Relativistic Nano-Plasma Photonics
Final Report Abstract
Nanostructured surfaces have manifold applications. Among others they are used to selectively increase absorption of light. In laser proton acceleration this approach attracts a lot of attention as nanostructured targets hold the promise to significantly increase maximum proton energies and proton numbers at a given laser energy. As for any other new technology, a high efficiency is a key for a potential future use. If an ultrashort laser pulse (~30 fs, >1 J) is focused onto a solid target foil, such that relativistic intensities (>10^18 W/cm²) are reached, matter is transformed immediately into a plasma. Electrons are accelerated in the laser field and a part of them form a dynamic sheath that, together with the target surface, generates an electric field of several megavolts per micrometer, in which positive ions (e. g. protons and carbon ions from the surface contamination layer) experience extreme acceleration. This process is called target normal sheath acceleration (TNSA). Using nanostructured surfaces increase laser absorption, i. e. more energetic electrons are generated which, in turn, can accelerate protons to higher energies. We investigated, under which conditions the use of nanostructured targets is beneficial. The targets were laser structured in-situ. This method of generation of periodic surface structures via a laser (LIPSS) is particularly simple and in principle allows the development of a high repetition rate target system. Structural analysis and simulations showed that these structures possess nearly optimal parameters for maximum laser absorption. We investigated the influence of those nanostructures on the proton spectrum for different laser intensities and temporal laser contrast. First of all, we could show that nanostructures remain functional even at highest intensities at the present contrast conditions in the sense that they increase the laser absorption as evident from an increase of produced X-ray Kα yield. For relatively low intensities, nanostructures significantly enhance both the conversion efficiency and proton energies. For example, at 5x1017 W/cm² the maximum proton energies were increased by a factor of four, the conversion efficiency from laser to proton energy was even enhanced by two orders of magnitude. However, at highest laser intensities with optimal laser plasma parameters no significant benefits from the nanostructures for ion acceleration were measured. This to some extend surprising result indicates fundamental limitations in the energy transfer processes. However, from a more general view it also proofs that as in many optimization problems, there are different paths to the optimum and combining them usually does not lead to an even better result. So far, these experiments performed at extreme conditions cannot be theoretically simulated in every respect and new input is given to understand underlying processes. It is therefore the merit of this project to have clarified under which conditions the use of nanostructures is beneficial and in which direction new theoretical investigations can be initiated.
Publications
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(2016), Relativistic laser nano-plasmonics for effective fast particle production, Plasma Physics and Controlled Fusion 58, 014038
A Andreev, K Platonov, J Braenzel, A Lübcke, S Das, H Messaoudi, R Grunwald, C Gray, E McGlynn and M Schnürer
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(2017). Prospects of target nanostructuring for laser proton acceleration, Scientific Reports 7, 44030
Andrea Lübcke, Alexander A. Andreev, Sandra Höhm, Ruediger Grunwald, Lutz Ehrentraut & Matthias Schnürer
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Interaction of ultrafast laser pulses with nanostructure surfaces, in: K. Wandelt (Editorin-Chief), Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, Kidlington, Oxford, UK (2017)
A. Lübcke, M. Schnürer, L. Ehrentraut, E. McGlynn, D. Byrne, S. Lowry, R. Wehner, and R. Grunwald
