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September 23, 2005

Slicing X-Rays by the Millionths of a Billionth of a Second
The Advent of Femtosecond Spectroscopy at the Advanced Light Source

Nothing stays the same; the physical world is in a perpetual state of flux. At the atomic scale — the making or breaking of chemical bonds or the transition of a system from one phase to another — changes can take place in a few hundred femtoseconds or less (a femtosecond is one millionth of a billionth of a second).

Robert Schoenlein (inset), a physicist with Berkeley Lab's Materials Sciences Division, leads the ultrafast spectroscopy research group that is pioneering the use of femtosecond x-rays produced at the Advanced Light Source. Matteo Rini (center) and Andrea Cavalleri used femtosecond spectroscopy techniques with visible light to measure the speed at which thin films of vanadium dioxide make the phase transition from an electrical insulator to a conductor. Scientists are now using femtosecond spectroscopy with x-rays to study the material's electronic structure during this transition.

Ideally, these changes would be studied with spectroscopic probing techniques that use x-rays pulsed on a femtosecond time-scale and tunable to the spectral range of specific critical elements. However, scientists have lacked a source for such x-rays — until now.

Scientists working at Berkeley Lab's Advanced Light Source (ALS) have reported the first femtosecond x-ray spectroscopy experiment. Using a beam of ALS x-rays that were "laser-sliced" into pulse lengths of about 150 femtoseconds, the research team performed time-resolved x‑ray absorption spectroscopy measurements on samples of vanadium dioxide. This material has been shown to change from an electrical insulator to a conductor in about 80 femtoseconds.

"We have demonstrated the ability to generate and use sliced x-rays in an energy range that cannot be reached with any other femtosecond x-ray source," said Andrea Cavalleri, a physicist now with Oxford University who was at Berkeley Lab during the experiments. "The laser-slicing technique we used at the ALS is currently the only proven method to generate broadband x-ray pulses of femtosecond duration."

As a member of the femtosecond spectroscopy group of physicist Robert Schoenlein in the Lab's Materials Sciences Division, Cavalleri conducted investigations into the superfast insulator-to-metal transition in thin films of vanadium dioxide. The ability of vanadium dioxide, a nonmagnetic semiconductor, to change from a transparent insulator to a reflective conductor was first reported back in 1959, but the speed at which the phase transition takes place was only recently clocked by Cavalleri and Matteo Rini, another member of Schoenlein's group.

Vanadium dioxide belongs to a class of materials including high-temperature superconductors and ferroelectrics, which hold great promise for future high-speed optical switches and other devices. In these materials there is known to be a strong relationship between structural and electronic effects, but a much better understanding of the physics behind these effects is needed before they can be commercially exploited.

The situation calls out for femtosecond x-rays, which can be used to probe electronic and magnetic effects as well as the short-range atomic structures of a material's specific elements. To date, however, femtosecond x-ray studies have been limited almost exclusively to time-resolved diffraction experiments using hard (high-energy) x-ray pulses.

Sending femtosecond pulses of laser light through a wiggler at the same time as the ALS's electron beam creates femtosecond slices within the electron beam that can be separated using a bend magnet. Femtosecond x-rays are generated from these slices of electrons.

The ALS is an electron synchrotron radiation source designed to accelerate electrons to relativistic speeds (near light speed) and energies of nearly two billion electron volts (2 GeV), focus them into a hair-thin beam, and send the beam around the curved path of a storage ring for several hours. Beams of photons, primarily x-rays, are extracted from the electron beam in the storage ring through use of bend, wiggler, or undulator magnetic devices.

In addition to being tunable to the chemical property wavelengths of specific elements, ALS x-rays are also bunched in pulses. The pulse lengths of the photon beams directly extracted from the electron beam are in the picosecond (trillionths of a second) time scale. These picosecond pulse lengths, however, can be sliced into femtosecond pulse lengths thanks to a technique originally proposed in 1996 by Alexander Zholents and Max Zolotorev, physicists at Berkeley Lab's Center for Beam Physics in the Accelerator and Fusion Research Division, and further developed under the leadership of Schoenlein.

The technique utilizes femtosecond pulses of light from an optical laser to produce energy modulations in an electron beam. In this case, the laser light pulses are sent through a wiggler magnetic device at the same time as the ALS's electron beam. Sending a relativistic beam of electrons through a magnetic wiggler (or undulator) oscillates the motion of the speeding electrons, causing them to lose energy in the form of light emission. The addition of the laser beam modulates the energy loss and light emission of the oscillating electrons.

Energy modulation makes it possible to select and spatially separate or "slice" femtosecond-sized bunches of electrons from the main electron beam. When these displaced electron bunches are sent through a bend magnet, they generate femtosecond pulses of x-ray light.

Beamline 6.0.1, dubbed the "Ultrafast X-Ray Facility," will be commissioned in the fall of 2005. It will be optimized for the generation of pulses of x-ray light on a femtosecond time scale.

"In making use of laser-sliced synchrotron pulses, we combine the broad tunability of bending-magnet radiation with the femtosecond time duration of mode-locked lasers," Cavalleri said. "For the vanadium dioxide experiment, we used this technique to follow the way electrons are being shuffled among different orbitals during a photo-induced phase transition. This could not have been done with previous femtosecond techniques, and it will be very important in future studies of ultrafast magnetic phenomena, and chemical dynamics at surfaces, in the gas phase, or in liquids."

The vanadium dioxide femtosecond spectroscopy experiments were carried out using the wiggler magnet at ALS Beamline 5.0.2, which powers most of the protein crystallography research at the ALS, the bend magnet for Beamline 5.3.1, and a high-power titanium-sapphire laser. A future ALS beamline, 6.0.1, dubbed "The Ultrafast X-Ray Facility," will be optimized for the generation of femtosecond x-ray pulses through an increase in flux.

Said Schoenlein, "The ultrafast x-ray facility will be based on an in-vacuum undulator" — the premier ALS magnetic insertion device — "and will have two branch lines, one for hard x-rays and the other for soft x-rays. The soft x-ray branch line will be completed in the next few weeks, and we expect to start commissioning before the end of September. The hard x-ray branch line will be completed next spring."

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