【作者】 戴晔；

【导师】 余昺鲲； 邱建荣；

【作者基本信息】 上海大学 ， 无线电物理， 2008， 博士

【摘要】 光子学是当今一个飞速发展的学科领域，它主要研究作为信息和能量载体的光子行为及其应用。使用光子作为信息载体，其信息容量比电子相比要大出3-4个量级，并且具有极快的响应能力和良好的空间相容性，因此开拓新的思路和技术对发展光子学具有十分重要的现实意义，并能促进材料科学、物理和信息等多个学科的发展。特别是近年来，利用飞秒激光在材料中写入功能化的光子学微结构得到越来越多的关注。飞秒激光由于具有超短的脉冲宽度和极高的脉冲峰值功率，使得其与材料的相互作用主要是非线性过程占主导，并且这种非线性过程有着阈值场强的限制，因而飞秒激光在材料中能够空间选择性的诱导各种微结构。本文就1kHz和250kHz两种不同脉冲频率的飞秒激光在多种透明介质中诱导光子学微结构等方面展开系统的研究，并取得具有创新性的成果。研究的内容包括发现玻璃中掺杂的Ce³⁺会影响飞秒激光诱导的银纳米颗粒的沉积、飞秒激光在Ag⁺掺杂的硅酸盐玻璃中诱导出具有荧光特性的银纳米颗粒、飞秒激光在玻璃和晶体等透明介质中诱导自组装周期孔洞结构以及飞秒激光在玻璃内部诱导出非线性光学晶体等几个领域。1．通过分析样品辐照前后及不同热处理温度的吸收光谱，发现在Ag⁺-Ce³⁺高浓度共掺杂的硅酸盐玻璃中，Ce³⁺在飞秒激光辐照下会通过多光子电离等非线性效应释放一个自由电子，该自由电子能同玻璃基体桥氧键断裂释放的自由电子一同还原Ag⁺转变为Ag₀，因此掺杂了Ce³⁺的玻璃在辐照参数相同的条件下，银纳米颗粒生长的速度快于未掺杂Ce³⁺的玻璃。在较高温度的热处理下，已经氧化的Ce⁴⁺又会被还原成Ce³⁺，从而消耗银原子，影响银纳米颗粒的生长。研究表明，在硅酸盐玻璃中高浓度掺杂的Ce³⁺会影响飞秒激光诱导的银纳米颗粒的沉积。2．利用飞秒激光诱导和热处理相结合，在Ag⁺掺杂的硅酸盐玻璃内部，成功地诱导出了具有荧光特性的银纳米颗粒。通过吸收和荧光光谱的分析，这种荧光银纳米颗粒来源于局域形成的高浓度Ag_n^m+离子原子混合体。这种混合体所发射的荧光波长会随着激发波长的增加而出现红移现象，这一特征非常相似于半导体量子点的光致发光特性。这些荧光银纳米颗粒有很好的稳定性，在室温条件下放置6个月，没有出现明显的变化。3．研究了飞秒激光在玻璃和晶体等透明介质中诱导自组装周期孔洞微结构的物理现象，并对这个结构的形成提出了机理解释。实验结果显示这种孔洞的周期和尺寸能随着激光辐照参数的变化而变化，这种结构来源于克尔自聚焦和等离子体自散焦共同作用形成的多次重复聚焦效应，在每个聚焦点由微爆炸而形成孔洞。研究还表明，空气和介质的折射率失配引起的界面球差效应也会对这种孔洞微结构产生影响。4．首次在Ba₂ TiSi₂O₈系列玻璃内部空间选择性地诱导出了非线性光学晶体。研究表明，具有与非线性晶体相似组分的玻璃，如Ba₂TiSi₂O₈、Sr₂TiSi₂O₈和Ba₂TiGe₂O₈，在250kHz高重复频率飞秒激光辐照下都能诱导出非线性晶体，这些晶体展示了十分优异的光学倍频性质。这些晶体形成的主要原因是高重复频率的飞秒激光辐照进玻璃后由于多光子效应形成等离子体强烈地吸收激光能量，从而在焦点区域形成热积累效应，随着温度的不断升高，玻璃便会逐渐转化为晶体。此外，玻璃中还可以掺杂不同的稀土离子，利用入射激光诱导出的晶体发出的倍频光激发这些稀土离子可以得到特定波长的激发光，这项技术能应用于多色显示，三维存储等领域。飞秒激光在透明介质中诱导光子学微结构这一研究领域将超快光学、材料科学以及激光光谱学结合起来，是一个多学科交叉的新兴领域。特别是国际上对飞秒激光加工光子学微结构的研究时间还比较短，研究还不够深入，缺乏系统性。开展这方面的研究一方面可以推进更新的有源或无源光子学器件的发展，另一方面可以利用飞秒激光的特性发掘材料的新功能，发现更多在普通条件下无法观察到的新奇的物理现象，促进非线性光学以及相关研究的发展。更多还原

【Abstract】 Photonics is now a rapidly developing discipline. It mainly studies a photon how to act as an information carrier for communication technology. The data storage capacity of photon as an information carrier is 3-4 larger than that of electron, and it has fast response and good spatial compatibility. Therefore, the development of photonics will significantly promote materials science, physics and other disciplines. Recently, much attention has been paid on the photonics microstructures induced by femtosecond laser. Due to its ultrashort pulse and ultrahigh peak power, nonlinear optical effects are dominant in the process of femtosecond laser interaction with dielectrics, and this process is strictly limited by the threshold intensity of incident laser. In this way, femtosecond laser may space-selectively induce various functional microstructures in materials.In this thesis, two different repetition （1 kHz and 250 kHz） femtosecond lasers are used to induce photonics microstructures in transparent dielectrics including glass and crystal. We focused on the following topics: the microstructures induced by femtosecond laser in Ag⁺-doped silicate glass, self-assembly void microstructure induced by femtosecond laser in borosilicate glass and CaF₂ crystal, the space-selectively precipitation of nonlinear optical crystal induced by high repetition femtosecond laser in similar glass matrix.We investigated the effect of Ce³⁺ on the precipitation of Ag nanoparticles in silicate glass via a femtosecond laser irradiation and successive annealing. Absorption spectra show that Ce³⁺ may absorb part of the laser energy via multiphoton absorption and release free electrons, resulting in an increase of concertration of Ag atoms, which influence precipitation of the Ag nanoparticles. In addition, we found that the formed Ago may reduce Ce⁴⁺ to Ce³⁺ during the annealing process, which inhibits the growth of the Ag nanoparticles.A kind of fluorescent Ag nanoparticles may be space-selectively induced in glass via femtosecond laser irradiation and successive annealing. Extinction spectra and fluorescence spectra show that a fraction of the Ag atoms generated from multiphoton reduction, and aggregated to form nanoparticles under thermal treatment. Red luminescence from the irradiated region is observed under blue or green laser excitation, which is very similar in fluorescence characteristics of semiconductor quantum dots. Therefore, this red fluorescence may be attributed to interband transitions within Ag nanoparticles. These fluorescent Ag nanoparticles are very stable at room temperature.Multiple refocusing of a tightly focused femtosecond laser due to the dynamic transformation between Kerr self-focusing and self-defocusing from plasma produced a quasi-periodic void microstructure in borosilicate glass and CaF₂ crystal. It is found that the diameter or interval of the periodic void increase with the increasing pulse energy of the laser. The detailed course for producing periodic voids is discussed by analyzing the damaged track induced by the tightly focused femtosecond laser,Nonlinear optical crystals Ba₂TiSi₂O₈, Sr₂TiSi₂O₈ and Ba₂TiGe₂O₈ may be arbitrarily induced by 250 kHz femtosecond laser inside the stoichiometric glass matrices of BaO-TiO₂-SiO₂, BaO-TiO₂-SiO₂ and BaO-TiO₂-SiO₂ compositions. The induced crystals show excellent nonlinear optical nonlinearity. A heat accumulation effect is proposed as an important contribution to crystal growth in the irradiated regions. The mechanism of crystal formation as follows: The glass will transform to plasma state via multiphoton ionization due to the ultrahigh field intensity of femtosecond laser, the optically dense plasma will also absorb the laser energy effectively. So there exists a temperature gradient field in the heat-affected zone. When the glass temperature exceeds the crystallization temperature T_c, the glass will be a phase change into a crystal. Finally, crystallization due to the heat accumulation is brought in the laser-irradiated regions.更多还原