Adaptive control, generation, and characterization of bright soft x-rays by quasi-phase-matching

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

Abstract

This programme uses recent advances made by the Oxford group in producing and controlling trains of amplified, ultrashort laser pulses, as well as a new method for producing high-energy photons in waveguides, and seeks to apply this to generating very bright sources of coherent, ultrashort, soft x-ray radiation - laser-like pulses of radiation at energies approaching x-ray photon energies, with extremely short time durations.Coherent soft x-ray radiation can be produced through a process called high harmonic generation . In this process multiple frequencies of an intense laser pulse can be generated by ionizing an atom and colliding the resulting electron into its parent ion, producing a pulse of radiation which can be as short as 100 attoseconds (1 attosecond = 1 billionth of a microsecond). Photon energies approaching x-ray energies can be generated this way. However, the efficiency of this process is very low, and gets much lower at higher energies, severely limiting many applications. The main cause for this low efficiency is the effect of dephasing . Simply put, this means that the laser pulse and the generated soft x-rays travel at different speeds, preventing the continuous growth of the soft x-ray. One way to overcome this dephasing is to employ a technique known as quasi phase-matching, whereby the harmonic generation process is switched on and off with a period equal to the coherence length , i.e. the distance over which the driving laser pulse and the generated radiation become out of phase. If this is achieved over N coherence lengths the soft x-ray signal will increase by a factor N-squared. Therefore, to maximize the power of this technique it is necessary to quasi phase-match over as many coherence lengths as possible. Switching the harmonic process on and off can be achieved in a number of ways. One method involves using a series ( or train ) of closely spaced, very short laser pulses travelling in the opposite direction to the generating laser pulse. Under EPSRC grant EP/C005449 the Oxford group has recently developed a method for producing trains of laser pulses with up to 100 pulses which can be accurately controlled. This has the potential to match up to 100 coherence lengths, which would increase the soft x-ray yield by a factor of 10,000! The adaptive nature of these pulse trains will also allow genetic algorithms to be employed to fully optimize the soft x-ray source. As part of this programme the group will seek to exploit the power of these new adaptive pulse trains to produce a soft x-ray source from harmonic generation with a brightness far exceeding anything previously recorded. Under grant EP/C005449 the Oxford group, in collaboration with a group at Queen's University Belfast (QUB), also discovered a new method for controlling the harmonic process. This relies on manipulating the way a laser pulse propagates through a narrow, hollow glass fibre. Through precise control of the laser pulse and how it couples into the fibre it is possible to excite modes in the fibre such that they create an interference pattern within the fibre which switches the harmonic generation on and off. The Oxford and QUB groups successfully used such a pattern to create the brightest source of water window x-rays from harmonic generation ever reported and, as part of this program will seek to develop this method even further. The development of such bright soft x-ray sources is crucial for a variety of scientific applications such as ultrafast measurements of chemical reactions, high-contrast biological imaging, advanced lithography, atomic and molecular spectroscopy, and ultrafast x-ray diffraction.
 
Description The development of bright sources of coherent (i.e. "laser-like") radiation in the soft x-ray region is of major interest to the scientific community, as evidenced by the current construction of several x-ray free-electron lasers (FELs) around the world. These machines, together with conventional synchrotrons, will lead to major advances. However, their high cost - of order £300M for a 4th generation synchrotron, and of order £1000M for an x-ray FEL - means that they will remain scarce and user access will always be limited. The development of compact sources of coherent short-wavelength radiation, able to be operated in university or industrial laboratories, is therefore vital if this important spectral region is to be exploited fully.

High-harmonic generation provides an alternative and compact source of laser-like X-rays which could be used in several applications which presently require national or international facilities. The arrangement is straightforward: a visible laser pulse is focused into a gas target and high-order harmonics of the visible laser are emitted in a beam.

A major challenge to be overcome for high-harmonic sources, however, is their low efficiency. One cause of this is that X-rays generated in the downstream parts of the gas cell are out of phase with those generated upstream, leading to destructive interference and hence a reduction of the X-ray intensity.

This research programme addressed that challenge by developing ways to increase the output by "quasi-phase-matching" (QPM); in this approach the harmonic generation is modulated in space to ensure that X-rays are only generated in regions of the gas target which are mutually in phase.

We showed that a train of counter-propagating laser pulses could be used to modulate the harmonic generation and that this increased the X-ray output by a factor of 40. We also devised and demonstrated new methods for generating trains of ultrashort laser pulses; we developed new techniques for diagnosing the properties of high-harmonic X-ray beams; and we devised new methods for quasi-phase-matching based on controlling the polarization of the driving laser radiation.
Exploitation Route The results obtained during our programme will be of interest to all groups working with, or on the generation of, coherent short-wavelength radiation.

For example, compact sources of coherent X-rays can be used to image systems with atomic scale spatial and temporal resolution. Much of this work is presently done at large-scale facilities, and so the the ability to do this with a laboratory-scale source would undoubtedly lead to many advances in the physical and biological sciences.

The ability to image systems with very high time resolution has the potential to revolutionize ultrafast science, with applications across the biological and physical sciences including: studies of enzyme and surface catalysis, photosynthesis, and electron dynamics in magnetic systems.

We also note that our proposed technique of optical rotation QPM is unique in that it offers a route to bright circularly-polarized soft X-rays, and as such could find widespread applications in studies of spin dynamics.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Environment,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology
 
Title High Harmonic Optical Generator (EP13722033.1; US patent application number US 2015/086148 A1) 
Description A high harmonic optical generator comprising a laser arrangement for emitting a beam of polarized radiation at a fundamental frequency and an optical waveguide having a hollow core for a gaseous harmonic generation medium for the generation of high harmonics of the fundamental frequency, the optical waveguide having an optical propagation axis along the hollow core, the laser arrangement is configured to couple the beam of polarized radiation along the propagation axis of the hollow core optical waveguide to provide a beam of optical driving radiation for the high harmonic generation, the optical driving radiation having a plane of polarization that rotates about the propagation axis. 
IP Reference European patent application number EP13722033.1 
Protection Patent application published
Year Protection Granted
Licensed No
Impact None at this stage.
 
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
URL https://www.facebook.com/events/1570531493258300