新型宽带太赫兹源物理机制的研究
摘要
太赫兹(Terahertz)科学技术是当前科学研究领域的热点方向。太赫兹科学在通信、传感、成像和基础科学方面有广泛的应用前景。利用飞秒(femtosecond)激光脉冲泵浦非线性光学介质的宽带太赫兹辐射源是一种重要的太赫兹辐射方法，已经得到了很多研究小组的关注。本篇论文主要研究几种新型宽带太赫兹辐射源的物理机制。
在分析非线性过程时，在不同的偏振方向物理过程不完全一致，所以测量太赫兹脉冲的完整矢量信息对分析非线性光学物理过程是很重要的。本文中发展了太赫兹二维电光取样的实验方法，利用探测晶体对于太赫兹偏振的依赖特性，分别测量正交偏振的太赫兹电场矢量，可以重构太赫兹脉冲的瞬时电场。重构瞬时太赫兹电场矢量对于非线性过程的矢量分析有重要的意义。
利用飞秒激光泵浦非线性晶体可以得到超宽带太赫兹脉冲，而且实验装置简单，是应用广泛的太赫兹光源。但是晶体在太赫兹波段的非线性光学性质还没有详细的研究，制约了这种太赫兹光源技术的发展。本文基于二维偏振太赫兹时域光谱技术，研究了非线性光学晶体在太赫兹频段的二阶非线性光学特性。并且使用BBO (β-Barium Borate)晶体为例，测量了BBO晶体在太赫兹频段的非线性光学特性，并得到重要结论，在紫外和可见光波段起重要作用的非线性系数d22，在太赫兹波段几乎可以忽略。这为以后优化晶体在太赫兹波段的能量转化效率、相位匹配条件提供了基础。
基于飞秒激光诱导等离子体辐射太赫兹脉冲的方法可以产生电场强度为400kV/cm，带宽为35THz的强电场、超宽带太赫兹脉冲。这种太赫兹辐射源为太赫兹波谱技术和基础物理学等领域提供了高强度的光源。基于空气等离子体产生和探测方法可以实现太赫兹远程传感，是这种太赫兹光源的另一个重要研究方向。飞秒激光在空气中传播因为克尔透镜效应电离空气形成一条等离子通道，这种现象称为飞秒激光成丝效应。飞秒激光成丝向前辐射太赫兹脉冲，这种太赫兹辐射源很好的解决了太赫兹远程传感的技术难题。但这种辐射源的内在物理机制还没有明确。本文基于二维偏振太赫兹光谱，研究了成丝产生太赫兹波的偏振态。在实验上首次发现了成丝产生椭圆偏振的太赫兹脉冲，相应的理论分析揭示了飞秒激光成丝辐射太赫兹脉冲的主要机制是光丝中的四波混频机制。
双色激光电场电离空气辐射太赫兹脉冲被认为是一种高能量、超宽带的太赫兹辐射源，这种新型太赫兹辐射源有很大的应用潜力。但这种辐射源的物理产生机制还没有达到统一的共识。本文基于二维太赫兹偏振光谱，系统研究了不同实验条件下太赫兹偏振态变化，澄清了双色场辐射太赫兹的主要机制是四波混频，并首次证明了空气等离子体中辐射太赫兹脉冲的偏振态为线偏振，并发现了交叉项χxyxy的重要作用。这项成果对于优化这种新型太赫兹辐射源的能量转化效率有重要的指导意义。
Terahertz science and technology is an attractive research topic nowadays. Theterahertz science has wide range application on communication, sensing, imagingand fundamental science research areas. The broadband terahertz radiation source isnormally achieved by pumping nonlinear optical media with the femtosecond laserpulse, which has attracted considerable interest worldwidely. The dissertationfocuses on the physical mechanism of several broadband terahertz radiation sources.
The nonlinear property of medium can be expressed by a nonlinearsusceptibility tensor. In different polarization orientation, the medium experiencesdifferent physical processes. Hence, the reconstruction of complete terahertz vectoris important to understand the physical mechanism. The two-dimensionalpolarization terahertz electro-optics sampling technique is developed in the thesis.The detection crystal is sensitive to the terahertz polarization, which is feasible tomeasure the orthogonal polarized terahertz electric field individually. Based on thenew setup, the accurate and complete terahertz instantaneous electric field can bereconstructed, which is used to vector analyse the nonlinear process. Thetwo-dimensional polarization terahertz electro-optics sampling setup is the keyexperimental tool in this dissertation.
Terahertz radiation based on the rectification effect in nonlinear crystal, thathasthe advantage of simple experimental setup and ultra-broadwidth terahertz pulse, iswidely-used terahertz radiation source. However, the nonlinear optics property hasnot been fully investigated, which limits the development of this field. In the thesis,the second order nonlinearity tensor in terahertz region is investigated based on the2D polarization sampling technique. In order to demonstrate the new method, thenonlinearity tensor of BBO (β-Barium Borate) crystal is systematically studied. Thesusceptibility coefficient d22, which has dominamt role in the visible and IR region,can be neglected in terahertz region. It helps to optimize the energy conversionefficiency and phase matching condition in the future research.
The terahertz pulse radiated from the air plasma can achieve400kV/cm electricfield and35THz bandwidth. This new type terahertz source has potential applicationon the advanced spectroscopy technique, and fundamental physical research. Thegeneration and detection both in the air plasma, which can be applied in terahertzremote sensing, is also an attractive topic in this field. The terahertz radiation fromfemtosecond laser filamentation shows a great potential in terahertz remote sensing.This type of terahertz pulse source is investigated experimentally in the dissertation.With the2D polarization terahertz time domain spectroscopy, the polarization fromthe filmentation is investigated in the thesis. The ellipitical polarization terahertzpulse from femtosecond filamentaton is first time observed. After the analysis, thefour wave mixing effect is revealed to be the reason for the terahertz pulse radiationfrom filamentation.
The terahertz radiation induced by the two-color laser field scheme isconsidered as a reliable high-power, broadband terahertz source. However, theunderlying physical mechanism is not in agreement yet. The mechanism of thismethod is systematically studied. The instantaneous terahertz electric field isreconstructed by the two-dimensional electrooptics sampling method. Our workclarifies and extends the understanding of the four wave mixing model in the2-colorscheme.The linear polarization is first time observed in the experiment, and the crossterm χxyxyis proved to be the most dominant term in the third-order susceptibilitytensor,which is meaningful to improve energy conversion efficiency of the new-typeterahertz radiation source.
在分析非线性过程时，在不同的偏振方向物理过程不完全一致，所以测量太赫兹脉冲的完整矢量信息对分析非线性光学物理过程是很重要的。本文中发展了太赫兹二维电光取样的实验方法，利用探测晶体对于太赫兹偏振的依赖特性，分别测量正交偏振的太赫兹电场矢量，可以重构太赫兹脉冲的瞬时电场。重构瞬时太赫兹电场矢量对于非线性过程的矢量分析有重要的意义。
利用飞秒激光泵浦非线性晶体可以得到超宽带太赫兹脉冲，而且实验装置简单，是应用广泛的太赫兹光源。但是晶体在太赫兹波段的非线性光学性质还没有详细的研究，制约了这种太赫兹光源技术的发展。本文基于二维偏振太赫兹时域光谱技术，研究了非线性光学晶体在太赫兹频段的二阶非线性光学特性。并且使用BBO (β-Barium Borate)晶体为例，测量了BBO晶体在太赫兹频段的非线性光学特性，并得到重要结论，在紫外和可见光波段起重要作用的非线性系数d22，在太赫兹波段几乎可以忽略。这为以后优化晶体在太赫兹波段的能量转化效率、相位匹配条件提供了基础。
基于飞秒激光诱导等离子体辐射太赫兹脉冲的方法可以产生电场强度为400kV/cm，带宽为35THz的强电场、超宽带太赫兹脉冲。这种太赫兹辐射源为太赫兹波谱技术和基础物理学等领域提供了高强度的光源。基于空气等离子体产生和探测方法可以实现太赫兹远程传感，是这种太赫兹光源的另一个重要研究方向。飞秒激光在空气中传播因为克尔透镜效应电离空气形成一条等离子通道，这种现象称为飞秒激光成丝效应。飞秒激光成丝向前辐射太赫兹脉冲，这种太赫兹辐射源很好的解决了太赫兹远程传感的技术难题。但这种辐射源的内在物理机制还没有明确。本文基于二维偏振太赫兹光谱，研究了成丝产生太赫兹波的偏振态。在实验上首次发现了成丝产生椭圆偏振的太赫兹脉冲，相应的理论分析揭示了飞秒激光成丝辐射太赫兹脉冲的主要机制是光丝中的四波混频机制。
双色激光电场电离空气辐射太赫兹脉冲被认为是一种高能量、超宽带的太赫兹辐射源，这种新型太赫兹辐射源有很大的应用潜力。但这种辐射源的物理产生机制还没有达到统一的共识。本文基于二维太赫兹偏振光谱，系统研究了不同实验条件下太赫兹偏振态变化，澄清了双色场辐射太赫兹的主要机制是四波混频，并首次证明了空气等离子体中辐射太赫兹脉冲的偏振态为线偏振，并发现了交叉项χxyxy的重要作用。这项成果对于优化这种新型太赫兹辐射源的能量转化效率有重要的指导意义。
Terahertz science and technology is an attractive research topic nowadays. Theterahertz science has wide range application on communication, sensing, imagingand fundamental science research areas. The broadband terahertz radiation source isnormally achieved by pumping nonlinear optical media with the femtosecond laserpulse, which has attracted considerable interest worldwidely. The dissertationfocuses on the physical mechanism of several broadband terahertz radiation sources.
The nonlinear property of medium can be expressed by a nonlinearsusceptibility tensor. In different polarization orientation, the medium experiencesdifferent physical processes. Hence, the reconstruction of complete terahertz vectoris important to understand the physical mechanism. The two-dimensionalpolarization terahertz electro-optics sampling technique is developed in the thesis.The detection crystal is sensitive to the terahertz polarization, which is feasible tomeasure the orthogonal polarized terahertz electric field individually. Based on thenew setup, the accurate and complete terahertz instantaneous electric field can bereconstructed, which is used to vector analyse the nonlinear process. Thetwo-dimensional polarization terahertz electro-optics sampling setup is the keyexperimental tool in this dissertation.
Terahertz radiation based on the rectification effect in nonlinear crystal, thathasthe advantage of simple experimental setup and ultra-broadwidth terahertz pulse, iswidely-used terahertz radiation source. However, the nonlinear optics property hasnot been fully investigated, which limits the development of this field. In the thesis,the second order nonlinearity tensor in terahertz region is investigated based on the2D polarization sampling technique. In order to demonstrate the new method, thenonlinearity tensor of BBO (β-Barium Borate) crystal is systematically studied. Thesusceptibility coefficient d22, which has dominamt role in the visible and IR region,can be neglected in terahertz region. It helps to optimize the energy conversionefficiency and phase matching condition in the future research.
The terahertz pulse radiated from the air plasma can achieve400kV/cm electricfield and35THz bandwidth. This new type terahertz source has potential applicationon the advanced spectroscopy technique, and fundamental physical research. Thegeneration and detection both in the air plasma, which can be applied in terahertzremote sensing, is also an attractive topic in this field. The terahertz radiation fromfemtosecond laser filamentation shows a great potential in terahertz remote sensing.This type of terahertz pulse source is investigated experimentally in the dissertation.With the2D polarization terahertz time domain spectroscopy, the polarization fromthe filmentation is investigated in the thesis. The ellipitical polarization terahertzpulse from femtosecond filamentaton is first time observed. After the analysis, thefour wave mixing effect is revealed to be the reason for the terahertz pulse radiationfrom filamentation.
The terahertz radiation induced by the two-color laser field scheme isconsidered as a reliable high-power, broadband terahertz source. However, theunderlying physical mechanism is not in agreement yet. The mechanism of thismethod is systematically studied. The instantaneous terahertz electric field isreconstructed by the two-dimensional electrooptics sampling method. Our workclarifies and extends the understanding of the four wave mixing model in the2-colorscheme.The linear polarization is first time observed in the experiment, and the crossterm χxyxyis proved to be the most dominant term in the third-order susceptibilitytensor,which is meaningful to improve energy conversion efficiency of the new-typeterahertz radiation source.
引文
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