Laser-Ion Interaction Produces Coherent X-Rays
Daniel S. Burgess
A team at Albert Ludwigs Universität Freiburg in Germany has proposed a means of generating femtosecond pulses of coherent x-rays with wavelengths less than 0.1 nm for chemical and biological imaging. If successful, the technique promises to enable the time-resolved imaging of matter on an atomic scale.
The interaction of intense laser pulses and multiply charged ions may serve as a coherent source of hard x-rays for the atomic-scale, time-resolved imaging of matter. The graph depicts the effective potential of the ionic potential and the laser field at maximum strength, the bound ground state (-Ip) of an electron wave packet and the tunneled component as a function of the propagation and polarization directions of the laser field. A fraction of the tunneled wave packet may return to the ionic core, releasing high-frequency radiation. Courtesy of Christoph H. Keitel, Albert Ludwigs Universität Freiburg. Reprinted with permission of Applied Physics Letters.
Christoph H. Keitel, a researcher on the project, explained that the process is initiated by the interaction of intense laser fields with a target of multiply charged ions. Electrons in the vicinity of the ions escape and return in time with the changing laser and ion fields. When the ionic core reabsorbs an electron, the latter releases high-frequency radiation, at higher frequencies with a higher ionic charge. Because the process is periodic, he said, the emitted radiation is coherent, and it is more coherent as the number of re-collisions increases.
Coherence would be highest from ensembles of a few trapped ions, Keitel said, but it is likely that producing radiation sufficiently intense for imaging applications would require a solid target, such as a micron-scale lead or sodium crystal. Ideally, the material and laser pulse would be selected so that the laser pulse frees six electrons while a seventh may return when the phase of the laser field changes, he said.
The researchers modeled the interaction of 1.2 x 1018-W/cm2 pulses of 248-nm radiation from a KrF laser and a crystal with an ion density of 1022/cm3. They calculated that such a setup would generate coherent subangstrom radiation at an intensity of a few hundred W/(cm2 keV), noting, however, that this is much weaker than the best sources of coherent -- but longer-wavelength -- x-rays. They suggest that recent developments in ion storage may afford better control of ion repulsion and thus enable higher gain.
Looking forward, it is theoretically possible to apply the technique to the generation of coherent gamma rays as well by using more intense laser pulses and more strongly ionized targets, such as U91+. Actually putting it into practice, however, is daunting.
"At the moment, I believe there is no laser source strong and brief enough to induce such dynamics in highly charged ions," Keitel said. "However, times are changing, and then we may resolve time-dependent processes in matter at even shorter wavelengths." He cautioned, however, that even if the technology permitted it, the generated signals might be too weak to be viable.
Currently, several experimental groups are considering testing the technique for the production of x-rays, but there are no concrete plans, Keitel said.
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