Nonlinear pulse propagation in nanoscale waveguides

The bottleneck of optical communication systems, which form the backbone of the internet, is formed by electronical signal processing. A promising approach to solve this issue is to route the data instead by all-optical means using nonlinear optical effects. To exploit the normally weak nonlinear optical effects short light pulses can be spatially confined to nanoscale waveguides. This thesis presents an experimental study of nonlinear pulse propagation in such waveguides using a near-field microscope, which enables the local investigation of guided light in a nondestructive fashion. Firstly, we track the changes in the spectral density of a short high-intensity pulse propagating caused by nonlinear effects during its propagation through a composite photonic circuit. This measurement approach allows us to unravel the effects occurring inside a desired section of the photonic circuit without the need of taking the other components into account. In contrast, to obtain the same information from transmission measurements, which represent the characteristics of the complete composite sample, all components have to be considered. Secondly, we show that solitons, which are waves where nonlinear optical effects balance linear dispersion, can be also observed in nanophotonic waveguides. Further, we present the first observation of their breakup into several wave packets, which is called a soliton fission, caused by a perturbation due to free carriers. This soliton fission event is instrumental for extreme spectral broadening and may therefore be used for implementing on-chip white-light sources. Thirdly, we demonstrate that plasmonic nanowires form a promising platform for ultrashort pulse propagation. In addition to the achievable high spatial confinement in a metallic waveguide, their dispersion properties can be very low. As a consequence, ultrashort pulses propagating in plasmonic nanowires can maintain a high peak amplitude since they can be spatially concentrated and do not reshape temporally.