Making the shortest light bursts leads to better understanding of nature

In a paper accepted for publication in the American Institute of Physics' journal Review of Scientific Instruments, a team of researchers describes an advanced experimental system that can generate attosecond bursts of extreme ultraviolet light. Such pulses are the shortest controllable light pulses available to science. With these pulses, according to the researchers, it's possible to measure the dynamics of electrons in matter in real-time. Advances in attosecond science may enable scientists to verify theories that describe how matter behaves at a fundamental level, how certain important chemical reactions – such as photosynthesis – work. Additional advances may eventually lead to the control of chemical reactions.
"Understanding how matter works at the level of its electrons is likely to lead to new scientific tools and to novel technologies," said Felix Frank, of Imperial College in London and one of the authors on the paper. "In the future, this knowledge could help us to make better drugs, more efficient solar cells, and other things we can't yet foresee."

The researchers were able to produce these pulses by a process called high harmonic generation (HHG). The fundamental technology driving their setup is a high-power femtosecond laser system (femtoseconds are three orders of magnitude longer than attoseconds). The near infrared femtosecond laser pulses are corralled through a waveguide and a series of specialized mirrors, causing them to be compressed in time. With their waveforms precisely controlled, these compressed pulses are then focused into a gas target, creating an attosecond burst of extreme ultraviolet radiation. The experimental system developed by the researchers is able to accurately measure the attosecond pulses and deliver them to a variety of experiments in conjugation with other precisely synchronized laser pulses. "Though it incorporates many novel features, our system builds on a decade of research conducted by physics groups around the world," said John Tisch, lead scientist developing the technology at Imperial College.

Flashes of light out of the mirror

(Phys.org) -- A team of the Laboratory of Attosecond physics at the Max Planck Institute of Quantum Optics developed an alternative way of generating attosecond flashes of light. 


Electrons at a glass surface send out flashes of light with durations of only a few attoseconds when they come under the influence of high-intensity laser pulses. One attosecond is one part in a billion of one part in a billion of a second. In the electric field of the laser, the electrons at the surface start to oscillate. Hereby the ultrashort attosecond flashes of light are generated. The team at the Laboratory of Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics (MPQ) in Garching has now advanced this innovative method. It has the potential to replace the current procedure of the generation of attosecond flashes of light. Presently these flashes are generated by electrons in noble gases. But the scientists are sure, that their method of the generation of attosecond flashes of light at surfaces has some advantages (Physical Review Letters).

Flashes of light with attosecond duration enable observations in a world yet widely unknown – the microcosm. With their help the first images of the extremely fast motion of electrons became possible. The short bursts of light are usually generated by the use of noble gas atoms. The electrons of these atoms absorb the energy of the laser light and subsequently emit it again in the form of ultrashort flashes of light. It holds: The shorter the burst of light, the sharper the images out of the microcosm.

But there are other ways of generating these short bursts of light. A team at the Laboratory of Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics (MPQ) in Garching has now advanced one of these methods. The scientists shot a laser pulse with a duration of only 8 femtoseconds and a power of 16 terawatts onto a glass target, which thereby turned into a relativistically oscillating mirror. One femtosecond corresponds to one part in a million of one part in a billion of one second and 16 terrawatt correspond to the power of round about 1000 nuclear power stations.

The 8 femtosecond laser pulse consisted of only 3 optical cycles and hence 3 cycles of its electric field. As soon as this electric field hits the glass surface a relativistic plasma forms. This means, that the electrons at the surface are accelerated out of the solid to velocities close to the speed of light and subsequently are decelerated and sent back to the surface again, as soon as the electric field changes its polarization. Thereby the electrons form an oscillating mirror. During the reflection at this moving mirror the pulsed laser light is converted from the near infrared spectral region down to the extreme ultraviolet (XUV, down to a wavelength of 17 nanometer) part of the spectrum. Hereby even shorter flashes of light with a duration in the attosecond regime are generated. These flashes of light occur as isolated bursts or trains of pulses, if filtered appropriately. Comparison with simulations of the method show that the ultrashort flashes of light have durations of around 100 attoseconds.

Compared to the conventional method of attosecond pulse generation these new flashes of light possess a higher number of photons and are hence more intense than their predecessors. This higher intensity allows for the splitting of these isolated bursts into two parts which enables the observation of processes in the microcosm with two attosecond flashes of light. This in turn permits a higher resolution than achievable up to now with the use of an attosecond burst in combination with a longer femtosecond laser pulse.
For ultrashort imaging this means that images with a greater richness of detail will become achievable in the future.

More information: P. Heissler, et al. Few-cycle driven relativistically oscillating plasma mirrors - a source of intense, isolated attosecond pulses, Phys. Rev. Lett. 108, 235003 (2012)

Tabletop laser-like device creates coherent multicolor beams of ultraviolet, T- and X-rays

For the first time, researchers have produced a coherent, laser-like, directed beam of light that simultaneously streams ultraviolet light, X-rays, and all wavelengths in between.

One of the few light sources to successfully produce a coherent beam that includes X-rays, this new technology is the first to do so using a setup that fits on a laboratory table.

An international team of researchers, led by engineers from the NSF Engineering Research Center (ERC) for EUV Science and Technology, reports their findings in the June 8, 2012, issue of Science.

By focusing intense pulses of infrared light--each just a few optical cycles in duration--into a high-pressure gas cell, the researchers converted part of the original laser energy into a coherent super-continuum of light that extends well into the X-ray region of the spectrum.

The X-ray burst that emerges has much shorter wavelengths than the original laser pulse, which will make it possible to follow the tiniest, fastest physical processes in nature, including the coupled dance of electrons and ions in molecules as they undergo chemical reactions, or the flow of charges and spins in materials.

"This is the broadest spectral-bandwidth, coherent-light source ever generated," says engineering and physics professor Henry Kapteyn of JILA at the University of Colorado at Boulder, who led the study with fellow JILA professor Margaret Murnane and research scientist Tenio Popmintchev, in collaboration with researchers from the Vienna University of Technology, Cornell University and the University of Salamanca.

"It definitely opens up the possibility to probe the shortest space and time scales relevant to any process in our natural world other than nuclear or fundamental particle interactions," Kapteyn adds. The breakthrough builds upon earlier discoveries from Murnane, Kapteyn and their colleagues to generate laser-like beams of light across a broad spectrum of wavelengths.

This picture shows an actual image of a coherent (laser-like) X-ray beam. In contrast to the incoherent (light-bulb-like) light emitted in all directions from a Roentgen X-ray tube, the X-rays produced by high harmonic generation emerge as well-directed, laser-like, beams.

The researchers use a technique called high-harmonic generation (HHG). HHG was first discovered in the late 1980s, when researchers focused a powerful, ultra-short laser beam into a spray of gas. The researchers were surprised to find that the output beam contained a small amount of many different wavelengths in the ultraviolet region of the spectrum, as well as the original laser wavelength. The new ultraviolet wavelengths were created as the gas atoms were ionized by the laser.

"Just as a violin or guitar string will emit harmonics of its fundamental sound tone when plucked strongly, an atom can also emit harmonics of light when plucked violently by a laser pulse," adds Murnane. "The laser pulse first plucks electrons from the atoms, before driving them back again where they can collide with the atoms from which they came. Any excess energy is emitted as high-energy ultraviolet photons."

Like many phenomena, when HHG was first discovered, there was little science to explain it, and it was considered more a curious phenomenon than a potentially useful light source. After years of work, scientists eventually understood how very high harmonics were emitted, however there was one major challenge that most researchers gave up on--for most wavelengths in the X-ray region, the output HHG beams were extremely weak.

Murnane, Kapteyn and their students realized that there might be a chance to overcome that challenge and turn HHG into a useful X-ray light source--the tabletop-scale X-ray laser that has been a goal for laser science since shortly after the laser was first demonstrated in 1960.

"This was not an easy task," says Murnane. "Unlike a laser--which gets more intense as more energy is pumped into the system--in HHG, if the laser hits the atoms too hard, too many electrons are liberated from the gas atoms, and those electrons cause the laser light to speed up. If the speed of the laser and X-rays do not match, there is no way to combine the many X-ray waves together to create a bright output beam, since the X-ray waves from different gas atoms will interfere destructively."

Popmintchev and JILA graduate student Ming-Chang Chen worked out conditions that enable X-ray waves from many atoms in the gas to interfere constructively. The key was to use a relatively long-wavelength, mid-infrared laser and a high pressure gas cell that also guides the laser light. The resulting bright, X-ray beams maintain the coherent, directed beam qualities of the laser that drives the process.

The HHG process is effective only when the atoms are hit "hard and fast" by the laser pulses, with durations nearing 10-14 seconds--a fundamental limit representing just a few oscillations of the electromagnetic fields. Murnane and Kapteyn pioneered the technology for generating such light pulses in the 1990s, and used those lasers to develop and utilize HHG-based light sources in the extreme-ultraviolet (EUV) region of the spectrum in the 2000s. However, while researchers were using those lasers and the HHG technique to measure ever-shorter duration light pulses, they were stymied in how to make coherent light at shorter wavelengths in the more penetrating X-ray region of the spectrum.

The new paper in Science, under lead author and senior research associate Popmintchev, demonstrates that breakthrough, showing that the understanding of the HHG process the researchers developed is broadly valid.

"We would have never found this if we hadn't sat down and thought about what happens overall during HHG, when we change the wavelength of the laser driving it, what parameters have to be changed to make it work," added Kapteyn. "The amazing thing is that the physics seem to be panning out even over a very broad range of parameters. Usually in science you find a scaling rule that prevents you from making a dramatic jump, but in this case, we were able to generate 1.6 keV - each X-ray photon was generated from more than 5,000 infrared photons."

When the researchers first started to work with ultrafast, mid-infrared lasers just a few years ago, they actually made a step backwards and generated bright extreme-ultraviolet light of longer wavelengths than they used to achieve in the lab.

"However, we discovered a new regime that helped us to realize, just on paper, that we could make this giant step forward towards much shorter electromagnetic wavelengths and generate bright, laser-like, soft and hard X-rays," adds Popmintchev. "What the experiments were suggesting back then looked too good to be true! It seemed that Mother Nature has combined together, in the most simple and beautiful way, all the microscopic and macroscopic physics. Now, we are already at X-ray wavelengths as short as roughly 7.7 angstroms, and we do not know the limit."

To truly control the beam of photons, the researchers needed to understand the HHG process at the atomic level and how X-rays emitted from individual atoms combine to form a coherent beam of light.

That understanding combines microscopic and macroscopic models of the HHG process with the fact that those interactions occur at very high intensity in a dynamically changing medium. The development of such a conceptual understanding took the last decade to develop.

The result was the realization that there is no fundamental limit to the energy of the photons that can be generated using the HHG process. To obtain higher-energy photons, the system paradoxically begins with laser light using lower energy photons--specifically, mid-infrared lasers.

The JILA researchers demonstrated the validity of that principle in their labs in Colorado, but to achieve their breakthrough, the researchers traveled to Vienna with their beam-generating setup. There, they used a laser developed by co-author Andrius Baltuška and colleagues at the Vienna University of Technology--the world's most-intense ultrashort-pulse laser operating in the mid-infrared, with a wavelength of four microns.

"Thirty years ago, people were saying we could make a coherent X-ray source, but it would have to be an X-ray laser, and we'd need an atomic bomb as the energy source to pump it," said Deborah Jackson, the program officer who oversees the ERC's grant. "Now, we have these guys who understand the science fundamentals well enough to introduce new tricks for efficiently extracting energetic photons, pulling them out at X-ray wavelengths . . . and it's all done on a table-top!"

In addition to achieving the high energy, the increasingly broad spectrum opens a range of new applications.

"In an experiment using such a source, one energy region from the beam will correspond with one element, another with another element, and so on to simultaneously look at atoms across entire molecules, and that will allow us to see how charge moves from one part of a molecule to another as a chemical reaction is happening," adds Kapteyn. "It'll take us awhile to learn how to use this, but it's very exciting."

More information: "Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers," by T. Popmintchev et al, Science, 2012.