Nonlinear And Ultra-fast Characterization Technologies Of Femtosecond Laser Interaction With Materials

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.