Using multi-plane light conversion for 2D, direct laser interference patterning

Functional surfaces characterised by periodic microstructures are sought in numerous technological applications. Direct laser interference patterning (DLIP) is a technique that allows the fabrication of microscopic periodic features on different materials, e.g., metals. The mechanisms effective during nanosecond pulsed DLIP of metal surfaces are not yet fully understood. In the present investigation, the heat transfer and fluid flow occurring in the metal substrate during the DLIP process are simulated using a smoothed particle hydrodynamics (SPH) methodology. The melt pool convection, driven by surface tension gradients constituting shear stresses according to the Marangoni boundary condition, is solved by an incompressible SPH (ISPH) method. The DLIP simulations reveal a distinct behaviour of the considered substrate materials stainless steel and high-purity aluminium. In particular, the aluminium substrate exhibits a considerably deeper melt pool and remarkable velocity magnitudes of the thermocapillary flow during the patterning process. On the other hand, convection is less pronounced in the processing of stainless steel, whereas the surface temperature is consistently higher. Marangoni convection is therefore a conceivable effective mechanism in the structuring of aluminium at moderate fluences. The different character of the melt pool flow during DLIP of stainless steel and aluminium is confirmed by experimental observations.

Starting from a simple concept, transferring the shape of an interference pattern directly to the surface of a material, the method of Direct Laser Interference Patterning (DLIP) has been continuously developed in the last 20 years. From lamp-pumped to high power diode-pumped lasers, DLIP permits today for the achievement of impressive processing speeds even close to 1 m2/min. The objective: to improve the performance of surfaces by the use of periodically ordered micro- and nanostructures. This study describes 20 years of evolution of the DLIP method in Germany. From the structuring of thin metallic films to bulk materials using nano- and picosecond laser systems, going through different optical setups and industrial systems which have been recently developed. Several technological applications are discussed and summarized in this article including: surface micro-metallurgy, tribology, electrical connectors, biological interfaces, thin film organic solar cells and electrodes as well as decorative elements and safety features. In all cases, DLIP has not only shown to provide outstanding surface properties but also outstanding economic advantages compared to traditional methods.

The friction of surface textured cylinder liner segments modified by Direct Laser Writing and Direct Laser Interference Patterning processes

Direct laser interference patterning via scanning optics using the Michelson-Morley configuration

Laser-based processing techniques have proven to be an effective option of modifying polymer surfaces. In this context Direct Laser Interference Patterning in conjunction with the polygon scanner technique is used to fabricate textured polystyrene surfaces through ultra-fast beam deflection. This is achieved by using a high-average power picosecond laser in combination with a polygon mirror-based scanning system. The two-beam DLIP optical configuration leads to the formation of line-like structures with a spatial period of 21.0 µm. The influence of the scanning speed and the repetition rate on the structure formation is investigated, allowing structure heights up to 23 µm. The formation of the micro-structure was found to result from swelling and ablation mechanisms. By applying scanning speeds of 350 m/s, a throughput of 1.1 m2/min is reported for the first time using this method.

This work addresses the fabrication of periodic micro patterns on stainless steel by means of Direct Laser Interference Patterning. A modular optics assembly is introduced which produces two‐beam interference patterns within a 5 × 100 μm rectangular shaped laser spot. The optical assembly considers optimal utilization of the available laser power in order to enable high processing rates. Using different optical elements, the spatial period of the periodic structures was set to 8 μm and 19 μm. The effect of the processing parameters on the morphology of the produced patterns is investigated by means of confocal microscopy, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy. It is found that the pulse repetition rate shows a strong influence on the structure height accompanied by considerable heat accumulation effects, which is reported for the first time for this method. In addition, aspect ratios slightly higher than one could be realized, which is also reported for the first time when using Direct Laser Interference Patterning with ns‐pulsed lasers. Particularly for the smaller periods, the melt flow impairs pattern formation due to the overmelting of the material (humping). Moreover, variations of the surface chemistry are observed, especially for higher accumulated laser fluences.

Direct laser interference patterning (DLIP) has emerged as a versatile tool for producing well-defined microstructures that mimic natural surfaces with the aim of obtaining functionalized surfaces on relevant technological materials. On the other hand, the fabrication of surface patterns with micro- and submicron resolution features necessitates of advanced monitoring and setup strategies in order to ensure repeatability as well as quality control. In addition, the monitoring systems also allow inline capabilities to enable a closed-loop control approach. A possible strategy, that has been already applied to different laser processes, is the utilization of the sound pressure generated by the laser beam hitting the surface and producing ablation that can be detected and analyzed using commercially available microphones. In this frame, this work focuses on the analysis of the acoustic information extracted from the audio signal for determining process-inherent characteristics in DLIP, allowing the calculation of interference volume using stainless steel and titanium as reference materials. The results show that the acoustic emission measured at the ablation spot can be correlated to the interference volume shape and thus allowing to approximate the size of the interference spot. The possible utilization of this approach as an auto-focus and auto-setup method during DLIP is discussed.

Large-angle Programmable Direct Laser Interference Patterning with Ultrafast Laser Using Spatial Light Modulator

Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance.

Laser-assisted modification of metals, polymers or ceramics yields a precise adjustment of wettability, biocompatibility or tribological properties for a broad range of applications. Due to a specific change of surface topography on micro- and nanometer scale, new functional properties can be achieved. A rather new scientific and technical approach is the laserassisted surface modification and structuring of metallic current collector foils for lithium-ion batteries. Prior to the thick film electrode coating processes, the formation of micro/nano-scaled surface topographies on current collectors can offer better interface adhesion, mechanical anchoring, electrical contact and reduced mechanical stress during cycling. These features in turn impact on the battery performance and the battery life-time. In order to generate the 3D surface architectures on metallic current collectors, two advanced laser processing structuring technologies: direct laser interference patterning (DLIP) and ultrafast laser-induced periodic surface structuring (LIPSS) were applied in this study. After laser structuring via DLIP and LIPSS, composite electrode materials were deposited by tape-casting on the modified current collectors. The electrode film adhesion was characterized by tensile strength measurements. The impact of various surface structures on the improvement of adhesive strength was discussed.

Surface structuring using nano-second lasers can be used to enhance certain properties of a material or even to introduce new ones. One way to create these structures efficiently is direct laser interference patterning using different polarization vector orientations of the interfering beams. However, experimentally measuring the fabrication process of these structures is very challenging due to small length and time scales. Therefore, a numerical model is developed and presented for resolving the physical effects during formation the predicting the resolidified surface structures. This three-dimensional, compressible computational fluid dynamics model considers the gas, liquid, and solid material phase and includes various physical effects, such as heating due to the laser beam for both parallel and radial polarization vector orientations, melting, solidification, and evaporation, Marangoni convection, and volumetric expansion. The numerical results reveal a very good qualitatively and quantitatively agreement with experimental reference data. Resolidified surface structures match both in overall shape as well as crater diameter and height, respectively. Furthermore, this model gives valuable insight on different quantities during the formation of these surface structures, such as velocity and temperature. In future, this model can be used to predict surface structures based on various process input parameters.

Laser-based technologies have become a relevant method for creating microstructured surfaces on materials for enhancing their functionalities.In this field, traditional monitoring methods can be supplemented by analyzing acoustic emissions (AE), offering insights into laser-material interactions for quality control.This study explores the dynamics of laser-induced shock waves and plasma from stainless steel targets under atmospheric conditions using Direct Laser Interference Patterning (DLIP) as well as Direct Laser Writing (DLW).Utilizing optical beam deflection technique, the propagation of a supersonic shock wave and its evolution into an acoustic wave could be measured, alongside the plasma plume.Acoustic emissions from the laser ablation were recorded at various distances, with the explosion blast wave model providing a good estimation of the shock front's temporal development.These emissions originate from the ablation plasma's lifecycle, including expansion, oscillation, and contraction.The performed research enhances the understanding of DLIP and DLW, suggesting new pathways for improved monitoring and control in laser surface patterning.

Transparent conductive oxides (TCOs) are used in solar cells not only to extract photogenerated carriers but also to allow sunlight to reach the photoactive material. Therefore, controlling the electrical and optical properties of such oxides is crucial for the optimization of the efficiency of solar cells. Herein, direct laser interference patterning (DLIP) method is used to control the surface morphology, optical and electrical properties of fluorine‐doped tin oxide (FTO) by applying femtosecond laser pulses. The topography characterization reveals periodic line‐like microstructures with a period of 3.0 μm and average heights between 20 and 185 nm, depending on the applied laser fluence levels. Laser‐induced periodic surface structures are observed on the valleys of the texture aligned perpendicularly to the laser radiation polarization. A relative increase in the average total and diffuse optical transmittance up to 5% and 500%, respectively, is obtained in the 400–800 nm spectral range as a consequence of the generated micro‐ and nanostructures. Calculations of two figures of merit suggest that the texturing of FTO might enhance the efficiency of solar cells, in particular dye‐sensitized (DSSCs). The findings of this study confirm that DLIP is a convenient technique for structuring electrodes for highly efficient optoelectronic devices.

Remote radiofrequency triggering of topography changes in a surface micropatterned PANI@PNIPAM nanocomposite

Patterning of nanocomposites by conventional lithography processes is problematic due to the different physical properties of matrix and filler. Direct laser interference patterning (DLIP) promises rapid micromachining of virtually any material with subwavelength resolution. A control of intense laser light modifies the filler particle size distribution in nanocomposite films. Herein, DLIP of periodic patterns with half‐pitch <1 μm in reactive magnetron‐sputtered hydrogenated amorphous diamond‐like carbon nanocomposite thin films with embedded silver nanoparticles (DLC:Ag) is demonstrated. Periodic patterns of regularly repeating areas with modified size distributions of silver nanoparticles are obtained by a variation of fluences and number of pulses of 515 nm wavelength femtosecond laser. Depending on the silver content in nanocomposite films, nanoparticle size distributions are either bimodal (for 14.1 at% Ag containing films) or unimodal (7.9 at%) with effective average diameters shifting from 13 to 69 nm, depending on the laser processing parameters. The importance of localized surface plasmon absorption and localized field enhancement at the DLC–Ag interface for lowering the ablation threshold of DLC:Ag nanocomposites below 6 mJ cm−2 at 1000 pulses is demonstrated. This is at least four times lower than the threshold for pure DLC obtained using the same DLIP setup.