An investigation of distributed coupling in a nonlinear semiconductor waveguide

In this thesis, we theoretically and experimentally analyze the nonlinear coupling properties of a model waveguide-grating structure. A planar Si-on-insulator (SOI) structure with a periodic distributed grating coupler is used as the model system. Unlike other optoelectronic materials, Si does not possess a direct band gap or a linear electrooptic effect. However, it may be possible to utilize silicon's large free carrier effect in optoelectronic devices. It may also be possible to integrate Si optical structures with Si based electronic circuits on monolithic substrates.;In our experimental study of the SOI structure, a distributed grating input coupler is used to transfer light from a Nd:YAG Q-switched laser (;By numerically modeling the structure, we attribute the temporal fluctuations in the mode profile to intensity dependent changes in the refractive index of the Si guiding layer. Contributions to the refractive index from both free carrier and thermal changes are considered. Rapid index variations causes the propagation wavevector in the interaction region to be power dependent. As a result, the coherent optical coupling properties of the structure are altered. We have determined that spatial changes in the resonance condition of the coupling structure accounts for most of the limiting effects. Damped oscillations observed in the temporal pulse profile are attributed to interference effects induced by a nonlinear phase shift in the guided mode.;The numerical computations indicate that the nonlinear coupling process is dominated by free carrier effects. However, competition between electronic and thermal processes becomes more acute with higher energies. By fully modeling the two dimensional carrier and thermal dynamics, we are able to fully characterize the nonlinear coupling process due to the material and geometric constraints.