The Formation Dynamics Of Subwavelength Perioripples On Metal Film Surface Induced By Femtosecond Laser

Laser-induced periodic surface structures(LIPSSs)on solid matter surfaces are universal phenomena which have been investigated intensely in the last five decades.Over the past fifty years,the mechanism of LIPSS formed on the material surface has been a perplexing problem.Especially in the past more than and 20 years,the subwavelength and deep subwavelength periodic surface structures induced by the femtosecond laser on the surface of metal,semiconductor and transparent materials has attracted wide attention.By studying the nanostructures formed by laser irradiation on the surface of material under different conditions through scanning electron microscope,the laser and surface scattering light interference model,laser and surface plasmon interaction model are all introduced to explain the formation mechanism of LIPSS.Ultrafast microscopic imaging is an effective method to study the ultrafast evolution of micro and nano structures on the surface of materials under femtosecond laser irradiation.In the project,we researched the subwavelength periodic ripples induced by a single femtosecond laser pulse on Ni-Fe film surface and their ultrafast dynamics.We also studied the ultrafast dynamics of the subwavelength periodic ripples formation on gold film irradiated by multiple femtosecond laser pulses.The innovative results are as follows:(1)The influences of surface plasmons and thermal effects on the formation of subwavelength periodic ripples on Au films irradiated by 800 nm femtosecond laser pulses were studied by collinear pump-probe imaging.The spatial and temporal resolution of the experiment was 300 nm and 1 ps,respectively.No periodic ripples were observed on the Au film during ablation by the first pump pulse,but during ablation by the second pump pulse the appearance of transient ripples was evident from a delay time of hundreds of picoseconds to several nanoseconds.These ripples,however,were not retained after solidification.When the sample was immersed in water during ablation,however,the ripples were retained.It is proposed that,during the second laser pulse irradiation,the surface defects produced by the first pulse induced surface plasmon polaritons on the Au film,which caused a modulated energy deposition and the formation of transient ripples.The weak electron-phonon coupling and significant residual heat erase these ripples after the molten surface was solidified(2)Multiple pulses irradiation will introduce unwanted surface defects and the influence of the analysis of the formation mechanism.So this paper studies periodic surface structures induced by a single laser pulse.It is a better way to the origin and mechanism of femtosecond laser induced periodic surface structures.The formation of periodic surface structures on Ni-Fe film irradiated by a single femtosecond laser pulse was studied.A concave lens with a focus length of-150 mm was placed in front of an objective(100×,NA=0.9),which makes the Gaussian laser field into ring distribution by the Fresnel diffraction.Periodic ripples formed on the ablation area after irradiation of a single femtosecond laser pulse,which depended on the laser polarization and laser fluence.We propose that the ring structure of the laser field leads to a similar distribution of transient permittivity on the sample surface,which further launches the surface plasmon polaritons.The interaction of the incident laser with surface plasmon polaritons dominates the formation of periodic surface structures.