【作者】 吴健；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2007， 博士

【摘要】 研究和控制发生在极短时间内的超快过程，例如化学反应、生物信息传递、电子跃迁等，既是超快物理研究的核心内容，也是人们认识自然界的有效途径。自从飞秒激光脉冲问世以来，飞秒科学技术已经在物理学、生物学、化学控制反应、光通讯等领域中得到了广泛应用。为了研究更快的过程，例如内核电子的运动过程，我们需要更短的时间标尺、更快的探测手段。突破飞秒极限，亚飞秒脉冲将帮助我们探测和控制发生在阿秒时间尺度的物理过程，是推动人们对自然界深入理解必不可少的工具。目前，主要有三种途径可以获得亚飞秒脉冲。利用近红外光谱区的超短飞秒激光脉冲，通过高次谐波方案，人们已经可以在x射线波段产生亚飞秒脉冲，但能量转换效率较低。与此相比，在可见和近红外光谱区域，我们可以利用分子调制高阶受激拉曼散射的方案产生亚飞秒脉冲序列，其能量转换效率可以接近于100％。另外，我们还可以利用其他非线性过程产生极宽频谱，通过频谱合成的方案获得亚飞秒脉冲输出。本论文工作致力于亚飞秒脉冲的产生及控制的研究，围绕产生亚飞秒脉冲的三个主要途径，主要的研究内容和创新点包括以下几个方面：首先，基于分子调制技术，我们对亚飞秒脉冲的产生和控制进行了深入的研究。针对分子调制过程中分子调制频率对产生的亚飞秒脉冲序列脉冲间距的限制性，我们提出了一种可控间距亚飞秒脉冲序列的产生和优化控制的方案。在不影响分子调制频率的情况下，该方案能够有效地控制亚飞秒脉冲序列的脉冲间距，在增大脉冲间距的同时能使更多的能量集中到更少的脉冲内，可以显著的提高亚飞秒脉冲的单脉冲能量以及峰值功率。根据实际应用的需要，为了进一步得到大间距的亚飞秒脉冲序列，我们提出了基于多个相关联的双光子跃迁的亚飞秒脉冲序列的产生和优化控制方案。利用多能级耦合的分子调制过程，该方案可以支持大间距的亚飞秒脉冲序列的产生，通过控制泵浦激光场之间的相对强度和延时可以实现对亚飞秒脉冲序列的优化控制，在实验实现方面具有简单、有效等特点。在标准的基于二能级系统的分子调制过程中，只有通过选择合适的双光子失谐量(小于零，即下失谐)，使系统处于反相态的情况下，才可能结合介质的正常色散特性在介质输出端自动获得亚飞秒脉冲。然而，基于调制相干的分子调制过程，我们发现不论双光子失谐量的正负(上失谐或下失谐)，调制相干技术都能周期性的产生亚飞秒脉冲。利用调制相干分子调制过程中系统相干和驱动光场之间相位的周期性变化的特点，该方案还有望能克服实验上遇到的拉曼自散焦以及自聚焦效应对分子调制过程的影响，推动分子调制产生亚飞秒脉冲方案在实验研究方面的进展。同时，我们还讨论了调制相干技术在调制转移方面的应用，这项技术可以有效地实现不同波长、不同强度的激光场之间的调制结构的转移，克服传统电光调制器件在损伤阈值、透光范围等方面固有的限制，进一步拓展了分子调制技术的应用前景。在基于分子调制过程产生亚飞秒脉冲序列所需要的泵浦源方面在实验上进行了探索。针对分子调制过程所需的高能量、窄线宽的准连续纳秒激光脉冲，我们完成了其关键技术，即连续锁腔部分的研究工作；同时，在二阶非线性介质中，我们首次实现了基于调制不稳定性的多色圆锥辐射的注入放大，并在二阶非线性介质中观察到了二维多色阵列辐射，完成了其上转换的相关研究。其产生的激光场具有宽频谱、波长连续可调、单脉冲能量高、多波长同时输出、相干性好等特点，可以作为近共振受激拉曼散射和瞬动分子调制产生高阶拉曼边带以及亚飞秒脉冲的泵浦源。其次，我们还开展了有关频谱合成产生亚飞秒脉冲的相关研究。利用Ti:Sapphire再生放大系统输出的飞秒激光脉冲，基于非共线的级联非线性混频过程，我们首次在二阶非线性介质中实现了波长范围覆盖了从紫外到近红外的多波长相干辐射的输出。利用其覆盖频谱范围宽以及相干性好等特点，我们对频谱合成产生超短脉冲方案在实验上进行了初步的探索。此外，我们围绕当前实验上产生亚飞秒脉冲的重要途径，即高次谐波，开展了一系列相关的研究。随着飞秒振荡器和腔增强技术的发展，我们已经可以在腔内获得足够高的光强，基于振荡器开展原来只有利用包括放大级在内的大功率激光系统才能完成的高次谐波的相关研究。为了解决腔内高次谐波在输出问题上遇到的困难，我们对非共线高次谐波产生方案进行了探索。我们发现，在小角度的情况下，非共线高次谐波方案能有效地解决腔内高次谐波产生的谐波输出问题，为基于振荡器的高重复频率的腔内高次谐波以及亚飞秒脉冲的产生开辟了一条新途径。在高次谐波以及亚飞秒脉冲产生所需要的泵浦种子源方面，我们在实验上开展了超短脉冲输出的Ti:Sapphire飞秒振荡器的相关研究。目前，已经完成了10-fs量级超短脉冲输出的飞秒振荡器的研制工作。为了进一步理解和控制高次谐波以及亚飞秒脉冲的产生，我们对其物理基础，即原子分子在超短强激光场中的动力学行为进行了系统的实验研究。我们发现了一系列新的物理现象，并提出和发展了一些新的实验手段。所取得的一系列结果可以为我们深入理解和控制高次谐波的产生提供参考，并且能激发进一步的研究。更多还原

【Abstract】 Observation and control of ultrafast events, such as chemical reactions, biological interactions, and electronic processes, is the core of ultrafast physics and enables us an insight of the underlying physics. Over the last two decays, femtosecond technologies have renovated the fields of chemistry, biology, physics, and optical communications. Furthermore, the generation of sub-femtosecond pulses will extend the horizon of ultrafast measurements to the time scale of electronic motions and allow us a stirring new view on the nature.Breaking the femtosecond barrier needs new approaches. Generation of sub-femtosecond pulses in the soft x-ray region of the spectrum has been demonstrated by using high harmonic generation. In the optical region of spectrum, we can produce sub-femtosecond pulses based on Raman approach through molecular modulation with near unit conversion efficiency. On the other hand, sub-femtosecond pulses can be obtained by directly Fourier synthesizing the coherent optical fields in a wide spectral range.This dissertation documents our main results on generation and control of sub-femtosecond pulses based on the approaches mentioned above.Firstly, based on the molecular modulation, the following studies have been carried out. For the first time, we show that the pulse spacing of sub-femtosecond pulse trains generated by molecular modulation can be increased in a controllable way, rather than a constant restrained by the oscillating frequency of the molecules. We also demonstrated that sub-femtosecond pulse train with a large pulse spacing can be generated by using three correlated Raman generators between three Raman levels with strong molecular modulation to control the generation and propagation of the cascade stimulated Raman sidebands. Using the modulated molecular modulation, we show that an oscillatory Raman coherence with periodically changed phase is established. It leads to periodic compression of the generated broadband sidebands in the same medium. Compared to the standard scheme using non-modulated Raman excitations where sub-femtosecond pulses can be generated only with negative Raman detunings, the modulated molecular modulation scheme supports sub-femtosecond pulse generation with positive, negative, or even opposite Raman detunings around the Raman resonance. Additionally, we further extended the ideal of modulated molecular modulation to control the modulation features of the Raman sidebands. We demonstrated that the modulation features can be transferred from a modulated laser field to a non-modulated one with different wavelengths and/or intensities. This provides an efficient way to overcome the intrinsic limitations of commercial modulators, including the limited transparent ranges and damage thresholds. In order to obtain pump fields for molecular modulation with narrow line-width, we carried out a study of cavity stabilization by means of dither-locked method based on phase sensitive detection. On the other hand, we show that the pulses form seeded amplification of colored conical emission and 2D multicolored transverse arrays can be used as pump for impulsive molecular modulation and near-resonant Raman scattering to support sub-femtosecond pulse generation.Secondly, based on the multicolored coherent laser fields resulted from cascaded non-collinearly frequency mixing processes, we demonstrated that it was possible to produce sub-femtosecond pulses by directly Fourier synthesizing the coherent optical fields in a wide spectral range.Thirdly, concentrated on high harmonic generation, the following studies have been performed. With the rapid development of cavity enhanced femtosecond laser technique, it is possible now to investigate high harmonic generation in an oscillator based cavity. We demonstrated that non-collinear high harmonic generation scheme with a small cross angle is a good idea to couple the harmonics out of the cavity without any additional influence. A Ti:Sapphire femtosecond oscillator with pulse width down to 10-fs was built. It can be used as the driving field for high harmonic generation. In order to get a better understanding on the underlying physics of high harmonic generation, we carried out a systematic investigation on the ionization and dissociation dynamics of atoms and molecules in ultrashort laser pulses. A considerable amount of new phenomena and physics on the ionization behavior of atoms and molecules in a high non-perturbative strong field regime were explored. This allows us a better way to understand and control the generation of high harmonics and sub-femtosecond pulses. It will also stimulate further investigations along this direction.更多还原