Abstract
Ultrafast laser-induced periodic surface subwavelength ripples, categorized based on the ripple period into near-subwavelength ripples (NSRs) and deep-subwavelength ripples (DSRs), are increasingly found in the variety of materials such as metals, semiconductors and dielectrics. The fabrication of hierarchical periodic NSRs and DSRs on the same laser-irradiated area is still a challenge since the connection between the two remains a puzzle. Here we present an experimental study of linearly polarized picosecond laser-induced hierarchical periodic NSRs and DSRs on stainless-steel surfaces. While experiencing peak power density higher than a threshold value of 91.9 GW/cm2, in the laser-scanned area appear the hierarchical periodic NSRs and DSRs (in particular, the DSRs are vertically located in the valley of parallel NSRs). A large area of the uniformly hierarchical periodic NSRs and DSRs, with the spatial periods 356 ± 17 nm and 58 ± 15 nm, respectively, is fabricated by a set of optimized laser-scanning parameters. A qualitative explanation based on the surface plasmon polariton (SPP) modulated periodic coulomb explosion is proposed for unified interpretation of the formation mechanism of hierarchical periodic NSRs and DSRs, which includes lattice orientation of grains as a factor at low peak power density, so that the initial DSRs formed have a clear conformance with the metallic grains.
Highlights
The laser-induced periodic surface structure (LIPSS) [1,2], a by-product effect of laser material processing applications is at its inception, but in the past few years has been a research hotspot that involves multiple branches of physics such as wave optics, non-linear optics, fluid dynamics and thermodynamics
Our experimental exploration was started by counted ps laser pulses stacking hitting a fixed point on the stainless-steel sample surface for investigation of the influences of space-time characteristics on laser-induced near-subwavelength ripples (NSRs)/deep-subwavelength ripples (DSRs), expanded to line scanning for fabrication of large area hierarchical periodic NSRs and DSRs
The results clearly suggest the peak power density plays an important role in the formation of subwavelength ripples
Summary
The laser-induced periodic surface structure (LIPSS) [1,2], a by-product effect of laser material processing applications is at its inception, but in the past few years has been a research hotspot that involves multiple branches of physics such as wave optics, non-linear optics, fluid dynamics and thermodynamics. The NSRs (sometimes called classical ripples) are found to be formed by a wide range of pulse durations from a few hundred nanosecond (ns) to femtosecond (fs) laser irradiation on the solid material surface, which makes it seem like a universal pulse laser-induced material response. The excitation of surface plasmon polaritons (SPPs) by incident laser pulses is widely used for the explanation of NSR formation [3]. Huang et al [3] compared experiments with metal, semiconductor and dielectric and assumed the grating-assisted surface plasmon (SP)-laser coupling should be responsible for the origination of NSRs. Reif et al [14,15] considered the ion sputtering and thin liquid film, and proposed the self-organized effect of the non-stable material to explain the formation of subwavelength ripples. Diverse mechanisms are proposed for explanation of the origins of DSRs, such as second and higher harmonic generation [10,20], SPPs [11] and self-organized effect [14,21]
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