Ablation Efficiency And Fluorescence Emission Of Silicon Under The Irradiation Of Femtosecond Double Pulses

In recent years, laser ablation technology has been widely applied in scientificexperimental studies and industrial fabrications. Because of the short duration ofultrashort laser pulse, a smaller heat-affect region will be produced in target surfacewhich greatly improved the accuracy and precision in material fabrication. Ultrashortlaser micro-fabrication has wide potential applications in many fields.In this article, the interaction mechanisms of femtosecond double-pulse ablationare studied by measuring the fluorescence emission from plasma and morphology,depth and volume of ablation crater, under different experimental conditions likepressure, fluence or pulse delay. Firstly, laser pulses with different fluences are usedto ablate Si target in air condition, fluorescence intensities are scanned by varying thedelay time between the double-pulse. At lower laser fluences, trend of fluorescencechange as a function of double pulse delay is similar to that has been observed inprevious-double experiments, i.e. fluorescene intensity will increase with increasingdelay time. This is attributed to the stronger coupling effect between the second pulseand liquid layer which is formed by the first one. With the fluence further increasing,stronger plasma is produced, density gradient increases with increasing delay timewhich will decrease the absorption efficiency of plasma to the second pulse, sofluorescence intensity will decrease with delay time. When the fluence is high enough,fluorescence intensity has no obvious dependence on delay-time, possibly because of saturation of the carrier density, saturation of conductivity, or critical point phaseseparation. In vacuum, the trend of fluorescence intensity change is similar to thetrend in air condition.In addition, the influence of gas environment pressure on double-pulse ablationis studied. It is found that the fluorescence intensity increase with increasing pressurein the range of0Pa-103Pa. When the pressure is103Pa, the intensity reaches itsmaximum value. The intensity starts to decrease in the range of103-105Pa. Weattribute this result to the complex combined effect of plasma energy coupling totarget sample and environmental gas.The volume, depth and morphology of ablation craters obtained under differentpressures are studied. Similar to fluorescence intensity, the depth of craters increaseswith pressure initially, but declines after it reaches a maximum value at104Pa. It canbe seen from the images of ablation crates that compared with those obtained athigher pressure condition, more ejected material is found around the crater at lowerpressure condition.