Morphology And Dynamics Control On Ablation Of Cu Using Shaped Femtosecond Pulse

The duration of femtosecond laser pulses is shorter than the typical time scale of electron-phonon thermal equilibrium.The energy absorption of femtosecond laser pulses during metal ablation is non-thermal and the ablation threshold is usually lower than that of long-pulses.Ultra-short pulse duration and ultra-high power density can greatly reduce the thermal damage of materials and improve the machining accuracy and quality.These characteristics make femtosecond laser pulses play an irreplaceable role in precision micromachining.The pulse shaping system can transform the transformation limit pulse into a "new pulse" with some specific properties,such as increasing the time-domain width of the pulse,generating pulse sequence,changing the time-domain structure of the pulse,so that the shaping pulse can be designed according to the special physical characteristics of the experimental material.In this paper,Sine phase shaped pulse three-pulse train and seven-pulse train are used to ablate single-crystal Cu materials.Sine phase is used to obtain complex shaping pulse sequence to ablate single-crystal Cu material.Sine phase shaped pulse control parameters A and T are used to ablate single-crystal Cu material.Cubic phase shaped pulse are used to ablate single-crystal Cu.Specific research contents are as follows:(1)Two kinds of time domain shaping pulse train(three pulse sequences and seven pulse sequences)generated by Sine phase modulation are used to ablate single-crystal Cu materials.The characteristics of ablation morphology are studied with different experimental conditions,such as pulse energy,number of sub-pulses and interval of sub-pulses,in vacuum environment(0.9 Pa).It is found that the pulse energy,the number of sub-pulses and the interval of sub-pulses all affect the width of ablation region.With the increase of the overall fluence of the pulse train,the width of the ablation region on the sample surface becomes larger,and the width of the ablation region formed by the two shaped pulses under the same fluence condition is similar,but the difference is not obvious.In addition,with the increase of sub-pulse interval,the width of ablation region does not change significantly.Inside the ablation area is a structure similar to a crater.The ablation crater consists of two different slope structures,namely the steep slope structure located in the center of the ablation crater and the circular gentle slope structure located at the edge of the ablation crater.The depth of gentle slope structure is almost constant,while the width of gentle slope increases slowly with the increase of subpulse interval.The depth of the steep slope decreases obviously with the increase of subpulse interval,while the width of the steep slope decreases slowly.The overall depth of ablation crater decreases obviously with the increase of sub-pulse interval,while the overall width increases slowly.The change trend of ablation crater depth is more obvious,but the change trend of width is less.The circular gentle slope structure at the edge of ablation crater is formed under the action of low energy flux and its depth almost does not change with the interval of subpulses.The steep slope structure in the central area of ablation crater is formed by the action of high energy flux and its depth changes obviously with the increase of sub-pulse interval.By fitting the ablation crater total depth curve,the coefficient K using two kinds of pulse trains are extracted experimentally.The plasma shielding effect dominating the ablation processes are modulated by changing sub-pulse interval,and the corresponding physical interpretation are proposed.(2)Fluorescence and ablation crater depth of ablated Cu are studied by using three Sine phase shaped pulse train.It is found that the fluorescence intensity is closely related to the fluence,time-domain structure and sub-pulse interval of the shaped pulse train,and there is also a certain correlation between the ablation depth and the variation of fluorescence intensity.The plasma shielding effect plays a very important role in the ablation process.When As=2.4 and As=3.4,the fluorescence intensity first increases and then decreases with the increase of sub-pulse interval.When As=4.2,the fluorescence intensity increases first,then decreases,and finally increases with the increase of sub-pulse interval.The variation law of ablation depth with sub-pulse interval is just opposite to the variation law of fluorescence intensity,indicating that the shielding effect of plasma has a negative mechanism for depth and a positive mechanism for fluorescence excitation.When As=4.2,the secondary eruption of plasma is generated in ablation process,and a two-layer molten condensation structure is formed.When the parameter T is constant and A added from 0.2 to 7.2,equivalent to an overall increase pulse temporal width,pulse number increase,once again,the plasma secondary eruption and formation of the shielding effect,the changing rule of the fluorescence intensity and ablation depth is still on the contrary,the melt condensation again,there is double circular structure.(3)The effect of subpulse interval of Sine phase shaping pulse on ablation depth and the control and optimization of related mechanism in ablation process were studied.Has chosen three pulse train(TPT)and seven pulse train(SPT),change the total fluence of the pulse and the pulse interval ?t,based on the electronic multiple heating and plasma shielding mechanism,through the control the number of pulse and pulse interval,the changing rule of the ablation depth of the crater is studied.When the sub-pulse interval is less than 0.4 ps,the multiple heating effect is stronger than the plasma shielding effect.When the sub-pulse interval is greater than 0.4 ps,the plasma shielding effect is the main mechanism.With the increase of the sub-pulse interval,the electron multiple heating effect weakens and the plasma shielding effect increases.The effect of electron multiple heating is more obvious when the seven pulse sequence ablates the sample material.In addition,the effect of the coherent phonon oscillation excited by the pulse train on the ablation depth is studied.The variation of ablation depth is the result of the superposition of the plasma shielding effect and the coherent phonon oscillation.The effects of different shaped conditions on the morphology and ablation depth of Cu materials were analyzed by using cubic phase shaped pulse ablation.Based on the electron multiple heating mechanism,the advantages of negative third phase in ablation were analyzed.