Vanessa Knittel

Tailoring Spatiotemporal Light Confinement in Single Plasmonic Nanoantennas

Plasmonic nanoantennas are efficient devices to concentrate light in spatial regions much smaller than the wavelength. Only recently, their ability to manipulate photons also on a femtosecond time scale has been harnessed. Nevertheless, designing the dynamical properties of optical antennas has been difficult since the relevant microscopic processes governing their ultrafast response have remained unclear. Here, we exploit frequency-resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near-infrared. Third-harmonic imaging is used in parallel to spatially monitor the plasmonic mode patterns. We find that the few-femtosecond dynamics of these nanodevices is dominated by radiative damping. A high efficiency for nonlinear frequency conversion is directly linked to long plasmon damping times. This single parameter explains the counterintuitive result that rod-type nanoantennas with minimum volume generate by far the strongest third-harmonic emission as compared to the more bulky geometries of bow-tie-, elliptical-, or disk-shaped specimens.

Bow-tie nano-antenna assisted generation of extreme ultraviolet radiation

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.

Nonlinear photoluminescence spectrum of single gold nanostructures.

We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence.

Colloidal ZnO quantum dots in ultraviolet pillar microcavities

Three dimensional light confinement and distinct pillar microcavity modes in the ultraviolet have been observed in pillar resonators with embedded colloidal ZnO quantum dots fabricated by focused ion beam milling. Results from a waveguide model for the mode patterns and their spectral positions are in excellent agreement with the experimental data.

Dispersion of the nonlinear susceptibility in gold nanoantennas

Optical nanoantennas are excellent tools for accessing the nonlinear response of noble metals owing to the strong enhancement of light-matter interaction occurring in the near-field. For example, excitation of gold nanostructures with intense radiation triggers both coherent and incoherent phenomena such as third-harmonic generation (THG) [1] and multi-photon photoluminescence (MPPL), respectively [2, 3]. In this work, we study the interplay between these frequency conversion mechanisms in order to unveil the effective 2(3) nonlinear response of metals.

Optical phase control of ultrafast single-electron nanocurrents

By accessing the nonperturbative strong-field regime, light-matter interaction is governed by the electric field transient of light as opposed to multiphoton effects. In our experiments, we illuminate a circuit with a nanoscale open junction (Fig. 1(a)) with intense ultrashort pulses at high repetition rate. With this setup, we are able to coherently drive electronic currents at optical frequencies and beyond [1].

Multi-Photon Photoluminescence Spectral Behavior of Single Gold Nanorods

The spectral shape and nonlinear order of optical emission from single gold nanorods is investigated. The results highlight the complex absorption cascade in the out-of-equilibrium electronic distribution after few-cycle excitation by near-infrared pulses.

Gold Bowtie Nanoantennas Generating UV

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.

Nonlinear ultrasonics in gold-cobalt bilayer structures probed with femtosecond surface plasmons

Giant (1%) picosecond strain pulses are generated in a fs-laser-irradiated cobalt transducer sandwiched between a gold layer and sapphire substrate. Ultrafast plasmonic interferometry reveals nonlinear acoustic propagation effects in the (111) gold film.

Analysis of Gold Nanoantennas for Harmonic Generation Utilising Plasmonic Field Enhancement

We present an analysis of the plasmonic field enhancement in gold nanoantennas based on FDTD calculations. In experiments up to the 7th harmonic-order is observed. Experimental issues are discussed and explained by a theoretical model.