【作者】 王东；

【导师】 刘劲松；

【作者基本信息】 华中科技大学 ， 光学工程， 2014， 博士

【摘要】 太赫兹大气光子学技术近些年蓬勃发展并且逐渐成熟起来。能够描述太赫兹波脉冲的物理量得到了十分广泛的研究,然而对太赫兹偏振态的研究工作却十分稀少。双色飞秒激光激发大气等离子体能够产生线偏振的太赫兹波。可以通过改变双色飞秒激光之间的时域延时来进行对太赫兹波偏振方向的相干控制。这个对太赫兹偏振方向角的相干控制的现象吸引了众多科研工作者的兴趣。本论文从两个方向出发来解释太赫兹波偏振方向的相干控制现象。一个是从宏观的角度来分析这个问题,另一个是从微观的角度来建立物理模型。这两个思路得到了统一的解析表达式来解释上述关于偏振方向相干控制的实验现象。在宏观的角度上,发展了一套分析同轴转动物理系统的新方法,并发现了同轴转动角系统的守恒特性。用这个守恒特性十分成功地分析了双色光激发大气等离子体产生太赫兹波的偏振控制现象。最终发现对太赫兹波偏振方向进行相干控制的不是时域延时,也不是基频光与倍频光的相对相位,而是基频光与倍频光的在光路方向上的相对转动角。当基频光与倍频光均是右旋偏振态的时候,基频光与倍频光在光轴方向上的相对转动角,表现为基频光与倍频光的相对相位,进而表现为基频光与倍频光的时域延时,因为这时这三者之间是正比关系。当基频光与倍频光均是线偏振态的时候,基频光与倍频光在光轴方向上的相对转动角,表现为基频光与倍频光的相对偏振方向角,这时的相对转动角与相对相位以及时域延时没有相关性。在微观的角度上,立足于量子光学的理论,建立了七能级系统的物理模型,阐述了双色激光的相对相位对所产生的太赫兹波偏振方向相干控制的微观机理。这个七能级系统是由两个拥有同一个基态的四能级系统组成的。其中一个四能级系统产生右旋态的太赫兹波光子,另一个四能级系统产生左旋态的太赫兹波光子。太赫兹光子最终的偏振态将由这两个自旋态光子的相干叠加所决定。采用光子产生算符的平均值来反映这种量子态叠加过程中的量子幅。最后得到了与宏观分析一致的解析表达式。随后用同轴转动角系统的守恒特性分析了矢量叠加的一些性质。从侧面证明了该守恒特性的正确性。最后用同轴转动角系统的守恒特性分析了双螺旋电极产生椭圆偏振的太赫兹波的过程。并基于同轴转动角系统的守恒特性对该实验给出了一个指导性方案。更多还原

【Abstract】 These years the technology of the THz air photonics has developed and is maturing. The physical quantities to describe the THz wave pulse have got the very extensive research. However the study about THz polarization is scarce. Linear polarized THz wave can be obtained from a gas plasma channel induced by two-color femtosecond laser pulses. The azimuthal orientation of the THz polarization can be coherent controlled by changing the optical delay of the two-color femtosecond lasers. This phenomenon about the coherent controllable THz polarization orientation angle has drawn much interest of the science researchers.This dissertation presents two way to explain the phenomenon about the coherent controlled THz polarization orientation angle. One way is to analyze the problem in macroscopic view, and the other way is to construct a physical model in microscopic view. From the two views the unification analytical expression is obtained to explain the phenomenon about the coherent controllable THz polarization angle in the end.In macroscopic view, a new method has been developed to analyze the physical system of turning by the same axis, and the conservation property of the azimuthal orientation angles turning by the same axis of the system has been found. With this conservation property it has been sucsessfully analyzed that the phenomenon of the controllable THz polarization generated from two-color laser-induced gas plasma. And then it has been found that the reason about the coherent controlled THz polarization is neither the optical delay, nor the relative phase of the two-color lasers. The reason is the relative rotation angle of the two-color lasers in the optical direction. When the fundamental light and the second-harmonic light are both right circularly polarized, the relative rotation angle of the two-color lasers in the optical direction is equivalent to the optical delay and then be equivalent to the relative phase, because these three physical quantities at this time are proportional to each other. However, when the fundamental light and the second-harmonic light are both linear polarized, the relative rotation angle is equivalent to the relative polarization angle of the two-color lasers, because the relative rotation angle has nothing to do with the optical delay or the relative phase. In microscopic view, based on the quantum optics, a seven-level quantum model has been constructed to state the microscopic mechanism of the phenomenon of the controllable THz polarization generated from two-color laser-induced gas plasma. This seven-level system is constructed of two four-level systems, while the two four-level systems share the same ground state. One four-level system is to generate the right-circularly polarized THz photon, and the other is to generate the left-circularly polarized THz photon. At last the polarization of the THz photon is the superposition of the two processes. We ues the average of the creation operator for photon to reflect the quantum amplitude of the superposition of the quantum state. In the end the unification analytical expression with the macroscopic view is obtained.Then the conservation property of the azimuthal orientation angles turning by the same axis of the system has been used to analyze the property of the vector superposition. In another way, the conservation property has been proved to be correct in this aspect.At last the conservation property has been used to analyze the generation of elliptically polarized THz wave with double helix electrodes. Based on the conservation property, guidance programmes for the experiment has been provided.更多还原