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X-ray, meet optical control …

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Researchers have shown that they can control matter directly with intense x-ray beams and can use one beam to control another. The experiments could open the door to new and interesting ways to use x-rays, said Thornton E. “Ernie” Glover of Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS), a principal investigator of the study.

The research team – led by Glover, Linda Young of Argonne National Laboratory in Illinois and Ali Belkacem of Berkeley Lab’s Chemical Sciences Division – were inspired by previous “optical control” experiments in the visible light regime.

“In those all-optical experiments, a ‘control’ light pulse manipulates a medium and changes how an optical ‘probe’ pulse experiences that medium,” Glover said. “Our goal was to discover whether similar control techniques could be extended to the x-ray regime: Can light be used to tailor how x-rays interact with matter? If so, what are the limits to such ‘optical control’?”

He added that, although the optical control method was introduced more than a decade ago, and others may have considered extending similar physics to the x-ray regime, “actually demonstrating this is an altogether different thing.” The experiment required femtosecond x-ray pulses easily tunable over a considerable wavelength range as well as high-power, femtosecond optical laser pulses. Also, significantly, the x-ray and optical pulses had to be synchronized on a femtosecond timescale.

Shown is a simplified sketch of the experiment at the femtosecond spectroscopy beamline. A laser oscillator generates femtosecond pulses that follow two paths. One path interacts with the synchrotron’s electron beam to generate femtosecond x-ray pulses; the other (after a delay to ensure synchronization) rejoins the x-ray pulses and co-propagates with them through the gas cell. Courtesy of Lawrence Berkeley National Laboratory.

The necessary technological capabilities were first reported at the ALS, where a team developed a beam-slicing technique enabling generation of femtosecond x-ray pulses at a synchrotron. (To date, only two other light sources – one in Switzerland and one in Germany – have implemented the beam-slicing approach to generating femtosecond x-rays.) Intrinsic to this technique is the availability of high-power optical pulses synchronized to the x-rays on a femtosecond timescale.

Using the ALS’ femtosecond spectroscopy beamline 6.0.2, the researchers sent ultrashort laser pulses and higher-frequency x-rays through a gas cell containing pressurized neon. The neon, which normally would absorb the x-rays, became transparent to them when excited by the laser pulses.

Using the femtosecond spectroscopy beamline 6.0.2 developed at Lawrence Berkeley National Laboratory’s Advanced Light Source, investigators have shown that they can control matter directly with intense x-ray beams and can use one beam to control another. These capabilities suggest a range of “new and interesting” ways to use x-rays.

Glover – whose group studies ultrafast x-ray science (UXS), the effort to understand and ultimately control the ways in which matter evolves at the atomic and molecular levels – explained why this is significant. First, he said, “both UXS and optical control are relatively new fields and, viewed broadly, the novelty of our study is finding a useful nexus of these two fields.”

Controlling interactions

From the ultrafast x-ray science perspective, the experiment was notable because it employed light in a way fundamentally different from previous studies. Prior experiments focused on using light to put a system in an excited state and x-rays to probe how the system changes. The current study used light to control the underlying x-ray/matter interaction cross section for x-rays passing through matter; also, the material was typically in its ground state.

By controlling the underlying interaction cross section, and thus the ways in which x-rays experience the material, the researchers hope to find new ways to use x-rays. In the current experiment, they used x-ray transparency to measure the duration of femtosecond x-ray pulses by recording the x-ray transmission while moving a femtosecond laser pulse through a femtosecond x-ray pulse. Such an approach to measuring ultrashort x-ray pulses is attractive, Glover said, because the instrumentation used is simpler than the photoelectron spectroscopy-based technique currently used.

An important extension of this, he added, is the ability to shape x-ray pulses on a femtosecond timescale. Combining this ability with the next generation of intense x-ray sources could lead to a range of new research areas, including quantum control in the x-ray regime. Indeed, this represents a possible avenue of investigation for Glover and colleagues, albeit down the road.

“We hope to start thinking in earnest about quantum control with x-rays,” he said, “but that will probably wait for a couple of years while the new x-ray FELs [free-electron lasers] come online, and researchers learn how to efficiently perform experiments at these new facilities.”

In the near-term, the researchers plan to explore whether light can be used to manipulate the phase of an x-ray pulse, and not just the amplitude, which they have already demonstrated.

“This question is of fundamental interest,” Glover said. “If such phase control is possible, it may spawn new applications such as new approaches to making x-ray lenses/optics and possibly even new approaches to how we use x-rays to view matter at the microscopic level.”

Photonics Spectra
Jun 2010
Advanced Light SourceAliAli BelkacemALSArgonne National LaboratoryBasic ScienceBeam SplicingBeamline 6.0.2BelkacemBoasErnieFELsfree-electron lasersGaryGary BoasGloverLawrence Berkeley National Laboratorylight sourcesLindaLinda YoungOptical ControlResearch & TechnologyTech PulseThornton E. (Ernie) GloverUltrafast X-ray Sciencex-rayXLSYoungLEDs

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