Study Of High Efficient Femtosecond Laser Subwavelength Fabrication On Metal Surfaces

The micro/nano structure fabrication technology is an important foundation for the material surface re-functionalization and its applications.With the rapid development of the laser technology,the femtosecond laser has been successfully used for the high precision fabrication and property modification of surfaces of various materials,by virtue of the unique advantages of ultra-short pulse width and ultra-high peak power,which brings many innovative developments in the fields of micro-optics,micromechanics and micro-fluids.Until now,the femtosecond laser micro/nano-fabrication technology has been developed into an essential part of advanced or high-tech manufacturing.However,this technology still has a problem that the structural accuracy strictly depends on the beam focusing conditions,which fundamentally gives limitations on its application.In other words,the contradiction between the high precision and high efficiency has become the key problem need to be solved in the femtosecond laser micro/nano-fabrication technology,which is extremely important and urgent.Fortunately,in recent years the research on the femtosecond laser subwavelength fabrication based on laser-induced periodic surface structure(LIPSS)has provided new ideas for solving the above problems.This technology can not only quickly fabricate submicron or even nanoscale structures,but also hardly suffer from the limitation of laser focusing condition,so it promises a huge application potential in the high efficient fabrication of nanostructures.Different from the random thermal motion of the material electrons in the traditional light-matter interaction process,this technology essentially utilizes the collective physical response of the electrons on the material surface to modulate the incident laser energy within a spot into the spatially subwavelength periodic distributions.For the femtosecond laser subwavelength fabrication technology based on LIPSS,many transient physical processes are involved during the femtosecond laser-material interaction,especially the collective response of electrons(related to the optical properties of the material surface)and subwavelength localized near-field enhancement effects.Therefore,there is still a long way to master this technology.Compared with semiconductors and transparent dielectric materials,metals have the characteristics of high mechanical strength and robustness,so the research on interactions of femtosecond laser with the metal surface has attracted great interests.At present,even though a variety of structures on metal surfaces have been obtained using the femtosecond laser subwavelength fabrication technology,to modulate the surface properties in the wettability,optical absorption and thermal radiation and etc.,there still exist many problems such as the simple structural morphology,the poor regularity,the low efficiency and unclear physical mechanisms,which cannot satisfy the application requirements of the surface functional devices.In order to solve the above problems,we need to conduct in-depth researches in both experiment and theory for the femtosecond laser subwavelength fabrication on the metal surfaces,and strive to promote the development and improve the application level of this technology.This thesis mainly studies the fabrication or suppression of the large-area regular subwavelength periodic structures with different morphologies on the metal surface in high efficient and controllable ways,through building a home-made experimental platform under irradiation of double femtosecond laser beams,in which,the emergence of new phenomena,new mechanisms and new effects has been also comprehensively investigated.The main contents are summarized as follows:(1)During the experiment for the formation of one-dimensional subwavelength periodic ripple structures on the metal surface irradiated by a single femtosecond laser beam,we have observed an anomalous phenomenon of the slantwise orientation of the structures,which is neither parallel nor perpendicular to the direction of the laser polarization.According to the measured spatial distribution profiles of the modulation depth and the laser polarization dependent slantwise degrees of the structure orientation,we have proposed a physical model to explain the experimental phenomena,in which a surface slope is caused by the multiple laser pulses to consequently make the SPP excitation the effective component of the incident electric field on it.And the correctness of our theoretical analysis has been validated by the experimental result that reducing the incident laser energy fluence can make the slanting structure disappear.(2)In order to break through the limitations of the subwavelength fabrication technology based on the single femtosecond laser beam,we have employed the collinear time-delayed double femtosecond laser beams with orthogonal polarizations,to achieve the high efficient fabrication of one-dimensional and two-dimensional subwavelength structures on metal surfaces.Through the experimental design of the expanding laser beam and the linear focusing of a cylindrical lens,we have substantially improved the timeliness,flexibility and regularity of the femtosecond laser subwavelength fabrication technology.Besides,both a dynamic range of the laser parameters and the key technical factors for fabricating different structures have been obtained in the experiment.We have also provided some new insights in the theory for the influence of the transient surface properties on the formation of different structures.In addition,the characterization and measurement of the new efficacy have been performed for the subwavelength periodic structures fabricated in our experiments.(3)In the experiment where the metal surface was irradiated by double femtosecond laser beams,we have observed a new phenomenon of the inhibited formation of the subwavelength periodic structures on the metal surface.A comprehensive study has been carried out about the influence of the laser parameters including the laser polarization direction,energy fluence ratio and scanning speed on the structure inhibition effect,and we have grasped the key factors for effective inhibiting the formation of the subwavelength structures.In addition,based on the micro-examination of the surface morphology,we have found that the sizes of the defects on the structure-inhibited surface is smaller than those on the mechanically polished surface.Consequently,this structure inhibition effect of the femtosecond laser has a great potential application in the nanoscale smoothing of the metal surfaces.