We study the nonlinear behavior and thermal stability of bowtie nanoantennas both theoretically and experimentally to understand their suitability for high harmonic generation in gaseous media [1, 5, 6]. The obtained results are compared to those in the literature.

High-order harmonic generation in xenon with oscillator repetition rates is studied. The necessary intensity is reached via plasmonic field enhancement at nanostructured arrays of bow-tie gold antennae. The theoretical analysis focuses on the thermal properties and the damage threshold of the bow-tie antennae. On the experimental side the number of contributing atoms is determined and optimized. Extreme ultraviolet radiation is successfully observed with photon fluxes almost an order of magnitude larger than previously reported.

We report on low-order harmonic generation utilising the plasmonic field enhancement in arrays of rod-type gold optical antennae. Furthermore, we examine their suitability to support high-order harmonic generation (HHG). The low-order harmonics are used as a tool to investigate the nonlinear properties of the antennae. Particular attention is paid to the thermal properties, which become significant at the peak intensities necessary for HHG. A theoretical model explains the experimental findings and enables future improvements. In experiments we observe up to the fifth harmonic order and measure a field enhancement sufficient to support high-order harmonic generation. Moreover, we find a damage threshold for the antennae.

We report on the generation of extreme ultraviolet radiation utilizing the plasmonic field enhancement in arrays of bow-tie gold optical antennae. Furthermore, their suitability to support high-order harmonic generation is examined by means of finite-difference time-domain calculations and experiments. Particular emphasis is paid to the thermal properties, which become significant at the employed peak intensities. A damage threshold depending on the antenna length is predicted and confirmed by our experimental findings. Moreover, the gas density in the vicinity of the antennae is characterized experimentally to determine the number of atoms contributing to the measured radiation, which is almost an order of magnitude larger than previously reported.

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations-or "nanobumps"-are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents.

Summary form only given. We study the nonlinear behaviour as well as the thermal stability of bowtie nanoantennas both theoretically and experimentally to understand the suitability for high harmonic generation (HHG) in gaseous media in the plasmonic enhanced field areas. The obtained results are compared to those in the literature.The experiments are performed in a vacuum chamber, and Xenon gas for harmonic generation can be fed onto the sample through a glass nozzle. To illuminate the antennas a homebuilt Ti:sapphire oscillator is focused with an achromatic lens onto an array of nanoantennas. The generated radiation is detected with a photomultiplier or a channeltron detector. The nanoantennas lead to a significant enhancement of the local laser intensity and we observe a strong third harmonic without gas flow on the sample, but at the same time thermal antenna damage due to melting. With Xenon gas flow we detect radiation at the wavelengths of driving laser harmonics as well as various plasma lines with comparable photon numbers. This is in contrast to the results by Kim et al., who measured harmonic radiation alone but in agreement with the results by Sivis et al. [3]. Therefore, the gas nozzle has been thoroughly characterized (fig. 1(a)) to determine the gas density at thesample, which is a crucial parameter in this generation scheme. A conservative calculation reveals a significantly higher gas density than in previous experiments [4], which would favour the HHG process. Further experiments e.g. on the coherence properties of the measured radiation are currently performed to determine its origin. Additionally, the nanoantenna's response to a laser pulse is calculated with the freely available finite difference time domain (FDTD) implementation Meep [5] to determine the near field intensity enhancement and optimise antenna parameters. By solving the diffusion equation [6] thermal effects are also considered in the simulation and a damage threshold is deduced, which mainly depends on the antenna arm length and matches well to the experimental findings. Based on these calculations an optimal antenna length of 160 nm with maximised peak intensity at a high damage threshold is found (fig. 1 (b)). In summary, we present simulation results for the design of nanoantennas to excite a plasmonic resonance for HHG directly from a laser oscillator as well as experimental results. The produced structures are used in our experiments in combination with a high and well known Xenon gas density. They show a significant enhancement leading to the observation of various plasma lines and low order harmonic radiation.

A double pass cw-pumped Ti:sapphire amplifier delivering 1.6µJ pulses at 1 MHz is presented. Furthermore a simple analytical model for the amplifier is deduced and concepts for further energy scaling are explored.

We demonstrate a chirped-pulse Ti:sapphire laser oscillator with both Kerr-lens and semiconductor- saturable-absorber-mirror-assisted mode locking generating 1.1 microJ pulses at 1 MHz pulse repetition rate. The pulses are coupled out of the laser cavity by means of an acousto-optical cavity dumper, have a spectral width that supports a Fourier limit of 74 fs, and currently have a chirped-pulse duration of 5 ps. After compressing the pulses, this laser will be an ideal tool for efficient high-harmonic generation directly from a laser oscillator.