【作者】 朱海生；

【导师】 陈险峰；

【作者基本信息】 上海交通大学 ， 光学， 2009， 硕士

【摘要】 随着飞秒激光的出现,激光与材料的相互作用研究进入了一个更为广阔的空间,激光的应用深入到更多的领域。本文主要介绍了一种新型的诱导铁电晶体畴反转的方法——飞秒激光诱导畴反转,并且对该方法进行了数值模拟和实验研究;另外,我们利用飞秒激光对铌酸锂和掺镁铌酸锂晶体表面进行烧蚀研究,这对铁电晶体抗损伤性能的深入了解以及飞秒激光微加工的应用有着重要的意义。在激光诱导铁电晶体畴反转的理论模型研究中,我们通过模拟激光电场对铁电晶体中决定畴反转的离子的作用,证实了激光诱导畴反转的可行性,同时发现,随着激光能量的递增,诱导畴反转的激光能量窗口与非反转窗口交替出现。此外,通过对模型中各种参数的研究得知,铁电晶体的阻尼系数、耦合弹性系数、温度、脉宽以及脉冲频率的减小,都会不同程度地降低畴反转的阈值,而通过减小激光束腰半径则能实现更小的畴结构。激光脉冲的载波包络位相（CEP）亦起到重要的作用,当CEP在0.5π附近时,激光能量能更充分利用在畴反转上,从而增大畴反转的概率。当模型中的各参数根据铌酸锂晶体的实际参数取值时,得到红外波段的飞秒激光诱导畴反转所需的脉冲能量范围为1～100μJ。在800nm的飞秒激光脉冲诱导下,实验得到了微米量级的铌酸锂晶体畴反转环状结构。此时的激光偏振方向与铌酸锂晶体z轴平行,相比之下,激光偏振方向改变90度后,畴反转的诱导变得相当困难。这说明激光电场对锂离子的作用是诱导畴反转的主因,与我们的理论模型相吻合。实验中的反转阈值能流流量在1～4 J /cm 2的范围内,作用的激光脉冲能量范围在30～60μJ,验证了数值模拟的预测值。另外,我们还对实验进行了优化,发现当聚焦激光的透镜焦距选择150mm,焦点离样品10mm时,畴反转现象较为明显。这种完全通过光进行微极化的方法对三维畴反转结构的实现具有重要的技术参考价值。另外,我们还研究了飞秒激光对铌酸锂晶体和掺镁铌酸锂晶体的表面烧蚀情况。通过拟合烧蚀面积与激光能量流量的对数关系得出两种晶体在单脉冲和多脉冲作用下的烧蚀阈值。比较发现掺镁铌酸锂晶体的单脉冲烧蚀阈值比铌酸锂晶体的高出50%,而在多脉冲作用下,掺镁铌酸锂烧蚀阈值随着脉冲个数增加而递减的速度比铌酸锂慢,再次证明其抗损伤能力较强。此外,关于不同掺镁成分的铌酸锂烧蚀研究表明,Mg（5mol%）: LiNbO3与Mg（6mol%）: LiNbO₃相比具有更高的抗损伤性。更多还原

【Abstract】 With the development of femtosecond laser, the application of laser and the research area of interaction between laser and materials have been widened. In this dissertation, a novel method for domain inversion of ferroelectric crystal, domain inversion induced by femtosecond laser, has been proposed with simulation study and experimental investigation. Besides, the surface ablation of lithium niobate and Mg-doped lithium niobate by femtosecond laser has also been studied, which can help to get a deeper understanding of the anti-damage characteristic of ferroelectric crystal and also has important reference on the microfabrication in such materials by femtosecond laser.In our simulation model of domain inversion, which describes the process of electric field of laser acting on the crucial ions in domain inversion, that the domain inversion can actually be induced by laser in domain inversion window has been found, which alternates with un-inversion window as the energy of laser growing. Study of the parameters of the model shows that the decrease of damping constant, coupling spring constant, temperature, duration of the pulse and frequency of the laser field, will more or less reduce the inversion threshold, while a smaller laser waist radius will bring to a smaller domain inversion area. Carrier Envelope Phase (CEP) also plays an important part in the inversion process. When the value of CEP is about 0.5π, more laser energy will be devoted onto the domain inversion which can help to induce the possibility of domain inversion Simulation of the model for lithium niobate clarifies that the domain inversion by infrared femtosecond laser has a threshold in the range of 1~100μJ.Ring-like domain inversion window of lithium niobate induced by 800nm femtosecond laser, which has a polarization parallel to z axis of the specimen, has been observed. However the inversion becomes harder when the laser polarization turns an angle of 90 degree, which means the domain inversion is caused by electric field as is simulated in the model. Results show that the inversion threshold is between 1~4J/cm2 corresponding to laser energy of 30~60μJ, which is agreeable with our theoretical predictions. Besides, the experiment has been improved by using lens with focal length of 150mm and adjusting the position of focal point 10mm away from the specimen. Such a domain inversion induced totally by laser creates a path to achieve domain inversion structure in 3-dimension.Finally, the surface ablation of lithium niobate and Mg-doped lithium niobate under irradiation by single and multiple femtosecond laser pulses has been studied, and their ablation thresholds have been found out by using the logarithm relation of the ablation area and the laser energy fluence. Results show that when lithium niobate is Mg (5mol%) doped, its single pulse ablation thresholds increases 50% and its multi pulse ablation thresholds reduces more slowly as the pulses number increases. Another research for the comparison of Mg (5mol%): LiNbO3 and Mg (6mol%): LiNbO3 shows that the former one has a better anti-damaged characteristic.更多还原