Formation Of Periodic Surface Structures Research Articles

Ageing of commercial austenitic, martensitic and duplex stainless steel surfaces after formation of laser-induced periodic surface structures by fs-laser irradiation

The time-dependent wetting behavior was investigated for commercially available austenitic, martensitic and duplex stainless steel surfaces after femtosecond (fs) laser irradiation. For this purpose, highly regular LIPSS with almost identical geometric properties were generated on the stainless steels in an air environment by near-infrared fs-laser processing (λ = 1025 nm, τ = 300 fs, frep = 100 kHz, v = 0.67 m/s, Δx = 6 μm, F=1.1 J/cm2) and compared to the polished initial surfaces as reference. The wetting with distilled water was evaluated as a function of aging time by contact angle measurements using the sessile drop method. The laser-induced morphological, topographical and chemical alteration of the surface was characterized by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, glow-discharge optical emission spectroscopy as well as high-resolution imaging and scanning transmission electron microscopy. Despite an almost identical LIPSS-based topography alteration, the stainless steels revealed very different ageing behavior. For permanently used samples, it was shown that the ageing effect is less pronounced than for samples that were stored in ambient air following fs-laser irradiation until the wetting measurement. The results obtained make an important contribution to understanding and controlling the ageing effect of metal surfaces and thus facilitate the transfer of LIPSS-based functional surface engineering and design to industrial processes.

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Simulation of femtosecond laser-induced periodic surface structures on fused silica by considering intrapulse and interpulse feedback

The formation of laser-induced periodic surface structures (LIPSSs) on fused silica upon irradiation with plane wave, double pulse, spot processing, and scanning processing (pulse duration tp = 35 fs, center wavelength λ = 800 nm, low repetition rate ≈1 kHz) is studied theoretically with an improved three-dimensional finite-difference time-domain plasma model. The model covers both intrapulse feedback under single shot and interpulse feedback under multi-shots, thus enabling better prediction of transient responses during laser–material interaction and the evolution of the ablated morphology and accumulated defects’ density with more shots. In simulations of a single plane wave, a double pulse can modulate LIPSS periodicity. In simulations of spot processing with Gaussian beam, an increase in the number of shots results in a noticeable ablation pattern where high-spatial-frequency LIPSS surrounds low-spatial-frequency LIPSS at a fluence of 2.8 J/cm2. Moreover, simulations of scanning processing with Gaussian beam showcase the broad applicability of this model, revealing that the orientation of the LIPSS depends on the polarization direction rather than the scanning path. This new model provides a powerful tool to simulate the formation of LIPSS on silica, particularly when temporally modulated laser is involved or predicting the evolution of morphology dependent on the number of shots.

Polarization and kinetics of optical emission during laser induced periodic surface structure formation on crystalline silicon

Polarization and kinetics of secondary optical emission, which occurs during the formation of laser-induced periodic surface structures (LIPSSs) on crystalline silicon by a femtosecond laser beam in air, were analyzed. It was found that the emission lines of ablated Si atoms and formed in plasma SiN molecules are unpolarized, and they decay on the nanosecond scale. The second harmonic generation (SHG) is enhanced by laser-induced surface plasmons, and the SHG polarization basically repeats the polarization of the laser. The results can be used for optical monitoring of LIPSS formation in real time, which is especially important for laser processing of large non-planar surfaces.

Investigating the transition from high to low spatial frequency periodic nanoripples on TiO2 (0 0 1) oriented surface upon irradiation with UV-femtosecond pulses

The formation of laser-induced periodic surface structures (LIPSS) on a TiO2 (001) oriented surface upon irradiation with UV-fs laser pulses was analyzed. Two systems of low spatial frequency LIPSS (LSFL) perpendicular to the direction of laser polarization are observed after N = 5 laser pulses. One was developed on the edges of the crater created by the first laser pulse, while the other appeared in the center of crater and spread towards the edges. When N was greater than 10, the two LIPSS systems merge. Both LSFL systems have a spatial period of approximately 200 nm and are due to the coherent superposition of surface waves scattered on inhomogeneities of a transient metallic surface. However they differ by the type of defects involved in the excitation process of surface waves. While a pronounced and localized rim triggered LSFL on the edge of the laser spot, LSFL in the central part nucleated at the shallow high spatial frequency LIPSS (HSFL), which are parallel to the laser polarization. A mechanism is proposed to explain the role of the HSFL in the formation of LSFL. Numerical simulations of the absorbed laser power density support the evolution from HSFL to LSFL as the number of laser pulses increased.

Oxidation-driven 2D LIPSS on amorphous Si films atop metal layers: The interplay of near-field and far-field effects

We investigate the formation of Laser-Induced Periodic Surface Structures (LIPSS) on amorphous Si films coated on Al using circularly polarized femtosecond lasers. By controlling laser fluence, we induce oxidation-based surface patterns, with their regularity governed by the interplay between near-field and far-field contributions. Achieving the right balance between these effects results in a consistent hexagonal pattern. Varying the underlying metal type further revealed its impact on pattern quality: strong far-field contributions led to irregular patterns, while dominant near-field effects resulted in extensive oxidation. Our study reveals the critical balance between near-field and far-field effects in optimizing 2D LIPSS patterns, emphasizing the role of the underlying metal.

Chemical and Topographical Changes upon Sub‐100‐nm Laser‐Induced Periodic Surface Structure Formation on Titanium Alloy: The Influence of Laser Pulse Repetition Rate and Number of Over‐Scans

Titanium and its alloys are known to allow the straightforward laser‐based manufacturing of ordered surface nanostructures, so‐called high spatial frequency laser‐induced periodic surface structures (HSFL). These structures exhibit sub‐100 nm spatial periods – far below the optical diffraction limit. The resulting surface functionalities are usually enabled by both, topographic and chemical alterations of the nanostructured surfaces. For exploring these effects, multi‐method characterizations were performed here for HSFL processed on Ti–6Al–4V alloy upon irradiation with near‐infrared ps‐laser pulses (1030 nm, ≈1 ps pulse duration, 1–400 kHz) under different laser scan processing conditions, i.e., by systematically varying the pulse repetition frequency and the number of laser irradiation passes. The sample characterization involved morphological and topographical investigations by scanning electron microscopy (SEM), atomic force microscopy (AFM), tactile stylus profilometry, as well as near‐surface chemical analyses hard X‐ray photoelectron spectroscopy (HAXPES) and depth‐profiling time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). This provides a quantification of the laser ablation depth, the geometrical HSFL characteristics and enables new insights into the depth extent and the nature of the non‐ablative laser‐induced near‐surface oxidation accompanying these nanostructures. This allows to answer the questions how the processing of HSFL can be industrially scaled up, and whether the latter is limited by heat‐accumulation effects.

Formation of two-dimensional laser-induced periodic surface structures on titanium by GHz burst mode femtosecond laser pulses

GHz burst mode femtosecond (fs) laser pulses, which consist of a series of pulse trains with ultra-fast intervals of several hundred picoseconds, have offered distinct features for material processing compared to conventional irradiation of laser pulses (single-pulse mode). We apply GHz burst mode processing to fabricate laser-induced periodic surface structures (LIPSS) on the material surfaces. In our previous work for silicon (Si), we have found that GHz burst mode can create unique two-dimensional (2D) LIPSS composed of both parallel and perpendicular to the laser polarization direction. We proposed that the formation of 2D-LIPSS is attributed to the synergetic contributions of electromagnetic and hydrodynamic mechanisms. To further investigate more detailed formation mechanisms and explore practical applications, we employ titanium (Ti), whose properties are significantly different from Si. We demonstrate that GHz burst mode fs laser pulses (central wavelength: 1,030 nm, intra-pulse width: 230 fs, intra-pulse repetition rate (an intra-pulse interval): 4.88 GHz (205 ps) and burst pulse repetition rate: 10 kHz) can also fabricate 2D-LIPSS on Ti surfaces. We attribute the dominant formation mechanism of 2D-LIPSS to the generation of hot spots with highly enhanced electric fields due to transient change of material properties during GHz burst pulse irradiation. Based on this speculation, properly tailoring the shapes of the burst pulse with an optimum intra-pulse number enables the creation of well-defined 2D-LIPSS. Furthermore, essentially homogeneous 2D-LIPSS can be formed in a large area by laser scanning of a focused fs laser beam with a stage scanning speed of 5 mm/s.

Numerical study of ultrafast optical instability causing structured light absorption in metal

We present the first-principle numerical study of nonlinear decay of a femtosecond laser pulse into a pair of surface plasmon polaritons (SPP) during reflection from a rough metallic surface. The ultrafast dynamics of the decay was studied at damaging laser fluences of about 1 J/cm2, and the principal role of the electronic collision rate growth was proved. The resulting strongly inhomogeneous heating of metal is an important stage of laser-induced phenomena like ablation, terahertz radiation generation, and periodic surface structures formation.

Investigating Laser-Induced Periodic Surface Structures (LIPSS) Formation in Silicon and Their Impact on Surface-Enhanced Raman Spectroscopy (SERS)

Laser-induced periodic surface structures (LIPSS) have gained significant attention due to their ability to modify the surface morphology of materials at the micro-nanoscale and show great promise for surface functionalization applications. In this study, we specifically investigate the formation of LIPSS in silicon substrates and explore their impact on surface-enhanced Raman spectroscopy (SERS) applications. This study reveals a stepwise progression of LIPSS formation in silicon, involving three distinct stages of LIPSS: (1) integrated low-spatial-frequency LIPSS (LSFL) and high-spatial-frequency LIPSS (HSFL), (2) principally LSFL and, (3) LSFL at the edge of the irradiated spot, elucidating the complex interplay between laser fluence, pulse number, and resulting surface morphology. Furthermore, from an application standpoint, these high-quality multi-scale periodic patterns lead to the next step of texturing the entire silicon surface with homogeneous LIPSS for SERS application. The potential of LIPSS-fabricated silicon substrates for enhancing SERS performance is investigated using thiophenol as a test molecule. The results indicate that the Au-coated combination of LSFL and HSFL substrates showcased the highest enhancement factor (EF) of 1.38×106. This pronounced enhancement is attributed to the synergistic effects of localized surface plasmon resonance (LSPR) and surface plasmon polaritons (SPPs), intricately linked to HSFL and LSFL characteristics. These findings contribute to our understanding of LIPSS formation in silicon and their applications in surface functionalization and SERS, paving the way for sensing platforms.

Laser-Induced Periodic Surface Structures on Layered GaSe Crystals: Structural Coloring and Infrared Antireflection.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Formation of Periodic Surface Structures on Semiconductors under Mid-Infrared Free-Electron Laser Irradiation

Formation of Periodic Surface Structures on Semiconductors under Mid-Infrared Free-Electron Laser Irradiation

GLOSSY CARBON ELECTRODES WITH LASER-INDUCED PERIODIC SURFACE STRUCTURES FOR ELECTROCHEMILUMINESCENT ANALYSIS

In this paper, the possibility of using glassy carbon electrodes modified with laser-induced periodic surface structures for conducting electrochemical and electrochemiluminescent studies and creating appropriate analytical systems is investigated. Modification of the surface of the electrodes is carried out with the help of a femtosecond laser. The surface morphology of the modified electrodes by atomic force microscopy shows the stable formation of periodic surface structures. Electrochemiluminescence studies indicate the possibility of effective use of such modified electrodes for the creation of electrochemiluminescent analytical systems.

Ultra-short pulsed laser ablation of decagonal AlCoNi and AlCoCuNi quasicrystals

Al70Co20Ni10 and Al70Co15Cu10Ni5 (at%) decagonal quasicrystals (DQCs) were subjected to femtosecond laser ablation at a laser wavelength of 800 nm, in deionized water (DI). The generated nanoparticles (NPs) confined within the liquid medium were studied for optical and structural properties. The structural analysis revealed the retention of the targets phase in the NPs. In case of Al70Co20Ni10, formation of Al3Ni2 phase was confirmed through the SAED patterns and HRTEM micrographs. Contrary to this, the formation of Al-Ni phase was not observed in Al70Co15Cu10Ni5. This is attributed to the presence of Cu in the latter sample, which restricts the mixing of Ni with Al. The HRTEM micrographs in both cases showed that Al2O3 was present at the edges of the NPs. The optical analysis confirmed the formation of Al2O3 in both cases. However, in the case of Al70Co15Cu10Ni5, a very weak signal corresponding to CuO was detected. This is attributed to the diffusion of Cu along with Al at the edge. The ablated region of the target’s surface was studied for the formation of laser-induced periodic surface structures (LIPSS) also known as ripples, and the type of ripples formed was identified.

Highly regular nanogratings on amorphous Ge films via laser-induced periodic surface sublimation

Despite the fact that monocrystalline germanium (Ge) was historically the first material where formation of laser-induced periodic surface structures (LIPSSs) was observed in 1965 and then extensively studied, practically relevant thin films of amorphous Ge (a-Ge) were occasionally ignored by these studies so far. Here, highly regular LIPSSs were observed on the surface of magnetron-sputtered a-Ge films under near-IR femtosecond laser exposure. Formation of these regular structures was explained by excitation and interference of the surface waves at the interface of photoexcited Ge that was confirmed by performed calculations and full-wave simulations. At the same time, remarkable structural regularity of these LIPSSs unveiled their ablation-free formation scenario involving laser-driven oxidation of the Ge followed by ultra-clean sublimation of the oxide. Laser patterning in vacuum was found to regulate oxidation/sublimation rates of the a-Ge and its oxide improving the structure quality and opening pathways for practical applications in optoelectronics, solar light harvesting and near-IR photonics.

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.

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 .

Femtosecond Laser‐Induced Periodic Surface Structuring of the Topological Insulator Bismuth Telluride

AbstractSurface structuring of topological insulator Bi2Te3 single crystals with femtosecond laser by varying pulses N and energy E is reported. Interesting effects related to laser‐induced periodic surface structures formation in this class of materials are evidenced. At low pulse energy, a clear formation of periodic, subwavelength ripples oriented orthogonally to the laser polarization is observed; those are restricted to an annular region surrounding a featureless central disk as the laser energy progressively increases. The structural analysis shows that some degree of crystallinity is preserved in the rippled area, but the central disk is amorphous resembling what is observed for germanium (Ge) and is associated with the hindering of surface structure formation due to a thick melted surface layer. Interestingly, at larger fluence or number of pulses, a transition to suprawavelength grooves occurs within the annular region covered by surface structures. The findings demonstrate a clear incubation behavior, suggesting that the formation of laser‐induced periodic surface structures is coherent with the general features of the process already reported for other materials. However, the disappearance of these structures in the central area, possibly resulting from the influence of the depth of the melt layer, indicates a mixed behavior for Bi2Te3.