摘要
等离子体光栅是指在空间上呈现周期性强度分布的等离子体。在过去的一些年间,全世界很多科研工作者对等离子体光栅进行了理论和实验研究,这使得等离子体光栅成为一个热门的研究领域。相对于近红外等离子体光栅而言,紫外超快激光诱导形成的等离子体光栅具有光栅条纹细,内部电子密度高等优势,所以本课题组通过高效的倍频技术,将双紫外激光束以交叉相互作用的方式形成了光场空间相干调制,并进一步在空气介质中形成等离子体光栅。在不同波长的激发下,等离子体光栅的等离子体密度、光栅厚度、条纹宽度,衍射效率等特性都会有所差异。比较紫外和近红外飞秒激光诱导等离子体光栅的一些相关参数,从而进一步验证发展紫外强场光物理的潜在优势,并为开展紫外强场等离子体研究提供有参考价值的数据。
目前所有报道的有关等离子体光栅的理论和实验研究结果大都集中于近红外波段,有关紫外等离子体光栅的研究结果寥寥无几,在理论方面,有研究者通过PIC模拟的方法对极高破坏阈值的超短脉冲布拉格光栅以及欠密等离子体中等离子体进行了研究。在实验方面,本课题组通过对紫外布拉格光栅的研究,我们可以发现光栅内部的电子通过被近红外强场驱动而进一步加速,同时这将极大增强局部范围内的电子密度。所以我们对其进行进一步的探索将具有重要的意义。
本论文包括以下方面内容：
首先,通过两束交叉的紫外飞秒激光在空气中发生干涉从而发生空间强度调制,并且会在相交区域周围局部范围产生一个周期性的强度分布,即紫外等离子体光栅。重点研究了光栅的厚度和条纹宽度,以及这两项参数与延时、偏振等影响因素之间的关系。
其次,我们将一束基频光从侧面与紫外等离子体光栅进行相互作用,我们发现基频光发生了衍射,于是我们对光栅的衍射特性进行了研究,并且推算出了光栅内部的电子密度。
最后,我们对两束近红外激光交叉形成等离子体光栅进行了补充测量,主要是将紫外和近红外等离子体光栅的一些重要特性进行比较和分析。
通过紫外和近红外等离子体光栅的比较研究,我们得到紫外等离子体光栅比近红外光栅薄,条纹比近红外光栅细,同时紫外等离子体光栅内部的电子密度要比近红外光栅大将近一个数量级等结果。因此通过紫外等离子体光栅的研究,有助于进一步揭示基于光丝效应的等离子体光栅产生的动力学过程及其应用。
In the past few years, a lot of researchers of the world investigate into plasma grating, and considerable attention has been attracted to the research of plasma grating. Many ach icvement s arc about near-infrared (IR) plasma grating. We can get a plasma grating appears in the air by optical interference modulation spatially through two crossed ultraviolet(UV) lasers. If the wavelength is different, grating parametersN electron density and diffraction of plasma grating will be different. We will know dynamic processes and its application based on plasma grating by filamentation through the differences between UV and IR plasma grating.
In our experiments, we get harmonic conversion in BBO crystal by the method of doubling-compensation-tripling in a collinear device, so we can obtain efficient doubling efficiency, the wavelength and power of UV laser are276nm and10GW. It is convenient to research on the plasma grating which is conformed by two crossed UV lasers.
Contrasting to IR plasma grating, the electron density in the UV plasma grating is much higher. So plasma grating is a characteristic of the researches on the ultrashort laser. We have make a study of electron density、grating thickness、the width of fringes and diffraction efficiency of the UV.plasma grating by mJ-UV lasers. Contrasting all these parameters between UV plasma grating and IR plasma grating, we will know great superiority of UV laser in optical physics research and we can provide valuable date for further research on the UV plasma grating.
The contents of this thesis are following:
Firstly, spatially intensity modulation is generated by the interference of two crossed UV lasers, a periodic intensity distribution is appeared in the subrange of crossed section, that is UV plasma grating. We primary study the thickness and width of fringe of grating, moreover, we also pay close attention to the relationship between delay、 polarization and these two parameters.
Secondly, a FW interacts with the plasma grating from the side, we find that diffraction effect appears of the FW. So we study on the features of grating diffraction, and we also calculate the electron density in the grating through diffraction efficiency.
Finally, we also investigate into IR plasma grating additionally, we compare and analyze some important properties about UV and IR plasma grating mainly.
Through comparative study in UV and IR plasma grating, we can know that thickness and width of fringe of UV plasma grating are thinner than IR plasma grating, and the electron density of UV plasma grating is two orders of magnitude higher than that of IR plasma grating.
引文
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