Abstract
Based on the quasi-phase matching (QPM) technique, the periodically poled Lithium niobate (PPLN) realized the quasi-phase matching condition in a nonlinear frequency conversion process. Nonlinear optical conversions from visible light wavelengths to near infrared wavelengths were widely developed by using this technique. From this point of view, the PPLN crystal could also used to generate terahertz wave. The thickness of PPLN crystal is limited by the poling technique. Therefore, the thickness of PPLN usually varies from 500μm to 1000μm. This kind of PPLN thicknesses forms a slab waveguide for THz wavelengths. Consequently, the multi-mode character shows up in such a PPLN THz waveguide. The main point of this dissertation is to understand the waveguide modes in a PPLN THz waveguide, and try to reduce the higher order modes to increase the conversion efficiency of the THz parametric generation (TPG). We used a 4cm long and 500μm thick PPLN crystal with grating period Λ=67μm. The PPLN crystal was pumped by a 1064-nm passive Q-switched Nd: YAG laser, which is capable of producing ~63-□J pulse energy. The THz wave can be monitored by measuring the idler spectrum in collinearly phase-matched TPG process. From the experimental results, the idler wavelength located around 1.067μm, and has several spikes in the spectrum. These spikes correspond to the modes of the THz wave in the multi-mode PPLN THz waveguide. To eliminate the mode numbers of the PPLN THz waveguide, several methods were used in this dissertation. One is to thin out the PPLN waveguide by a surface polishing technique; the other is to sandwich the THz PPLN waveguide in a pair of cladding materials, such as MgO: lithium niobate (LN) and congruent lithium niobate (CLN). For the THz PPLN waveguide, the waveguide confinement could strongly affect the TPG process. On the other hand, the modes having a better mode overlapping, higher THz gain, and lower loss, can grow up in the waveguide PPLN TPG. In this dissertation we used different kinds of design to do the experiments, and compared the experiment results with theoretical results. Finally, a 192μm thick PPLN crystal sandwiched between two CLN crystals gave a single (quasi-single) mode in the quasi-phase matched TPG process.