Spatial Control Of Femtoseond Vector Optical Field And Its Application In Micro/Nano-fabrication

With the development of the science and technology, the application of the femtosecond laser in fabrication of micro/nano-size microstructures tends to more and more extensive. The ultrashort pulse-duration and ultrahigh peak power density of the femtoseocnd laser strongly suppress the thermal diffusion effect in the laser proceeding process, that is, due to the unique advantage of "cold machining", femtosecond laser is suitable for the fabrication of hyperfine microstructure. femtoseocnd laser micro/nano-fabrication has a promising future in microelectronics, microreactors and microdevices and so on.Polarization state, as an elementary feature of the light, is almost one of the most hot research frontier. Vector optical field is a new concept which is proposed in recent years and has an inhomogeneous spatial polarization state distribution and provide a new way to study the interaction of the light with the matter. This will induce various new features, new phenomena and new effects. However, due to the difficulties in generating the generating vector optical field, the relative research is just beginning. Among all the researches, the application of the femtosecond vector optical field on laser micro/nano-fabrication has attracted more and more attention in recent years. Up to now, most of the research on the laser microfabrication is based on the homogeneous polarized light. However, the patterned microstructures fabricated by the traditional laser beam are simple, it has restricted the development of the study on the technology of the laser microfabrication. In view of the unique characters of the femtosecond vector optical field and the potential application value in the region of the femtosecond laser microfabrication, This work is on the generation of the femtosecond vector optical field and the theoretical and experimental study on the laser micro/nano-fabrication with femtoseocnd vector optical field, and we will start the work from the following several aspects:1. By the method of wave-front shaping, attaching the computer-generated holographic grating on the phase-only spatial light modulator and transforming by a 4f system, a vector optical field with arbitrarily spatial polarization state distribution could be obtained. The relation of the spatial polarization state distribution of the generated femtosecond vector optical field with the temporal shape of the pulses is analyzed, and a method is proposed to measure the pulse-duration of the generated vector beams with a single-shot autocorrelator. We also analyze the influence of every components of this system on the pulse-duration of the pulses and make an optimization design of the optical components, so as to reduce the broadening effect. In addition, some work has been done on the generation of the some special beams such as the type of the first kind of multi-order Bessel.2. Theoretical analysis of physical mechanism of the self-orgornized periodic grating structures induced by the femtosecond laser on the surface of the materials. The research is on the exciting the free carriers to reach the critical density through the process of multi-photon ionization and avalanche ionization by using the femtosecond laser on the surface of the materials and the process of the surface plasma formation. We also study the the physical model of the interference of the excited surface plasma wave and the incident wave. This model states the surface plasma wave induced by the femotsecond laser have a compont along the longitudal direction, the propagating direction is parallel to the polarization of the incident light. The interference of the incident light with the excited surface plasma wave make the free carriers peridicaly distributed, the free electrons emitting from the surface makes the Coulomb Explosion resulting in the final periodic grating structures formation. This method could interprete the phenomena of the laser-induced sub-wavelength periodic grating structures.3. Making use of the unique intensity and polarization state distribution of the focusing femtosecond vector light field in the focusing field, which is different from the homogeneous polarized field, the research from manipulating the polarization state distribution of the vector optical field is started and pay attention to the different laser fabricated results when the incident optical field has the different polarization state distribution. Firstly, the ultrashort laser will induce sub-wavelength periodic grating structures on the surface when the incident intensity resaerches the ablated threshold of the material, and the orientation of the grating structures is perpendicular to the polarization direction of the incident laser. In this thesis, through manipulating the two-dimensional polarization distribution at the focusing field, two-dimensional sub-wavelength grating structures on the surface of silicon are fabricated, a theoretical analysis is also given on the grating microstructure formation process and the relationship with the energy. In addition, due to the high heat conductivity and high thermal diffusion, the metallic copper was used to analyze the role of the number of the pulses on the microstructures formation process, it is observed that the first Bragg scattering play an important role in the formation of the surface grating structures.4. This experimental setup is applied in the wave-front shaping to generate parallel vector beams with different local field distributions through the production of holographic grating attached on the spatial light modulator. By modulating the intensity and local field distributions of the parallel vector optical beams, and focused by a single lens, multiple peaks with the similar intensity distribution is formed at the focal plane. The laser beam incident onto material can produce various patterned microhole arrays. Through observating the scanning electron microscopy (SEM) image of the experimental results and compared with the numerical simulation, this paper proves that this method can effectively improve the microhole arrays preparation efficiency and enhance the aspect ratio of the microhles.