Third harmonic imaging of plasmonic nanoantennas Andreas Trügler, Ulrich Hohenester Karl-Franzens-Universität Graz, Austria Work performed together with: T. Hanke, J. Cesar, R. Bratschitsch, A. Leitenstorfer Lehrstuhl für Moderne Optik und Quantenelektronik, Univ. Konstanz

Goal of this work Agenda Tailoring spatiotemporal light confinement in single nanoantennas… 200 nm Agenda - Optical antennas, experiments Simulation of metallic nanoparticles THG mapping of particle plasmons 200 nm

Nanoantennas as nonlinear emitters Hyper-Rayleigh scattering at surface imperfections, see M. Stockman et al., PRL 92, 057402 (2004) Linear optics: Resolution given by wavelength l Nonlinear optics (THG): Resolution given by l / 3 Excitation vs. Detection: Wavelength difference Strong χ(3) nonlinearity for gold, see T. Hanke et al., PRL 103, 257404 (2009)

Imaging with optical antennas Substrate THG intensity ~ | E |6 E THG Pump laser pulse 0.97 eV, 24 fs Array of nanoantennas By scanning the excitation spot over the sample and observing the THG signal (in the farfield), we obtain a map of the electric fields of the particle plasmons. See S. Kim et al., High-harmonic generation by resonant plasmon field enhancement, Nat. Lett. (2008); T. Hanke, R. Bratschitsch, A. Leitenstorfer @ Univ. Konstanz, Germany (2011).

THG mapping of particle plasmons Third harmonic generation (THG) map (left) and sample (right) Excitation with fs – pulses and with a bandpass filter for wavelengths 1100 – 1500 nm T. Hanke, R. Bratschitsch, A. Leitenstorfer @ Univ. Konstanz, Germany (2011).

THG mapping of particle plasmons T. Hanke, R. Bratschitsch, A. Leitenstorfer @ Univ. Konstanz, Germany (2011).

THG intensity for particle plasmons Lowest antenna volume gives highest THG intensity !?

Boundary element method (BEM) Discretization of surface integral into „boundary elements“ Collocation method … surface charges located at centers of boundary elements from boundary conditions… F. J. García de Abajo et al., PRB 65, 115418 (2002); U. Hohenester et al., PRB 72, 195429 (2005).

THG mapping of particle plasmons Simulation of antenna structures Result from experiment Size of each triangle ca. 300 nm, discretisation with 20.000 surface elements

THG intensity for particle plasmons Lowest antenna volume gives highest THG intensity !? Scattering intensity generated by electromagnetic fields at the surface… Incoherent optics: Biggest volume gives highest intensity… Coherent optics: Lowest volumes gives highest intensity…

THG autocorrelation Autocorrelation allows to measure dephasing time of particle plasmons THG autocorrelation intensity depends on time delay between femtosecond pulses

THG autocorrelation Autocorrelation allows to measure dephasing time of particle plasmons harmonic fields Insert harmonic fields together with plasmon damping time: two interacting pulses damping in / out of phase ratio gives 32:1 THG autocorrelation intensity depends on time delay between femtosecond pulses

THG autocorrelation Autocorrelation allows to measure dephasing time of particle plasmons Dephasing times: rod 5.5 fs ellipse 3.5 fs disc 2.0 fs THG autocorrelation intensity depends on time delay between femtosecond pulses Weak plasmon damping effective build-up of the plasmon oscillation Knowledge of the plasmon damping time alone suffices to predict the nonlinear intensity !

THG intensity vs. plasmon dephasing THG intensity directly scales with plasmon dephasing ! Long dephasing times correspond to large THG intensities 30 47 64 81 98 115 132 149 166 183 200 high nonlinear emission connected to small antenna volumes radiative damping! rod length: 300 nm gap: 50 nm

THG intensity vs. plasmon dephasing THG intensity directly scales with plasmon dephasing ! Long dephasing times correspond to large THG intensities

High intensity linked to smallest antenna volume! Summary & Acknowledgement Theoretical Nanoscience Ulrich Hohenester Jürgen Waxenegger KFU Graz, Austria Temporal scale: Measuring few-fs plasmon damping times Spatial scale: Mapping of third-harmonic emission Radiative damping: Lowest volumes generates strongest third-harmonic emission Moderne Optik und Quantenelektronik Alfred Leitenstorfer Rudolf Bratschitsch Tobias Hanke Vanessa Knittel Julijan Cesar High intensity linked to smallest antenna volume!