基于飞秒激光与物质相互作用的非线性与超快表征技术研究
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摘要
近年来,飞秒激光由于其优异的特性与广阔的应用领域,在很多研究方向上都倍受科学家们的青睐。飞秒激光技术自20世纪60年代诞生以来得到了长足的进步与发展,并日益突显出了它在物理学、化学、生物学、医学等各种研究领域中的重要性。飞秒激光与物质之间相互作用的机理也一直受到科学家们的广泛关注。随着飞秒激光超快技术、双光子吸收、多光子吸收、飞秒激光微纳米加工、高密度三维信息存储、飞秒激光治疗手术等应用技术的蓬勃发展,飞秒激光与物质的相互作用机理已经成为一个新兴的多学科交叉前沿研究领域。
飞秒激光的定义就是脉冲宽度在飞秒量级上的超短脉冲激光,它的特点是平均能量比较低,但单脉冲峰值能量非常高。基于超短脉冲宽度和超高能量密度这两个特质,飞秒激光在很多领域和方向的应用上具有长脉冲或连续波激光所无法比拟的固有优势。本论文介绍了飞秒激光与材料的基本光物理反应过程,并对飞秒激光与物质相互作用的非线性超快表征技术进行了研究,分别采用了不同的技术对飞秒激光与物质的相互作用机理做出了详尽的描述,主要研究内容如下：
首先我们利用双光子荧光技术研究了有机树枝状大分子的光物理特性。树枝状大分子(Dendrimer)是近二十年发展起来的一类具有特殊结构的新型化合物,早在1941年的时候,Flory等人就已经提出通过多功能基单体的聚合来制备高度支化的大分子的可能性。但是直到上世纪80年代中期,随着这类具有高度有序结构的大分子化合物的合成方法逐渐建立并完善之后,树枝状大分子的研究才蓬勃地发展起来。与传统的有机化合物相比较,树枝状大分子具有很多优异的特点,如：精确的分子结构、高度的几何对称性、大量的外围官能团、分子内空腔、相对分子质量可控性、非平面空间构型、纳米量级尺寸等。我们在实验中介绍了新型的三苯胺基树枝状大分子的合成和制备过程,并采用钛蓝宝石飞秒激光脉冲,记录了在甲苯溶液中三苯胺树枝状大分子的双光子诱导荧光反应过程。在飞秒激光辐射下,树枝状大分子都发射出强烈的蓝绿色荧光。我们还得到了这两种树枝状大分子的双光子激发态荧光截面,通过对双光子吸收截面的测量,我们发现这两个树枝状大分子都表现出了明显的双光子吸收。相对于类似的三(三苯胺)结构(tristriphenylamine)树枝状大分子,这两种聚合三苯胺(polytriphenylamin)树枝状大分子在甲苯溶液中显示了较大的双光子激发态荧光截面。这一现象表明,随着树枝状大分子结构的增大,出现了强协同增加效应,这主要是由于树枝状大分子的分支间耦合以及光捕捉能力的增强。
接着我们利用荧光上转换技术对有机材料的聚集诱导发光(Aggregation Induced Emission)机理进行了分析。通常情况下,大部分荧光材料在溶液中具有较强的荧光发射,但聚集后其荧光发射现象就会减弱甚至淬灭,这就是“聚集导致淬灭”(ACQ)现象。在2001年,唐本忠教授研究组首次报道了Silole分子。这种材料在溶液中几乎观察不到荧光,而一旦形成纳米粒子或制备成薄膜状态后,其荧光发射显著增强,这就是“聚集诱导发光”(AIE)现象。聚集诱导发光现象的发现为开发固态的高发光效率有机材料提供了一条新的有效的途径,并且因其具有独特的光学现象,在有机电致发光、荧光传感器、PH值传感器以及生物检测方面表现出广阔的应用前景。因此,开发和研究具有聚集诱导发光现象的材料体系倍受科学家们的重视。在实验工作中,我们运用时间分辨荧光光谱技术对于样品材料TPCNDSB (Cyano-Substituted Oligo (p-phenylenevinylene))在随着极性增加的不同溶剂中、以及晶体形态下与飞秒激光相互作用所产生的特性进行了表征。通过对于在不同波长范围内的荧光动力学特性的测量,我们对时间分辨发射光谱进行了重构,通过这一技术我们清晰的揭示了：在较低极性的溶剂(如甲苯)中,仅仅在具有高量子产率的本地激发态存在发射；反之在较高极性的溶剂(如DMF)中,存在着一个从本地激发态到内部电荷转移态的快速跃迁,导致了较低的量子产率；而在晶体形态下,样品材料TPCNDSB分子的特殊结构导致了较低极性的局部环境并且限制了扭转运动,所以就不存在从本地激发态到内部电荷转移态的跃迁,从而导致了高效率的聚集诱导发光现象。这一研究结果意味着,在具有施主-受主单元的染料分子中,普遍是由于内部电荷转移态的限制而导致了聚集诱导发光现象的产生。这些关于聚集诱导发光现象的研究工作,对于制备新型OLED材料和激光材料的发展有着非常积极的意义。
最后我们利用超快成像技术研究了飞秒激光与透明介质相互作用过程中,在皮秒、纳秒、微秒量级时间尺度上,物质内部发生的物理形变与非线性光学过程。当飞秒激光在透明介质内部发生高度聚焦时,能够使得物质发生电离,产生超连续白光、等离子体、自聚焦等物理现象。实验通过泵浦探测时间分辨超快成像技术对这些现象进行了表征与并对于其中的机理进行了分析。我们设计并搭建了全新的三维时间分辨超快成像系统,用来实现对不同方向上的透明物质内部飞秒激光诱导击穿现象的成像。通过对实验结果的观察和分析我们可以得出飞秒激光与物质作用过程中在皮秒时间尺度上的反应过程,还验证了在透明物质内部飞秒激光的自聚焦现象和成丝现象。
Ever since the birth of the femtosecond laser technology in the 1960s', there have been significant development and increasingly important application in the fields of physics, chemistry, biology and many other areas of research. The mechanism of the interaction between femtosecond laser and materials has also gained great concern by scientists. With the rapid development of the ultra-fast femtosecond laser technology, two-photon absorption technology, multiphoton absorption technology, femtosecond laser micro-nano-fabrication, high-density three-dimensional information storage, laser surgery technology, etc.,the research of the mechanism of the interaction between femtosecond laser and materials has become an emerging frotier research of interdisciplinary field.
The femtosecond laser is defined as the laser that has a pulse width of the femtosecond order. Femtosecond laser is characterized for its low average energy, and especially high single pulse peak energy. Based on the two advantageous qualities of ultra-short pulse and ultra-high energy density, femtosecond laser has inherent advantages in many areas and directions of laser applications that the long pulse laser or continous wave laser can not match. In this paper, we will describe the basic photophysical reaction process of interaction between femtosecond laser and the materials, and will also investigate the nonlinear ultrafast characterization techniques to study the interaction between femtosecond laser and materials. We have investigated different techniques that were used to study the interaction between femtosecond laser and materials, and made a detailed description of the mechanism. the main contents are as follows:
Firstly, we use two-photon fluorescence technology to investigate the photophysical properties of organic dendrimers. Dendrimer is a new class of compounds with a special structure that have been invented and developed in the past two decades. As early as the year 1941, Flory and his colleagues have already claimed the possibility of the synthesis of a new kind of multi-functional monomers and highly branched polymerization macromolecules. But not until the mid-80s of last century, with the establishment and gradually development of the synthesis of such a highly ordered structure macromolecular compounds, the research of dendrimer has vigorously developed. Compared with traditional organics, dendrimer has many excellent features,such as:a precise molecular structure, a high degree of geometric symmetry, a large number of peripheral functional groups, the intramolecular cavity, molecular weight control, non-flat space configuration, nanometer size, et cetera. In our experiment, the novel triphenylamine-based dendrimers were synthesized and characterized by methods of FT-IR, elemental analysis,1H NMR spectroscopy, and MALDI-TOF mass spectrometry. The linear photophysical properties including absorption spectrum, one-photon induced fluorescence spectrum and the fluorescence lifetimes in different solvents were investigated. The two-photon induced fluorescence behaviour was recorded in toluene solution, employed by a Ti:sapphire femtosecond laser pulse. The dendrimers both emit strong blue-green fluorescence under irradiation. Two photon excited state fluorescence cross-sections were also obtained. The two dendrimers displayed a large two-photon absorption. The polytriphenylamine dendrimer shows larger two photon excited state fluorescence cross-sections in toluene relative to the tristriphenylamine analogue, indicating that there is cooperative enhancement originating from inter-branch coupling and an increase of light-harvesting ability with increasing dendrimer size.
Secondly, we Investigate the Aggregation Induced emission (AIE) mechanism of the cyano-substituted oligo(p-phenylenevinylene) 1,4-bis [1-cyano-2-(4-(diphenylamino) phenyl) vinyl] benzene (TPCNDSB) by time resolved fluorescence technique. Typically, most of the fluorescent materials will show a strong fluorescence emission in solutions, but will show a decreasing and even quenching process of fluorescence when being aggregated. This phenomenon is called Aggregation Caused Quenching"(ACQ). In 2001, Professor Tang Benzhong and his research group have reported the Silole molecules for the first time. We can hardly observe the fluorescence of this material when it is in solution. But once it is prepared in the form of nano-particles or thin films, the fluorescence emission was significantly enhanced. This phenomenon is called the "Aggregation Induced Emission"(AIE). The discovery of this phenomenon of aggregation induced emission provides a new and effective way for the development of solid organic materials of high luminous efficiency. Because of its unique optical properties in the fields of light emitting, fluorescent sensors, PH sensors and biological detection, aggregation induced emission materials have a broad prospects in applications. Therefore, the development and research of aggregation induced emission materials have drawn much attention of scientists. In our experiment, by reconstructing the time resolved emission spectra (TRES), it is found that in solvent of low polarity, the emission is mainly from the Local Emission (LE) state with high quantum yield, but in high polarity solvent, the emission is mainly from the Intramolecular Charge Transfer (ICT) state, which is a relatively dark state, with low Quantum yield. In Crystal form, the restriction of transfer from LE state to ICT state results in efficient AIE.
Finally, We use the ultrafast imaging technology to investigate the interaction between femtosecond laser and transparent media. In this process, in the picosecond, nanosecond, microsecond time scale, the physical deformation of material and nonlinear optical processes were also investigated. Femtosecond laser is highly focused in transparent materials,which can make the material ionized, resulting in supercontinuum, plasma, self-focusing and other phenomena. This study used time-resolved ultrafast pump probe imaging techniques were used to characterize these phenomena.
飞秒激光的定义就是脉冲宽度在飞秒量级上的超短脉冲激光,它的特点是平均能量比较低,但单脉冲峰值能量非常高。基于超短脉冲宽度和超高能量密度这两个特质,飞秒激光在很多领域和方向的应用上具有长脉冲或连续波激光所无法比拟的固有优势。本论文介绍了飞秒激光与材料的基本光物理反应过程,并对飞秒激光与物质相互作用的非线性超快表征技术进行了研究,分别采用了不同的技术对飞秒激光与物质的相互作用机理做出了详尽的描述,主要研究内容如下：
首先我们利用双光子荧光技术研究了有机树枝状大分子的光物理特性。树枝状大分子(Dendrimer)是近二十年发展起来的一类具有特殊结构的新型化合物,早在1941年的时候,Flory等人就已经提出通过多功能基单体的聚合来制备高度支化的大分子的可能性。但是直到上世纪80年代中期,随着这类具有高度有序结构的大分子化合物的合成方法逐渐建立并完善之后,树枝状大分子的研究才蓬勃地发展起来。与传统的有机化合物相比较,树枝状大分子具有很多优异的特点,如：精确的分子结构、高度的几何对称性、大量的外围官能团、分子内空腔、相对分子质量可控性、非平面空间构型、纳米量级尺寸等。我们在实验中介绍了新型的三苯胺基树枝状大分子的合成和制备过程,并采用钛蓝宝石飞秒激光脉冲,记录了在甲苯溶液中三苯胺树枝状大分子的双光子诱导荧光反应过程。在飞秒激光辐射下,树枝状大分子都发射出强烈的蓝绿色荧光。我们还得到了这两种树枝状大分子的双光子激发态荧光截面,通过对双光子吸收截面的测量,我们发现这两个树枝状大分子都表现出了明显的双光子吸收。相对于类似的三(三苯胺)结构(tristriphenylamine)树枝状大分子,这两种聚合三苯胺(polytriphenylamin)树枝状大分子在甲苯溶液中显示了较大的双光子激发态荧光截面。这一现象表明,随着树枝状大分子结构的增大,出现了强协同增加效应,这主要是由于树枝状大分子的分支间耦合以及光捕捉能力的增强。
接着我们利用荧光上转换技术对有机材料的聚集诱导发光(Aggregation Induced Emission)机理进行了分析。通常情况下,大部分荧光材料在溶液中具有较强的荧光发射,但聚集后其荧光发射现象就会减弱甚至淬灭,这就是“聚集导致淬灭”(ACQ)现象。在2001年,唐本忠教授研究组首次报道了Silole分子。这种材料在溶液中几乎观察不到荧光,而一旦形成纳米粒子或制备成薄膜状态后,其荧光发射显著增强,这就是“聚集诱导发光”(AIE)现象。聚集诱导发光现象的发现为开发固态的高发光效率有机材料提供了一条新的有效的途径,并且因其具有独特的光学现象,在有机电致发光、荧光传感器、PH值传感器以及生物检测方面表现出广阔的应用前景。因此,开发和研究具有聚集诱导发光现象的材料体系倍受科学家们的重视。在实验工作中,我们运用时间分辨荧光光谱技术对于样品材料TPCNDSB (Cyano-Substituted Oligo (p-phenylenevinylene))在随着极性增加的不同溶剂中、以及晶体形态下与飞秒激光相互作用所产生的特性进行了表征。通过对于在不同波长范围内的荧光动力学特性的测量,我们对时间分辨发射光谱进行了重构,通过这一技术我们清晰的揭示了：在较低极性的溶剂(如甲苯)中,仅仅在具有高量子产率的本地激发态存在发射；反之在较高极性的溶剂(如DMF)中,存在着一个从本地激发态到内部电荷转移态的快速跃迁,导致了较低的量子产率；而在晶体形态下,样品材料TPCNDSB分子的特殊结构导致了较低极性的局部环境并且限制了扭转运动,所以就不存在从本地激发态到内部电荷转移态的跃迁,从而导致了高效率的聚集诱导发光现象。这一研究结果意味着,在具有施主-受主单元的染料分子中,普遍是由于内部电荷转移态的限制而导致了聚集诱导发光现象的产生。这些关于聚集诱导发光现象的研究工作,对于制备新型OLED材料和激光材料的发展有着非常积极的意义。
最后我们利用超快成像技术研究了飞秒激光与透明介质相互作用过程中,在皮秒、纳秒、微秒量级时间尺度上,物质内部发生的物理形变与非线性光学过程。当飞秒激光在透明介质内部发生高度聚焦时,能够使得物质发生电离,产生超连续白光、等离子体、自聚焦等物理现象。实验通过泵浦探测时间分辨超快成像技术对这些现象进行了表征与并对于其中的机理进行了分析。我们设计并搭建了全新的三维时间分辨超快成像系统,用来实现对不同方向上的透明物质内部飞秒激光诱导击穿现象的成像。通过对实验结果的观察和分析我们可以得出飞秒激光与物质作用过程中在皮秒时间尺度上的反应过程,还验证了在透明物质内部飞秒激光的自聚焦现象和成丝现象。
Ever since the birth of the femtosecond laser technology in the 1960s', there have been significant development and increasingly important application in the fields of physics, chemistry, biology and many other areas of research. The mechanism of the interaction between femtosecond laser and materials has also gained great concern by scientists. With the rapid development of the ultra-fast femtosecond laser technology, two-photon absorption technology, multiphoton absorption technology, femtosecond laser micro-nano-fabrication, high-density three-dimensional information storage, laser surgery technology, etc.,the research of the mechanism of the interaction between femtosecond laser and materials has become an emerging frotier research of interdisciplinary field.
The femtosecond laser is defined as the laser that has a pulse width of the femtosecond order. Femtosecond laser is characterized for its low average energy, and especially high single pulse peak energy. Based on the two advantageous qualities of ultra-short pulse and ultra-high energy density, femtosecond laser has inherent advantages in many areas and directions of laser applications that the long pulse laser or continous wave laser can not match. In this paper, we will describe the basic photophysical reaction process of interaction between femtosecond laser and the materials, and will also investigate the nonlinear ultrafast characterization techniques to study the interaction between femtosecond laser and materials. We have investigated different techniques that were used to study the interaction between femtosecond laser and materials, and made a detailed description of the mechanism. the main contents are as follows:
Firstly, we use two-photon fluorescence technology to investigate the photophysical properties of organic dendrimers. Dendrimer is a new class of compounds with a special structure that have been invented and developed in the past two decades. As early as the year 1941, Flory and his colleagues have already claimed the possibility of the synthesis of a new kind of multi-functional monomers and highly branched polymerization macromolecules. But not until the mid-80s of last century, with the establishment and gradually development of the synthesis of such a highly ordered structure macromolecular compounds, the research of dendrimer has vigorously developed. Compared with traditional organics, dendrimer has many excellent features,such as:a precise molecular structure, a high degree of geometric symmetry, a large number of peripheral functional groups, the intramolecular cavity, molecular weight control, non-flat space configuration, nanometer size, et cetera. In our experiment, the novel triphenylamine-based dendrimers were synthesized and characterized by methods of FT-IR, elemental analysis,1H NMR spectroscopy, and MALDI-TOF mass spectrometry. The linear photophysical properties including absorption spectrum, one-photon induced fluorescence spectrum and the fluorescence lifetimes in different solvents were investigated. The two-photon induced fluorescence behaviour was recorded in toluene solution, employed by a Ti:sapphire femtosecond laser pulse. The dendrimers both emit strong blue-green fluorescence under irradiation. Two photon excited state fluorescence cross-sections were also obtained. The two dendrimers displayed a large two-photon absorption. The polytriphenylamine dendrimer shows larger two photon excited state fluorescence cross-sections in toluene relative to the tristriphenylamine analogue, indicating that there is cooperative enhancement originating from inter-branch coupling and an increase of light-harvesting ability with increasing dendrimer size.
Secondly, we Investigate the Aggregation Induced emission (AIE) mechanism of the cyano-substituted oligo(p-phenylenevinylene) 1,4-bis [1-cyano-2-(4-(diphenylamino) phenyl) vinyl] benzene (TPCNDSB) by time resolved fluorescence technique. Typically, most of the fluorescent materials will show a strong fluorescence emission in solutions, but will show a decreasing and even quenching process of fluorescence when being aggregated. This phenomenon is called Aggregation Caused Quenching"(ACQ). In 2001, Professor Tang Benzhong and his research group have reported the Silole molecules for the first time. We can hardly observe the fluorescence of this material when it is in solution. But once it is prepared in the form of nano-particles or thin films, the fluorescence emission was significantly enhanced. This phenomenon is called the "Aggregation Induced Emission"(AIE). The discovery of this phenomenon of aggregation induced emission provides a new and effective way for the development of solid organic materials of high luminous efficiency. Because of its unique optical properties in the fields of light emitting, fluorescent sensors, PH sensors and biological detection, aggregation induced emission materials have a broad prospects in applications. Therefore, the development and research of aggregation induced emission materials have drawn much attention of scientists. In our experiment, by reconstructing the time resolved emission spectra (TRES), it is found that in solvent of low polarity, the emission is mainly from the Local Emission (LE) state with high quantum yield, but in high polarity solvent, the emission is mainly from the Intramolecular Charge Transfer (ICT) state, which is a relatively dark state, with low Quantum yield. In Crystal form, the restriction of transfer from LE state to ICT state results in efficient AIE.
Finally, We use the ultrafast imaging technology to investigate the interaction between femtosecond laser and transparent media. In this process, in the picosecond, nanosecond, microsecond time scale, the physical deformation of material and nonlinear optical processes were also investigated. Femtosecond laser is highly focused in transparent materials,which can make the material ionized, resulting in supercontinuum, plasma, self-focusing and other phenomena. This study used time-resolved ultrafast pump probe imaging techniques were used to characterize these phenomena.
引文
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