Defocusing Femtosecond Laser Ablation On Silicon

In recent years,with the progress of science and industry,laser machining has reached micro-nano scale.Femtosecond laser processing is widely used in scientific research,medicine and industrial manufacturing because of its simple operation,high efficiency and high precision.Femtosecond laser ablation is the basis of femtosecond laser processing.Due to the ultrashort pulse width and very high peak power compared to long pulse processing,femtosecond pulse has brought distinct advantages such as high precision machining,small heat affected area and so on.The ablation phenomena and mechanisms are also more complex,which have been widely studied and discussed recently.Based on the single pulse and multi-pulse experiments of femtosecond laser on monocrystalline silicon,the influences on surface ablation of different laser parameters and environmental parameters on the focal plane,the front focal plane and the back focal plane were studied.To find the optimal processing parameters the ablation threshold of monocrystalline silicon was measured to study the amorphization,oxidation and ablation phenomena under different pulse energies and.In the process of the femtosecond laser scribing,the generation of homogeneous amorphous white line and the phenomenon of recrystallization by secondary scanning were observed.In the process of defocusing femtosecond laser experiments,the phenomenon of twisting caused by the plasma due to tight focus is observed.The concentric ring structure and the inconsistent experimental phenomena produced by two different types of objective lens are found.Through experiments and simulation,we found that these phenomena and differences are the result of lens diffraction and spherical aberration.In the study of defocused femtosecond laser ablation using oil lens,we also found similar concentric rings structure.It is demonstrated that this is due to the fact that the second pulse passes through the bubbles generated by the first pulse in the highly viscous liquid and is formed by refraction and interference.