Research On Material Modification By Femtosecond Pulse Trains Based On Electron Dynamic Control

Femtosecond(fs) lasers, with ultrahigh power intensities and ultrashort irradiation periods, have incomparable advantages in fine processing of materials. A femtosecond laser pulse train which contains several subpulses can be made by shaping the femtosecond pulse energy distribution in temporal domains. By changing the numbers of the subpulses, laser pulse delay, laser fluence, and pulse energy distribution ratio, localized transient free electrons dynamics in the material can be controlled, which induces great changes in physical and chemical properties.The main research contents and innovations of this thesis are summarized as follows:(1) We developed an efficient fabrication method of high-quality concave microarrays on fused silica substrates based on temporal shaping of fs laser pulses. This method involves exposures of fs laser pulse trains followed by a wet etching process. Compared with conventional single pulses with the same processing parameters, the temporally shaped fs pulses can enhance the etch rate by a factor of 37 times.(2) By changing the laser pulse delay, laser fluence, and pulse energy distribution ratio, the profile of the concave microarray structures can be tuned, which improves the flexibility and controllability of the manufacture. The experimental results indicate that for the etch rate enhancement by the fs pulse train, 350 fs is the optimum pulse delay and 1:1 is the optimum energy ratio.(3) Micro-Raman spectroscopy was conducted to characterize the internal structure of the sample. The Raman spectra of original fused silica and the fs-laser-modified zones in the material were plotted. By calculating the peak areas of different Raman peaks, we found that the relative content of planar 3-membered rings in fs double pulse irradiated zone was obviously higher than that in single pulse irradiated zone. Since such 3-membered rings structure is more vulnerable to the attack by HF, the strong etch rate enhancement induced by fs double pulse can be verified.(4) The fs laser pulse train enhanced chemical etching can be theoretically explained using the plasma model: The free electron density in the material can be controlled during the irradiation by fs laser pulse train, which induces great changes in localized transient physical properties. In the area being irradiated by fs pulses, the reflectivity of the dielectric material changes obviously and tremendously reshapes the ultrafast laser field, which strongly affects the photon-electron interactions. Thus, the photon absorption efficiency is enhanced greatly and it results in the obvious changes in the Si–O chemical bonds. Finally, in the irradiated zone, the chemical activity in reactions with HF improves significantly.(5) The fabrication of large-area, high-quality microlens arrays has been realized by the method of femtosecond laser pulse train irradiation followed by chemical etching.