Femtosecond Pulses Produced From Synchrotron Light Source At Berkeley Lab's Advanced Light Source

Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), working at the Advanced Light Source (ALS), extracted femtosecond pulses of synchrotron light directly from the electron beam in the synchrotron's storage ring. With further refinement, their technique will essentially provide scientists with a stop-action x-ray camera that can capture the motion of atoms during physical, chemical, and biological reactions on a time-scale that is almost incomprehensibly short. Roughly speaking, a femtosecond is to one second what one second is to 30 million years.

"Since our approach creates a femtosecond time structure on the electron beam, standard radiating devices such as a bend-magnet, a wiggler, or an undulator, can be designed to emit femtosecond x-ray pulses with desired properties such as bandwidth, tunability, and brightness," says Robert Schoenlein, a physicist with Berkeley Lab's Materials Sciences Division and senior author of a paper reporting this work in the March 24 issue of the journal Science.

The scheme for generating femtosecond pulses directly from a synchrotron was originally conceived by Alexander Zholents and Max Zolotorev, of Berkeley Lab's Center for Beam Physics (CBP), who are co-authors of the Science paper. Other co-authors are Berkeley Lab director and femtosecond spectroscopy pioneer Charles Shank, Swapon Chattopadhyay of the CBP, Thornton Glover and Philip Heimann of the ALS, and H.H.W.Chong, a graduate student at the University of California at Berkeley.

Schoenlein, Zoloterev and Glover were also members of a Berkeley Lab research team that in 1996 produced the first directed beams of femtosecond x-rays. This earlier work was also done at the ALS but on a branch beamline off the 50 MeV (million electron volt) linear accelerator that feeds electrons into the ALS' booster synchrotron. The latest femtosecond pulses were produced within the storage ring. This is an enormous advantage as it enables the researchers to capitalize on the exceptionally high flux and brightness of the ALS storage ring beam.

The ALS is an electron synchrotron designed to accelerate electrons to energies of 1.9 GeV (billion electron volts) and hold them for several hours in a tightly constrained beam inside a storage ring approximately 200 meters in circumference (660 feet). As the electron beam circles through this storage ring, beams of ultraviolet and low energy or "soft" x-ray light can be siphoned off from it through the use of either bend, wiggler, or undulator magnetic devices. This light, which can be used for a wide variety of scientific applications, is on the order of a hundred million times brighter than the light from the most powerful x-ray tubes.

Because the electron beam in the ALS' storage ring is formed from discrete bunches of electrons rather than a continuous stream of particles, all of the light it generates is pulsed, with an optimal repetition rate of about 500 million pulses per second and each pulse lasting a mere 30 to 40 picoseconds (trillionths of a second). While fast enough to capture the action of a great many processes, a picosecond time-scale is not fast enough for the study of such ultrafast events as the making and breaking of electronic bonds during chemical reactions, or atomic motion associated with a phase transition (solid to liquid to gas). Many vital biological processes such as eyesight also take place on a sub-picosecond timescale.

"X-rays are ideal for investigating the atomic structure of matter because they interact directly with nuclei and core electrons," says Schoenlein. "The production of directed beams of x-rays on a femtosecond time-scale has been highly prized by the scientific community because, at room temperature, atomic motion takes place, in most cases, on a time-scale of approximately 100 femtoseconds."

Schoenlein and his colleagues produced their femtosecond pulses by sending a burst of light from a femtosecond optical laser through a wiggler insertion device at the same time as the electron bunches that make up the storage ring beam passed through it. Wigglers (and undulators) are magnetic arrays with alternating north and south poles that vibrate the motion of speeding electrons causing them to lose energy in the form of emitted light. Under the right conditions, the simultaneous presence of a pulse of laser light will modulate the energy loss of some of these oscillating electrons.

In this experiment, the energy modulation enabled an ultrashort slice of electrons to be spatially separated from the rest of the bunches in the ALS storage ring beam. When the displaced electron slice was then sent through a bend magnet, it generated its own light. Theoretically, this light should appear at approximately the same femtosecond pulse-length as the laser light that created it. However, time-of-flight effects due to the distance between the wiggler in which the laser/electron interaction occurred and the bend magnet resulted in pulses of about 300 femtoseconds.

Says Schoenlein, "The location of our bend-magnet in this experiment was less than optimum. By locating the bend-magnet immediately following the wiggler, we should be able to obtain an x-ray pulse of about 100 femtoseconds."

Schoenlein also says that the use of an undulator rather than a bend magnet to extract light from the electron slice will greatly improve the flux and brightness of a directed femtosecond x-ray beam. Unlike a bend magnet, which produces sweeping beams of light, like that from a beacon, undulator light is more like the beam from a laser, highly coherent and tunable to a specific wavelength.

A dedicated bend magnet beamline and experimental station for femtosecond x-ray spectroscopy studies is now under construction at the ALS and should be operational by this summer. Plans are being developed for an undulator beamline providing femtosecond x-ray pulses of even higher brightness and flux.

Creating femtosecond x-rays at the ALS won't interfere with other experiments at the facility as only a small fraction of the electron bunches in the storage ring beam are modulated by the laser and the beam quickly recovers. In their Science paper, the authors say that with 300 electron bunches in the storage ring, they can generate femtosecond x-ray pulses at repetition rates as high as 100,000 pulses per second.

The removal of an ultrashort slice of electrons from the storage ring beam leaves behind a hole in the beam's bunches that acts like a "dark pulse" of coherent infrared light. In principle, this infrared pulse could also be used for femtosecond time-scale spectroscopy.

In a recent review of the ALS sponsored by the Department of Energy, a blue-ribbon panel chaired by Yves Petroff, Director General of the European Synchrotron Radiation Facility, predicted that the combination of the ALS with a femtosecond laser will at last enable scientists to realize the long-time dream of direct x-ray experiments on a sub-picosecond time-scale.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.