【作者】 张思思；

【导师】 袁孝；

【作者基本信息】 华中科技大学 ， 光学工程， 2009， 硕士

【摘要】 在高精密物理实验中对激光脉冲对比度的要求相当高。对比度不够高的激光会导致主脉冲与已产生的预等离子体(而不是靶)相互作用,因此提高高能激光对比度非常重要。等离子体镜利用脉冲前沿产生的稠密等离子体反射主脉冲可以有效提高脉冲对比度,反射特性是高脉冲对比度有效性的重要指标之一。本论文从理论分析和实验两方面对临界等离子体密度和等离子体镜面反射率进行了研究。利用超短超强激光与物质相互作用的机理,对电离机制、电离系数、影响电离速率的因素以及超短脉冲与熔石英相互作用过程中的等离子体镜镜面反射率进行了分析与计算,并利用800nm、60fs、30mJ的钛蓝宝石激光器设计实验光路并进行了实验。理论分析表明,主要电离机制为多光子电离和雪崩电离,材料禁带宽度越大电离速率越大,等离子体电子密度越容易达到临界密度。研究还发现反射率上升时间约为100fs,并随入射能量密度增加而减小,为等离子体动态膨胀提供了解释方法。实验结果表明当入射强度超过材料的损伤阈值(1014W/cm2)时,S、P偏振光反射率迅速从10%分别增加到70%和50%,随着入射强度增加,反射率保持稳定。强度低于1016W/cm2时反射率随入射强度增加而迅速增加,实验结果与模拟计算等离子体的镜面反射率吻合地很好。当入射强度高于2×1016W/cm2时,反射率剧烈波动。通过分析激光近场照射基片上的光斑和等离子体动态膨胀度,发现高强度下近场强度强烈调制和等离子体的动态膨胀导致反射波前严重畸变、非镜面反射和吸收增加是造成等离子体剧烈膨胀的主要原因。本论文中的研究结果在激光器对比度改善等领域具有一定参考意义。更多还原

【Abstract】 The precise physical experiments such as ultra-fast X-ray research and ICF have extraordinarily high requirement for the pulse contrast ratio due to avoiding the main part of the pulse interacts with the preplasma instead of the target. Therefore enhancement of the contrast ratio becomes very important. Plasma-Mirror can effectively improve the laser pulse contrast ratio with the plasma generated by the pulse pedestal reflecting the main part, and characteristics of the reflection is the main indicator of its feasibility.In this thesis the density of plasma and specular reflectivity of plasma mirror both in theoretical and experimental aspects are researched. The coefficient of the ionization, the factors affecting the ionization rate, specular reflectivity of plasma mirror are analyzed and caculated. The Ti: Sapphire laser with pulse duration of 60fs and wavelength of 800nm is used to investigate the reflection characteristics of the fused silica.The main ionization mechanism is Multi- photonization and avalanche ionization, the ionization rate and plasma density increases as the band gap of the material increases. In addition, the rise time of reflectivity is about 100fs and it decreases as the incident fluent grows, which provides a method to explain the hydrodynamic expansion of the plasma.Experiment result shows that the reflectivity increased to 70% and 50% from 10% for S and P polarization respectively when the incident intensity is above threshold of silica (1014W/cm2), and with the continuous growth of the incident intensity reflectivity maintains as a plateau. It is found that when intensity below 1016W/cm2 the specular reflectivity is as a function of the incident intensity and the experimental corresponds to simulative result. It is also observed that reflectivity fluctuates and drops to only a few percent for very high intensity. Through analysis of the spot on the subtract and the hydrodynamic expansion, strong modulation of laser intensity distribution at near-field and expansion of the plasma, which leads to significant distortions in the reflected wave front, the augment of the non-specular reflection and absorption, are concluded to be the reasons inducing the reflectivity fluctuation.These results can be of a certain reference value in the improvement of the laser contrast ratio in the future.更多还原