This thesis presents the realization and characterization of passive and active photonic and nanoplasmonic waveguides for applications in all-optical circuitry. The key results focus on generating visible light in nanoscale silicon waveguides through nonlinear interactions and demonstrating ultrafast all-optical modulation through nonlinear loss mechanisms.

Nanofabrication processes are developed to interface silicon photonic waveguides and silicon-based nanoplasmonic waveguides, along with a technique to integrate nanoplasmonic waveguides onto a macroscopic characterization beam. Passive propagation and nonlinear interactions are investigated in silicon-on-insulator photonic waveguides to provide a detailed understanding of nonlinear interactions present in silicon at lambda=1550nm and the relevant timescales of the interactions. Extensive investigations into third-harmonic generation in silicon photonic waveguides are performed, and conversion efficiencies up to 2.8×10⁻⁵ are measured.

Measurements of the passive performance of silicon-based nanoplasmonic waveguides revealed a propagation length of 2.0um at λ=1550nm and a coupling efficiency of 38% to silicon photonic waveguides. The concepts of nonlinear light generation and ultrafast modulation are then applied to sub-wavelength silicon-based nanoplasmonic waveguides. Third-harmonic generation with conversion efficiencies up to 2.3×10⁻⁵ is demonstrated in a nanoplasmonic waveguide with a footprint of 0.43μm². Accurate investigations of ultrafast nonlinear interactions in silicon-based nanoplasmonic waveguides integrated onto a macroscopic characterization beam are performed using pump-probe time-domain measurements.

Ponderomotive acceleration of two-photon absorption-generated free-carriers in silicon-based nanoplasmonic waveguides is examined and it is demonstrated that electrons can be accelerated to energies exceeding the threshold for impact ionization. Measurements reveal that the highly confined nanoplasmonic field drives an electron avalanche, and white light emission resulting from the avalanche is observed to scale exponentially with the input power. The electron avalanche effectively sweeps free-carriers from the nanoplasmonic waveguide on a timescale of ~2ps, allowing for a reduction in the free-carrier recovery time by more than two orders of magnitude compared to silicon photonic waveguides.