兩種可產生較強兆赫波脈衝方法之研究

The Studies of Two Methods for Generation of Stronger Terahertz Pulses

鄭宸語(Cheng, Chen Yu)

指導教授 ： 潘犀靈

國立清華大學/電機資訊學院/光電工程研究所/碩士(2015年)

摘要

此工作中，我們研究兩種可產生較強兆赫波脈衝的方法:(1)傾斜脈衝前緣泵浦高摻雜濃度鈮酸鋰晶體與(2)在碲化鋅晶體中同調控制雙色光脈衝雷射激發之光整流效應。首先，以傾斜脈衝前緣泵浦5 mol% 鎂摻雜鈮酸鋰晶體化學計量配比之兆赫波源，用於時析兆赫光譜量測時，可有效提升次兆赫頻段之訊噪比，如量測聚四氟乙烯 (Teflon) 及石英玻璃的光學常數低至50 GHz。在0.11.5 THz範圍，我們發現兆赫波吸收在此晶體中比低摻雜濃度者沒有明顯增加，高摻雜濃度鈮酸鋰晶體更可減緩兆赫波轉換過程之飽和現象，以提升兆赫波之轉換效率。這些現象可解釋為由於高摻雜濃度導致較多的缺陷，導致較多的聲子被缺陷散射，減少兆赫波於晶體中傳播所產生的電磁極化子 (polariton) 透過受激放射聲子的過程被轉變成為聲子對，降低了兆赫波吸收而減緩了飽和作用。
其次，我們也研究了在碲化鋅晶體中同調控制雙色光脈衝雷射激發之光整流效應產生兆赫波的理論模型和實驗。我們觀察到由不同色光脈衝激發之兆赫波脈衝的相位反轉現象，以及不同能量比例與時間差之雙色光脈衝造成之光整流效應之變化。我們認為這主要是產生兆赫波之位移電流的機制，一般是由光子能量高於半導體能隙之脈衝激發後，由電磁波中的電場之偏振與頻率決定電荷在晶體結構的移動模式。用雙色光脈衝激發時的藍光 (400 nm，鈦藍寶石雷射的倍頻光) 之電場也會影響位移電流之產生，且兩種頻率之電磁波在晶格中有相反的非對稱特性，最終導致光整流效應中位移電流減弱。

關鍵字

兆赫波產生 ； 傾斜脈衝前緣 ； 鈮酸鋰晶體 ； 光整流效應 ； 光譜儀 ； 雙色光 ； 相位匹配 ； 石英玻璃 ； 聚甲基丙烯酸甲酯 ； 聚四氟乙烯 ； 高電阻係數矽晶片

並列摘要

In this work, we studied two methods to generate stronger THz pulses: (1) by tilted-pulse-front pumping in highly-doped stoichiometric lithium niobate (sLN) and (2) by optical rectification (OR) in ZnTe by using dual-color pulses. Firstly, we build up the system with 5 mol% Mg: sLN. When this unique source was applied in THz-TDS, it can increase signal-noise ratio in sub-THz region, such as the extraction of the optical constant of Teflon and fused silica to 50 GHz. Further, the saturation effect which limits efficiency of THz generation can be much more unobvious than from the sLN with small Mg doping level with negligible increase of THz absorption in 0.1-1.5 THz region. Our explanation is that more phonons are scattered by the more defects in the highly-doped crystal. However, THz phonon-polaritons decay into pair of phonons. But, the less number of the phonons decreases the decay rate of the THz polaritons.
Besides, to explore the possibility of enhancement in THz generation, we demonstrate the experimental and theoretical THz pulses generation by applying OR in ZnTe crystal semiconductor with coherent-controlled two-color laser. The phase inverse phenomenon of THz signals generated by fundamental wave (FW) and the second harmonic (SH) pulses can be observed. With different power ratios and delay times between two-color pulses, the competition of the THz generations between FW and SH pulses was revealed. The possible explanation of the competition is that the shift current which is one of the OR process is modulated by the different color pulse. Since the shift current is established by the excitation of the photons whose energy is above the energy bandgap of crystal and the state of the electromagnetic field of same frequency wave, the SH pulses whose photon energy is below energy bandgap also affect the generation of the shift current. Consequently, the shift current is reduced by the different color pulses.

並列關鍵字

Terahertz generation ； tilted-pulse-front ； lithium niobate ； optical rectification ； spectroscopy ； dual-color ； phase-matching ； fused silica ； PMMA ； Teflon ； high-resistivity silicon wafer

參考文獻

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