Table-top femtosecond X-ray dynamical imaging

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


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

Planned Impact

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.
Description We have developed three new methods that address the central question framed in the proposal.

First, we have devised a scheme for the characterisation of ultrashort optical pulses that can deal with significant null-space in the spectrum of the pulse. This is important for the control and utilisation of optical wave-field synthesisers, which are emerging tools for the generation of ultrafast XUV radiation by controlling the form of the pulse driving HHG.

Second, we have developed a novel approach to quasi-phase-matching (QPM) high-harmonic generation (HHG) based on the excitation of different modes of a photonic crystal fiber, such that the two modes, propagating with different group velocities, interfere near the end of the fiber, generating an increased brightness by a factor of about 60 compared to a single pulse of the same total energy at the same harmonic. This new scheme therefore has the potential to provide a new generation of high-brightness table-top XUV light sources, since it enables the use of lower pulse energy, higher repetition rate drive lasers.

Third, we have invented a new scheme for wide-field, high resolution holography that allows estimation of the amplitude and phase of the field scattered from a transparent (or in principle reflective) object, without a need to know the probe or reference pulse shape. It derives from a result from a prior EPSRC grant, in which we came up with a rapidly converging iterative method to extract both fields from a set of data. The method has an improved extraction of the object from a high noise background than previous methods.
Exploitation Route Our work on the development of new schemes for characterizing ultrashort pulses will very likely be of use to those using ultrafast laser pulses in science and technology.

Our results on the generation of bright extreme ultraviolet (XUV) beams by quasi-phase-matching high-harmonic generation in PCFs could provide a new generation of bright table-top XUV sources. In the near term this work will likely be taken forward by university research groups, but there is potential for it to be taken forward by manufacturers of high-repetition rate fibre lasers.

It is most likely that in the near term our new scheme for wide-field, high resolution holography will be taken forward by academic researchers.
Sectors Aerospace, Defence and Marine,Electronics,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

Description HICONO Studentship
Amount £1,600,000 (GBP)
Funding ID 641272 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 10/2015 
End 09/2019
Description Advanced hollow core fibre technology 
Organisation University of Bath
Department Department of Biology and Biochemistry
Country United Kingdom 
Sector Academic/University 
PI Contribution We have investigated extreme nonlinear optics within gas-filled photonic crystal fibres.
Collaborator Contribution Our partners have provided advanced photonic crystal fibres.
Impact Several conference submissions that are currently under review.
Start Year 2016
Description Femtosecond OPA development 
Organisation ICFO - The Institute of Photonic Sciences
Country Spain 
Sector Academic/University 
PI Contribution We have developed novel femtosecond OPAs for applications in strong field physics.
Collaborator Contribution Our partners advised on the design of these complex optical systems.
Start Year 2014
Description Femtosecond OPA development 2 
Organisation Science and Technologies Facilities Council (STFC)
Country United Kingdom 
Sector Public 
PI Contribution We have developed novel femtosecond OPAs for applications in strong field physics.
Collaborator Contribution Our partners advised during the optimizations phase of these complex systems.
Impact doi:10.1117/12.2250775
Start Year 2015
Description Light Fantastic: Outreach at the Museum of the History of Science 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact David Lloyd, alongside colleagues from the Department of Physics, presented demonstrations on the phenomena of wave interference, as part of a public outreach event held at the Museum of the History of Science, Oxford. The demonstrations were hands-on activities where participants could explore interference through Young's slits and spray paint. Experimental data recorded in Professor Simon Hooker's laboratory was used to show that interference is an important tool in current research.
Year(s) Of Engagement Activity 2016
Description Physics Flash Talks 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact David Lloyd presented a 10 minute "Flash Talk" to a public audience, as part of an evening of outreach activities. The theme of the talk was x-ray imaging and microscopy, and how such methods were being used in Professor Simon Hooker's laboratory in Oxford. The purpose of the event was to provide snapshots of the research being conducted in the Department of Physics through the medium of brief presentations. A video of the talk was recorded and subsequently used as part of an outreach collaboration between the University of Oxford and the Times Educational Supplement.
Year(s) Of Engagement Activity 2013