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Start of funding 01.01.2010

Exploiting nanoplasmonic field enhancement for compact XUV and X-ray light sources

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XUV pulses, produced by laser-driven high harmonic generation (HHG), offer unprecedented access to the attosecond temporal and nanometre spatial scales. By exploiting the plasmonic field enhancement in ordered nanostructures, this project aims to produce a more compact XUV source as well as address problems, such as low flux and tunability, associated with traditional techniques. The research focuses on the processes leading to HHG occurring in nanostructures of different size, shape and composition for the formation of XUV radiation with few-cycle driving laser pulses. The aim is to understand, optimise and use the processes leading to nanolocalized plasmonic fields within these structures for HHG, ultimately reaching into the attosecond regime.

Final report:
The aim of this project was the close interaction between nanofabrication (at the Molecular Foundry at LBNL) and laser experiments (at MPQ) to optimize structures for EUV light generation by nanoplasmonic enhancement. The proposal was based on a report in the journal Nature that EUV light can be created by high-harmonic generation (HHG). HHG is a well-established process for producing ultrashort, extreme-ultraviolet pulses by direct frequency up-conversion of femtosecond, near-infrared pulses in noble gases. We obtained a couple of Au bow-tie nanostructures from the Molecular Foundry for our tests. While the structures were excellently made, unfortunately, EUV light generation turned out to be a most challenging task. We have successfully produced lower order harmonics on the bow-tie structures and also used AFM to characterize the structures before and after laser interaction. It remained unclear during the project phase, why the structures did not work. It should be noted that recent work by the group of Ropers at the University of Goettingen suggests that the EUV light generation is based on incoherent atomic line emission rather than a coherent HHG process.

We have nevertheless produced some publications from our theoretical work that accompanied the experiments [1-2]. We studied the coherent process in detail. In particular, we could identify the role of the carrier-envelope phase (CEP) and chirp on the generation of isolated attosecond pulses with such a source. For suitable pulse forms and spectral filtering, sub-femtosecond light pulses could be generated from such a source. Increasing the number of emitters can significantly enhance the coherent high-harmonic generation process versus non-coherent contributions. It should be noted that the experimental verification of a coherent process for EUV light generation is still missing. Nevertheless, our theoretical insight is applicable also for direct electron emission in the near-field of nanostructures and has initiated a rich research program on attosecond nanophotonics.

1. S.L. Stebbings et al., Probing ultrafast nano-localized plasmonic fields via XUV light generation, Proc. SPIE 7757, 77571F-1 (2010).

2. S.L. Stebbings et al., Generation of isolated attosecond extreme ultraviolet pulses employing nanoplasmonic field enhancement: optimization of coupled ellipsoids, New J. Phys. 13, 073010 (2011).