Browsing by Author "Metzger, Bernd"

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    Ultrafast nonlinear plasmonics : from dipole nanoantennas to hybrid complex plasmonic structures
    (2014) Metzger, Bernd; Giessen, Harald (Prof. Dr.)
    The aim of this thesis is to study and investigate the nonlinear optical response of complex plasmonic nanostructures under illumination with high intensity ultrashort laser pulses. In order to perform nonlinear spectroscopy experiments we develop a new laser source for the generation of widely tunable ultrashort laser pulses. The experimental setup consists of a high-power Yb:KGW solitary mode-locked oscillator, of a nonlinear photonic crystal fiber for spectral broadening, of a prism sequence for pulse compression and a 4f Fourier transform pulse shaper for amplitude and phase modulation. This setup allows for the generation of Fourier-limited widely tunable sub-20 fs laser pulses and constitutes and ideal light source for the nonlinear optical experiments of plasmonic nanostructures. In contrast to most previous studies of the nonlinear optical response of plasmonic nanostructures here we perform spectrally-resolved nonlinear optical spectroscopy, which means that we tune the fundamental narrow-band laser over a broad spectral range and are therefore able to measure the frequency-dependence of the nonlinear optical response of complex plasmonic nanostructure arrays. In the nonlinear spectroscopy experiments we find that the spectral position of highest conversion efficiency for third harmonic generation in rod-type gold nanoantenna arrays is equivalent with the spectral peak position of the plasmonic near-field amplitude. The absolute conversion efficiency of nonlinear optical effects strongly depends on the linear optical response and the properties of the plasmonic modes. In particular, the resonance frequency, the linewidth and the absolute oscillator strength critically influence and determine the overall nonlinear optical response. By comparing measured linear and nonlinear spectra of complex coupled plasmonic nanostructure arrays to results of a classical coupled oscillator model we find excellent agreement, which shows that the highly complex electrodynamic processes can even be understood in a classical intuitive fashion. Furthermore, we investigate the enhancement of third harmonic generation in complex plasmonic nanostructure arrays, which exhibit plasmonic Fano resonances. The plasmonic Fano resonance is the result of the interference of a bright and a dark mode and due to the long lifetime of the latter it exhibits the potential to further enhance the conversion efficiency of nonlinear optical effects. In particular we find that a nonlinear near-field polarization which is generated in a dark mode does not radiate to the far-field due to destructive interference. Beyond pure plasmonic nanostructures we study the enhancement of third harmonic generation in highly nonlinear indium tin oxide nanocrystals boosted by the intense electric near-field of plasmonic gap-antennas. When comparing corresponding hybrid and bare plasmonic gap-antenna arrays we find an enhancement in the third harmonic response by a about a factor of two. However, the origin of the higher third harmonic signal strength can be attributed to an enhanced plasmonic near-field and to the high optical nonlinearity of gold, rather than to the optical nonlinearity of the indium tin oxide. Finally, we introduce a spectroscopic method for measuring the frequency-dependent second-order response using ultrabroadband strongly chirped laser pulses. The dispersion suppresses nonlinear frequency mixing, hence the second-order response of a material can be unambiguously retrieved. We demonstrate this method by measuring the frequency-dependent second harmonic response of the metals gold, aluminium, silver and copper in the wavelength range of about 900 to 1150 nm and compare the results to classical second harmonic spectroscopy. The second harmonic spectra indicate that interband transitions in the metals influence the overall nonlinear optical response.