Temporally Shaped Femtosecond Laser Ablation Of Nonmetallic Materials Based On Electron Dynamics Control

Femtosecond laser micro/nano fabrication is a frontier and interdisciplinary field, involving mechanics, thermodynamics, optics and materials. It is widely applied in national defense, medical treatment, information and etc. Due to its ultrahigh peak power, femtosecond laser-materials interaction is a nonlinear non-equilibrium process, which ranges from nanometer to millimeter and from femtosecond to microsecond. The mechanism of femtosecond laser micro/nanofabrication is totally different from the traditional fabrication. Hence, investigating the techniques of femtosecond laser micro/nano fabrication possesses significant contribution to the science of physics and manufacture.With the development of microminiaturization, the demand for fabrication quality is continuously increasing. Femtosecond laser micro/nano fabrication is confronted with new challenges, such as more accurate fabrication quality, higher fabrication efficiency, and better fabrication controllability. In theoretical aspects, many classical theories are no longer applicable during the process of femtosecond laser-materials interaction, the transient(femtosecond) and localized(nanometer) changes of materials’ properties become dominant, and quantum mechanics should be taken into consideration for better prediction. In experimental aspects, femtosecond laser micro/nano fabrication is a process related with multi-parameters, including wavelength, temporal shape, pulse width, repetition and etc. However, the dispersion during laser transmission will inevitably distort the temporal shape and widen the pulse width, which has great influence on fabrication accuracy and controllability. Besides, the surface quality, effiencicy and controllability of femtosecond laser micro/nano fabrication need further investigation. Therefore, as to the abovementioned scientific problems, our group proposes the femtosecond laser temporal shaping methodology based on electron dynamics control, dealing with the pulse distortion and wideness. In order to realize a new method of femtosecond laser micro/nano fabrication with higher quality, higher efficiency and better controllability, the materials’ transient localized optical and thermodynamics properties are controlled, and the materials’ phase change is selectively triggered by adjusting the interaction between photon and electron.The thesis presents the study of temporally shaped femtosecond laser fabrication of non-metal materials(dielectrics, polymer and semiconductor). The main contents are as follows: 1) Taking the Gaussian beam as an example, we establish a dispersion model for femtosecond laser transmission in medium, which is employed to theoretically simulate and analyze the influence of the second order dispersion and the third order dispersion on the pulse width and pulse temporal shape of femtosecond laser. 2) We establish a plasma model for the interaction between the temporally shaped femtosecond laser and dielectrics, which is used to deeply investigate the free electrons’ excitation, heating and the changes of materials’ transient localized optical properties. 3) A temporally shaped femtosecond laser micro/nano fabrication system is setted up to systematically explore the effects of temporally shaped femtosecond laser parameters on the ablation size(diameter, depth) and surface quality(recast, microcrack). 4) We theoretically derive the relationship between two-photon polymerization feature size and laser irradiation parameters, which is verified by temporally shaped femtosecond laser two-photon polymerization, and feature size of λ/12 is achieved. 5) We propose a new and high-throughput method of temporally shaped femtosecond laser fabrication of microchannels and discuss the influence of parameters on the microchannels fabrication efficiency. 6) We study the new phenomenon of anisotropy during temporally shaped femtosecond laser direct writing on silicon surface, and propose that the orientation and geographic morphology of the periodic surface structures can be controlled by adjusting the scan direction, speed and laser polarization direction.The main innovations of the thesis are summarized as follows:1) We propose a new method of temporally shaped femtosecond laser ablation dielectrics, achieving high-accuracy and high-quality micro/nano fabrication by adjusting electron excitation, ionization, recombination and phase change. Firstly, a new phenomenon of ablation enhancement is observed between 100 fs and 200 fs by adjusting the pulse separation time. Secondly, the ablation diameter and depth can be controlled by changing the subpulse number in a pulse train. The last, materials’ phase change can be selectively triggered by adjusting the pulse energy distribution ratio, minimizing the recast and improving the surface ablation quality.2) We propose a new method of temporally shaped femtosecond laser ablation microchannels in dielectrics, realizing high-throughput micro/nano fabrication by adjusting the electron density distribution. Firstly, we propose a microchannels fabrication method using liquid assisted femtosecond laser pulse train. Compared with the conventional method under the same experimental condition, the ablation efficiency is increased by 56 times, and the aspect ratio of microchannels fabricated by single scan is increased by 3 times. Secondly, we propose a microchannels fabrication method using femtosecond laser pulse train irradiation followed by chemical etching. Compared with the conventional method under the same experimental condition, the etching rate of the irradiation zone is increased by 10 times. In addition, the cross section shape of the microchannel is improved by decreasing the scan speed, and optimization fabrication is realized. The last, we propose that the influence of the laser polarization on the chemical etching of the laser irradiation zone can be eliminated by using femtosecond laser pulse trains, achieving isotropic chemical etching during microchannels fabrication.3) A new phenomenon of anisotropy is observed during temporally shaped linearly polarized femtosecond laser direct writing on silicon surface. The surface micro/nano structures can be controlled by adjusting the plasma formation and distribution. In fixed-spot irradiation, the geographic morphology and periodicity of the periodic surface structures can be manipulated by adjusting the subpulse delay, while in laser direct writing, the anisotropy phenomenon is firstly proposed, and we also reveal that the geographic morphology and orientation of the periodic surface structures can be manipulated by changing the laser polarization, scan direction and scan speed.