【作者】 李敏；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 太赫兹波由于其低光子能量,能够携带丰富的物理化学信息,以及皮秒量级脉宽和短周期量级引起了研究者的广泛关注。太赫兹波是频率范围在0.1～10THz,波长范围在0.03～3毫米,介于微波与红外之间的电磁波。众所周知,双色飞秒脉冲在空气中传输时,由于克尔自聚焦和等离子体自散焦的动态平衡所产生的光丝通道是一种有效的太赫兹波辐射源,在产生过程中,有效的控制所产生的太赫兹波的强度以及偏振一直是人们关注的问题。飞秒光丝产生的分子排列引起了分子非线性特性的改变,因此可以改变空气中的非线性过程,是一种调控光丝中非线性过程的重要手段。我们将分子排列过程引入到太赫兹的产生过程中,实现对太赫兹强度以及偏振的调控。本论文主要以空气中的分子排列为研究手段,对双色飞秒脉冲产生太赫兹辐射的强度以及偏振受分子排列的影响进行了深入而细致的研究工作,同时对光丝中的非线性过程受分子排列的影响进行了分析。具体的工作包括：1.实验上提出了一种简单有效的方法来判断双色飞秒脉冲成丝辐射太赫兹波的物理机制。该方案通过准确判断双色飞秒脉冲之间的相位差,将产生的太赫兹波的强度与双色飞秒脉冲相位差的函数关系呈现出来,表明太赫兹波的电场强度与双色飞秒脉冲的相位差的正弦成正比关系,从而证明了双色飞秒脉冲成丝产生太赫兹波的物理机制是光电流模型。这对于进一步寻找高功率太赫兹波辐射源有着重大的意义。2.实验上实现了一种基于分子排列的太赫兹产生控制方法。分子排列效应改变了气体分子的折射率,对于后续脉冲来说,平行的分子排列使得气体折射率增大,起到凸透镜的作用,使经过它的后续脉冲进一步聚焦,电离增强,辐射的太赫兹强度增强；垂直的分子排列使得气体折射率减小,起到凹透镜的作用,使经过它的后续脉冲的聚焦作用被平衡掉一部分,电离减弱,辐射的太赫兹强度降低。另一方面,飞秒激光导致的分子排列引起空气分子非线性性质的改变,进而影响了双色飞秒脉冲在空气分子中传播时的相速度,使得双色飞秒脉冲的相位差发生改变,对所产生的太赫兹波的强度以及偏振进行了有效的控制。因此,飞秒激光诱导的分子排列可以做为一种瞬时的太赫兹波波片,有效调控所产生的太赫兹波的偏振状况,使我们从实验上既可以得到椭圆偏振的太赫兹波,也可以探测到线偏振的太赫兹波。3.实验上实现了一种基于等离子体和外加偏置静电场的太赫兹控制手段。在双色飞秒脉冲形成的等离子体光丝两侧加入偏置静电场,使得等离子体光电流增强,增强了太赫兹辐射。利用泵浦光脉冲引入等离子体,然后加入外加电场,既可以实现对太赫兹强度的控制,也可以对产生的太赫兹偏振进行调节。因此,这两种方法都可以做为增强太赫兹辐射的有效方法。更多还原

【Abstract】 It has attracted more and more research interest in terahertz generation with its low photon energy, abundant physical and chemical information, picosecond pulse-width and short few cycle pulse. The terahertz wave is between microwave and infrared wave with its frequency0.1-10THz, and the wavelength0.03-3mm. As known, the counterbalance between the Kerr self-focusing and the plasma self-defocusing effects produced the femtosecond dual-color pulse filament, which could be used as an efficient terahertz radiation source. During this process, it is quite necessary to find efficient method to control the terahertz intensity and polarization. Previous research has shown that the molecular alignment induced by the femtosecond pulse could modify its nonlinearity, and further adjust the nonlinear process conveniently.In this dissertation, based on the molecular alignment, we investigate the modulation of the terahertz intensity and polarization and the nonlinear process during the plasma channel. Mainly includes the following:1. Experimentally demonstrate the physical mechanism of the terahertz generation derived from the dual-color pulse. This project measures the phase difference of the dual-color pulse accurately, and shows the function relationship between the terahertz electric field and the dual-color pulse phase difference. It proves that the physical mechanism of the terahertz radiation can be explained by the photo current model as the terahertz electric field is proportional to the sin θ in our results, where θ is the phase difference of the dual-color pulse. It has important significance to find higher power terahertz radiation source.2. The terahertz generation from dual-color pulse filament was controlled by molecular alignment induced (de)focusing effect. Molecular alignment effect modulated the refractive index of the air. For the subsequent dual-color pulse, parallel molecular alignment increased the refractive index acting as a positive lens to increase the ionization of the dual-color pulse, and the terahertz generation increased. On the contrary, the perpendicular molecular alginment decreased the refractive index acting as an negitive lens for the subsequent dual-color pulse. The ionization of the dual-color pulse was decreased and the terahertz generation decreased. On the other hand, the molecular alignment leads to the change of the nonlinearity, and the phase velocity also changed during the propagation process, which adjusts the dual-color pulse phase difference and further controls the intensity and polarization of the generated terahertz. Both linearly and elliptically polarized terahertz radiations can be obtained as the field-free molecular alignment in air functioned as a transient dynamic wave plate.3. The pre-existing plasma and a biased DC electric field was experimentally used to control the intensity and the polarization of the THz radiation. The biased DC electric field increased the plasma photo current to produce stronger terahertz radation. The pre-existing plasma controled the intensity and the polarization by the additional refractive index induced by its presence. These provided efficient methods to improve the terahertz generation.更多还原