Controllable Manufacturing of 2D Sub-wavelength Periodic Array Structures by Dual Time-delayed Femtosecond Lasers
Laser Induced Periodic Surface Structures(LIPSSs)are a universal phenomenon produced on almost all types of materials when irradiated by linearly polarized femtosecond laser pulses with the energy fluence near the damage threshold.The LIPSSs featuring period in(deep)subwavelength scale and orientation of laser-wavelength dependence can be easily manufactured in a maskless and single-step process,providing a simple and flexible surface nano-structuring technology on diverse materials.However,the irregularity and lack of diversity of the one-dimensional(1D)LIPSSs morphology produced by a single beam of femtosecond laser degrade the application performance and scope of the surface functionalizations.Recently,the temporally delayed femtosecond lasers irradiation was reported to not only improve significantly the regularity of the 1D LIPSSs formed on metals and semiconductors but also manufacture diverse types of two-dimensional(2D)LIPSSs,including the arrays of nanosquares,nanodots,nanotriangles and nanoholes,but the profile transition among them have been less explored.In this paper,cylindrical focusing of double 400 nm femtosecond lasers with orthogonal polarizations and a time delay of 1.5 ps is utilized to controllable manufacturing of diverse types of 2D LIPSSs on molybdenum,whose morphology features were characterized by both the microscopic observation and Fourier frequency analysis.This investigation focuses on the morphology evolution among the diverse 2D-LIPSSs with the total laser fluence and the fluence ratio of the double lasers.In the experiments,the scanning velocity of the double lasers on Mo surface was fixed as v=0.01 mm/s.At the total fluence of 0.164 mJ/cm2,the double lasers with identical energies bring forth 2D square arrays structures,but with different unit profiles including nanosquares,nanocircles(ellipses)and nanostrips.The 2D square arrays structure is composed of two groups of 1D periodic grooves with orientations perpendicular to the double laser polarizations and periods of approximately 280 nm and 300 nm,respectively.With increasing the fluence ratio to 1.42∶1,the available 2D-LIPSSs on local regions transform into the hexagonal arrays of nanotriangles composed of three groups of periodic grooves with periods of approximately 270 nm,290 nm and 320 nm,respectively.The orientation of one group of grooves is perpendicular to the polarization of the high-fluenced laser while the two other groups are neither perpendicular nor parallel to the double laser polarizations.When continuously to increase the fluence ratio,the two groups of periodic grooves with orientations non-dependence of the double laser polarizations gradually become shallower in depth.When the fluence ratio exceeds 2.37∶1,they disappear entirely,so that the double laser induced surface structure turns to 1D nanograting profile,featuring the extremely long-range uniform distribution.With optimizing the laser parameters,the large-area of 2D arrays of nanocircles and nanotriangles with high uniformity and quality are achieved.The ablation depth evolution of periodic grooves among the diverse LIPSSs implies they are originated from the periodically selective removals of material in different spatial directions induced by the orthogonally polarized double lasers.The selective removals of material are a result of the SPPs excitation and interference with the incident lasers.We utilize Finite Difference Time Domain method to numerically simulate collinear and noncollienar excitation of SPPs on the surface of one group of periodic grooves under the double orthogonally polarized lasers irradiation.Through theory analysis,the origins of 2D arrays of nanocircles and nanotriangles are attributed to two and three noncollinear SPPs excitation during the transiently correlated surface dynamics of dual laser-material interaction,respectively.The time delay between the double lasers is the crucial parameter influencing the structure uniformity in the spatial arrangement.The structural unit profile difference for the same 2D array structures are attributed to the heterogeneous laser energy depositions on different local regions.This investigation demonstrates the dual correlated femtosecond lasers irradiation provides a robust and versatile way for efficient scale-up manufacturing 2D-LIPSSs on metals,promising potential applications in the fields of optics,optoelectronic,tribology,thermology,biomedicine,etc.
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