Research On Third Harmonic Generation Based On Plasmonic Slot Waveguides

The mid-infrared(mid-IR) photonics is stimulating research field owing to their potential applications in a wide variety of areas, including free-space communications, chemical or biological sensors. Third harmonic generation(THG) devices working in this special waveband can find promising applications in the area of signal processing, high speed optical performance monitoring, geometrical structure imaging of microscopic samples, optical data storing three-dimensionally and biological materials studying. Therefore, searching for efficient THG devices from mid-IR to near-IR regions is necessary and valuable.Brief review of the basic theory and application of the infrared waveband and THG was carried out in this thesis. From the coupled wave equations, we deduced that high THG conversion efficiency depends on three main factors, including interactive material with high third-order nonlinear susceptibility, phase-matching condition(PMC) and large pump-harmonic modal overlap. Plasmonic slot waveguide is the most promising structure due to its deep nanoscale mode confinement and huge field enhancement accordingly. Furthermore, the nonlinear parameters of the waveguide can be engineered by employing the waveguide geometry. Based on the coupled wave equations of the third harmonic generation, we proposed several plasmonic slot waveguide structures to achieve efficient THG from mid-IR to near-IR regions. To obtain large pump-harmonic modal overlap, we designed specific waveguide structures to reduce the negative part contribution of the higher-order mode at third harmonic wave to the pump-harmonic modal overlap.First, we designed two asymmetric plasmonic slot waveguides. The silicon used here is to isolate the influence of the field from the top metal layer to field in the DDMEBT slot. Furthermore, due to the silicon slot layer, the PMC can be achieved by properly engineering the waveguide geometry. Then we utilized hybrid plasmonic slot waveguide structure to achieve higher THG efficiency. Due to the specific field distribution of the guided mode, the negative part of the higher mode at third harmonic is forced to distribute into the silicon slab region. Therefore, the corresponding pump-harmonic modal overlap is greatly enhanced. Based on the study of the asymmetric plasmonic slot waveguide structure and the hybrid plasmonic slot waveguide structure, we proposed a double-slot metal waveguide to obtain high THG efficiency, which provide guidance for designing specific metal core fibers. At last, we study the two-photon absorption(TPA) effect on the THG efficiency in details and provide solutions to reduce the TPA effect.