Analysis of gold nanoantennas utilising plasmonic field enhancement for high-order harmonic generation

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.