Table-top femtosecond X-ray dynamical imaging

How can we make a movie of atoms - or even electrons - moving inside molecules? This is a fundamental problem in many fields of physics, chemistry and biology.

For this, we need pulses of light with a duration which is much shorter than the characteristic times of the movements of the atoms or electrons. For the case of atoms this is typically a few femtoseconds (1fs is one billionth of a nanosecond); electrons move even faster, on the attosecond scale, where (1 attosecond is one thousandth of a femtosecond!). We also need very short wavelengths, such as those of X-rays, so to achieve the necessary resolution at the nanometre scale. Meeting these requirements is a formidable challenge, but the pay-off in terms of applications, ranging to medical science to material engineering, is enormous.

Cutting-edge imaging experiments of this type have already been achieved by using X-ray sources in huge facilities. However, their large scale and operating cost prevents them from becoming a widespread tool.

There is a more convenient and compact way of producing very short X-ray pulses. If we shine short pulses of visible light on a jet of gas, such as argon, the atoms of the gas respond to the presence of this light by emitting bursts of extreme ultraviolet and soft X-ray radiation by a process called "high harmonic generation" (HHG). The applicability of these pulses for probing electronic dynamics in atoms and molecules has been tested in a series of pioneering experiments. However, the brightness of HHG sources is far from being comparable with that of large-scale facilities.

We will investigate the prospects for making HHG a fully viable technique for taking "molecular movies" with a system small enough for an ordinary R&D laboratory. We have identified solutions for overcoming current limitations: in particular, we will work on choosing the best possible visible light for producing HHG radiation, as well as on employing techniques of "phase-matching", i.e. controlling how the light propagates through the jet, to increase the efficiency of generation.

HHG beams are akin to an X-ray laser, with which they share properties of coherence. This implies that, if we collect the full information on the amplitude and the phase of the light far from our target, we can use sophisticated computer codes to reconstruct the shape of this object. This avoids using lenses for X-rays, which are difficult to manufacture. Further, by tuning the wavelength of the X-ray beam it is possible to select and image only a specified atomic element in the object. We will demonstrate the utility of the bright HHG beams we plan to develop in proof-of-principle experiments on aluminium alloys. These alloys - which are of crucial importance to the aerospace, automotive, and electronic industries - derive their strength from the formation of inhomogeneities during heat treatment. However, the relation between their microscopic structure and mechanical properties is not well understood; our demonstration experiments may open a new route for exploring these important issues.

From a fundamental viewpoint, the electromagnetic field contains the maximum possible information about an object that can be obtained in an optical experiment. Hence we will also investigate methods able fully to characterize the X-ray field scattered from an object, allowing the spatial and structural dynamics of the object to be tracked.

In summary, we plan to take major steps towards laboratory-scale imaging at atomic spatial and temporal scales by developing bright, compact pulsed soft-X-ray sources and measurement methods that return the full details of the radiation field incident on, and scattered from, the object under study. This research programme therefore has the potential to deliver a step change in what is possible in spatio-temporal imaging at the nanoscale

The long-term goal of this work is the development of very compact sources of bright, ultra-fast X-ray pulses and the development of new imaging techniques able to probe matter in time and space on the atomic scale. The small size of HHG sources would bring into university-scale laboratories research which is presently restricted to national facilities, greatly increasing the access to ultrafast imaging and probing techniques. Further, the X-ray pulses generated are at least two orders of magnitude shorter than possible with conventional RF-driven sources, providing unique capabilities which will open entirely new scientific avenues.

There is little doubt that success in this area could revolutionize work in many areas of the medical, biological and physical sciences and in so doing will bring clear benefits to wider society. As one example, during this research proposal we will demonstrate spatial imaging of the size, shape and properties of aluminium (Al) particles in Al alloys. These alloys are of crucial importance in the aerospace, automotive, and electronic industries. However, despite this, the relation between alloy performance and the kinetics and morphology of precipitate formation are not well understood. By developing novel laboratory-scale imaging techniques we hope to provide a new tool for materials research which would have immediate impact in engineering and technology and would enable breakthroughs in technologies such as novel materials, and data transmission, switching and storage.

The longer term objective of our proposed work on dynamical imaging is the development of laboratory-scale techniques able to track the motion of nano-scale structures on sub-femtosecond time-scales. This ability would revolutionize our understanding of the dynamics of myriad processes of vital importance across the sciences, including: enzyme and surface catalysis, photosynthesis, and electron dynamics in magnetic systems. Understanding, and potentially controlling, dynamical systems of this type will lead to the development of new techniques for generating and storing energy; understanding disease and designing drugs; and controlling matter at the quantum level. There is simply no doubt that developing these capabilities will bring enormous benefits to society.