Optimising laser driven electron nanobunches from ultrathin foil interactions: Coherent synchrotron emission and relativistic electron mirrors

Summary of research for a general audience

One of the most exciting frontiers of science is the study of phenomena that take place on the timescale of attoseconds (as, 10^-18 s). To imagine such an incredibly short period of time, consider that light travels from here to the moon in one second, but only travels 0.0003mm in one femtosecond (fs, 10^-15 s). To put it in context, that is about 1/300th the width of a human hair in 10^-15 s (or 1000 as). Attoseconds are the timescales on which atomic processes/transitions occur - for example, an electron circles the hydrogen atom in ~24 as (the so called 'atomic unit of time').

To investigate, and in future control, the dynamics of such ultrafast processes measurement tools of unprecedented quality and precision are required - pulses of light with attosecond duration. This is much shorter than a single cycle of any visible light wave (violet~1.3fs, red~2.5fs), requiring instead extreme-ultraviolet (XUV)/ X-ray radiation to be controlled with clinical accuracy to achieve ultrashort durations. However, the pay-off for this effort is substantial; researchers can investigate the microcosm with a degree of spatial clarity and on shorter time scales than previously possible, thus allowing them to see events that are ordinarily 'blurred' using conventional XUV/X-ray sources such as synchrotrons.

Attosecond pulses must be synthesized using wavelengths shorter than those in the visible region of the spectrum and therein lies a significant problem - wavelengths shorter than the visible spectrum i.e. ultraviolet and X-rays, are strongly absorbed in most materials. It is therefore impossible to build an attosecond laser using conventional laser building techniques. Instead next generation methods are required. Two principle media are currently being studied at laser laboratories around the world - intense laser-gas interactions and relativistic laser plasmas formed using solid density targets - for the production of attosecond pulses.

In the proposed research we focus on the second medium - relativistic laser plasma. The underlying mechanism under investigation for the generation of intense attosecond pulses is the production of relativistic electron nanobunches during high power optical laser interactions with ultrathin carbon foils. This novel concept is based on our recent work showing that dense bunches of electrons with sub 10nm scale (nm = nanometer = 10^-9m) can be formed and rapidly accelerated on the front surface by the relativistically intense driving laser field and subsequently emerge from the rear surface of ultrathin carbon foils. Two resulting mechanisms will be studied in detail in this research - Coherent Synchrotron Emission (CSE) and Relativistic Electron Mirrors (REM). Only recently demonstrated, CSE and REM offer a novel window onto the relativistic laser plasma interaction and our work will not only reveal the microscopic dynamics of these mechanisms but also show a direct path to the generation of bright attosecond pulses.

Impact Summary

Our work has the potential to impact strongly on both the economy and society. Ultimately with this proposal we are developing our knowledge of the fundamental physics governing intense laser - ultrathin foil interacts with a view to creating novel sources with specific and tailored characteristics for the broader scientific community. Typically history has shown that future applications develop rapidly after a "new tool" has been demonstrated and optimised - the optical laser in 1960 is a case in point.

The use of lasers to produce bright pulses of extreme ultraviolet/X-ray radiation and charged particles is leading the way towards downsizing the current generation of large scale accelerator based facilities. These facilities, such as synchrotrons and linear accelerators, are simply too expensive to have in every university.

One of the key areas of impact of the work in this proposal will be for researchers and users of ultrafast XUV/X-ray and charged particle sources. Our proposal seeks for the first time to perform fully quantitative measurements on beam characteristics (such as efficiency, angular distribution of both XUV and electron beams from ultrathin foils) and how these vary with key interaction parameters so that the wider community can avail of these advances. While this will be achieved through standard methods (academic journals and presentation at conferences) we maintain a proactive stance to bringing our research to the wider scientific community and new audiences via novel multimedia dissemination methods.

Another key area where we will achieve significant impact is through our collaboration with major facilities in the US (Los Alamos National Laboratory (New Mexico), University of Austin (Texas), Ohio State University (Ohio)) and Germany (Helmholtz institute at Jena). All of these high power laser facilities perform high quality, impact-led research and, as evidenced by our statements of support, are very keen to collaborate with us on this project.