| 题名 | 超短脉冲激发的气体等离子体产生太赫兹辐射的研究 |
| 作者 | 白亚 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 刘鹏 |
| 关键词 | 周期量级脉冲，载波包络相位，太赫兹辐射，Gouy相移 |
| 其他题名 | Terahertz Generated from Gas Plasma Induced by Ultrashot Intense Laser Pulse |
| 中文摘要 | 随着飞速发展的超短脉冲激光技术和参量放大技术，可轻松获得飞秒时间尺度，毫焦耳能量，波长可调谐的超快光源。通过非线性过程对光谱的展宽和啁啾色散补偿的脉宽压缩，可使脉冲包络内只包含少数个光周期振荡。超短脉冲光场激发的非线性光学过程被广泛研究。特别是近几十年兴起的太赫兹辐射的研究工作。双色场或周期量级光场因为自身具有的光场非对称特征，可以通过电离过程将驱动光频率下转换至太赫兹波段。对于周期量级光场，载波包络相位（CEP）的概念与光场的非对称性特征关联，是描述光场的重要参量。太赫兹辐射产生过程由于依赖于光场的非对称特征，因而可以应用于CEP测量。与固体为媒介产生太赫兹辐射的方法相比，等离子体成丝方法因不存在损伤阈值的限制，可以利用较高的激光能量泵浦，提高太赫兹产生的强度。 本文的主要研究工作关注超短脉冲激发气体等离子体的太赫兹辐射产生。基于实验室的钛宝石激光放大系统，和自建的载波包络相位稳定的红外超短脉冲激光系统（中心波长1.8μm；脉宽~9fs，约1.5个光周期），对太赫兹的产生特性和机理进行了深入研究。主要成果有： 1. 实验研究了周期量级脉冲激发的太赫兹辐射产生。发现了太赫兹波形随初始CEP的变化以2π为周期有规律的改变。采用正交偏振的电光取样方法，获得太赫兹波形。探测波形只依赖于太赫兹的极性而与电光晶体的放置角度无关。提出了可以直接通过太赫兹波形极性和幅度来确定光场CEP的方法。 2. 基于CEP被动稳定周期量级激光脉冲进行波形可控太赫兹辐射的产生。太赫兹的波形不仅可以通过调节脉冲的初始CEP来控制，还可以通过改变丝长度的办法控制。 3. 利用传输模型的计算指出，太赫兹波形变化的原因来自载波相位移动和传输过程中变化的包络对载波相位的调制。提出了在色散介质中传播光场的修正Gouy相移的概念，解释了其物理本质，并首次揭示了等离子体光丝中各种相位变化的起因。并且对Gouy相移和等离子体色散分别对CEP的贡献进行了阐明。 4. 双色场产生太赫兹的极性依赖于双色场的相对相位。通过对聚焦条件的调整和利用传输非线性色散的作用对双色场相对相位进行调制。模型计算得到了双色场相移变平的条件，以及脉冲能量增加对相移的影响。随后利用相位变平缓的条件，得到了太赫兹发生极性反转的失相长度增长，最大长度近三倍于相移线性变化情形。对远场太赫兹辐射的计算，得到在满足相位变平条件下，远场太赫兹辐射在光轴前向的分布得到增强。 |
| 英文摘要 | As the rapid development of ultrafast laser technology and optical parametric amplification (OPA), the wavelength tunable pulse with duration of femotosecond time scale and energy up to several milli-Joule could be obtained. Through nonlinear process for spectra broadening and chirp compensation, few-cycle pulse could be obtained. Ultrafast laser pulse induced nonlinear process was widely researched, especially the emerging field of terahertz (THz) radiation in the last decades. Due to the asymmetry of two-color field or few-cycle pulse, THz is generated by strong field ionization process due to nonlinear frequency down conversion process. For laser pulse only has a few optical oscillations its carrier-envelop phase (CEP), associates with field asymmetry, becomes an important parameter. While generated THz radiation depends on field asymmetry, which allows determination of CEP from both emission field polarity and amplitude. THz field with high intensity is possible from plasma based source, which has no limitation of damage threshold. In the present thesis, we focus on THz radiation from the plasma filament produced by ultrashot laser pulse. The feature and mechanism of THz generation was studied based on Ti:shapphire laser amplifier facility and home-built CEP stabilized infrared few-cycle laser system, pulse duration ~8.4fs, about 1.5 optical cycles, at center wavelength 1.8μm. The main innovative results listed as follows: 1. We experimentally studied THz radiation pumped by few-cycle laser pulse. We observed that the THz waveforms are dependent on the CEP periodically, with period equal to π. Crossed polarizes electro-optic (EO) sampling geometry is applied to record temporal waveforms of far field THz emission. The measured signal polarity is only dependent on the THz field with a freely orientated EO crystal, which allows directly determination of the CEP by measurement of the amplitude and polarity of the THz pulse. 2. Waveform-controlled THz radiation is generated from air filament produced by CEP stabilized few-cycle laser pulse. The generated waveform could be controlled by varying the filament length and the initial CEP of driving field. 3. The pulse propagation model in transparent media is used in model calculation. The waveform evolution of THz radiation in the filament is due to the carrier phase shift and variation of pulse envelope during propagation. We propose the concept of modified Gouy phase shift for propagating few-cycle pulse in dispersion media. The origins of various phase shifts in the filament are elucidated. The contribution of Gouy phase shift and plasma dispersion to CEP is elucidated. 4. Pumped by two-color field generated THz field polarity dependent on relative phase between two frequencies. By adjusting the focusing geometry and nonlinear dispersion the relative phase of two-color field was modulated. We found that the relative phase flattens by the plasma formation and the conditions of focusing geometry. The dephasing length of THz fields that maintaining its polarity is elongated. Under the circumstance of phase flatten, with maximum dephasing length almost three times as large as usual cases. The far field THz radiation is enhanced near the propagation axis. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15753]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 白亚. 超短脉冲激发的气体等离子体产生太赫兹辐射的研究[D]. 中国科学院上海光学精密机械研究所. 2013. |
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