Monday, June 29, 2009

Laser-created temporal lens could lead to movies of molecular processes

( -- Finding a way to observe and record the behavior of matter at the molecular level has long been a holy grail among physicists. That ability could open the door to a wide range of applications in ultrafast electron microscopy used in a large array of scientific, medical and technological fields.

Now, a team at the University of Nebraska-Lincoln has figured out a possible way to do that. Working in collaboration with Nobel laureate Ahmed Zewail (chemistry, 1999) of the California Institute of Technology in Pasadena, they developed mathematical models to show that laser beams create ultra-high-speed "temporal lenses" that would be capable of making "movies" of molecular processes. The finding was published in the June 15-19 online edition of the Proceedings of the National Academy of Sciences.

The "lenses" in question are not made of glass like those found in standard tabletop microscopes. They're created by laser beams that would keep pulses of electrons from dispersing and instead focus the electron packets on a target. The timescales required, however, are hardly imaginable on a human scale -- measured in femtoseconds (quadrillionths of a second) and attoseconds (quintillionths of a second).

The physicists modeled two types of lenses. One was a temporal "thin" lens created using one laser beam that could compress electron pulses to less than 10 femtoseconds. The second was a "thick" lens created using two counterpropagating laser beams that showed the potential of compressing electron pulses to reach focuses of attosecond duration.

Friday, June 19, 2009

Europe's big lasers: the exawatt roadmap

Europe is thinking big – big lasers, big science, big budgets. Over the next decade, a trio of planned pan-European research facilities will give scientists access to unprecedented laser powers and intensities, opening the door to exotic science that will shed light on the origins of the universe and, it is hoped, provide the foundations for a sustainable energy future.

The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.

"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.

Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (10^15 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."

This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)

Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 10^18 W) and intensities (10^24 W/cm2).

Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10^–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).

The European X-ray Free Electron Laser (European XFEL) is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.

DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."

Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

Tuesday, June 16, 2009

Study gives clues to increasing X-rays' power

In a paper to be published in an upcoming edition of Physical Review Letters, UNL Physics and Astronomy Professor Anthony Starace and his colleagues give scientists important clues into how to unleash coherent, high-powered X-rays.

"This could be a contributor to a number of innovations," Starace said.

Starace's work focuses on a process called high-harmonic generation, or HHG. X-ray radiation can be created by focusing an optical laser into atoms of gaseous elements - usually low-electron types such as hydrogen, helium, or neon. HHG is the process that creates the energetic X-rays when the laser light interacts with those atoms' electrons, causing the electrons to vibrate rapidly and emit X-rays.

But the problem with HHG has been around almost as long as the onset of the method in 1988: The X-ray light produced by the atoms is very weak. In an effort to make the X-rays more powerful, scientists have attempted using higher-powered lasers on the electrons, but success has been limited.

"Using longer wavelength lasers is another way to increase the energy output of the atoms," Starace said. "The problem is, the intensity of the radiation (the atoms) produce drops very quickly."

Instead of focusing on low-electron atoms like hydrogen and helium, Starace's group applied HHG theory to heavier (and more rare) gaseous atoms having many electrons - elements such as xenon, argon and krypton. They discovered that the process would unleash high-energy X-rays with relatively high intensity by using longer wavelength lasers (with wavelengths within certain atom-specific ranges) that happen to drive collective electron oscillations of the many-electron atoms.

"If you use these rare gases and shine a laser in on them, they'll emit X-Rays with an intensity that is much, much stronger (than with the simple atoms)," Starace said. "The atomic structure matters."

Starace said that unlocking the high-powered X-rays could lead one day, for example, to more powerful and precise X-ray machines. For instance, he said, heart doctors might conduct an exam by scanning a patient and creating a 3D hologram of his or her heart, beating in real time.

Nanoscientists, who study the control of matter on an atomic or molecular scale, also may benefit from this finding, Starace said. Someday, the high-intensity X-rays may be used to make 3D images of the microscopic structures with which nanoscientists work.

"With nanotechnology, miniaturization is the order of the day," he said. "But nanoscientists obviously could make use of a method to make the structures they're building and working with more easily visible."