| Micro-/nano- manufacturing is one of the most important branches of high-end manufacture industry(HMI), which provides significant technical supports for high-end equipment, integrated circuits, new energies, new materials, and biomedicine. Femtosecond(fs) laser fabrication is considered one of the most important micro-/nano- manufacturing technologies because it offers solutions to many problems in fabrication of key components in many fields. In the field of fs laser manufacturing, laser-induced damage in materials has always been recognized as research focus, which carries rich physical significance. Laser-induced periodic surface structure(LIPSS/ripples) is a universal phenomenon in the field of laser-induced damage. These structures can be obtained on almost all kinds of materials after laser irradiation, which has become a new approach for micro-/nanostructures fabrication. By changing the morphology of surface micro-/nano- structures, the optical, thermal, electrical, mechanical, and other properties of a solid surface can be adjusted, which has been widely applied in micro-machines, optoelectronics, and surface and interface engineering. Hence, the study on how to control the morphology of surface micro-/nano- structures is very important, not only due to its scientific significance but also for its application potentials. Because a fs laser can easily achieve very high peak power density, the transient localized changes of optical and thermodynamic properties of the material are critical for the formation of micro-/nano- structures. However, traditional fs pulses are unable to adjust this ultrafast process. The control of the morphology of micro-/nano- structures still remains a challenge. Additionally, recent development of new nanomaterials also brings new challenges to micro-/nano- fabrication.With ultrashort irradiation periods(<1ps) and ultrahigh power intensities(>1012w/cm2), ultrafast laser fabrication presents unique advantages of nonlinear nonequilibrium processing. The first feature is nonlinear effects(i.e. ionization mechanisms), which can be adjusted to control the laser-material interactions by shaping the spatial and temporal energy distribution of the ultrafast laser pulses. Another important feature of fs laser fabrication is nonequilibrium processing. Because a fs pulse duration is much shorter than the electron-lattice relaxation time(10-10-10-12s), laser energy absorption is completed before the lattice changes, which results in electron-lattice nonequilibrium. Due to the significant electron-lattice nonequilibrium state, laser-material interactions are actually determined by laser-electron interactions. Hence, fs laser fabrication process can be adjusted by controlling electrons dynamics. Accordingly, our research group proposed the core idea of electrons dynamics control(EDC). More specifically, we proposed that by shaping a fs laser pulse in temporal and spatial domains, we are able to control photon-electron interactions, and then to control the localized transient electrons dynamics, and further to modify localized transient materials properties, and then to adjust material phase change, and eventually to implement the novel fabrication method.In this thesis, based on the core idea of EDC, theoretical and experimental studies have been performed on fabrication of controllable surface micro-/nano- structures on dielectrics, semiconductors and graphene-based materials by temporally fs laser pulse shaping. Meanwhile, the corresponding applications in molecule detection and bioinspired surfaces with special wettability have also been investigated. The main contents and results are described as the following aspects:1. Several methods have been proposed for adjusting subwavelength ripples’ periods, orientations, and ablation areaes in fused silica by using designed fs laser pulse trains to control transient localized electron dynamics and corresponding material properties. Firstly, the feasibility of this method is proved theoretically based on plasma model and formation mechanisms of ripples. Secondly, the morphology evolution of ripples in fused silica by fs pulse trains has been investigated. The major findings include: 1) by increasing the pulse delays from 0 to 100 fs, the ripple periods are changed from ~550nm to ~255nm and the orientation is rotated by 90°; 2) various repeatable periods of ripples can be obtained in the range of 200-800 nm by adjusting the subpulse number of a pulse train; 3) double-grating structures composed of low spatial frequency LIPSS(LSFL) and high spatial frequency LIPSS(HSFL) can be fabricated in one step by quadruple-pulse trains. The abovementioned results demonstrate that the pulse train technique is a simple and efficient method for fabrication of surface micro-/nano- structures with controllable morphology, which have pertential applications for the fabrication of photonic devices, as well as other applications.2. We have developed a technique for fabricating three-dimensinal nanopillars on fused silica by temporally asymmetric fs laser pulse trains with three subpulses. Three types of periodic structures can be obtained by using pulse trains with designed pulse delays, in which the three-dimensional nanopillar arrays with ~100–150 nm diameters and ~200 nm heights are first fabricated in one step. Compared to traditional fs laser pulses, the temporally asymmetric pulse train is composed of three subpulses separated by two pulse delays t1 and t2, which can be adjusted separately. It is found that the two pulse delays play different roles in processing. The type of surface morphology can be defined by pulse delay t1, while further morphology optimization can be realized by carefully adjusting pulse delay t2. This pulse train technique offers new possibilities for three-dimentional, controllable, and high- efficiency fabrication of transparent materials at the nanometer scale.3. We have developed a noval method for high surface-enhanced Raman scattering(SERS) by fabrication of large-scale hydrophobic hierarchical micro-/nano- structures on silicon in water using fs laser pulse trains. The resulting hierarchical surface structures exhibit both micrometer-scale and nanometer-scale structures. The micrometer-scale structures consists of ripples with 2 μm periods, which are covered by nanometer-scale ripples with 100 nm periods. The patterned surface exhibits hydrophobicity with a contact angle of ~ 135°. This substrate can be used to concentrate molecules in highly diluted solutions at a specific position for the following molecular detection. Then, silver nanoparticles are deposited on the patterned surface to created “hot spots” for local field enhancements, which can lead to a high enhancement factor. Hence, the fabricated hydrophobic substrate makes it possible to concentrate molecules over the sensing area, where hot spots generated by silver nanoparticles are used to carry out molecule detection. The presented hydrophobic SERS substrate suggests the possibility of detecting few molecules when the initial construct is highly diluted.4. We have investigated the response of freestanding and flexible graphene film to fs laser irradiation, and developed a simple, scalable and high-throughput method for fabrication of large-area three-dimensional rose-like microflowers with controlled size, shape and density by fs laser micromachining. The major findings include: 1) the laser beam spot radius fitted by D2 method from the experiments of graphene film is unexpected larger than the actual size of beam spot size, which indicates that the lateral diffusion of heat plays an important role in fs-laser-induced damage in graphene film; 2) the novel biomimetic microflower that composed of numerous turnup graphene nanoflakes can be fabricated on graphene film by only a single fs laser pulse, which is efficient enough for large-area patterning; 3) by using a simple scanning technique, large-area patterned surfaces at centimeter scales with controllable densities of flower patterns can be obtained, which can exhibit adhesive superhydrophobicity. The fabricated biomimetic microflowers on the graphene films can find broad applications in bionics, microfluidics, micro/nano-droplets manipulation, single droplet analysis, Janus interface materials for actuators, and in fabrication of lab on a chip that need to retain tiny quantities of liquid without leaking.This thesis is based on the research projects supported by National “973” Program of China(No. 2011CB013000) and the National Natural Science Foundation of China(NSFC)(Nos. 90923039, 91323301 and 51025521). The main innovations of the thesis are all published in journals indexed by SCI. |
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