Argonne sets world record for shortest wavelength ever from free-electron laser

Scientists and engineers at Argonne National Laboratory’s Advanced Photon Source (APS) achieved "saturation" of self-amplified spontaneous emission in a mirrorless free-electron laser at a wavelength more than 1,000 times shorter than the previous record. This accomplishment demonstrated that free-electron lasers based on this process may one day provide laser-quality X-ray beams and open exciting new horizons for research in dozens of scientific fields.

The beam of light produced in the experiment had a wavelength of 385 nanometers, placing it in the ultraviolet region of the spectrum. The success of the particular process employed at Argonne is gauged by whether the free-electron laser effect has "saturated," meaning the point at which the maximum power has been yielded by the electron beam and converted to coherent synchrotron radiation. The Argonne device clearly exhibited saturation of the process.

"In the history of synchrotron radiation research, which is only about 45 years old, you can count the true breakthroughs on the fingers of one hand," said David Moncton, Argonne’s Acting Associate Laboratory Director for the Advanced Photon Source. "This is one of them." Stephen Milton and Efim Gluskin lead the research team. "Synchrotron radiation" refers to the light emitted by high-energy electrons when their flight paths are bent by magnets. The world’s most powerful X-ray sources, including Argonne’s Advanced Photon Source, use this method to produce their X-rays.

The next generation of X-ray sources for scientific research will be based on the free-electron-laser concept, the latest extension and refinement of synchrotron radiation. Unlike a more conventional laser, the Argonne free-electron laser uses a powerful electron accelerator in combination with long arrays of very precise magnets of alternating polarity and needs no mirrors for operation. A further significant feature of this free-electron laser is that by merely changing the electron beam energy, the light is continuously tunable over a broad range of wavelengths, thus breaking additional barriers to traditional lasers.

With further development, free-electron lasers of this sort promise to provide extremely bright, laser-like X-ray beams with ultrashort pulse durations that will enable scientists to study the properties and structures of materials in far greater detail and in far less time than is possible today. Examples include:

• "Snapshots" or "movies" of chemical and biological reactions too fast to be observed with today’s sources.
• Holographic images of proteins and other molecules.
• The ability to make an image of a single protein molecule with a single X-ray pulse. Scientists would no longer be limited to making images of only proteins that form crystals.
• The ability to study "warm, dense matter," a state between one in which all the electrons surrounding a collection of atoms are highly excited and one in which all the electrons and atoms have become so excited that the electrons are stripped from the atoms and the whole collection becomes a hot plasma.

Today, X-rays are the most widely used scientific probe for studying the structures and interactions of crystalline materials at the atomic and molecular levels. But many materials do not form crystals, and many reactions take place too quickly to study adequately, even at the APS.

An ideal step forward for X-ray researchers would be to use lasers that produce intense, coherent, X-ray beams. Unfortunately, conventional lasers cannot produce beams of light much shorter in wavelength than ultraviolet light. This is, in part, because conventional lasers rely on mirrors, which become less efficient as the wavelength of light gets much shorter than the ultraviolet band. X-rays, however, have wavelengths that are more than 1,000 times shorter than ultraviolet light.

Scientists at laboratories around the world have been trying to step around this problem by developing a new version of the free-electron laser that can operate at X-ray wavelengths. This principle relies on a process called "self-amplified spontaneous emission" and does not require mirrors. Instead, a self-amplified spontaneous emission free-electron laser requires a high-quality electron beam and a long array of high-quality magnets, called an "undulator."

When the electron beam passes through the undulator, the magnets vibrate the electrons from side to side, causing them to emit light as synchrotron radiation. The higher the electron energy, the higher the resulting light energy.

As electron bunches propagate down the undulator, they are bathed in the same light they generate. As they wiggle back and forth through the magnets and interact with the electric field of this light, some gain energy (speed up) and some lose energy (slow down), depending upon their phase relationship with the light and the magnetic fields.
As a result, the electrons begin to form microbunches separated by a distance equal to the wavelength of the light they generate. The light waves emitted by the electron bunches will line up in phase — meaning that the waves’ peaks and valleys overlay each other — to reinforce and amplify the light’s brilliance and intensity.

Eventually, a favorable runaway instability develops, akin to the feedback squeal of a public address system with its volume turned up too high. The light intensity grows exponentially along the undulator until the process "saturates," and the generated light is at its highest intensity. By the time the light beam emerges, its initial intensity has been amplified more than a billion times.

At high enough electron energies and with an adequately long undulator system, a free-electron laser could theoretically produce an X-ray beam with a peak brightness more than one billion times greater than the brightest X-ray beams available today.

As presently configured, the Advanced Photon Source free-electron laser should be capable of reaching wavelengths as low as 60 nanometers with the primary limitation being the energy of the linear accelerator. To reach X-ray wavelengths one needs electron beam energies comparable to those produced by the world’s largest linear accelerator, located at the Stanford Linear Accelerator Center (SLAC). To capitalize on this national resource, however, a host of additional technical challenges must still be solved.

Argonne is collaborating with SLAC, the University of California at Los Angeles, and Brookhaven, Los Alamos and Lawrence Livermore national laboratories. The collaboration is aimed at demonstrating the feasibility of the proposed Linac Coherent Light Source, a proof-of-principle fourth generation light source to be built at SLAC.

For more information please contact David Baurac