Ultrashort Pulsed Laser Ablation Research Articles

In-Situ Analysis of Heat Accumulation during Ultrashort Pulsed Laser Ablation

In-Situ Analysis of Heat Accumulation during Ultrashort Pulsed Laser Ablation

Innovative micro-tool manufacturing using ultra-short pulse laser ablation

Abstract The continuously growing demand for micro-electro-mechanical systems (MEMS) with decreasing feature sizes necessitates adjusted manufacturing solutions. Micro-milling fulfills the high requirements for precision and reliability placed on the final product. The employed tools have diameters in the range of 50–300 μm and are commonly manufactured by grinding. However, a reduction in tool diameter leads to a decrease in stiffness in the fourth power and the introduced load in grinding causes a high scrap rate. Furthermore, the flexibility in tool design is limited to the attributes of the grinding wheel. These restrictions in micro-tool manufacturing can be avoided by using ultra-short pulse (USP) laser ablation. This process allows a force-free 2-D and 3-D machining across a broad range of materials without wear formation. By using a USP laser, a wide range of customer-oriented applications in the micrometer scale can be addressed. Also, it leads to precise ablation with minimal thermal and mechanical damage. This paper provides knowledge on the manufacturing of micro-tools made of tungsten carbide with small diameters and high aspect ratio. For the desired tool geometries, necessary process parameters are evaluated and their physical limits are shown and discussed. An innovative CAM-system has been programmed to allow the manufacturing of advanced geometries using a 4-axis laser machine test bench developed by ETH Zurich. A 515 nm wavelength laser beam is capable of providing flawless tools with diameters as low as 50 μm and aspect ratios up to 6. Due to precise calibration, the tool run-out is decreased to

Observation of single ultrashort laser pulse generated periodic surface structures on linelike defects

The formation of self-assembled laser induced periodic surface structures (LIPSSs) after ultrashort pulsed laser ablation is still a matter of controversy in the literature. There is agreement that at least two different physical driving forces lead to ripples with distinguishable spatial periodicity. High spatial frequency LIPSSs with periodicity well below the incident wavelength are discriminated from low spatial frequency LIPSSs (LSFLs) revealing longer periodic structures. In general, both types of LIPSS appear after multipulse irradiation with the linear polarization direction on all material classes from metals to dielectrics. However, single-pulse induced LSFLs at 540 ± 35 nm periodicity with subpicosecond pulse are observed at linelike surface defects, e.g., scratches and grain boundaries. Depending on the difference in orientation between the electric field vector and the scratch direction, LIPSSs evolve upon ablation with 515 nm and 1 ps pulses near the threshold. This corroborates the theory proposed by Sipe et al. [Phys. Rev. B 27, 1141–1154 (1983)], where the impinging electromagnetic wave interacts with a collectively excited surface electron wave of the respective material at a surface defect. The observations on oxygenfree pure copper, zirconia, and a stainless steel substrate are discussed. Moreover, LSFLs generated with circular polarization at defects after single pulse ablation of wide bandgap zirconia ceramic are presented. In application, this phenomena affects the attainable surface quality, where LSFLs appear at defects such as scratches, grain boundaries, and, generally, material inhomogeneity. The absorptivity and ablation characteristic change leading to an altered material-laser interaction at the surface. This could be the root cause of conelike protrusion structures observed on stainless steel.

Laser-Microstructured Copper Reveals Selectivity Patterns in the Electrocatalytic Reduction of CO2

Summary The strategy of engineering the local chemical environment to direct selectivity in the electroreduction of CO2 toward value-added products is only qualitatively understood. The unfeasibility of local concentration measurements and the limited applicability of simulations to practical systems hinder more precise guidelines. Here, we quantify the impact of the (electro)chemical environment on the selectivity pattern by using microstructured copper (Cu) electrodes prepared by ultra-short pulse laser ablation. We created regularly distributed micro-probes and assessed their product distributions at distinct overpotentials. The regular geometry enabled the accurate simulation of the local pH and CO2 concentration. Selectivity maps useful for mechanistic and applied studies emerged. They revealed clear patterns for C1–C3 products, suggesting novel insights, such as the presence of two reaction mechanisms for propanol. The effect on the selectivity pattern of operating parameters, such as enhanced mass transport and electrolyte composition, was also predicted by the maps.

Evolution of microstructures on stainless steel induced by ultra-short pulsed laser ablation

Ultra-short pulsed laser ablation of stainless steel is accompanied by the evolution of different microstructures. Depending on the fluence, accumulated energy and number of laser passes cones from impurities, laser induced periodic surface structures, cone-like protrusion (CLP), and thermal bumps evolve at the surface. These often unwanted morphologies can be induced or inhibited by carefully choosing the strategy and laser parameters. The investigated range reveals a small processing window for defined 515 nm sub 1 ps ablation leading to low surface roughness using circular polarization. Hitherto, the origin and dependencies of CLP are still not well understood and for the first time a precursor ripple structure reported. These precursor ripples reveal supra-wavelength periodicity with about $$2\,\upmu \hbox {m}$$ spacing and evolve earliest after the second layer of ablation. Potentially, low spatial frequency laser-induced periodic surface structure generated with the first laser pass with pulse and hatch overlap are the root cause of CLP evolution. Moreover, the CLP growth is grain orientation and strongly polarization state dependent. Preferentially, CLP start to evolve at the {110} planes of the face-centered cubic crystals of the inspected austenitic stainless steel and linear polarized laser radiation revealing a 1:1 aspect ratio of $$10\,\upmu \hbox {m}$$ . A nanoindentation study at the interface near region on cross-sections reveals robust mechanical properties of this CLP structure.

Laser-fabricated axicons challenging the conventional optics in glass processing applications.

Laser-based fabrication can be an alternative technology to mechanical grinding and polishing processes. However, the performance of these elements in real applications still needs to be validated. In this paper, we demonstrate that the subtractive fabrication technology is able to produce high-quality axicons from fused silica, which can be efficiently used for glass processing. We comprehensively investigate axicons, fabricated by ultrashort pulsed laser ablation with subsequent CO2 laser polishing, and compare their performance with commercially available axicons. We show that laser-fabricated axicons are comparable in quality with a precision commercial axicon. Furthermore, we demonstrate the intra-volume glass modification and dicing, utilising mJ-level laser pulses. We show that the tilting operation of the laser-fabricated axicons results in the formation of directional transverse cracks, which significantly enhance the 1 mm-thick glass dicing process.

Assessment machining of micro-channel textures on PCD by laser-induced plasma and ultra-short pulsed laser ablation

Micro-channels are one of the most common surface textures for applications, however, the machining of micro-channels on polycrystalline diamond (PCD) is still facing challenges due to its intrinsic properties of high brittleness and hardness. In this paper, fabrication of micro-channels by laser induced plasma (LIP) and laser ablation directly (LAD) machining methods on PCD surface is experimentally investigated. For comparing the processing capacity, the influence of different laser process parameters (including overlap, power, frequency, scanning speed and the number of passes) on the geometry and material removal rate of micro-channels is assessed. Results show that the process parameters have a significant influence on the formation of micro-channels, and LIP exhibits higher efficiency compared to LAD in the case of the plasma formation with the high overlap and laser power. The micro-channel depth and width produced by LIP increase with the increasing overlap, laser power and number passes, and that reduce with the increasing frequency and scanning speed; the MRR produced by LIP increases with the increasing overlap, laser power and scanning speed, and that reduces with the increasing frequency and number of passes. Meanwhile, LIP reduces the oxidation reaction inside the micro-channels compared to LAD due to the shielding effect of distilled water.

Time-resolved visualization of high-repetition ultrashort pulse laser ablation.

A pump-probe setup including a Robert-cell-type delay stage is calculated and built in the presented study. The goal is to visualize laser beam material interactions upon highly repetitive ultrashort pulse irradiations by shadowgraph imaging, which makes a valuable contribution to clarify the occurring interaction phenomena in this field. Ultrashort laser pulses (λ=1030nm; τ H =400fs) are irradiated onto a bright-rolled stainless steel metal plate (AISI 316). The high-speed shadowgraph sequences are captured for the time-resolved imaging of plasma and shockwave evolution during material ablation. The captured time frame ranges from the time just before the next pulse irradiates the interaction zone until 2 µs after pulse irradiation. The first part of the experimental study features the shockwave dynamics and evolution of the laser plasma/ablation plume as induced upon single-pulse irradiations. It is shown that the expansion velocity of the shockwave decreases from 10 km/s shortly after pulse irradiation to 6.1 km/s at 41 ns after pulse irradiation. The second part deals with laser pulse trains by irradiating up to 10 pulses at 500 kHz pulse repetition frequency to the substrate. For increasing pulse numbers, the shadowgraphs show a steady increase in height and width of the laser plasma/ablation plume that were measured at 2.4 mm in height and 1.2 mm in width after the 10th pulse.

Contactless surface flattening of additive manufactured nickel-base alloy parts by ultra-short pulsed laser ablation

Nickel-base alloy samples produced by selective laser melting (SLM) exhibit high surface roughness of 14.2 μm \± 4.8 μm standard deviation at the sidewalls. Ultra-short pulsed (USP) laser ablation at a pulse duration of 10 ps is applied to reduce the surface roughness. This flattening method does not require a melting phase. The influence of the tilt angle and the orientation of the SLM layers on the surface topography of the sample is investigated. Orthogonal laser ablation of Ni-base alloy samples leads to irregular surface topography and exposure of sharp spikes. Surface flattening by laser ablation is demonstrated applying a tilt angle of 80° in combination with a projected pulse fluence of 0.21 J cm-2. A significant influence of the tilt angle and SLM layer orientation on the surface roughness is observed.

Milling applications with GHz burst: Investigations concerning the removal rate and machining quality

GHz-bursts were reported to be highly efficient for ultra-short pulse laser ablation compared to single pulses. However, most comparisons were made for a drilling process or the machining of dimples. We investigated GHz-bursts (5.4 GHz) on copper, brass, stainless steel, silicon, zirconium oxide, soda-lime glass and sapphire for surface structuring applications. Inconsistent with the published results neither a higher removal rate, nor an improvement in the machining quality in case of the metals and silicon was observed, in the contrary, a tremendous drop in the specific removal rate of 90% for the metals and 60% for silicon, compared to single pulses, was measured when a 25 pulse burst (maximum of laser system) was applied. The situation differs for zirconium oxide, where only a moderate influence was observed and for soda-lime glass and sapphire where the specific removal rate increased by a factor of 2.3 and 6, when the number of pulses per burst were raised from 1 to 25.

Percussion Drilling Hole in Cu, Al, Ti, and Ni Alloys Using Ultra-Short Pulsed Laser Ablation.

Comparative study on micro-deep hole drilling by picosecond ultra-short pulse laser in four metals, including copper, aluminum alloy, titanium alloy, and nickel alloy, was presented. Destructive testing was performed to measure the depth, shape, and side-wall morphology of micro-deep holes in this study. Diameters and depths of micro-deep holes in four metals ablated using the same processing parameters were compared. The multiple thresholds of metals had been calculated. Relationships between hole dimensions and multiple thresholds (the average ablation threshold, the ablation threshold and the thermal threshold) and physical properties (for example, the heat capacity, the melting temperature, the optical penetration depth and the absorbance, etc.) of the four metals are examined. Furthermore, the surface and side-wall morphologies of the micro-deep holes in four metals were demonstrated. The influence mechanisms, especially the side-wall morphology, were discussed, such as the thresholds, conical emission, self-cleaning effect, physical properties of metals, the energy transmission, the plasma effect, thermochemical reaction, and so on.

Synthesis of multi-color luminescent ZnO nanoparticles by ultra-short pulsed laser ablation

Crystalline ZnO nanoparticles (NPs) are synthesized by ultra-short femtosecond (fs) pulsed laser ablation (PLA) of a zinc plate in deionized water, and are investigated by optical absorption and time resolved luminescence spectra in combination with the morphology and structure analysis. The comparison with previous experiments based on short nanosecond (ns) PLA highlights that pulse duration is a crucial parameter to determine the size and the optical properties of ZnO NPs. While short PLA generates NPs with average size S‾ of ~30 nm, ultrashort PLA allows to achieve much smaller NPs, S‾⩽10 nm, that evidence weak quantum confinement effects on both the absorption edge and the UV excitonic emission. Moreover, the defective nature of synthesized ZnO NPs gives rise to two visible photoluminescence bands: the first, centered around 2.3 eV and commonly associated with oxygen vacancies, unexpectedly blue-shifts on decreasing the NPs size; the second, peaked at 2.8 eV, shows a Lorentzian shape characteristic of a mobile exciton. Overall, these results demonstrate that PLA is a successful method to synthesize ZnO NPs with controlled luminescence properties, thus increasing their application in lighting technologies.

Refractive hard x-ray vortex phase plates.

In this Letter, we report on the creation of hard x-ray beams carrying orbital angular momentum of topological charge -ℏ and -3ℏ at a photon energy of 8.2keV via spiral phase plates made out of fused silica by ultrashort-pulsed laser ablation. The phase plates feature a smooth phase ramp with a 0.5μm nominal step height and a surface roughness of 0.5μm. The measured vortex beams show submicrometer-sized donut rings and agree well with numerical modeling. Fused silica phase plates are potentially suited to manipulate the electromagnetic field in highly intense x-ray beams at x-ray free-electron laser sources.

Laser-assisted micro-grinding of Si3N4

One of the main challenges in micro-machining of hard to cut materials is the accuracy of the machined part because of high grinding forces which cause tool deflection and rapid tool wear. Non-conventional machining processes have been developed as alternatives to conventional processes for optimizing the machining efficiency and achieving the desired tolerances. This paper addresses a novel Laser-Assisted Micro-Grinding (LAMG) process. For the first time, an ultra-short pulse laser is used to structure micro-grinding tools and a Si3N4 workpiece prior to the micro-grinding. The induced grinding forces and achieved surface roughness as well as the tool deflection and wear by the LAMG process are compared to the Conventional Micro-Grinding (CMG) process. Additionally, the ultra-short pulsed laser ablation process was simulated to find the proper laser parameters corresponding to the desired structure and laser-cut depth. The results proved an enhanced performance of LAMG process in micro-grinding of Si3N4. Using the simulation, the depth of laser cut prior to structuring could be predicted. Structuring, either the workpiece or the tool, significantly reduced the grinding forces. However, a rougher surface resulted via tool structuring. The grinding forces reduced up to almost 40% using 30% structured workpiece surfaces. Structuring the micro-tool (10% structuring) resulted in 60% reduction in the grinding forces with around 40% rougher surface. The accuracy of the part was highly improved by the laser structuring. The actual depth of cut using CMG was 30% lower than the nominal depth of cut. Utilizing LAMG the total nominal depth of cut was removed from material. The comparison between the topography of structured and non-structured tools indicated higher tool wear and shorter tool life with the non-structured tool.

Laser ablation mechanism of silicon nitride with nanosecond and picosecond lasers

Silicon nitride (Si3N4) has exceptional mechanical, thermal and chemical properties and is therefore, advantageous for many applications. However, the employment of Si3N4 is impeded by the high finishing cost. Short and ultrashort pulse laser ablation are considered as a non-contact machining processes and are utilised in recent years to overcome constrains of conventional machining of Si3N4. However, there is still a lack of research on the effects of different laser parameters such as pulse duration on the mechanism of laser ablation. In this study, laser-material interaction of two different types of solid-state lasers with 10 ps and 100 ns pulse durations was investigated at the constant power of 50 W on a Si3N4 workpiece. The width of thermal damages and heat-affected zone (HAZ) is extremely reduced within laser input energy density (EL−Input) less than 40 J/mm2 for both types of laser. It is found that in case of the nanosecond laser ablation, molten droplets inside the laser cut prevent diffusion of the laser beam at EL−Input higher than 62.5 J/mm2. This effect could not be observed in ablation with the picosecond laser. Compared to the nanosecond laser, the picosecond laser is more efficient for ablating the workpiece material at laser input energy densities higher than 120 J/mm2.

Effect of surface roughness on the ultrashort pulsed laser ablation fluence threshold of zinc and steel

Abstract The single and multiple pulse laser ablation threshold of zinc and steel at picosecond laser pulse duration is studied as a function of initial surface roughness at laser wavelengths of 515 and 1030 nm. The initial surface topographies and the resulting crater morphologies are analyzed using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Reflectivity measurements of the initial surfaces show increased absorptivity with increasing surface roughness. It was found that the single pulse ablation threshold increases with increasing effective surface area; the latter resulting from surface roughness. Rougher surfaces tend to have a higher degree of incubation as well. From the experimental and simulation results, it appears that the absorbed energy contributes more to residual heat than to material ablation when effective surface area increases.

Ultra-Short Pulsed Laser Ablation of Epoxy Mold Compound or Copper Frames for Partial Cut QFNs

Ultra-Short Pulsed Laser Ablation of Epoxy Mold Compound or Copper Frames for Partial Cut QFNs

Experimental and theoretical investigation of ultrashort pulsed laser ablation of diamond

Diamond is a wide bandgap material, which exhibits an abrupt increase of its free-electron density, when excited by an ultrashort laser pulse. The generation of free electrons transforms the insulator diamond to a conducting material with metallic optical behavior. This transformation process can be described by the multiple rate equation (MRE) model. The introduced MRE model considers strong-field excitation in the Keldysh picture as well as collisional excitation. The light attenuation results from the strong-field absorption and free-carrier absorption described in the Drude picture. Thus, the electron density and intensity distribution as functions of time, penetration depth, and laser beam radius are calculated. Furthermore, the model predicts the evolution of optical properties and estimates the ablation threshold value by the diameter and depth regression method. The calculated ablation threshold is compared to experimental results on a single crystalline chemical vapor deposited diamond by applying the diameter and depth regression method. Experimental and theoretical results are discussed with regard to the pulse duration. The discussion focuses on single pulse ablation but also addresses the multishot domain, which is essential for laser machining. At 1030 nm, the experimental single pulse ablation threshold fluence is determined to be 8.2 and 12.9 J/cm2 for pulse durations of 400 and 700 fs, respectively. This is in compliance with the simulation results.

Toward application of hierarchical structures by ultrashort pulsed laser ablation

Surface structuring is a versatile tool to modify the physical interplay at the interface to gaseous or liquid media. The two industrial used approaches are, in general, a chemical modification via coating or/and an introduced topography. Here, the structuring is carried out with a direct technique using an ultrashort pulsed laser with infrared and green wavelengths. Two hierarchical structural units are generated at the surface on copper, aluminum, and an amorphous alloy. Structures in the micrometer regime are produced via 2.5D layered laser ablation. Laser induced periodic surface structures (LIPSSs), an inherent phenomenon of self-generated periodic features, can be utilized to generate subdiffraction limit features. LIPSSs are demonstrated in the high spatial frequency regime with a periodicity of 320 and 600 nm for green and infrared radiations, accordingly. This work concerns the use of direct laser structuring for potential applications. Specifically, a change in the optical and wetting properties is addressed. Broadband absorbing behavior and static contact angles of up to 152° are reported. Laser ablation processes and experimental routines are presented to upscale the structuring process. Production rates up to 100 mm2/h for 2.5D ablation and 0.6 m2/h for LIPSSs can be reached.

In-Situ Analysis of Ultrashort Pulsed Laser Ablation with Pulse Bursts

In-Situ Analysis of Ultrashort Pulsed Laser Ablation with Pulse Bursts