, to shoot a movie of how a ring-shaped molecule breaks when exposed to light and to study atom-level processes in melting tungsten that could inform nuclear reactor designs.

The technique works by shooting bunches of electrons at a target object and recording how electrons scatter when they interact with the target's atoms. The electron bunches define the shutter speed of the electron camera. The shorter the bunches, the faster the motions they can capture in a crisp image.

"It's as if the target is frozen in time for a moment," says SLAC's Emma Snively, who spearheaded the new study.

SLAC's Emma Snively and Mohamed Othman at the lab's high-speed "electron camera," an instrument for ultrafast electron diffraction (MeV-UED). Credit: Jacqueline Orrell/SLAC National Accelerator Laboratory

For that reason, scientists want to make all the electrons in a bunch hit a target as close to simultaneously as possible. They do this by giving the electrons at the back a little boost in energy, to help them catch up to the ones in the lead.

So far, researchers have used radio waves to deliver this energy. But the new technique developed by the SLAC team at the MeV-UED facility uses light at terahertz frequencies instead.

Why terahertz?

A key advantage of using terahertz radiation lies in how the experiment shortens the electron bunches. In the MeV-UED facility, scientists shoot a laser at a copper electrode to knock off electrons and create beams of electron bunches. And until recently, they typically used radio waves to make these bunches shorter.

However, the radio waves also boost each electron bunch to a slightly different energy, so individual bunches vary in how quickly they reach their target. This timing variance is called jitter, and it reduces researchers' abilities to study fast processes and accurately timestamp how a target changes with time.

The terahertz method gets around this by splitting the laser beam into two. One beam hits the copper electrode and creates electron bunches as before, and the other generates the terahertz radiation pulses for shortening the electron bunches. Since they were produced by the same laser beam, electron bunches and terahertz pulses are now synchronized with each other, reducing the timing jitter between bunches.

Down to the femtosecond

A key innovation for this work, the researchers say, was creating a particle accelerator cavity, called the compressor. This carefully machined hunk of metal is small enough to sit in the palm of a hand. Inside the device, terahertz pulses shorten electron bunches and give them a targeted and effective push.

From left: SLAC's Emma Snively, Michael Kozina and Mohamed Othman at the lab's MeV-UED instrument. Credit: Jacqueline Orrell/SLAC National Accelerator Laboratory

As a result, the team could compress electron bunches so they last just a few tens of femtoseconds, or quadrillionths of a second. That's not as much compression as conventional radio-frequency methods can achieve now, but the researchers say the ability to simultaneously lower jitter makes the terahertz method promising. The smaller compressors made possible by the terahertz method would also mean lower cost compared to radio-frequency technology.

"Typical radio-frequency compression schemes produce shorter bunches but very high jitter," says Mohamed Othman, another SLAC researcher on the team. "If you produce a compressed bunch and also reduce the jitter, then you'll be able to catch very fast processes that we've never been able to observe before."

Eventually, the team says, the goal is to compress electron bunches down to about a femtosecond. Scientists could then observe the incredibly fast timescales of atomic behavior in fundamental chemical reactions like hydrogen bonds breaking and individual protons transferring between atoms, for example, that aren't fully understood.

"At the same time that we are investigating the physics of how these electron beams interact with these intense terahertz waves, we're also really building a tool that other scientists can use immediately to explore materials and molecules in a way that wasn't possible before," says SLAC's Emilio Nanni, who led the project with Renkai Li, another SLAC researcher. "I think that's one of the most rewarding aspects of this research."

Terahertz waves create short and stable electron pulses

Short and sharp: terahertz radiation has been used to improve the quality of ultrashort electron pulses. (Courtesy: Shutterstock/Anteromite)

https://physicsworld.com/a/terahertz-waves-create-short-and-stable-electron-pulses/

Terahertz radiation has been used to reduce the timing jitter of ultrashort pulses of relativistic electrons. This was achieved independently by two teams, one in China led by Dao Xiang at Shanghai Jiao Tong University and the other in the US led by Emilio Nanni at SLAC National Accelerator Laboratory. Their work could allow researchers to generate high-quality electron beams at far lower costs than current radio-frequency (RF) techniques, allowing for advanced studies of atomic-scale structures and femtosecond-scale processes.

Electron beams comprising ultrashort pulses are rapidly advancing the capabilities of today’s most cutting-edge imaging techniques. Currently, they are generated using RF techniques, which can compress bunches of electrons so that their “heads” and “tails” are separated by less than 10 fs as they move. Yet as researchers’ demands for ever shorter and brighter pulses continue to grow, the RF equipment used to generate them is barely keeping up. Not only is the apparatus bulky and expensive; it causes the distribution of pulse arrival times at a target to become spread out, increasing the timing uncertainty, or jitter, of the beam. So far, this has hindered the progress of many studies requiring the best possible ultrashort pulses.

Velocity boosts

Using similar approaches, both Nanni’s and Xiang’s teams discovered that bunches of electrons produced by a photocathode can be compressed, and their timing jitter reduced, by replacing RF signals with terahertz waves. Their setups included two terahertz sources that are linearly polarized in parallel directions. The waves interact with electron bunches as they pass through two separate waveguides. During this interaction, each pulse imparts velocity boosts that increase as the beam passes through. This compresses the electron bunches by speeding up their tails more than their heads. In addition, since the same laser is used to drive the photocathode and both terahertz sources, each radiation-bunch interaction is highly synchronized, reducing the jitter of the beam

In each case, beams were compressed to lengths of under 40 fs – not quite as short as those reachable through RF structures used today for electron pulse generation. However, the jitters produced in both studies were reduced to just around 30 fs – a significant improvement on previous techniques. All the while, the apparatus necessary for this is far more affordable than today’s bulky RF-based generators.

Although the teams did not collaborate, their similar methodologies both clearly demonstrated the unprecedented timing resolution afforded using terahertz radiation. With further improvements, their approach could soon satisfy the growing demand for intense, ultrashort electron beams. Possible applications include ultrafast electron diffraction, which can be used to capture images of atomic-scale structures. It could also be used to study the femotosecond-scale processes that play out within materials including semiconductors, yielding important new insights into their physics.

The Shanghai Jiao Tong University and SLAC studies are described in separate papers in Physical Review Letters.

Synopsis: Making Electron Pulses Shorter and Steadier

Terahertz radiation can be used to produce short and well-timed pulses of electrons—which could benefit electron diffraction schemes used to image ultrafast atomic and molecular dynamics.

Sophia Chen

https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.124.054802

Short electron pulses play a central role in imaging techniques such as ultrafast electron diffraction, which can capture fleeting atomic dynamics inside semiconductors and other materials. To take increasingly faster snapshots, researchers have sought to produce shorter pulses. Using radio frequencies (rf), they have compressed pulse duration to less than 10 femtoseconds (fs). These rf schemes, however, are expensive and lead to timing “jitter,” fluctuations in the pulse timing that can smear out the temporal resolution. Now, two independent groups—the first led by Emilio Nanni of SLAC National Accelerator Laboratory, the second by Dao Xiang of Shanghai Jiao Tong University—have demonstrated a cheaper method that compresses the electron pulses with terahertz, rather than rf, radiation, producing pulses that are both short and consistently timed.

The two groups produce electron pulses by illuminating a photocathode gun with a laser. As the electrons move through a cavity, terahertz radiation generated by the same laser accelerates electrons from the pulse tail and slows down those at the pulse head. As a result, the pulses are compressed from 100 to 30 fs. While this duration is longer than that achievable in rf schemes, the timing jitter of the pulses is considerably improved. Jitter occurs because, as an electron bunch moves down the cavity, it tends to drift in time with respect to the next bunch. Since the terahertz radiation is produced by the same laser that initializes the electron beam, the radiation and the bunch are synchronized. As the radiation compresses each electron bunch it also resets the bunch's timing. The technique improved timing jitter by more than twofold compared with rf schemes.

This research is published in Physical Review Letters.

Three SLAC Scientists Receive DOE Early Career Research Grants

                                    SLAC's 2017 DOE Early Career Award winners, from left:                                                                     Frederico Fiuza, Emilio Nanni and Zeeshan Ahmed

Zeeshan Ahmed, Frederico Fiuza and Emilio Nanni will each receive about $2.5 million over five years to pursue cutting-edge research into cosmic inflation, plasma acceleration and using terahertz waves to accelerate particles.

https://www.newswise.com/doescience/?article_id=679415&returnurl=aHR0cHM6Ly93d3cubmV3c3dpc2UuY29tL2FydGljbGVzL2xpc3Q=

Three scientists at the Department of Energy’s SLAC National Accelerator Laboratory will receive DOE Early Career Research Program grants for research to find evidence of cosmic inflation, understand how plasmas excite particles to high energies and develop a way to accelerate particles in much shorter distances with terahertz radiation.

Zeeshan Ahmed, Frederico Fiuza and Emilio Nanni were among 59 scientists selected out of about 700 applicants for the grants, which were announced August 9. They will receive about $500,000 per year for five years for salary and research expenses. You can see brief descriptions of the award winners’ work here.

The grants support the development of individual research programs of scientists who received their doctoral degrees up to 10 years earlier. Recipients must be full-time DOE national laboratory employees or tenure-track assistant or associate professors at a U.S. academic institution, and their research topics must fall within one of six Office of Science focus areas.

Zeeshan Ahmed: More Firepower on the CMB Sky

Ahmed, a project scientist and Panofsky Fellow at the Kavli Institute for Particle Astrophysics and Cosmology at SLAC, led the design, testing, construction and deployment of the SLAC and Stanford BICEP3 telescope at the South Pole in 2015 as a postdoctoral scholar on Chao-Lin Kuo's observational cosmic microwave background (CMB) research team at Stanford. 

BICEP3 belongs to the third generation of instruments scientists are using to look for patterns in the CMB as evidence of cosmic inflation – the rapid expansion of the early universe following the Big Bang.  

“We invented that quantum leap from the second to third generation of BICEP telescopes, and a lot of the technology that went into it was developed at SLAC,” Ahmed says. “BICEP3 has been taking high-quality data and the highest throughput of data of this kind for the past two years. In a year or two, the data collected with BICEP3 will produce deep maps that surpass the sensitivity of all previous CMB maps for inflation science.”

Prior to joining Stanford, Ahmed conducted graduate research on the second-generation Cryogenic Dark Matter Search, CDMS-II, leading the primary analysis of the final dataset. Ahmed received his PhD in physics from California Institute of Technology in 2012.

The Early Career grant will support his work on a fourth-generation technology for CMB cameras that can map a much larger part of the sky – 40 percent instead of the current 1 percent – at close to the same depth that BICEP3 will achieve at the end of its science program.

Whereas BICEP3 technology widened camera apertures, the fourth generation will focus on building larger arrays of camera sensors using scalable signal transduction techniques.

“If you think of a sensor as a unit of sensitivity, or firepower on the sky, we want to go an order of magnitude beyond the firepower we currently have – from tens of thousands of sensors to hundreds of thousands of sensors,” Ahmed says. “This would allow us to cover a large part of the sky very deeply and make maps that are useful for many scientifically compelling problems, such as inflation, measuring neutrino mass, finding relic particles that haven’t been detected yet, better understanding dark energy and large-scale structure, and more.”

To do so, Ahmed is working closely with SLAC and Stanford professor Kent Irwin’s groups in SLAC’s Technology Innovation and Particle Astrophysics directorates to explore questions such as how to get information out of the sensors more efficiently and how to package them better.

“It’s absolutely gratifying to receive this award,” he says. “It’s extremely competitive, but I think we do have a very compelling research case, and I’m glad the DOE supports this work as one of the directions to pursue for the next-generation CMB experiment.”

“Zeeshan has made tremendous contributions to BICEP3, the most sensitive B-mode machine operating at the frequency that has the best chance to see a signal,” Kuo says. “He is now on a great trajectory, aiming to bring all the pieces together to make the next-generation CMB focal planes. It is gratifying for me to see this recognition for him.”

Irwin adds, “Zeeshan is playing an invaluable leadership role in planning and executing SLAC’s plan for participation in CMB-S4. This includes planning pre-project R&D and preparing for project execution.

“One important area of scientific work for him has been in the development of next-generation readout electronics for CMB detectors, including CMB-S4, the BICEP series and Simons Observatory. His leadership in this area has been invaluable in coordinating electronics development, cryogenics testing and scientific implementation. This recognition for Zeeshan is exciting and very appropriate!”

Frederico Fiuza: Simulating ‘A Sea of Charged Particles’

Fiuza creates numerical experiments in plasma physics as a staff scientist and leader of the theory group in the lab’s High Energy Density Science division.

In this role, he models processes in plasma, or ionized gas, that accelerate particles to high energies. The work has a wide range of applications, from illuminating astrophysical phenomena to exploring controlled fusion energy and shrinking particle accelerators for medical therapy.

“In addition to using simulations, I like to work very closely with experimental teams and design laboratory experiments where these models can be verified and we can test our predictions,” Fiuza says. “Basically, we use high-power lasers to heat matter to high energies, dissociate the electrons from the atoms and explore the physics of the resulting sea of charged particles.”

The simulations he builds capture the fundamental physics of plasmas at small scales. The very small-scale physics is particularly demanding to model, Fiuza says, and there’s still quite a bit researchers don’t understand.

“There are a lot of directions one can explore,” Fiuza says. “Plasmas can support electric fields much stronger than those in conventional accelerators, which are limited by the breakdown of materials. Because plasma is already broken down into electrons and ions, we can achieve much higher electric fields, but we need to control them in a way that produces particle beams with very precise characteristics.”

Fiuza was introduced to plasma physics in a computational physics course as an undergraduate student. For the final project, the professor asked him to model a “surfatron” – an idea for accelerating particles to high energies as if they were surfers riding plasma waves.

“I was surfing quite a bit at the time, so it was all really exciting to me,” Fiuza says. “That really got plasmas in my head and I wanted to know more about it.”

That same professor—Luis Silva— became Fiuza’s PhD advisor. Fiuza earned his doctoral degree in plasma physics from Instituto Superior Tecnico in Lisbon, Portugal. In 2009, he was a visiting scholar in the Plasma Simulation Group at the University of California Los Angeles. From there he went to Lawrence Livermore National Laboratory as a Lawrence Fellow, and came to SLAC in 2015.

He will use the early career award for research aimed at understanding exactly how plasmas lead to efficient particle acceleration. Fiuza will work with researchers to propose and design laser experiments that test these models.

“This award means that for the next five years, I know that I will have the support to carry out the research that I’m really excited about and will have a team that can work with me on these goals,” Fiuza says.

Siegfried Glenzer, director of the High Energy Density Science Division, says, “Frederico’s work is world-leading. His insights into plasma physics and how particle acceleration works have greatly influenced the way we do laser experiments in pursuit of grand-challenge goals. His simulations have shown us how we can determine the physics important for the origin of cosmic rays. This award will allow him to significantly advance the field.”

Emilio Nanni: Exploring a Technological Desert

Nanni is an associate staff scientist in the lab’s Technology Innovation Directorate (TID), where he is working on ways to accelerate electrons to high energies in much shorter distances than possible today.  This particular approach uses terahertz radiation – a wavelength that falls between visible light and radio waves – to accelerate electrons through finely milled metal structures; a dozen of the experimental prototypes fit in your hand. But there are a number of challenges to overcome before the technology can be scaled up to high energies and leave the lab bench for deployment in the wider world.

“We’re trying to shrink the size of accelerators for a whole host of applications, from experiments in high-energy physics, biology and chemistry to new tools for medical treatment,” Nanni says.

“Terahertz radiation is at a sweet spot, where its wavelength is short enough to potentially achieve high gradients – high rates of acceleration in a short distance – at a frequency where metal structures are highly conductive. It can also offer very high pulse rates. That’s especially important to SLAC because we would like to be able to use it in experiments at the lab’s Linac Coherent Light Source X-ray free-electron laser.” LCLS is a DOE Office of Science User Facility.

Nanni came to SLAC in 2015 from MIT, where he earned his PhD and did postdoctoral research on generating terahertz waves and using them to accelerate electrons, among other things. By the time he arrived, he had already applied for and won a SLAC Laboratory Directed Research and Development grant to develop a way to use a laser system to generate very intense terahertz pulses.

The Early Career grant will help him “lay the foundation for terahertz accelerator technology,” according to a description of his work posted by DOE.

“I was blown away,” Nanni says of the award. “It’s such a unique opportunity to pursue what you’re passionate about and chart your own course, and it’s a big recognition of the people who have supported you along the way. Science is a team effort, and having good people around you helps you refine your thoughts and pursue valuable goals that are challenging.”

Michael Fazio, associate lab director in charge of TID, says, “This is very groundbreaking work. There is a big gap between the frequencies at which we operate accelerators today and some of the really far-out work being done with laser-driven acceleration. That gap is a technological desert, and Emilio has some very good ideas about how to exploit it. His work is very connected to what we do and to what the DOE is interested in.”

Craig Burkhart, head of TID’s RF Accelerator Research Division, adds, “Emilio arrived here with a well-formulated vision for a research program, and this Early Career Award will give him the opportunity to explore that. It’s a rare opportunity, and one that he richly deserves.”

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SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy's Office of Science. For more information, please visit slac.stanford.edu.

SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.