Formation Of Laser-induced Periodic Surface Structures Research Articles

Formation of the Submicron Oxidative LIPSS on Thin Titanium Films During Nanosecond Laser Recording.

Laser-induced periodic surface structures (LIPSSs) spontaneously appearing on the laser-treated (melted or evaporated) surfaces of bulk solid materials seem to be a well-studied phenomenon. Peculiarities of oxidative mechanisms of LIPSS formation on thin films though are far less clear. In this work, the appearance of oxidative LIPSSs on thin titanium films was demonstrated under the action of commercially available nanosecond-pulsed Yb-fiber laser. The temperature and energy regimes favoring their formation were revealed, and their geometric characteristics were determined. The period of these LIPSSs was found to be about 0.7 λ, while the modulation depth varied between 70 and 110 nm, with high stability and reproducibility. It was shown that LIPSS orientation is rather easily manageable in the regimes of our interest, which could provide a way of controlling their properties.

Generation, control and erasure of dual LIPSS in germanium with fs and ns laser pulses

Laser-induced periodic surface structures (LIPSS) can readily be fabricated in virtually all types of materials and benefit from an efficient parallel patterning strategy that exploits self-organization. The wide range of different LIPSS types with different spatial scales and symmetries is continuously growing, addressing numerous of applications. Here, we report on the formation of two fundamentally different types of LIPSS on germanium upon exposure to femtosecond laser pulses (λ = 800 nm, 130 fs), featuring different periods and orthogonal orientations. On the one hand, the well-known low-spatial frequency LIPSS (LSFL) with a period ≈ λ and perpendicular orientation to the laser polarization are formed, which can be extended homogeneously in 2D by sample scanning. Additionally, extremely smooth ripples with a period ≈λ/2 and parallel orientation were generated at lower pulse numbers. We show that this new kind of ripples, named parallel high-spatial frequency LIPSS (HSFL-∥), can be superimposed onto LSFL by increasing the pulse number, forming complex dual LIPSS with nanohill-like morphology. While exposure to multiple nanosecond laser pulses is found to trigger also the formation of LSFL, HSFL-∥ cannot be formed under these conditions, which points out the role of ultrafast excitation in the formation of the latter. By performing time-resolved reflectivity measurements, we are able to resolve the melting and solidification dynamics, revealing melting of a very shallow surface layer (<20 nm) and melt durations of a few ns for both pulse durations pulses at the fluences employed for LIPSS formation. Finally, we demonstrate erasure of both types of LIPSS by exposure to single nanosecond pulses at high fluences, which paves the way for erasable multi-level data storage.

Corrosion performance of DLC coatings with laser-induced graphitized periodic surface structure

Two types of diamond-like carbon (DLC) thin films, hydrogenated amorphous carbon (a-C:H) and super-hard tetrahedral amorphous carbon (ta-C), were irradiated with femtosecond laser pulses, and the corrosion resistance of the DLC films with laser-induced periodic surface structure (LIPSS) and modified to nanocrystalline graphite (nc-G) was electrochemically evaluated from anodic polarization measurements. The polarization curve for a glassy carbon (GC) plate, which is a type of nc-G, was measured for comparison. The polarization curve for stainless steel with LIPSS was also measured, and it was confirmed that LIPSS increased the surface area of the irradiated part, which resulted in an increased apparent corrosion current density. Polarization curve measurements for irradiated DLC indicated that the corrosion current density was reduced, despite the effect of an increased surface area by the formation of LIPSS, and the polarization curves tended to approach those for GC plates. Therefore, the irradiated surface layer of DLC is modified to nc-G, which results in excellent corrosion resistance similar to that of GC.

Large-Area Fabrication of Laser-Induced Periodic Surface Structures on Fused Silica Using Thin Gold Layers.

Despite intensive research activities in the field of laser-induced periodic surface structures (LIPSS), the large-area nanostructuring of glasses is still a challenging problem, which is mainly caused by the strongly non-linear absorption of the laser radiation by the dielectric material. Therefore, most investigations are limited to single-spot experiments on different types of glasses. Here, we report the homogeneous generation of LIPSS on large-area surfaces of fused silica using thin gold layers and a fs-laser with a wavelength λ = 1025 nm, a pulse duration τ = 300 fs, and a repetition frequency frep = 100 kHz as radiation source. For this purpose, single-spot experiments are performed to study the LIPSS formation process as a function of laser parameters and gold layer thickness. Based on these results, the generation of large-area homogenous LIPSS pattern was investigated by unidirectional scanning of the fs-laser beam across the sample surface using different line spacing. The nanostructures are characterized by a spatial period of about 360 nm and a modulation depth of around 160 nm. Chemical surface analysis by Raman spectroscopy confirms a complete ablation of the gold film by the fs-laser irradiation. The characterization of the functional properties shows an increased transmission of the nanostructured samples accompanied by a noticeable change in the wetting properties, which can be additionally modified within a wide range by silanization. The presented approach enables the reproducible LIPSS-based laser direct-writing of sub-wavelength nanostructures on glasses and thus provides a versatile and flexible tool for novel applications in the fields of optics, microfluidics, and biomaterials.

Periodic surface functional group density on graphene via laser-induced substrate patterning at Si/SiO2 interface

Controlling the spatial distribution of functional groups on 2D materials on a micrometer scale and below represents a fascinating opportunity to achieve anisotropic (opto)electronic properties of these materials. Periodic patterns of covalent functionalization can lead to periodic potentials in the monolayer; however, creating such superstructures is very challenging. Here, we describe an original approach to the periodic functionalization of graphene induced by substrate patterning using a pulsed laser. Laser-induced periodic surface structures (LIPSS) are produced on silicon wafers with thermally-grown oxide layers. The irradiation conditions for the formation of LIPSS confined at the SiO2/Si interface have been unravelled. LIPSS imprint their periodicity to the reactivity of the monolayer graphene placed on the substrate via modulation of its local doping level. This method is clean, straightforward and scalable with high spatial resolution.

Inducing LIPSS on multilayer thin metal films by femtosecond laser beam of different orientations

The occurrence of laser-induced periodic surface structures (LIPSS) has been known for a while. Multilayer thin films, like Al/Ti, are suitable for LIPSS formation and attractive for applications—due to their wearing behavior and corrosion resistance; LIPSS generation may improve their properties as well. LIPSS properties depend not only on the material but also on the beam characteristics, like wavelength, polarization and scanning directions, etc. After exposing with NIR femtosecond pulses from Coherent Mira 900 laser system in several beam exposures, we have analyzed the samples of thin metal film systems with Tescan Mira3 SEM and NTegra AFM. The formation of LIPSS is most probably due to the generation of surface plasmon polariton, through the periodic distribution of energy in the interaction zone which lead to thermal processes in layers and interfaces. Two types of LIPSS were generated, which differ in shape, orientation and in ablation pronounced or not. For consecutive interactions in the same direction, LIPSS maintained its orientation, while for orthogonal passes LIPSS with mutually orthogonal orientation were generated. LIPSS period fluctuated between 320 and 380 nm and structures with pronounced ablation have significantly smaller width. Probable mechanism is that for greater accumulated energy pronounced ablation takes place giving LIPSS in the form of trenches or grooves, while for less accumulated energy the buildup of the material—probably due to pronounced oxidation—lead to LIPSS in the form of hills or ridges.

Picosecond laser-induced periodic surface structures (LIPSS) on crystalline silicon

In this paper, laser-induced periodic surface structures (LIPSS) are created by a 532 nm picosecond laser with the pulse duration of 8 ps and the pulse repetition rate of 1 kHz on crystalline silicon. The number of the pulse and the fluence of the laser were changed in the experiment. With the same laser fluence, the uniformity of the ripples is improved by increasing the pulse number. With the same pulse number, the period of the ripples is increased by increasing the laser fluence. Morphologies of LIPSS are measured by AFM. Ripples oriented perpendicular to the polarization of the laser light with two different periods are found in one spot. The period of the LIPSS in the center is 1.38 μm, while it is 528.5 nm at the periphery of the spot. The height from peak to valley of the LIPSS in the center is 59.82 nm, while it is 33.58 nm at the periphery of the spot. It is found that, the laser-induced surface modifications can be classified into three different categories, including LIPSS molten, LIPSS and LIPSS not appear. The threshold of the laser fluence and pulse numbers for the formation of LIPSS is determined. LIPSS created on the crystalline silicon surface can improve material surface properties, e.g. reflectivity or wettability. Those properties have a far-reaching significance for potential application in many fields, such as solar energy utilization, specific wavelength absorption, image security, material lubrication and anti-corrosion.

Laser-Induced Periodic Surface Structuring of Poly(trimethylene terephthalate) Films Containing Tungsten Disulfide Nanotubes.

We report the study of the formation of Laser Induced Periodic Surface Structures (LIPSS), with UV femtosecond laser pulses (λ = 265 nm), in free-standing films of both Poly(trimethylene terephthalate) (PTT) and the composite PTT/tungsten disulfide inorganic nanotubes (PTT-WS2). We characterized the range of fluences and number of pulses necessary to induce LIPSS formation and measured the topography of the samples by Atomic Force Microscopy, the change in surface energy and contact angle using the sessile drop technique, and the modification in both Young’s modulus and adhesion force values with Peak Force-Quantitative Nanomechanical Mapping. LIPSS appeared parallel to the laser polarization with a period close to its wavelength in a narrow fluence and number of pulses regime, with PTT-WS2 needing slightly larger fluence than raw PTT due to its higher crystallinity and heat diffusion. Little change was found in the total surface energy of the samples, but there was a radical increase in the negative polar component (γ−). Besides, we measured small variations in the samples Young’s modulus after LIPSS formation whereas adhesion is reduced by a factor of four. This reduction, as well as the increase in γ−, is a result of the modification of the surface chemistry, in particular a slight oxidation, during irradiation.

Laser-induced graphitized periodic surface structure formed on tetrahedral amorphous carbon films

Femtosecond laser-induced periodic surface structure (LIPSS), graphitization and swelling observed on ultra-hard, hydrogen-free tetrahedral amorphous carbon (ta-C) films are examined and compared with those on hydrogenated amorphous carbon (a-C:H) films, nitride films, and glassy carbon plates. The threshold fluence for LIPSS formation on ta-C is approximately twice as high as that for other specimens, and the LIPSS period Λ near the threshold is very fine at ca. 80 nm. Λ gradually increases with increasing fluence, and rapidly increases to ca. 600 nm at a high fluence. The ablation rate also increases rapidly at this fluence. In addition, ta-C and a-C:H are graphitized by irradiation and expand in volume. The surface layer of ta-C film changes to nanocrystalline graphite as the fluence increases and the crystallinity is improved; however, at higher fluence, the crystallinity deteriorates suddenly similar to that at low fluence. At high fluence, the rapid increase in Λ and the ablation rate, and the sudden deterioration in crystallinity are determined as common phenomena for these disordered carbons. LIPSS formation and swelling over a large area by scanned spot irradiation produces submicron height flat hills with conductivity and surface functionality on the insulating surface.

Generation and Characterization of Laser Induced Periodic Surface Structures on Plastic Injection Molds

In injection molding, high pressure is required to fill the mold, due to the high viscosity of thermoplastic polymers, the reduced thickness of the cavity and the low mold temperature. In this work, we investigate the functionalization of mold cavities for Injection Molding by means of Laser-Induced Periodic Surface Structures (LIPSS) generated on the ground surface of AISI 420 stainless steel through a picosecond laser. The LIPSS formation is examined with reference to the process parameters, highlighting how they affect the generated pattern. The improvements of the functionalized mold surface on the Injection Molding process are experimentally characterized and discussed. In particular, the slipping of molten PET was investigated as a function of nano-structuring orientation and injection velocity. The results demonstrate that LIPSS parallel to flow induce strong wall slip of the polymer melt, allowing a maximum reduction of the injection pressure of 10%.

Formation of laser-induced periodic surface structures on different materials: fundamentals, properties and applications

Abstract The use of ultra-short pulsed lasers enables the fabrication of laser-induced periodic surface structures (LIPSS) on various materials following a single-step, direct-writing technique. These specific, well-ordered nanostructures with periodicities in the order of the utilised laser wavelength facilitate the engineering of surfaces with functional properties. This review paper discusses the physical background of LIPSS formation on substrates with different material properties. Using the examples of structural colours, specific wetting states and the reduction of friction and wear, this work presents experimental approaches that allow to deliberately influence the LIPSS formation process and thus tailor the surface properties. Finally, the review concludes with some future developments and perspectives related to forthcoming applications of LIPSS-based surfaces are discussed.

Femtosecond laser-induced periodic surface structures formation on bismuth thin films upon irradiation in ambient air

The formation of laser-induced periodic surface structures (LIPSS) on a bismuth thin film with a femtosecond laser is reported for the first time. The surface structures were generated at normal incidence in air environment with fluence below the ablation threshold. The effect of the number of pulses on the LIPSS morphology and on their chemical composition was investigated. The results show the generation of low spatial frequency LIPSS (LSFL), accompanied by regularly distributed nanoparticles. The LIPSS are oriented perpendicular to the laser polarization with a period ΛLSFL that doesn't show a significant variation with the number of pulses (∼1µm). In contrast to previous studies for LIPSS formation on bismuth films under ns laser irradiation [Opt. Mater. Express 7, 1777 (2017)], in the present case no oxidation was observed.

Picosecond laser-induced surface structures on alloys in liquids and their influence on nanoparticle productivity during laser ablation.

The productivity of nanoparticles formed by laser ablation of gold-silver and iron-gold alloy as well as copper and iron-nickel alloy targets in water is correlated with the formation of laser-induced surface structures. At a laser fluence optimized for maximum nanoparticle productivity, it is found that a binary alloy with an equimolar ratio forms laser-induced periodic surface structures (LIPSS) after ablation, if one of the constituent metals also form LIPSS. The ablation rate of nanoparticles linearly depends on the laser fluence if LIPSS is not formed, while a logarithmic trend and a decrease in productivity is evident when LIPSS is formed. To cancel LIPSS formation and recover from this decrease, a change to circularly polarized light is performed and an increase in nanoparticle productivity of more than 30% is observed.

Guiding of LIPSS formation by excimer laser irradiation of pre-patterned polymer films for tailored hierarchical structures

LIPSS (Laser Induced Periodic Surface Structures) formation at pre-patterned photoresist films was investigated using a partial polarized excimer laser beam (λ = 248 nm, tp = 20 ns). These line and dot pre-patterns were fabricated by the lithography in a photoresist films and have widths of 1–3 µm and 1–5 µm, respectively. LIPSS with a period of 195 nm and a direction along the laser polarization were formed at very low laser fluences (threshold 7 mJ/cm2) and high pulse numbers. With increasing fluence first LIPSS appear at the top surface, extend than to the sidewalls and finally at fluences higher than (F ∼ 17 mJ/cm2) found only at the sidewalls while the top surface is smooth. The height of the LIPSS increases with the pulse number and saturates at approximately 65 nm. The degree of LIPSS order depends on the alignment of the laser polarization to the pattern direction and shape. For parallel alignment of polarization and pattern edges, a high degree of order was found over several hundreds of micrometers along the stripe structure. This order mechanism can accept alignment mismatches up to 10°. This approach may serve as a general guideline for forming well-ordered LIPSS on large areas.

Generalized Plasmonic Modelling of the Effect of Refractive Index on Laser-Induced Periodic Nanostructures

Laser-induced periodic surface structures (LIPSS) have been studied theoretically employing generalized plasmonic modelling on several dielectric materials such as SiO2, Al2O3, ZnO, AlAs and diamond exposed to 800 nm wavelength multi-pulse femtosecond laser irradiation. The study of the optical properties of the materials during laser irradiation reveals a formation of a metallic like pseudo-material on the irradiated layer during excitation. A study of the grating periodicity of the nanostructures shows that the materials having a high refraction index allow LIPSS formation with a wide range of grating periodicities. Results also show High Spatial Frequency LIPSS formation with periodicities 3 to 8 times lower than the laser wavelength.

Surface behavior of 16Cr3Al ODS steel—Effects of high laser intensity 1014 W/cm2 in ambiences of air, helium and vacuum

The behavior of 16Cr3Al ODS steel (oxide dispersion strengthened steel), widely employed structural fusion material, under high-intensity laser radiation with intensity up to 1014 W/cm2 was investigated in air, helium and vacuum surrounding. Employed system was 65 fs laser at 804 nm, with applied pulse energy up to 5.25 mJ. Morphological effects were studied – cracking, crater parameters (depth, cross-section), LIPSS (laser-induced periodic surface structures) formation at the crater periphery, hydrodynamic effects, as well as chemical variations on the surface. Ablation thresholds were also determined for all three ambiences (for 100 applied pulses), and they were 0.30 J/cm2, 0.23 J/cm2 and 0.39 J/cm2 in air, helium and vacuum, respectively. Plasma occurred in all experiments and it was most prominent in vacuum due to strongest laser-material coupling.

Tribological performance of metal-reinforced ceramic composites selectively structured with femtosecond laser-induced periodic surface structures

The impact of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on tribological properties was investigated for metal-reinforced ceramic composites (Al2O3-ZrO2-Nb). For this purpose, the metallic niobium (Nb) phase was selectively structured with LIPSS in an air environment with different values of the fs-laser peak fluence by near-infrared fs-laser radiation (λ = 1025 nm, τ = 300 fs, frep = 1 kHz), taking advantage of the different light absorption behavior of ceramic and metal. The tribological performance was evaluated by reciprocating sliding tests in a ball-on-disc configuration using Ringer's solution as lubricant. The surfaces were characterized before and after laser irradiation by optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy and by measuring the contact angle with Ringer's solution. The LIPSS formation resulted in an increased wetting of the surface with the lubricant. Moreover, the selectively structured composite surfaces revealed a coefficient of friction significantly reduced by a factor of ~3 when compared to the non-irradiated surface. Furthermore, the formation of a laser-induced oxidation layer was detected with NbO as the most prominent oxidation state. Selectively structured composites with outstanding mechanical properties and enhanced tribological performance are of particular interest for biomedical applications.

Monitoring of Evolving Laser Induced Periodic Surface Structures

Laser induced periodic surface structures (LIPSS) are generated on titanium and silicon nitride surfaces by multiple femtosecond laser pulses. An optical imaging system is used to observe the backscattered light during the patterning process. A characteristic fringe pattern in the backscattered light is observed and evidences the surface modification. Experiments are complemented by finite difference time domain numerical simulations which clearly show that the periodic surface modulation leads to characteristic modulations in the coherently scattered light field. It is proposed that these characteristic fringe pattern can be used as a very fast and low-cost monitor of LIPSS formation formation during the manufacturing process.

LIPSS on thin metallic films: New insights from multiplicity of laser-excited electromagnetic modes and efficiency of metal oxidation

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold. Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed.

Synergistic Effect of Fullerenes on the Laser-Induced Periodic Surface Structuring of Poly(3-Hexyl Thiophene).

Ordered and homogeneous laser-induced periodic surface structures (LIPSS) could be fabricated in poly(3-hexyl thiophene):[6,6]-phenyl C71-butyric acid methyl ester (P3HT:PC71BM) blends by using wavelengths in the ultraviolet (UV) range (266 nm). The absorption coefficient of PC71BM, which is maximum in its UV–Visible absorption spectrum around 266 nm, enhanced the overall absorption of the blend. In addition, PC71BM itself was capable of developing homogeneous LIPSS by laser irradiation at λlaser = 266 nm. Therefore, we proposed that the synergistic effect of PC71BM on the LIPSS formation in P3HT:PC71BM (1:1) was due to a templating effect for the LIPSS formation of the PC71BM itself, which added to the overall increment of the absorption of the blend. LIPSS formation at ambient conditions in this wavelength range led to chemical modification of both P3HT and PC71BM, which rendered to non-conducting samples. Irradiation in vacuum significantly reduced radiation damage, rendering to the characteristic electrical conductivity pattern observed in P3HT LIPSS samples irradiated in the visible range. This effect could be of potential interest in order to obtain LIPSS in low absorbing polymers.