Sivis M.,University of Gottingen |
Duwe M.,University of Gottingen |
Abel B.,Leibniz Institute of Surface Modification |
Ropers C.,University of Gottingen
Nature Physics | Year: 2013
Strong-field phenomena in optical nanostructures have enabled the integration of nanophotonics, plasmonics and attosecond spectroscopy. For example, tremendous excitement was sparked by reports of nanostructure-enhanced high-harmonic generation. However, there is growing tension between the great promise held by extreme-ultraviolet and attosecond-pulse generation on the nanoscale, and the lack of successful implementations. Here, we address this problem in a study of highly nonlinear optical processes in gas-exposed bow-tie nanoantennas. We find multiphoton- and strong-field-induced atomic excitation and ionization resulting in extreme-ultraviolet fluorescence, as well as third- and fifth-harmonic generation intrinsic to the nanostructures. Identifying the intensity-dependent spectral fingerprint of atomic fluorescence, we gauge local plasmonic fields. Whereas intensities sufficient for high-harmonic generation are indeed achieved in the near-field, the nanoscopic volume is found to prohibit an efficient conversion. Our results illustrate opportunities and challenges in highly nonlinear plasmonics and its extension to the extreme ultraviolet. © 2013 Macmillan Publishers Limited. All rights reserved.
Risselada H.J.,Leibniz Institute of Surface Modification
Structure | Year: 2015
In this issue of Structure, Reddy and colleagues combined various experimental data to build a realistic near-atomic model of the complete lipidic influenza A virion. Here, we illustrate the advances made by this pioneering simulation study and discuss ongoing challenges. ©2015 Elsevier Ltd. All rights reserved.
Arnold T.,Leibniz Institute of Surface Modification
Vakuum in Forschung und Praxis | Year: 2010
Plasma Jet Machining (PJM) is a surface figuring technology based on atmospheric plasma assisted chemical etching or deposition, respectively. In both cases a sub-aperture plasma jet source is used combined with a CNC multi-axes system for the processing of curved surfaces. It is under development for the surface figuring of a variety of optical materials by IOM for about 15 years. PJM is capable to figure deep aspheric or free-form substrates with high material removal rate and high spatial resolution. Based on chemical reactions between plasma generated radicals and the surface PJM does not introduce any damage to the processed surface and sub-surface region in contrast to abrasive techniques. Deterministic deposition of SiOx layers and subsequent proportional transfer using ion beams or polishing is another plasma jet based technique for surface figuring that extends the range of machinable materials. The article gives an overview on the current state of PJM development in IOM and shows examples of its application. © 2010 WILEY-VCH 10 ViP Verlag GmbH & Co. KGaA, Weinheim.
Scherzer T.,Leibniz Institute of Surface Modification
Macromolecular Chemistry and Physics | Year: 2012
Acrylates and methacrylates are photopolymerized without photoinitiator by exposure to 172 nm radiation. The kinetics of the polymerization is studied using real-time Fourier-transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. It is shown that layers with a thickness of ≈500 nm can be polymerized very rapidly. The effect of structure, viscosity, functionality, and absorption of the acrylates as well as the influence of temperature and oxygen concentration on the reactivity are studied. A strong conversion gradient is observed in layers up to ≈2 μm thickness, which reflects the intensity gradient within the layer. However, the penetration of the polymerization into the layer exceeds the initial penetration depth of the VUV radiation, which indicates strong bleaching of the acrylates during irradiation. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Naumov S.,Leibniz Institute of Surface Modification |
Buchmeiser M.R.,University of Stuttgart
Organometallics | Year: 2012
The experimentally observed high α-addition selectivity of 1,6-heptadiynes to modified Grubbs-Hoveyda initiators was elucidated with quantum chemical calculations. For these purposes, the two possible pathways of initiation in the Ru alkylidene triggered cyclopolymerization (CP) of 1,6-heptadiynes, resulting in either five-membered (α insertion) or six-membered (β insertion) repeat units, were treated as a multistep process. The first reaction cascade entails the activation of the precatalyst RuX 2(IMesH 2)(CH-2-(2-PrO-C 6H 4)) (1: X = F, Cl, Br, I, CF 3COO; IMesH 2 = 1,3- dimesitylimidazolin-2-ylidene), reaction with a 1,6-heptadiyne (π-1 complex formation), and further transformation into the first metallacyclobutene (MCB-1) followed by ring opening. The second reaction cascade entails again the formation of a π complex (π-2) through binding of the second alkyne moiety of the 1,6-heptadiyne and further transformation into MCB-2 followed by ring opening of MCB-2. The energies of the transition structures for both MCB-1 and MCB-2 formation (TS-1 and TS-2), which are considered the rate-determining steps in CP, are systematically lower for an α insertion of a monomer than for a β insertion. In addition, the geometrical parameters of the most stable structure of the βπ-2 complex are systematically less favorable for MCB-2 formation than in the case of an απ-2 complex, resulting in very high activation energies for βMCB-2 formation. Finally, the formation of βMCB-2 needs an additional step: namely, the endergonic formation of the intermediate βMCB-2. Since a halogen exchange to pseudohalides in Grubbs-Hoveyda initiators is required to turn them into active initiators in CP, the effect of electronegativity (EN) of the X ligands on the stability of the π-1 complex was calculated for X = I, Br, Cl, CF 3COO, F. There, an increase in EN results in lower energies for the α-insertion-derived π-1 complexes. For α insertion, the barriers to the MCB-1 intermediate formation, i.e. the energies of the transition states (TS-1(α)) for MCB-1 formation, decrease in the order I > Br > Cl > CF 3COO < F. All findings are consistent with the experimentally observed preference for α insertion in the cyclopolymerization of 1,6-heptadiynes with modified Grubbs-Hoveyda initiators and with the necessity for using pseudohalide variations of the Grubbs-Hoveyda initiator. © 2012 American Chemical Society.