Geometrical Morphologies Adjustment Of Micro/Nanostructures On The Silicon Surface Induced By Femtosecond Laser

Some fancy phenomena in nature, such as colorful butter fly wings, multicoloredpeacock feathers, and self-cleaning lotus are a source of inspiration for scientists andresearchers. They found that the ordered uniform micro/nanostructures on the surface canimprove material properties for optical absorption, wetting, and self-cleaning, etc. Thoseproperties have a far-reaching significance for potential application in many fields, such assolar energy utilization, wave frequency absorption, metal lubrication, anti corrosive,self-cleaning, military steatlth and so on. Therefore, the development of efficient andeconomic techniques for producing surface micro/nanostructures and its geometricalmorphologies adjustment are an important work.The surface technique for micro/nanofabrication has various methods. Recently, withdevelopment of laser technique, especially femtosecond laser has promoted rapidlydevelopment in the field of micro/nanofabrication. Due to the characteristics of highfabricating precision, minimum microcracks, low thermal diffusioneffect and defectthreshold,femtosecond laser micromachining technique has been developed one oftechniques for micro/nanofabrication. During the processing of femtosecond laserfabrication, the pulse duration is considerably shorter than the electron-lattice relaxationtime (on the time scale of a few to tens of picoseconds). During the absorption offemtosecond pulse, the lattice temperature remains unchanged. Femtosecond laser-materialinteraction is highly nonequilibrium. Hence, the processing of femtosecond laserfabrication depends on the interaction between photons and electons.Because of the unique mechanism of laser processing of materials, the research studiesabout the laser micro/nano manufacturing have been carried out gradually.However, asystematic study of the physicalmechanism of the surface micro/nanostructuring as well asthe optimal conditions for acontrollable micro/nanostructuring using femtosecond laserdirect inducing micro/nanostructure method are still under inprovement. In this thesis,several kinds of ordered structure on the silicon surface have been inducedby femtosecondlaser irradiation, and the formation mechanism of each type of the micro/nanostructurehasbeen also discussed. The ablated morphologies and patterns adjustment formed on a silicon surface based on electron dynamics control by using shaped fs laser pulse trains andspatial light modulator. All these experimental and theoretical results lay a foundation offemtosecond laserproducing wanted micro/nanostructure on the solid surface. The maincontents, researchconclusions and contributions to innovation are summarized as following:(1) Femtosecond laser induced large area, high precision, uniform ripples on thesilicon surface, which has potential application in many fields, such as solar energyutilization, wave frequency absorption, self-cleaning, military steatlth and so on. Comparedto the conventional lithography technique, it is much simple due to no mask assistance. Wealso obtained large area ripples with periods about150nm by femtosecond laser inducedsilicon in the water.(2) A cross-patterned surface periodic structure in femtosecond laser processing ofcrystalline silicon was revealed under a relatively low shots (4<N <10) with the pulseenergy slightly higher than the ablation threshold. The experimental results indicated thatthe cross-pattern was composed of mutually orthogonal periodic structures (ripples).Ripples with a direction perpendicular to laser polarization (R⊥) spread in the wholelaser-modified region, with the periodicity around780nm which was close to the centralwavelength of the laser. Other ripples with a direction parallel to laser polarization (R‖)were found to be distributed between two of the adjacent ripples R⊥, with a periodicityabout the sub-wavelength of the irradiated laser,390nm. The geometrical morphology oftwo mutually orthogonal ripples under static femtosecond laser irradiation could becontinuously rotated as the polarization directions changed, but the periodicity remainedalmost unchanged. The underlying physical mechanism was revealed by numericalsimulations based on the finite element method. It was found that the incubation effect withmultiple shots, together with the redistributed electric field after initial ablation, plays acrucial role in the generation of the cross-patterned periodic surface structures.(3) Formation of the elliptical-shaped craters and geometrical morphologiesadjustment on silicon surface is investigated comprehensively using single shot of afemtosecond laser. It is observed that the ablation cratersare elongated along the major axisof the polarization direction while its orientation is parallel to the polarization direction.Theablation area grows and the morphologies of the craters evolve from an ellipse to nearly a circle with the increasing of fluence. The underlying physical mechanism is revealedthrough numerical simulations which are based on the finite-difference time-domaintechnique. It is suggested that the initially formed craters or surface defect lead to theredistribution of electric field on silicon surface, which plays a crucial role in the creationof the elliptical-shaped craters.And the field intensity becomes enhanced along the incidentlaser polarization direction, which determines the elliptical crater orientations.(4) The ablated morphologies adjustment formed on a silicon surface based onelectron dynamics control by using shaped fs laser pulse trains and spatial lightmodulator.The pulse train effect on femtosecondlaserprocessing of silicon was investigatedfrom the viewpoint of electrondynamics. Duringfemtosecond laser (800nm,50fs) pulsetrain(double subpulses per train) ablation of silicon with a singleshot at the total fluencies(9.5and3.5J/cm2), the bottom morphologies of the ablated craters change from sharp toflat with an increase in the pulse delay. An oscillation relationship between ablation size(ablation depth, diameter and recast height, width) and pulse separationwas observed in therange of0-1000fs, while a stable relationship occurs when the pulse delay is larger than1000fs. For multi-shot with double subpulses irradiated silicon, the ablated regionenhancement occurred at Δt=200fs and Δt=600fs, and it just occurred at Δt=600fs withtriple subpulses. It is assumed that this phenomenonis due to the enhancementofphoton-electron coupling efficiency and transition of thephase-change mechanism byadjusting the free-electron densityduring pulse train ablation. The subsequent subpulse ofthe train significantly impacted free electron distribution generated by the previoussubpulse, which might influence the formation mechanism of the crater morphology.Thelocal transiently electron dynamic properties have a significantly change when the fs laeserbeam modulate multi-beams pattern, the corresponding to ablated pattern on the siliconsurface was obtained.This thesis is based on the research projects supported by the National Natural ScienceFoundation of China (NSFC)(Nos.91323301and51025521), and the National “973”Program of China (No.2011CB013000). The main innovations of the thesis are allpublished in international journals such as Applied Surface Science and Applied Optics,indexed by SCI.