Ultrashort Laser Pulse Irradiation Research Articles

Applying Ultrashort Pulsed Direct Laser Interference Patterning for Functional Surfaces

Surface structures in the micro- and nanometre length scale exert a major influence on performance and functionality for many specialized applications in surface engineering. However, they are often limited to certain pattern scales and materials, depending on which processing technique is used. Likewise, the morphology of the topography is in complex relation to the utilized processing methodology. In this study, the generation of hierarchical surface structures in the micro- as well as the sub-micrometre scale was achieved on ceramic, polymer and metallic materials by utilizing Ultrashort Pulsed Direct Laser Interference Patterning (USP-DLIP). The morphologies of the generated patterns where examined in relation to the unique physical interaction of each material with ultrashort pulsed laser irradiation. In this context, the pattern formation on copper, CuZn37 brass and AISI 304 stainless steel was investigated in detail by means of a combination of experiment and simulation to understand the individual thermal interactions involved in USP-DLIP processing. Thereby, the pattern’s hierarchical topography could be tailored besides achieving higher process control in the production of patterns in the sub-µm range by USP-DLIP.

Efficient production of design textures on large-format 3D mold tools

Laser surface structuring is becoming increasingly important in the industry for tool and mold making. While structured surfaces contribute to minimizing friction in combustion engines or to increasing efficiency of light-emitting diode-based lighting systems, surface texturing is evolving a quality feature of products with regard to optical and haptic properties. Currently used manufacturing processes for tool texturing like photochemical etching are limited in precision and in flexibility. To establish a digital process chain and to increase the design flexibility, laser ablation with (ultra) short pulse laser radiation is becoming an increasingly important technology. In the research project “eVerest,” all necessary parts of a laser texture processing are integrated into the machine and operating concept, e.g., the virtual design of the product including unrolling and visualization of the textures. Finally, new process strategies and advanced machine and system technologies are developed.

Ablation in Externally Applied Electric and Magnetic Fields.

To harness light-matter interactions at the nano-/micro-scale, better tools for control must be developed. Here, it is shown that by applying an external electric and/or magnetic field, ablation of Si and glass under ultra-short (sub-1 ps) laser pulse irradiation can be controlled via the Lorentz force , where is velocity of charge e, is the applied electrical bias and is the magnetic flux density. The external electric E-field was applied during laser ablation using suspended micro-electrodes above a glass substrate with an air gap for the incident laser beam. The counter-facing Al-electrodes on Si surface were used to study debris formation patterns on Si. Debris was deposited preferentially towards the negative electrode in the case of glass and Si ablation. Also, an external magnetic field was applied during laser ablation of Si in different geometries and is shown to affect ripple formation. Chemical analysis of ablated areas with and without a magnetic field showed strong chemical differences, revealed by synchrotron near-edge X-ray absorption fine structure (NEXAFS) measurements. Harnessing the vectorial nature of the Lorentz force widens application potential of surface modifications and debris formation in external E-/B-fields, with potential applications in mass and charge spectroscopes.

Analysis of ultrashort laser pulse irradiation with 2d thin metal films using the fuzzy lattice Boltzmann method

Analysis of ultrashort laser pulse irradiation with 2d thin metal films using the fuzzy lattice Boltzmann method

Subwavelength character of the ordered destruction of condensed media by circularly polarized ultrashort pulse laser radiation

Subwavelength character of the ordered destruction of condensed media by circularly polarized ultrashort pulse laser radiation

Design of multi-beam optics for high throughput parallel processing

The utilization of ultrashort pulsed laser radiation enables the precise generation of microstructures and the processing of a wide variety of materials. However, for massive parallel processing, the raw intensity distribution of the beam emitted by the laser source usually has to be transformed into an arrangement of multiple laser beams with specific intensity distributions. Here, sophisticated, application-adapted optical systems serve as a key technology. This paper gives an overview of the optical design fundamentals of multi-beam optics. It also investigates the causes for one of the main challenges in the use of multi-beam optical systems: the distortion of the spot-array on the work piece. The distortion leads to individual spot position errors and can heavily limit the processing accuracy. Finally, active and passive concepts and technologies to reduce or eliminate distortion in multi-beam systems are presented.

High Pulse Repetition Frequency Micro Hole Drilling of Silicon Using Ultrashort Pulse Laser Radiation

High Pulse Repetition Frequency Micro Hole Drilling of Silicon Using Ultrashort Pulse Laser Radiation

Laser-assisted micro-milling of austenitic stainless steel X5CrNi18-10

This paper presents a novel Laser-Assisted Micro-Milling (LAMM) process of austenitic stainless steel X5CrNi18-10. The LAMM process is compared with the conventional micro-milling process. Ultra-short pulsed laser radiation is utilized for the structuring of the workpiece surface prior to the micro-milling process. Different laser structures are produced on the workpiece surface at a constant laser scanning speed with various laser powers and laser line spans. The high performance of the developed process is shown by experimental investigations. The effect of laser structuring on the micro-milling forces and temperature indicated the superior performance of the new LAMM process. Cutting forces and temperature could be reduced by up to 70% and 50%, respectively. The results of conventional micro-milling showed that increasing the cutting speed, at a constant undeformed chip thickness, reduced the micro-milling forces. Increasing the cutting speed from 50 to 250 m/min halved both the trust and normal forces, while it slightly improved the surface roughness. On the other hand, increasing the feed per tooth degraded the surface roughness and increased the cutting forces. Furthermore, in conventional milling the workpiece was subjected to high plastic deformation during the cutting process, while side flow, smeared material, metal debris, and cavities were observed on the workpiece surface.

Selective rear-side ablation of aluminum thin layers with ultrashort-pulsed laser radiation

In recent years, several applications for laser ablation of thin metal layers from the fused silica substrate side have been studied. The rear-side ablation is a highly effective ablation method for thin layer structuring and reveals a high structuring quality. Therefore, the present work dealt with the selective rear-side ablation of thin aluminum layers (10–50 nm) on fused silica with ultrashort-pulsed laser radiation (λ = 1028 nm, $$ \tau_{\text{H}} $$ = 0.2–10 ps and w0,86 = 15.2 µm). The influences of pulse duration and layer thickness on the ablation thresholds as well as the incubation coefficients were determined. For layer thicknesses of 30 and 50 nm, a decrease of the ablation threshold with increasing pulse duration was determined. Whereas, the ablation threshold remained constant for layer thicknesses of 10 and 20 nm. Different morphologies were observed depending on the process parameters. The rear-side ablation of aluminum proceeded over the melting phase and no lift-off process had taken place. In addition to experimental investigations, calculations were carried out to determine the theoretical threshold fluences. The theoretical values were compared to experimental data. With the help of these investigations, the quality of the structuring of aluminum layers can be improved.

Ab initio approach to lattice softening of an Al slab driven by collective electronic excitations after ultrashort laser pulse irradiation

Recent advances in ultrashort laser pulse techniques have opened up a wide variety of applications in both fundamental physics and industrial fields. In this work, $ab$ $initio$ molecular dynamics simulations based on time-dependent density functional theory revealed a steady deceleration of lattice distortion propagation in an aluminum slab with increasing laser pulse intensity. Analysis of the interatomic force revealed a significant reduction in the harmonic terms and non-monotonic growth of anharmonicity. This behavior was characterized by spatially non-uniform force screening by plasmons, which is missing from Born--Oppenheimer molecular dynamics, and is consistent with the current interpretation of laser-induced periodic structure patterning. This work provides a semi-quantitative criterion for modifying the phonon properties of non-equilibrium systems.

Fabrication of Tuning-Fork Gyroscope Resonator using Directed Chemical Etching of Fused Silica

The article presents the results of the experimental and practical work devoted to the study of directed chemical etching of fused silica locally exposed to ultra-short pulsed laser radiation as a way to produce sensitive elements of precision instruments of orientation, stabilization and navigation as resonators of tuning fork gyroscopes. The essence of the directed chemical etching method is presented. The created experimental setup structure and components are purposed and the laser processing and chemical etching operational modes are described. The laser processing mode obtained as a result of the experiment was used to fabricate the tuning fork gyroscope resonator. The etching rate of exposed fused silica in 5 % hydrofluoric acid is 27.5 to 68.8 µm/hour. That is much faster than the etching rate of the non-affected material. Based on this results the application of the method in the precision instruments fabrication area can be considered promising and subjected to further study.

Laser micromachining of silicon and cemented tungsten carbide using picosecond laser pulses in burst mode: ablation mechanisms and heat accumulation

This paper presents results obtained by studying material removal of silicon and cemented tungsten carbide using high-frequency ultrashort-pulsed laser radiation. In laser-induced material removal, ablation mechanisms and heat accumulation effects are considered. Depending on the fluence and number of pulses in a burst, structures are created on silicon and cemented tungsten carbide in order to be able to determine the ablated volume. A single pulse in the burst represents the conventionally pulsed laser radiation. Depending on the number of pulses, the ablated volume per pulse rises in the burst. Furthermore, an increase in the number of pulses in the burst results in a repetitive decrease as well as an increase in the ablated volume. Investigations at different ambient pressures establish that this phenomenon could be changed for cemented tungsten carbide under fine vacuum. The simulations demonstrate that the laser-induced heat accumulation in burst mode contributes significantly to the removed volume.

Modelling of the heat accumulation process during short and ultrashort pulsed laser irradiation of bone tissue.

An analytical model is presented that qualitatively describes the cooling of a biological tissue after irradiation with short and ultrashort laser pulses. The assumption that the distribution of temperature at the initial moment of surface cooling repeats the distribution of the absorbed laser energy allowed us to use the thermal conductivity approximation in both cases. The experimental results of irradiation of dry bone with nanosecond and femtosecond laser pulses are compared with the calculated data. The necessity of taking into account the change in the optical parameters of hard tissue in the field of laser irradiation during its treatment by nanosecond and femtosecond laser pulses and the key role of residual heating in its carbonization around the exposure region is shown. The application of the model to a particular biological tissue can significantly simplify the search for optimal parameters of lasers for surgical procedures.

Investigation of heat accumulation effects during deep hole percussion drilling by high power ultrashort pulsed laser radiation

In the last few years, commercially available ultrashort pulsed (usp) laser systems have reached average powers of several 100 W, which makes them also interesting for enhancing traditional applications. For example, laser drilling, a conventionally melt-dominated process, would benefit from the advantages of an usp ablation process. Due to the small processing area in laser drilling, substantial heat accumulates already at a few Watts of average power. This heat accumulation creates melt but also increases the mean ablation rate at least 1 order of magnitude and could lead to a productive drilling process. In this study, the increase of the mean ablation rate in percussion drilling due to heat accumulation is examined for various metals and sample thicknesses for high average powers of up to 300 W and a pulse energy in the milliJoule range. Those investigations have not yet been performed in such detail. It is shown that by doubling the pulse energy the drilling time can be decreased by 2 orders of magnitude due to heat accumulation. This behavior is valid for various metals like steel or aluminum, despite their varying material parameters. By analyzing the influence of different repetition rates and focal diameters, it is shown that the predominant parameter to characterize the heat accumulation is the average power. No significant difference is observed if the average power starting from 20 W is set up by a high pulse energy or repetition rate. Stainless steel has a different behavior compared to the other investigated metals when the pulse duration is changed from 2 to 20 ps, which is caused by a modified plasma generation. For stainless steel, the drilling time is found to be describable with one empirical formula for the entire range of examined average power and sample thickness.

Early Stage Material Motion and Transient Optical Properties of Metals after Ultrashort Laser Pulse Irradiation

Early Stage Material Motion and Transient Optical Properties of Metals after Ultrashort Laser Pulse Irradiation

Ultrafast Optical Control of Magnetic Interactions in Carrier-Density-Controlled Ferromagnetic Semiconductors

Investigation of the interaction of ultrashort laser pulses with magnetically ordered materials has become a fascinating research topic in modern magnetism. Especially, the control of magnetic order by sub-ps laser pulses has become a fundamentally important topic with a high potential for future spintronics applications. This paper will review the recent success in optically controlling the magnetic interactions in carrier-density-controlled ferromagnetic semiconductor EuO doped with Gd ions. When the Gd concentration is low, the magnitude of the magnetic interaction is enhanced by the irradiation of ultrashort laser pulses, whereas it is attenuated when the Gd concentration is high. In ferromagnetic Eu1−xGdxO, we thereby demonstrate the strengthening as well as the weakening of the magnetic interaction by 10% and within 3 ps by optically controlling the magnetic exchange interaction. This principle—ultrafast optical control of magnetic interaction—can be applied to future ultrafast opto-spintronics.

Generation of a large compressive strain wave in graphite by ultrashort-pulse laser irradiation.

We have studied strain wave generation in graphite induced by an intense ultrashort laser pulse. The study was performed in the intensity regime above the ablation threshold of graphite. The aim was to maximize the strain and, thus, also the internal pressure (stress). Laser pulses with a 1 ps temporal duration melt the surface of graphite resulting in a molten material which initially exists at the solid density. As the molten material expands, a compressive strain wave starts propagating into the crystal below the molten layer. The strain pulse was studied with time-resolved X-ray diffraction. At a temporal delay of 100 ps after laser excitation, we observed >10% compressive strain, which corresponds to a pressure of 7.2 GPa. This strain could be reproduced by hydrodynamic simulations, which also provided a temperature map as a function of time and depth.

Experimental research on noninvasive reconstruction of optical parameter fields based on transient radiative transfer equation for diagnosis applications

The noninvasive reconstruction of optical parameter fields in participating phantom is investigated experimentally based on the measured time-resolved radiative signals, which are detected by the time-correlated single-photon counting system. The discrete ordinate method is employed to solve the forward transient radiative transfer equation to simulate the time-resolved radiative transfer in the physical phantom exposed to ultra-short pulse laser irradiation. On top of that, the sequential quadratic programming algorithm based on the generalized Gaussian Markov random field is applied to solve the inverse problem. To improve the computational efficiency, an adjoint equation technique is adopted to determine the gradient of objective function. Good agreement was obtained between the predicted optical parameter fields and realistic ones. All the reconstruction results demonstrate that the proposed reconstruction methods are proved to be accurate and efficient experimentally.

Engineering Cell Adhesion and Orientation via Ultrafast Laser Fabricated Microstructured Substrates.

Cell responses depend on the stimuli received by the surrounding extracellular environment, which provides the cues required for adhesion, orientation, proliferation, and differentiation at the micro and the nano scales. In this study, discontinuous microcones on silicon (Si) and continuous microgrooves on polyethylene terephthalate (PET) substrates were fabricated via ultrashort pulsed laser irradiation at various fluences, resulting in microstructures with different magnitudes of roughness and varying geometrical characteristics. The topographical models attained were specifically developed to imitate the guidance and alignment of Schwann cells for the oriented axonal regrowth that occurs in nerve regeneration. At the same time, positive replicas of the silicon microstructures were successfully reproduced via soft lithography on the biodegradable polymer poly(lactide-co-glycolide) (PLGA). The anisotropic continuous (PET) and discontinuous (PLGA replicas) microstructured polymeric substrates were assessed in terms of their influence on Schwann cell responses. It is shown that the micropatterned substrates enable control over cellular adhesion, proliferation, and orientation, and are thus useful to engineer cell alignment in vitro. This property is potentially useful in the fields of neural tissue engineering and for dynamic microenvironment systems that simulate in vivo conditions.

Laser-based plastic-metal-joining with self-organizing microstructures considering different load directions

Innovative multi-material lightweight construction enables reducing dead weight while maintaining and preferably boosting the components' performance. The implementation of multi-material parts (e.g., plastic-metal-components) requires reliable joining processes since a direct connection between these materials is not feasible due to their different physical and chemical properties. In order to avoid additional weight through adhesive bonding, riveting or fasteners, thermal direct joining with a modified metal surface is a promising approach. Within a first process step, the metal surface is modified by laser-microstructuring. To enlarge the boundary surface and create undercut structures, random self-organizing micro- and nanostructures are generated with ultrashort pulsed laser radiation on stainless steel samples. In the subsequent direct thermal joining process, both joining partners are clamped together. The metal is heated up with diode laser radiation, and through heat conduction, the polymer melts and flows into the generated cavities. After cooling-down, a firm joint between both materials is created, which is based on mechanical interlocking and increased specific adhesion between the joining partners. The mechanical strength of the joint depends strongly on the load direction. In the presented contribution, the strength of the joint between stainless steel and glass-fiber-reinforced and non-reinforced thermoplastics (PP) is investigated for three different load directions (tensile shear, tensile and peel).