The Ablation And Surface Modification Mechanisms Of Pulsed Laser Interaction With Optical Materials

In this dissertation,the mechanisms of ablation and modification of pulsed laser irradiation of optical materials are studied theoretically and experimentally.Based on laser paramaters,material properities and constrained conditions,the formation process and structural characteristics of laser-induced self organized micro/nano structures are studied,and the applications of such micro/nano structures in optics are explored.First,the thin chrominum film is utilized as the absorber medium to carry out the experiment of laser-induced frontside etching of silicon by nanosecond laser.The influences of laser fluence,pulse number,film thickness and surface modification on the etching features are studied.The results show that the surface morphologies of the Cr/Si system can be classified into three categories according to the incident laser fluence:(1)the chrominum film melting and dewetting under low fluence region;(2)the etching under middle fluence region and(3)silicon ablation under high fluence region.The etching rate is increasing linear with the incident laser fluence in the medium fluence range.At low fluence region,period ripples and some partial ordered dot pattern are observed on the film.The surface modifications in the near surface due to the process of interaction between chrominum film and the silicon substrate after laser irradiation plays a stimulative role in realizing the follow-up process of silicon etching without absorber.Second,the laser-induced period surface structures(LIPSS)are constructed by nanosecond Excimer laser irradiation on photoresist film.The influences of laser fluence,pulse number and film thickness on the ablation morphology and ablation mechanism are studied.The results show that LIPSS formed in a small fluence range and the formation threshold is between the threshold of film melting and ablation.The ablation depth increases with the increasing laser fluence.The threshold of LIPSS formation and the film ablation both decreases with the increasing pulse number.Third,a method to form the hierarchical peridic structures is proposed by restricting the LIPSS formation region.The results show that the laser energy distribution deposited on the film surface is changed by mask guidance,and the LIPSS on the film surface developes from linear fringe to a point-chain structure.With the guidance of prefabricated thin film cylindrical and strip structures on the film surface,the formation position of LIPSS develops and extends from the top of the thin film column/strip to the lateral wall with the increase of laser energy density,forming a highly multi-layered periodic structure.With the increase of the number of laser pulses,the height of LIPSS on the film surface gradually increases and eventually becomes stable,but the formation position of LIPSS will not be changed.Finally,the formation of bubbles near the surface of soda-lime glass induced by single pulse femtosecond laser irradiation below the laser ablation threshold is studied.The results show that the bubble forming process is related to nonlinear processes of laser photo absorption,laser-induced spallation processes,material heating and thermal driven,viscoplasticmechanical process.The size of the bubbles is much smaller than the beam radius.The size of the bubbles can be controlled by adjusting the incident laser pulse energy.The formation of bubbles can be controlled by laser irradiation after different types of surface modifications of glass.The composition of surface modification layer has an important influence on the formation or inhibition of bubbles.The outcome of this dissertation will be helpful for applications of ns laser and fs laser in laser micro-machining of optical materials and provides a new method for laser-induced selforganized structures fabrication in future.