Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number,which in turn signifcantly afects successive laser-material interactions.By recording the dyn...Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number,which in turn signifcantly afects successive laser-material interactions.By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy,here we present direct visualization of the excitation of air plasma induced by the refected laser during the second pulse irradiation.Te interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction,showing anisotropic expansion dynamics in diferent directions.We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability.In the scenario,the interaction of air plasma and silicon plasma disappears;the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling.Te results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number,which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.展开更多
The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during t...The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface(metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.展开更多
Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide(MoS_(2));however,most of them were confined to the level below the damage threshold.Here the electro...Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide(MoS_(2));however,most of them were confined to the level below the damage threshold.Here the electron dynamics of MoS_(2) under intense ultrafast laser irradiation was investigated by experiments and simulations.Two kinds of ablation mechanisms were revealed,which led to two distinct types of electron dynamics and final ablation morphology.At a higher fluence,the emergence of superheated liquid induced a dramatic change in the transient reflectivity and micro-honeycomb structures.At a lower fluence,the material was just removed by sublimation,and the ablation structure was relatively flat.X-ray photoelectron spectroscopic(XPS)measurements demonstrated that thermal decomposition only occurred at the higher fluence.Furthermore,a theoretical model was developed to deeply reveal the ultrafast dynamics of MoS_(2) ablation.The simulation results were in good agreement with the temporal and spatial reflectivity distribution obtained from the experiment.The electron and lattice temperature evolution was also obtained to prove the ablation mechanism.Our results revealed ultrafast dynamics of MoS_(2) above the damage threshold and are helpful for understanding the interaction mechanism between MoS_(2) and intense ultrafast lasers,as well as for MoS_(2) processing applications.展开更多
基金Tis research was supported by the National Key R&D Program of China(grant no.2017YFB1104300)and the National Natural Science Foundation of China(grant nos.91323301,11704028).
文摘Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number,which in turn signifcantly afects successive laser-material interactions.By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy,here we present direct visualization of the excitation of air plasma induced by the refected laser during the second pulse irradiation.Te interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction,showing anisotropic expansion dynamics in diferent directions.We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability.In the scenario,the interaction of air plasma and silicon plasma disappears;the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling.Te results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number,which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.
基金National Natural Science Foundation of China(NSFC)(51605029,91323301)
文摘The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface(metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.
基金supported by the National Natural Science Foundation of China(Grant No.11704028)the National Key R&D Program of China(Grant No.2017YFB1104300).
文摘Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide(MoS_(2));however,most of them were confined to the level below the damage threshold.Here the electron dynamics of MoS_(2) under intense ultrafast laser irradiation was investigated by experiments and simulations.Two kinds of ablation mechanisms were revealed,which led to two distinct types of electron dynamics and final ablation morphology.At a higher fluence,the emergence of superheated liquid induced a dramatic change in the transient reflectivity and micro-honeycomb structures.At a lower fluence,the material was just removed by sublimation,and the ablation structure was relatively flat.X-ray photoelectron spectroscopic(XPS)measurements demonstrated that thermal decomposition only occurred at the higher fluence.Furthermore,a theoretical model was developed to deeply reveal the ultrafast dynamics of MoS_(2) ablation.The simulation results were in good agreement with the temporal and spatial reflectivity distribution obtained from the experiment.The electron and lattice temperature evolution was also obtained to prove the ablation mechanism.Our results revealed ultrafast dynamics of MoS_(2) above the damage threshold and are helpful for understanding the interaction mechanism between MoS_(2) and intense ultrafast lasers,as well as for MoS_(2) processing applications.
