臺灣博碩士論文加值系統

一般常見產生兆赫波的方法是，將pump與signal光源垂直入射準相位匹配(Quasi-Phase-Matched, QPM)非線性晶體來產生兆赫波，而我們提出的方法是利用橫向偏振pump(signal)光源，以布魯斯特角入射準相位匹配(Quasi-Phase-Matched, QPM)光學接觸GaAs(optically-contacted GaAs, OC-GaAs)，利用差頻(Difference Frequency Generation, DFG)的非線性頻率轉換原理來產生兆赫波。希望能藉由這個方法，降低準相位匹配(Quasi-Phase-Matched, QPM)光學接觸GaAs(optically-contacted GaAs, OC-GaAs)接面處air-gap和端面處的反射損失，藉由提高pump與兆赫波的穿透率，來提高兆赫波的轉換效率。而pump(signal)入射角與極化方向對於有效非線性係數(effective nonlinear coefficient)的影響，也會在接下來的章節做說明。當pump(signal)以布魯斯特角73度入射到準相位匹配(Quasi-Phase-Matched, QPM)光學接觸GaAs(optically-contacted GaAs, OC-GaAs)時，必須要旋轉GaAs的晶格方向，使此晶格方向與pump(signal)極化方向之間的夾角為33.5度，才能得到最大的有效非線性係數。而準相位匹配(Quasi-Phase-Matched, QPM)光學接觸GaAs(optically-contacted GaAs, OC-GaAs)接面處air-gap和端面處所造成的反射損失，將會利用橫向偏振光測量1片和6片GaAs在不同入射角時的穿透率變化來做說明，並且在入射角為布魯斯特角時，air-gap和端面處所造成的反射損失將可以被消除。同時於此論文也利用非線性耦合方程式(nonlinear coupled wave equations)來計算理論的轉換效率，並且比較在垂直入射與布魯斯特角入射時，兆赫波轉換效率的差異。但在實際的實驗中，經過反覆的量測後，還是沒辦法量測到兆赫波的訊號。在確定實驗架構上沒有問題的情況下，思考了幾個實驗上可能的問題，例如pump(signal)的光強度，pump(signal)與兆赫波的交互作用長度 (interaction length) ，以及兆赫波的光束品質(M2)，發散角和全反射的問題，將會在本論文最後做一些討論，並提出一些建議來改善這些問題。

1 Introduction
1.1 Introduction

2 Pump Laser source
2.1 Introduction
2.2 The properties of EKSPLA Nd:YAG 1064 nm Laser
2.2.1 The structure of EKSPLA Nd:YAG 1064 nm Laser source
2.2.2 The pulse energy, pulse width, and repetition rate of EKSPLA Nd:YAG 1064 nm Laser
2.2.3 The beam quality M-squared parameter (M2) of EKSPLA Nd:YAG 1064 nm Laser
2.3 Optical Parametric Generator (OPG) and Optical Parametric Amplifier (OPA)
2.3.1 The performance of Optical Parametric Generator (OPG) and Optical Parametric Amplifier (OPA)

3 OC-GaAs with Brewster angle TM-wave pumping
3.1 Introduction
3.2 Tunable frequency region of THz wave by different period lengths
3.3 Transmission of GaAs at different incident angle
3.4 Effective nonlinear coefficient (deff) values at different incident angle in (1 1 0) GaAs wafer
3.4.1 Calculation of deff values
3.5 deff values and TM-polarized Pump wave at Brewster angle

4. Terahertz generation and optical loss in OC-GaAs with Brewster angle pumping
4.1 Introduction
4.2 Principle of THz generation
4.3 Propagation of pump wave and THz generation
4.4 Absorption in GaAs crystal
4.4.1 Water absorption of THz radiation
4.5 Nonlinear coupled wave equations on different frequency generation
4.5.1 Theoretical calculation result
4.6 THz generation experiment
4.7 Possible problems and discussion for my THz generation experiment
4.7.1 Pump and signal comparison with other papers
4.7.2 Pump, signal, and THz interaction length
4.7.3 THz beam quality (M2), divergence angle, and total internal reflection
4.7.4 Other loss and the detection limit of detector
4.7.5 Summary

5. Summary and Suggestions
5.1 Summary and Suggestions

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