Fundamental Researches Of Femtosecond Laser Inducing Micro/Nano Structures On The Metal Surface

The ordered micro/nano structure on the solid surface, which can endow materials with unique properties such as super-hydrophobic, super-adhesion, super-light- absorbancy and so on, has comprehensive application foreground in the field of industry, military, aviation, aerospace and medicine. For this reason, the development of efficient and economic techniques for producing surface micro/nano structure is an important work. Recently, femtosecond (fs) laser direct inducing ordered surface structure has been demonstrated to be a promising method for producing of surface micro/nano structure. However, a systematic study of the physical mechanism of the surface micro/nanostructuring as well as the optimal conditions for a controllable micro/nanostructuring using fs laser direct inducing micro/nano structure method are still lack. In this thesis, several kinds of ordered structure on the metal surface have been induced by fs laser irradiation, and the formation mechanism of each type of the micro/nanostructure has been also discussed. All these experimental and theoretical results lay a foundation of fs laser producing wanted micro/nano structure on the solid surface. The main contents, research conclusions and contributions to innovation are summarized as following:1. Under a scanning electron microscope (SEM), different morphology characteristic of grating structure on the metal surface induced by linear-polarized (LP) laser with different pulse widths was identified. The grating on the uniform melted metal surface induced by nanosecond laser shows a regular sine-wave outline, and the period of this grating is just slightly shorter than the laser wavelength. By contrast, the period of the grating induced by fs laser is much shorter than laser wavelength, in addition the period is gradually increase and the duty ratio of crest vs valley is decrease following increasing the laser fluence.2. We proposed that using surface plasmons mechanics effect to explain the formation mechanism of the laser induce grating structure: using the ions collision pressure effect to explain the formation mechanism of sine-waveform grating structure induced by nanosecond laser, using the electrostatic force ablation model which considering the electron plasmons effect to explains the formation mechanism of fs laser induced metallic grating structure.3. The different types of micro/nanostructure on the metal surface induced by a circular-polarized (CP) femtosecond laser were observed under SEM. When laser normal incident, in addition to the grating structure with a linear-grating vector which has been reported, we observed two novel structures, one is grating structure with circular-grating vector, another is spike structure surrounded with nanoprotrusions. When laser oblique incidence, two-dimensional (2D) orthogonal grating structure will appeared on the metal surface. In the same incident angle, the surface structures show exactly mirror symmetry for left and right CP light.4. Four kinds of micro/nano structure, such as one-dimensional grating structure, 2D grating structure, nanocone structure with the size of 200nm and nanohollow structure were formed on the blind-hole wall which induced by multipulses fs laser ablating Ti. In particular, the location elevation angle of two-dimensional grating structure on the hole wall was examined by a confocal laser scanning microscope, and the formation mechanism of this 2D grating structure was successfully interpreted as laser and surface plasmons interaction on a angular surface.5. By the scanning technology, large area micro/nano structures (such as grating structure, microbump structure which covered by grating structure, and periodic microporous structure) on titanium alloy surface have be obtained under single fs laser beam irradiating. These structured surfaces show quite difference on their response characteristics of the reflective spectrum. What is worth mentioning that fs laser with focused spot size of 70μm would periodic producing 30μm hole under scan spot moves in uniform motion, and these microporous structure can realize the metal surface of broad-spectrum nearly all absorption.