Laser-induced Periodic Surface Structures Research Articles

Laser machining of free-standing silicon nitride membranes

Patterning free-standing Silicon Nitride membranes has enabled the development of various SiN-based devices such as quantum resonators, biosensors, temperature detectors, and plasmonic devices. Patterning SiN membranes usually involves expensive and time-consuming cleanroom techniques that limit flexibility in the development of new SiN membrane designs. While ultrafast laser machining has been proposed to create micron to submicron features in these membranes, laser machining of large area features has not been reported as it often results in membrane failure due to stress concentration during laser machining. We show that continuous laser machining can be used to create a crack and to extract the fracture toughness of the membrane (1.9 ± 0.3 MPa.m1/2). To successfully patterned SiN membranes without failure, a low stress laser machining method is developed, where stress concentration can be minimized by adjusting the laser drilling sequence, a result confirmed by finite element simulations. Complex shapes can thus be machined over a large area with a very high success rate. Laser parameters are optimized to improve the quality of the laser-cut edges without the formation of laser-induced periodic surface structure. Transmission Electron Microscopy and Electron Energy Loss Spectroscopy indicate a modification of the Si/N ratio over a small region next to the laser machined features. These results demonstrate that low stress ultrafast laser machining can be used to create patterns in SiN membranes quickly and economically, enabling the rapid development of new membrane designs.

Formation and modulation of multi-pulse femtosecond laser-induced periodic surface structures by electromagnetic and partial differential equation simulations.

In order to demonstrate the formation of laser-induced periodic surface structures (LIPSS), simulations were performed to investigate the effect of multiple femtosecond laser pulses with different laser energy densities on a Ti6Al4V surface. In this work, a set of partial differential equations calculating the electron and lattice temperature variations, followed by coupling with an electric field, is used to analyze the evolution of the periodic surface structure induced by the interaction of the femtosecond laser with the material. As the number of pulses increases, the surface structure of the material changes from none to produce LIPSS structure and from low spatial frequency LIPSS (LSFL) structure to high spatial frequency LIPSS (HSFL) structure. In order to compare the results, single-point laser scanning ablation experiments were carried out at femtosecond laser energy. The experimental results are consistent with the simulation results.

Formation of laser-induced periodic surface structures on Zr-based bulk metallic glasses with different chemical composition

Bulk metallic glasses (BMG) are amorphous metal alloys known for their unique physical and mechanical properties. In the present study, the formation of femtosecond (fs) laser-induced periodic surface structures (LIPSS) on the Zr-based BMGs Zr46Cu46Al8, Zr61Cu25Al12Ti2, Zr52.5Cu17.9Al10Ni14.6Ti5 (Vit105) and Zr57Cu15.4Al10Ni12.6Nb5 (Vit106) was investigated as a function of their different chemical composition. For this purpose, LIPSS were generated on the sample surfaces in an air environment by fs-laser irradiation (λ = 1025 nm, τ = 300 fs, frep = 100 kHz). The surface topography was characterized by scanning electron microscopy and atomic force microscopy. Moreover, the impact of LIPSS formation on the structure and chemical surface composition was analyzed before and after fs-laser irradiation by X-ray diffraction and X-ray photoelectron spectroscopy as well as by transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. Despite the different chemical composition of the investigated BMGs, the fs-laser irradiation resulted in almost similar properties of the generated LIPSS patterns. In the case of Zr61Cu25Al12Ti2, Vit105 and Vit106, the surface analysis revealed the preservation of the amorphous state of the materials during fs-laser irradiation. The study demonstrated the presence of a native oxide layer on all pristine BMGs. In addition, fs-laser irradiation results in the formation of laser-induced oxide layers of larger thickness consisting of an amorphous ZrAlCu-oxide. The precise laser-structuring of BMG surfaces on the nanoscale provides a versatile alternative to thermoplastic forming of BMG surfaces and is of particular interest for the engineering of functional material surfaces.

Self-organized submicron structures in photoresist films by UV-laser irradiation at water-confined conditions

A new kind of self-organized pattern formation process has been found during laser irradiation of polymer films in water confinement just below the laser ablation threshold, resulting in a randomly oriented pattern with a period of about 475 nm. The morphology, orientation, period, and amplitude of these patterns are inconsistent with both laser-induced periodic surface structures that typically consist of linear grooves with periods smaller the laser wavelength and wrinkling patterns that feature a much larger period and appear at layered systems. Excimer laser (λ = 248 nm, tp = 25 ns) exposure of 650 nm thick photoresist films on silicon wafers cause the growth of irregular submicron patterns. The pattern morphology that is examined with imaging techniques is correlated to processing parameters. The amplitude of these laser-induced self-organized (LISE) submicron structures are strengthened with pulse number and laser fluence. The experimental results are discussed together with simulations of laser heating the photoresist film in water confinement. The proposed pattern formation mechanism of such laser-induced self-organized submicron structures at temporal excitation of a confined polymer surface comprises the formation of an oriented roughness based on LIPSS that are developed to wrinkled structures due to the transient formation of a soft subsurface area that provides conditions for wrinkling of the water cooled, stiff polymer surface by laser-induced stress fields. Size, amplitude, and morphology of the LISE pattern provide good properties for applications in such fields as wetting, friction, optics, and bioactivity.

High-Resolution Laser Interference Ablation and Amorphization of Silicon.

The laser interference patterning of a silicon surface via UV femtosecond pulse irradiation, resulting in 350 nm periodic structures, is demonstrated. The structuring process was performed using a laser with a 450 fs pulse duration at a wavelength of 248 nm in combination with a mask projection setup. Depending on the laser fluence, single-pulse irradiation leads to amorphization, structure formation via lateral melt flow or the formation of voids via peculiar melt coalescence. Through multipulse irradiation, combined patterns of interference structures and laser-induced periodic surface structures (LIPSS) are observed.

Effect of film thickness on laser induced surface structures formation on optically transparent polymer films

Laser-induced periodic surface structures can be prepared on polymer thin films, despite the absence of light absorption at the wavelength of the irradiating laser, by using a bilayer approach. Nanostructuring of a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer, by laser irradiation at 532 nm, covered by a thin layer of a non-absorbing polymer, in this case, the ferroelectric copolymer poly(vinylidene fluoride - trifluoro ethylene),P(VDF-TrFE), allows the development of a grating-like structure on the surface. The range of thicknesses of the upper layer, which leads to the formation of LIPSS, has been determined. It is verified that for a thickness larger than 40 nm, no periodic structures are formed, neither in the upper layer nor in the bottom P3HT one. This can be explained considering a mechanical restriction which prevents P3HT rearrangement. The upper nanostructured surface layer exhibits ferroelectric properties as proven by piezoresponse force microscopy measurements.

Anisotropic reversing motion in laser-induced periodic surface structures

Observation of the evolution process of laser-induced periodic surface structure (LIPSS) will provide clues to its formation mechanism. We constructed a pulse-by-pulse image capture setup using oil-immersion optical microscopy with which we observed the evolution process on a coverslip of borosilicate glass, finding three LIPSS patterns: stripe patterns parallel and perpendicular to the electric field of the laser pulse, and a dot pattern. Furthermore, “reversing” motion was observed, that is, bright and dark interchange from pulse to pulse. The reversing motion had remarkable anisotropy, moving only perpendicular to the electric field of the laser pulse.

Influence of zirconium and copper sub-layer in cell integrations on femtosecond laser-processed Ti thin films

The creation of novel biocompatible Ti-based thin films with a Zr or Cu sub-layer modified by ultrafast laser processing is studied. To prepare bioactive surfaces, ultrafast laser processing is focused on the formation of laser-induced periodic surface structures (LIPSS) with the production of oxide phases at the surfaces. Two differently designed multilayer thin films Ti/Cu/Ti and Ti/Zr/Ti were deposited on the silicon using the ion sputtering method. The Ti thin film contains Cu or Zr sub-layer (thickness of 10 nm) at the 10 nm below the surface. The composition and surface morphology variations for these systems, deposited and laser-processed under the same experimental conditions, were caused only by different thermo-physical properties of the sub-layer (Cu or Zr). The surface morphology in the form of LIPSS, led to improved cell adhesion and stable cells/thin films interface compared to as-deposited samples. Field-emission scanning electron microscopy and MTT analysis revealed that laser processing of both systems increased cell adhesion, proliferation, and metabolical activity of L929 mouse fibroblast cells compared to non-modified flat surfaces. Overall, the biocompatibility of Zr-containing thin films is better than Ti/Cu/Ti system. Further, laser processing and formation of LIPSS makes Ti/Zr/Ti thin films excellent candidate for biomedical .

Formation of low- and high-spatial frequency laser-induced periodic surface structures (LIPSSs) in ALD-deposited MoS2

The formation of laser-induced periodic surface structures (LIPSSs) on the atomic layer-deposited (ALD) molybdenum disulfide (MoS2) upon femtosecond laser processing is studied experimentally. Laser-processing parameters such as average laser power and the scan speed at which the formation of the periodic nanostructures takes place are identified. Optical and scanning electron microscopy are applied to identify the parameter regions for the different LIPSS formations and transitions between them. High- and low-spatial frequency LIPSS (HSFL and LSFL) with two distinct periods λLSFL ≈ 1.1 μm and λHSFL ≈ 83 nm can be observed. The HSFL are dominating at higher and the LSFL at lower laser average powers. Formation of LIPSS is found to inhibit laser ablation at lower scan speeds.

Influence of re-deposited nanoparticles on the surface elemental composition of femtosecond laser machined copper surfaces

Nanoparticles re-deposited during the laser machining process have been suspected to modify the intrinsic surface chemistry of machined metallic surfaces. However, only very few studies have focused on elucidating their effect. In this work, under two different machining conditions (laboratory air & controlled humid air), we fabricated laser-induced periodic surface structures, LIPSS, on polished Cu surfaces with three different amounts of re-deposited Cu nanoparticles to quantify how re-deposited Cu nanoparticles affect the surface chemistry of the machined samples. In carrying out these experiments, we demonstrate that the re-deposition of nanoparticles during laser machining is affected by the volume of the laser machining environment, which impacts the surface chemistry of laser-machined surfaces. Our results indicate that evacuation of the vacuum chamber before and after laser machining using a vacuum scroll pump did not affect the adsorption of carbonaceous molecules. Further, we present the effect of removing re-deposited nanoparticles through ultrasonic cleaning on the surface chemistry of the test surface. The cleaning exposes the LIPSS surface entirely to airborne carbonaceous species, resulting in a more significant increase in the total surface carbon content. Finally, we evaluated the quality of the nanoparticles generated during laser machining by assessing their stability to provide a first insight into the utility of those nanoparticles.

Tailoring the wettability of surface-textured copper using sub-THz bursts of femtosecond laser pulses

In this work, copper surfaces were textured with sub-Terahertz bursts of femtosecond pulses. The wettability of Cu textured surfaces was investigated by measuring the static water contact angle (WCA) as a function of the number of sub-pulses and the intra-burst frequency. A superhydrophobic, antiadhesive response was observed when using bursts with a high number of sub-pulses (equal to or higher than 16) or a high intra-burst frequency (equal to or higher than 0.09 THz). Such trend was ascribed to the generation, under specific laser irradiation conditions, of a double-scale hierarchical texture on the sample surface, formed by sub-micro patterns with fine periodic ripples (LIPSS, Laser-Induced Periodic Surface Structures) and random nanoparticle decoration. Such texture enhances the hydrophobic behavior given by inherent adsorption of adventitious hydrocarbons on laser-processed and thermally-treated metal targets.

Effect of Femtosecond Laser-Irradiated Titanium Plates on Enhanced Antibacterial Activity and Preservation of Bacteriophage Stability.

Titanium (Ti) is widely recognized for its exceptional properties and compatibility with medical applications. In our study, we successfully formed laser-induced periodic surface structures (LIPSS) on Ti plates with a periodicity of 520-740 nm and a height range of 150-250 nm. To investigate the morphology and chemical composition of these surfaces, we employed various techniques, including field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Additionally, we utilized a drop-shape analyzer to determine the wetting properties of the surfaces. To evaluate the antibacterial activity, we followed the ISO 22196:2011 standard, utilizing reference bacterial cultures of Gram-positive Staphylococcus aureus (ATCC 25923) and Gram-negative Escherichia coli (ATCC 25922). The results revealed enhanced antibacterial properties against Staphylococcus aureus by more than 99% and Escherichia coli by more than 80% in comparison with non-irradiated Ti. Furthermore, we conducted experiments using the Escherichia coli bacteriophage T4 (ATCC 11303-B4) and the bacterial host Escherichia coli (ATCC 11303) to investigate the impact of Ti plates on the stability of the bacteriophage. Overall, our findings highlight the potential of LIPSS on Ti plates for achieving enhanced antibacterial activity against common bacterial strains while maintaining the stability of bacteriophages.

Ultrafast laser-induced plasma anisotropy in pristine and surface pre-structured zinc telluride, probed by terahertz pulses.

We use THz probe pulses to detect and analyze the dynamics of charge transport anisotropies generated by ultrafast laser two-photon absorption in Zinc Telluride (ZnTe) semi-insulating crystal showing smooth and laser structured surfaces. The detected anisotropy consists in a modulation of the THz transmission as a function of the orientation of the <001 > axis of ZnTe. The change in THz transmission after pump excitation is attributed to free carrier absorption of the THz field in the laser-induced electron-hole plasma. Pre-structuring the surface sample with laser-induced periodic surface structures (ripples) has strong influence on free carrier THz transmission and its associated anisotropic oscillation. Within the relaxation dynamics of the laser-induced free carriers, two relaxation times have to be considered in order to correctly describe the dynamics, a fast relaxation, of about 50 picoseconds in pristine sample (90 picoseconds in sample pre-structured with ripples), and a slow one, of about 1.5 nanoseconds. A theoretical model based on classical Drude theory and on the dependence of the two-photon absorption coefficient with the crystal orientation and with the laser polarization is used to fit the experimental results.

Anisotropy of the Tribological Performance of Periodically Oxidated Laser-Induced Periodic Surface Structures

Laser-induced periodic surface structures (LIPSS) enable advanced surface functionalization with broad applications in various fields such as micro- and nanoelectronics, medicine, microbiology, tribology, anti-icing systems, and more. This study demonstrates the possibility of achieving anisotropy in the tribological behavior of C45-grade steel structured by nanosecond laser radiation using the LIPSS method. The lateral surface of the steel roller was irradiated with a pulsed Nd:YAG laser at an optimum intensity I = 870 MW/cm2 for the formation of LIPSS. Two sets of samples were formed with LIPSS that were perpendicular and parallel to the roller’s rotational motion direction. The Raman intensity maps revealed that the LIPSS structure consisted of periodically arranged oxides at the top of hills. At the same time, the valleys of the LIPSS structures were almost not oxidized. These results correlated well with scanning electron microscopy energy dispersive X-ray spectroscopy mapping and atomic force microscopy measurements. A comparison of Raman and X-ray photoelectron spectroscopy spectra revealed that both the magnetite phase and traces of the hematite phase were present on the surface of the samples. Tribological tests were performed in two cycles with periodic changes in the normal clamping force and sliding speed. It was found that the LIPSS structures which were formed perpendicularly to the sliding direction on the roller had a significantly greater impact on the friction processes. Structures oriented perpendicular to the direction of motion had a positive influence on reducing the energy consumption of a friction process as well as increasing the wear resistance compared to LIPSS formed parallel to the direction of motion or ones having a non-texturized surface. Laser texturing to produce LIPSS perpendicular to the direction of motion could be recommended for friction pairs operating under low-load conditions.

Bessel beam generated LIPSS on NiTi wires

Ultrashort laser pulses are used to generate Laser-Induced Periodic Surface Structures (LIPSS). To apply LIPSS on small confined areas, the laser source must be focused to a small spot diameter. Contrary to commonly used high numerical aperture objectives, we present a Bessel beam-based approach to produce LIPSS covered lines with a width of 4 to 10 µm. Therefore, an 800 nm Ti:Sa-laser with a pulse duration of 110 fs and a repetition rate of 5000 Hz was used. The setup convinces with an extended depth of focus. Thus, high reproducibility of the process was proven, while no focus control and no specific requirements for the surface quality are needed. The applicability of this method is demonstrated by structuring the shell surface of nickel–titanium wires.

High orientation consistency and adjustable convex width of laser-induced periodic surface structures using picosecond laser pulse trains

High orientation consistency and adjustable convex width of the low-spatial-frequency laser-induced periodic surface structures (LSFLs), crucial to the functional surface characteristics, have remained elusive. This paper proposes a new method to fabricate LSFLs with high orientation consistency on the rough surface of titanium by combining laser polishing and laser induction with LSFLs with a tunable convex width via laser melting as the post-treatment. Picosecond pulses trained with a 50-ns interval are applied to regulate the thermal incubation effect and achieve laser polishing and laser nanoscale melting. The melting time of titanium for laser polishing and laser nanoscale melting is determined to be on a microsecond time scale and around 100 ns, respectively. Experimental studies show that the surface texture of titanium lowers the orientation consistency of LSFLs and that its divergence angle is 30°. Picosecond pulses with a sub-pulse number of three are applied to achieve surface polishing and the formation of the rudiment of the LSFLs, followed by the picosecond laser induction. As a result, the divergence angle of LSFLs decreases from 30° to 12°. On this basis, aiming at the problem of the narrow adjustability of the convexity ratio of LSFLs, a nanoscale melting processing method based on picosecond pulse trains with a sub-pulse number of four is proposed, and LSFLs with the tunable convexity ratios from 0.3 to 0.87 are obtained.

Superior wear resistance in cast aluminum alloy via femtosecond laser induced periodic surface structures and surface hardening layer

Cast Al alloys have been increasingly used in aviation, shipping, aerospace, and automotive industries as important raw materials of engine parts. However, its poor wear resistance hampered their application in some tribological environments. To overcome the current issue, in present work, we proposed a femtosecond laser (fs-laser) surface processing strategy to fabricate pillar-shaped laser induced periodic surface structures (LIPSS), as well as a work-hardening layer (hardness increment reached 51.2% compared with that of substrate) with an affected depth of ∼ 75 μm, on the surface of cast Al alloy in one step. In this case, the wear performance of processed samples showed an obvious improvement with a 49.4% reduction of the cross sectional area of wear track. Our work broke the existing understanding about the strengthening mechanism of fs-laser surface modification technology, in which surface texture or shock wave induced by fs-laser were viewed as two isolated strengthening effects. For the first time, we achieved the combination of these two strengthening effects, and proved the effectiveness of this strategy. The present results provided theoretical bases for the further development of novel fs-laser surface technologies with better performance.

Hierarchical Surface Structures and Large-Area Nanoscale Gratings in As2S3 and As2Se3 Films Irradiated with Femtosecond Laser Pulses.

Chalcogenide vitreous semiconductors (ChVSs) find application in rewritable optical memory storage and optically switchable infrared photonic devices due to the possibility of fast and reversible phase transitions, as well as high refractive index and transmission in the near- and mid-infrared spectral range. Formed on such materials, laser-induced periodic surface structures (LIPSSs), open wide prospects for increasing information storage capacity and create polarization-sensitive optical elements of infrared photonics. In the present work, a possibility to produce LIPSSs under femtosecond laser irradiation (pulse duration 300 fs, wavelength 515 nm, repetition rate up to 2 kHz, pulse energy ranged 0.03 to 0.5 μJ) is demonstrated on a large (up to 5 × 5 mm2) area of arsenic sulfide (As2S3) and arsenic selenide (As2Se3) ChVS films. Scanning electron and atomic force microscopy revealed that LIPSSs with various periods (170-490 nm) and orientations can coexist within the same irradiated region as a hierarchical structure, resulting from the interference of various plasmon polariton modes generated under intense photoexcitation of nonequilibrium carriers within the film. The depth of the structures varied from 30 to 100 nm. The periods and orientations of the formed LIPSSs were numerically simulated using the Sipe-Drude approach. A good agreement of the calculations with the experimental data was achieved.

Optoelectronic performance of indium tin oxide thin films structured by sub-picosecond direct laser interference patterning

A route to increase the efficiency of thin film solar cells is improving the light-trapping capacity by texturing the top Transparent Conductive Oxide (TCO) so that the sunlight reaching the solar absorber scatters into multiple directions. In this study, Indium Tin Oxide (ITO) thin films are treated by infrared sub-picosecond Direct Laser Interference Patterning (DLIP) to modify the surface topography. Surface analysis by scanning electron microscopy and confocal microscopy reveals the presence of periodic microchannels with a spatial period of 5 µm and an average height between 15 and 450 nm decorated with Laser-Induced Periodic Surface Structures (LIPSS) in the direction parallel to the microchannels. A relative increase in the average total and diffuse optical transmittances up to 10.7% and 1900%, respectively, was obtained in the 400–1000 nm spectral range as an outcome of the interaction of white light with the generated micro- and nanostructures. The estimation of Haacke’s figure of merit suggests that the surface modification of ITO with fluence levels near the ablation threshold might enhance the performance of solar cells that employ ITO as a front electrode.

Cell Response on Laser-Patterned Ti/Zr/Ti and Ti/Cu/Ti Multilayer Systems

Arranged patterns obtained via ultrafast laser processing on the surface of Ti/Cu/Ti/Si and Ti/Zr/Ti/Si thin-film systems are reported. Two differently designed multilayer thin films Ti/Cu/Ti/Si and Ti/Zr/Ti/Si were deposited on silicon using the ion sputtering method. The bioactive surfaces of these systems involve the formation of laser-induced periodic surface structures (LIPSS) in each of the laser-written lines of mesh patterns on 5 × 5 mm areas. The formation of nano- and micro-patterns with an ultra-thin oxide film on the surfaces was used to observe the effects of morphology and proliferation of the MRC-5 cell culture line. To determine whether Ti-based thin films have a toxic effect on living cells, an MTT assay was performed. The relative cytotoxic effect, as a percentage of surviving cells, showed that there was no difference in cell number between the Ti-based thin films and the control cells. There was also no difference in the viability of the MRC-5 cells, except for the Ti/Cu/Ti/Si system, where there was a slight 10% decrease in cell viability.