週期性晶格反轉鈮酸鋰實現了有效率的準相位匹配非線性混頻轉換過程，各種波長在可見光至近紅外光的光參數過程拜此技術所賜，已有廣泛的應用。就技術層面而言，週期性晶格反轉鈮酸鋰晶體亦可應用在兆赫波的產生，也就是。因為週期性晶格反轉鈮酸鋰晶體的厚度受製程限制，通常是五百微米到一千微米之間，這種厚度對於所產生的兆赫波而言是一簡易的平板波導，因此產生的兆赫波應具有多模態的特性。本篇論文的重點即在觀察在平板波導中準相位匹配兆赫波參數產生的模態結構，並設法減少高階模態的出現，以提升此兆赫波參數產生的效率。
實驗中我們使用了波長為1064奈米、脈衝能量約為63微焦耳的被動式Ｑ開關Nd: YAG雷射來激發一塊四公分長、週期67微米、厚度500微米的週期性晶格反轉鈮酸鋰晶體，以進行準相位匹配的兆赫波參數產生。藉由idler頻譜的量測，可以確定兆赫波的產生，並且了解波導兆赫波參數產生的模態結構。實驗結果顯示idler波長約在1.067微米附近，頻譜上也可以觀察到許多尖峰，這些尖峰就對應到多模模態兆赫波的每個模。為了降低這些模態的個數，我們使用了下列幾種方式：一、利用平面拋光的技術來使週期性晶格反轉鈮酸鋰波導變薄。二、利用參雜氧化鎂的鈮酸鋰作為包覆層，黏合在週期性晶格反轉鈮酸鋰上下兩面。三、使用單晶鈮酸鋰黏合在週期性晶格反轉鈮酸鋰的上下兩面作為包覆層。
這些方法的目的在於限制波導的傳導，因為波導的傳導限制會強烈的影響參數產生中的交互作用，只有符合相位匹配以及與混合波重疊良好的模態才會被波導的傳導限制所增強，因此可以達到減少模態的目的。在論文中我們針對不同的設計，進行實驗和理論的交互比對，並使用不同厚度的週期性晶格反轉鈮酸鋰反覆試驗，最後以190微米的厚度配合上述的第三個方法，達到接近單模輸出的準相位匹配的兆赫波參數產生。

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.

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