| The terahertz (THz) regime, lying between microwave (0.3 THz) and infrared (10 THz) frequencies, is rich with emerging possibilities ranging from molecular spectroscopy to sensing and communications. The generation of THz radiation by difference frequency mixing of two optical pumps (lasers) in a nonlinear material has been widely used due to its room temperature operation, potentially large output powers, and frequency tunability. Single mode waveguides can enhance the efficiency of the difference frequency mixing process by providing strong optical confinement, collinear propagation without diffraction, improved overlap of pumps and THz product, and waveguide dispersion for phase matching. These conditions, however, cannot all be met in a simple cladding-core-cladding waveguide that is single mode for both near-infrared (NIR)/visible pumps and THz product. A nested waveguide configuration can achieve these conditions and continuous phase matching between single mode NIR pumps and THz product.;In this thesis, a new approach to achieve efficient THz difference frequency generation (DFG) is presented. This multilayer or nested waveguide approach makes it possible to take advantage of a broad range of material and structural combinations to achieve all of the above conditions necessary for efficient THz conversion. Theoretical and experimental work will be shown on a lithium niobate-based nested waveguide structure, using NIR (∼1.5 mum) pumps. Using low optical pump powers, particularly in comparison to other current approaches, we experimentally demonstrated for the first time all guided wave and continuously phase matched DFG of 1.325 THz light, with a power-normalized conversion efficiency 23 times larger than the best previously reported results. In addition, numerical simulations of a AlGaAs version of this guided wave THz generator show a conversion efficiency ∼2500 times larger than previously reported. |
