Study of Terahertz Pulse Generation due to Coherent Phonon-Polariton Effect in ＜110＞ ZnTe Crystal

利用同調聲子-極化子在碲化鋅＜110＞方向 產生兆赫脈衝輻射之研究

杜建明(Chien-Ming Tu)

指導教授 ： 齊正中

國立清華大學/理學院/物理學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00365

摘要

In this dissertation, we investigate the phenomenon and mechanism of Terahertz (THz) Electro-Magnetic pulse generation due to coherent phonon-polariton excitation by propagating ultrafast optical pulses in <110> ZnTe crystal. The phonon-polariton is the hybrid mode of THz photon and TO phonon of ZnTe, which is excited through nonlinear processes involving Optical Rectification (OR) and Impulsive Stimulated Raman Scattering (ISRS). Due to strongly coupling between THz photon and TO phonon of ZnTe, the THz generation process is not simply due to OR only, which gives rise to the tale-telling two-to-three-cycle THz waveform. In addition, we have observed a long duration quasi-monochromatic damped oscillations (QMDO) following the typical THz pulse shape.
All theoretical calculations are based on the Non-Linear Wave Conversion (NLWC) without Slowly Varying Envelope Approximation (SVEA) for THz wave, considering pump non-depletion approximation and the case of planar wave in semi-infinite slab. We also show the calculated THz waveforms and spectrums in various conditions, such as different optical central wavelengths ( = 750 and 800 nm), optical pulse durations ( = 50 to 800 fs) and optical pulse’s traveling distance inside ZnTe, (i.e. the thickness of ZnTe ranging from 0.0 to 4.0 mm). The calculated THz waveforms (excited by both = 750 and 800 nm) are composed of a main few-cycle THz and QMDO when the QMDO meets the phase-matching conditions. With the broadening of optical pulse, the QMDOs shrink in time domain and only the main THz remain.
We construct the system of THz generation and detection by ultrafast optical pulse. <110> ZnTe crystals are employed as THz emitter and Electro-Optic Sampling (EOS) detector. The measured THz waveforms are very agreement with our theoretical calculations. By varying the thickness of both ZnTe emitter and EOS detector, we rule out the possibility of etalon effect due to thin ZnTe slab for the QMDOs. The dependence of optical-pulse-duration for the QMDOs is also observed in our experimental results. By comparing with the phonon-polariton dispersion of ZnTe, we have confirmed that the QMDOs are the results from coherent phonon-polariton excitation through the phase-matching condition.

關鍵字

兆赫 ； 碲化鋅 ； 超快光學 ； 聲子-極化子 ； 光譜學

並列摘要

在本論文中，我們以超快光脈衝(Ultrafast optical pulse)照射在碲化鋅<110>單晶(<110> ZnTe)，產生兆赫電磁脈衝(Terahertz Electro-Magnetic pulse)並研究同調聲子-極化子(Coherent phonon-polariton)對兆赫電磁脈衝的影響。聲子-極化子為兆赫波光子(Terahertz photon)與碲化鋅之橫向光模聲子(TO phonon)的混合態，而其乃透過光整流(Optical Rectification: OR)與Impulsive Stimulated Raman Scattering (ISRS)所激發。由於兆赫波光子與碲化鋅之橫向光模聲子之間的耦合，僅光整流效應不足以解釋兆赫波的產生-過去普遍認為透過光整流效應會產生僅幾個週期的兆赫波脈衝(Few cycle THz pulse)。在實驗中，我們所觀察到的兆赫波的波形，除了常見的few cycle THz pulse之外，還有長時間的近似單頻震盪(Quasi-monochromatic damped oscillations: QMDO)緊接其後。
在理論計算中，我們由非線性光轉換(Non-Linear Wave Conversion)的模型(Model)出發並考慮”無激發光耗損”(Pump non-depletion approximation)。對於兆赫波，我們不考慮非線性光學中常用的”緩慢變化包絡近似”(Slowly Varying Envelope Approximation: SVEA)但假設兆赫波以平面波的形式在半無限長的晶體(Semi-infinite slab)裡傳播。我們亦計算了在不同的光中心波長( = 750 and 800 nm)、不同的光脈衝寬度( = 50 to 800 fs)以及不同的晶體厚度(0.0 to 4.0 mm)之下的兆赫波波形與頻譜。所計算兆赫波波形皆由few cycle THz pulse與QMDO所組成，而且QMDO滿足相位匹配(Phase-matching)。我們亦發現，當光脈衝變寬時，QMDO會縮減、消失而且僅剩下few cycle THz pulse殘存。
在實驗方面，我們架設了兆赫波產生-偵測系統(THz generation-detection)並以碲化鋅<110>單晶作為電光取樣(Electro-Optic Sampling: EOS)偵測晶體。在實驗中，所量測到的兆赫波波形與理論計算的波形非常吻合。對於實驗所量測的QMDO，藉由改變碲化鋅<110>單晶的厚度，我們排除了由於有限的晶體厚度所造成的Etalon效應。而QMDO對光脈衝寬度的相依性，亦在實驗中得到了證實。藉由比較碲化鋅的聲子-極化子的色散關係(Phonon-polariton dispersion)，我們確認QMDO是由透過相位匹配所產生的同調聲子-極化子所造成的結果。

並列關鍵字

Terahertz ； ZnTe ； Ultrafast optics ； Phonon-polariton ； Spectroscopy

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