Scientists have generated the shortest pulses of high-energy X-rays to date by using a powerful laser to stimulate inner shell electrons - the electrons closest to an atomic nucleus. These extremely short pulses, clocking in at 60-100 attoseconds (quintillionths of a second), could be used to study quantum-scale phenomena that occur so quickly they are assumed to be unobservable.
"When we use a laser with a femtosecond pulse (1,000 times longer than attosecond) to investigate atoms, we can observe electrons as a cloud that surrounds it. That's the most detail anyone has been able to achieve so far," said Uwe Bergmann, a professor at the University of Wisconsin-Madison and affiliate researcher at Lawrence Berkeley National Laboratory (Berkeley Lab). Bergmann is lead author on a new paper about the process, now published in Nature. "But at the timescale of attoseconds, we could start to ask how exactly does the cloud move around," said Bergmann.
Fundamental theories of quantum mechanics state that it is impossible to pinpoint the location of an electron at a given time. The team's laser system could allow scientists to investigate the inner workings of atomic interactions like never before.
"Femtosecond laser pulses let us study atomic bonds breaking and forming," elaborated author Vittal Yachandra, a senior scientist in Berkeley Lab's Molecular Biophysics and Integrated Bioimaging (MBIB) Division. "Now we're going inside to look at not just the making or breaking of a bond, but what actually is a bond?"
The principle of attosecond characterization is similar to traditional photography in that the "flash" has to be fast enough to capture a rapidly moving subject. But when the subject is not only very fast, but also very tiny, the wavelength of the light has to be small. Bergmann and an international team of colleagues used the two most powerful X-ray lasers in the world - the Linac Coherent Light Source at SLAC National Accelerator Laboratory and the SPring-8 Ångstrӧm Compact free electron Laser (SACLA) in Japan - to excite inner shell electrons in copper and manganese atoms. These lasers generate femtosecond pulses of photons in the hard X-ray range, the high-energy end of the X-ray light spectrum where wavelengths are just a few billionths of a meter - the same size as individual atoms. The light stimulates the metal's closely held electrons to also emit a beam of hard X-rays.
This is the fundamental process of a laser, which is actually a descriptive acronym for Light Amplification by Stimulated Emission of Radiation. Scientists have built many powerful laser systems that can produce extremely short pulses of light or very high-energy light, allowing them to study many properties of molecules and materials. But building a laser system that can achieve both at the same time is challenging.
And, in fact, the team was surprised to discover they had invented a new way to do so. Bergmann and his Berkeley Lab colleagues Vittal Yachandra, Jan Kern, and Junko Yano in the MBIB Division, have been working together for many years on studies about the atomic-scale activity of important enzymes that contain metals, such as those that drive photosynthesis and respiration. They were using the X-ray lasers at SLAC and SACLA to investigate properties of molecules with manganese through inner-shell lasing when they first detected evidence of an unexpected laser signal coming off the metals. To their surprise, they found that the combination of the extremely powerful lasers pointed at the metal samples (equivalent at the brief moment of contact to all the sunlight hitting Earth focused down to a square millimeter) and the fact that they were focused in on inner shell electrons (the electrons that require the most energy to excite) is an ideal recipe for very strong laser effects that can also lead to attosecond lasing.
It took the team several years to identify what was happening, then rigorously verify their findings, culminating in this paper.
"We discovered it while trying something else - it's a phenomenon we did not expect to happen, even scientists working on attosecond lasing hadn't observed it," said author Jan Kern, who is a senior scientist in MBIB.
Looking to the future, the team is excited for the phenomenon to be studied more deeply and integrated into tools at existing laser facilities so the scientific community to begin exploring the vast potential of attosecond hard X-rays.
"Strong inner-shell lasing can complement and enhance other approaches to generate attosecond pulses that are currently underway at the X-ray laser facilities, and the implementation is relatively straightforward," said co-author and MBIB Director Junko Yano. "It is also intriguing to speculate what unexpected discoveries might lie ahead with this approach, as did so when strong lasing effects were first discovered by the pioneers of laser physics exactly 70 years ago, leading to one of the most powerful and widely used tools in modern technology."
This work was supported by the DOE Office of Science. The LCLS is a national user facility located at the SLAC National Accelerator Laboratory.
