Ultrafast Nonlinear Spectroscopy of Hybrid Plasmonic Systems

In this thesis, the nonlinear optical properties as well as the ultrafast temporal dynamics of hybrid plasmonic systems are studied. The model system under investigation simultaneously provides localized plasmonic modes in a metallic nanowire grating and photonic modes in an adjacent dielectric waveguide slab. The coupling of the modes leads to the formation of polaritonic eigenstates and significant variations in the linear optical spectra as well as in the dephasing dynamics. In the first part, the ultrafast temporal dynamics on the femtosecond timescale are studied by a nonlinear autocorrelation technique. The linear optical response of the polaritonic system has a major influence on the shape of the autocorrelation function. By employing a harmonic oscillator-based model, the dephasing times are extracted. Due to the hybridization, significantly prolonged dephasing times are observed. Furthermore, the full spectral response, i.e., amplitude and phase, is retrieved from the experimental data which renders the nonlinear experiments distinctly different from purely linear experiments. The second part concentrates on the nonlinear response of the hybrid plasmonic structure. From third harmonic generation spectroscopy it is found that the shape of the nonlinear optical spectra is completely different when compared with the linear spectra. By carefully varying the structure geometry as well as the constituent materials, the origin of the nonlinearity can be unambiguously identified from the shape of the nonlinear spectra. The observations are confirmed by several theoretical considerations. In the last part, the prolonged dephasing times of the polaritonic eigenmodes are finally utilized for coherent control experiments. Here, one eigenmode is excited by a first ultrashort laser pulse. Subsequently, a few tens of femtoseconds after the excitation, the eigenmode as well as its nonlinear response is either turned on or off by a second laser pulse dependent on the exact time delay between the pulses. A third laser pulse provides the necessary temporal resolution of the whole control process. Due to the high signal contrast between the suppressed and the reexcited case this technique is suitable for ultrafast plasmonic switching.