Investigation of Heat Accumulation in Femtosecond Laser Drilling of Carbon Fiber-Reinforced Polymer

Carbon fiber-reinforced polymer (CFRP) has indispensable applications in the aerospace field because of its light weight, corrosion resistance, high specific modulus and high specific strength, but its anisotropy brings great difficulties to precision machining. Delamination and fuzzing, especially the heat-affected zone (HAZ), are the difficulties that traditional processing methods cannot overcome. In this paper, single-pulse and multi-pulse cumulative ablation experiments and drilling of CFRP have been carried out using the characteristics of a femtosecond laser pulse, which can realize precision cold machining. The results show that the ablation threshold is 0.84 J/cm² and the pulse accumulation factor is 0.8855. On this basis, the effects of laser power, scanning speed and scanning mode on the heat-affected zone and drilling taper are further studied, and the underlying mechanism of drilling is analyzed. By optimizing the experimental parameters, we obtained the HAZ < 10 μm, a cylindrical hole with roundness > 0.95 and taper < 5°. The research results confirm that ultrafast laser processing is a feasible and promising method for CFRP precision machining.

Subthreshold Laser Ablation Measurements by Langmuir Probe Method for ns Irradiation of HfO2 and ZrO2

The unbiased Langmuir probe (LP) method was used to perform measurements on HfO₂ and ZrO₂ samples around the laser ablation threshold on a wide range of irradiation conditions. Important changes in the lifetime (from ms to μs) and the shape of the charge particle current were seen with the increase of the laser fluence. The ablation threshold was estimated by evaluating the overall average ablated charge as a function of the laser fluence. Above the ablation threshold, the generation of high kinetic species is seen, which can reach several keV. An important jump in ion acceleration potential is observed for values above 1 J/cm², which coincides with the dominant presence of negative ions in the plasma. The evolution of several plasma parameters (ion density, expansion velocity, electron temperature, Debye length) was investigated and correlated with the fundamental ablation mechanism involved in various irradiation regimes. The LP data were correlated with COMSOL simulations on the maximum surface temperature reached during irradiation. Important correlations between the evaporation and melting processes and ablation threshold fluence and ion acceleration phenomena are also reported.

Interaction Energy Dependency on Pulse Width in ns NIR Laser Scanning of Silicon

Laser ablation of semiconductor silicon has been extensively studied in the past few decades. In the ultrashort pulse domain, whether in the fs scale or ps scale, the pulse energy fluence threshold in the ablation of silicon is strongly dependent on the pulse width. However, in the ns pulse scale, the energy fluence threshold dependence on the pulse width is not well understood. This study elucidates the interaction energy dependency on pulse width in ns NIR laser ablation of silicon. The level of ablation or melting was determined by the pulse energy deposition rate, which was proportional to laser peak power. Shorter pulse widths with high peak power were likely to induce surface ablation, while longer pulse widths were likely to induce surface melting. The ablation threshold increased from 5.63 to 24.84 J/cm² as the pulse width increased from 26 to 500 ns. The melting threshold increased from 3.33 to 5.76 J/cm² as the pulse width increased from 26 to 200 ns, and then remained constant until 500 ns, the longest width investigated. Distinct from a shorter pulse width, a longer pulse width did not require a higher power level for inducing surface melting, as surface melting can be induced at a lower power with the longer heating time of a longer pulse width. The line width from surface melting was less than the focused spot size; the line appeared either as a continuous line at slow scanning speed or as isolated dots at high scanning speed. In contrast, the line width from ablation significantly exceeded the focused spot size.

Femtosecond UV Laser Ablation Characteristics of Polymers Used as the Matrix of Astronautic Composite Material

Ultrafast laser processing has recently emerged as a new tool for processing fiber-reinforced polymer (FRP) composites. In the astronautic industry, the modified epoxy resin (named 4211) and the modified cyanate ester resin (known as BS-4) are two of the most widely used polymers for polymer-based composites. To study the removal mechanism and ablation process of different material components during the ultrafast laser processing of FRPs, we isolated the role of the two important polymers from their composites by studying their femtosecond UV laser (260 fs, 343 nm) ablation characteristics for controllable machining and understanding the related mechanisms. Intrinsic properties for the materials' transmission spectrum, the absorption coefficient and the optical bandgap (E_g), were measured, derived, and compared. Key parameters for controllable laser processing, including the ablation threshold (F_th), energy penetration depth (δ_eff), and absorbed energy density (E_abs) at the ablation threshold, as well as their respective "incubation" effect under multiple pulse excitations, were deduced analytically. The ablation thresholds for the two resins, derived from both the diameter-regression and depth-regression techniques, were compared between resins and between techniques. An optical bandgap of 3.1 eV and 2.8 eV for the 4211 and BS-4 resins, respectively, were obtained. A detectable but insignificant-to-ablation difference in intrinsic properties and ablation characteristics between the two resins was found. A systematic discrepancy, by a factor of 30~50%, between the two techniques for deriving ablation thresholds was shown and discussed. For the 4211 resin ablated by a single UV laser pulse, a F_th of 0.42 J/cm², a δ_eff of 219 nm, and an E_abs of 18.4 kJ/cm³ was suggested, and they are 0.45 J/cm², 183 nm, and 23.2 kJ/cm³, respectively, for the BS-4 resin. The study may shed light on the materials' UV laser processing, further the theoretical modeling of ultrafast laser ablation, and provide a reference for the femtosecond UV laser processing characteristics of FRPs for the future.

Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., S_a parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average S_a value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser

Femtosecond laser ablation has become one of the important structural processing methods for the third-generation semiconductor material, silicon carbide (SiC), and it is gradually being employed in the manufacture of microelectromechanical systems and microelectronic devices. Experimental study has been performed on infrared single and multiple pulses (1035 nm) femtosecond laser ablation of SiC at various processing parameters. Diameters of laser ablation spots on 4H-SiC were measured to estimate the absorption threshold for material modification and structural transformation, which were 2.35 J/cm² and 4.97 J/cm², respectively. In the multiple-pulse scribing ablation for microgrooves, the ablation threshold dropped to 0.70 J/cm² due to the accumulation effect when the effective pulse number reached 720. The calculated average of the thermally stimulated ablation depth of 4H-SiC is 22.4 nm, which gradually decreased with the raising of the effective pulse number. For obtaining square trenches with precise and controllable depths and a smooth bottom in 4H-SiC, the effects of processing parameters on the material removal rate and surface roughness are discussed. The ablation rate per pulse is almost constant, even if the effective pulse number varies. The reduction of laser spot overlapping ratio in x direction has a greater weakening effect on the material removal rate than that in y direction. The precise amount of material removal can still be controlled, while modulating the surface roughness of the ablated features by changing the hatch rotation angle. This research will help to achieve controllable, accurate, and high-quality machining results in SiC ablation, using infrared femtosecond laser.

Ultrashort Pulsed Laser Drilling of Printed Circuit Board Materials

We report on a comprehensive study of laser percussion microvia drilling of FR-4 printed circuit board material using ultrashort pulse lasers with emission in the green spectral region. Laser pulse durations in the pico- and femtosecond regime, laser pulse repetition rates up to 400 kHz and laser fluences up to 11.5 J/cm2 are applied to optimize the quality of microvias, as being evaluated by the generated taper, the extension of glass fiber protrusions and damage of inner lying copper layers using materialography. The results are discussed in terms of the ablation threshold for FR-4 and copper, heat accumulation and pulse shielding effects as a result of pulse to pulse interactions. As a specific result, using a laser pulse duration of 2 ps appears beneficial, resulting in small glass fiber protrusions and high precision in the stopping process at inner copper layer. If laser pulse repetition rates larger than 100 kHz are applied, we find that the processing quality can be increased by heat accumulation effects.

Simple and robust method for determination of laser fluence thresholds for material modifications: an extension of Liu's approach to imperfect beams

The so-called D-squared or Liu's method is an extensively applied approach to determine the irradiation fluence thresholds for laser-induced damage or modification of materials. However, one of the assumptions behind the method is the use of an ideal spatial Gaussian beam that can lead in practice to significant errors depending on beam imperfections. In this work, we rigorously calculate the bias corrections required when applying the same method to Airy-disk like profiles. Those profiles are readily produced from any beam by insertion of an aperture in the optical path. Thus, the correction method gives a robust solution for exact threshold determination without any added technical complications as for instance advanced control or metrology of the beam. Illustrated by two case-studies, the approach holds potential to solve the strong discrepancies existing between the laser-induced damage thresholds reported in the literature.  It provides also an appropriate tool for new studies with the most extreme laser radiations.

Laser-Material Interactions of High-Quality Ultrashort Pulsed Vector Vortex Beams

Diffractive multi-beams based on 1 × 5 and 2 × 2 binary Dammann gratings applied to a spatial light modulator (SLM) combined with a nanostructured S-wave plate have been used to generate uniform multiple cylindrical vector beams with radial and azimuthal polarizations. The vector quality factor (concurrence) of the single vector vortex beam was found to be C = 0.95 ± 0.02, hence showing a high degree of vector purity. The multi-beams have been used to ablate polished metal samples (Ti-6Al-4V) with laser-induced periodic surface structures (LIPSS), which confirm the polarization states unambiguously. The measured ablation thresholds of the ring mode radial and azimuthal polarizations are close to those of a Gaussian mode when allowance is made for the expected absolute intensity distribution of a ring beam generated from a Gaussian. In addition, ring mode vortex beams with varying orbital angular momentum (OAM) exhibit the same ablation threshold on titanium alloy. Beam scanning with ring modes for surface LIPSS formation can increase micro-structuring throughput by optimizing fluence over a larger effective beam diameter. The comparison of each machined spot was analysed with a machine learning method—cosine similarity—which confirmed the degree of spatial uniformity achieved, reaching cosθ > 0.96 and 0.92 for the 1 × 5 and 2 × 2 arrays, respectively. Scanning electron microscopy (SEM), optical microscopy and white light surface profiling were used to characterize and quantify the effects of surface modification.

On the Ablation Behavior of Carbon Fiber-Reinforced Plastics during Laser Surface Treatment Using Pulsed Lasers

This contribution discusses the ablation phenomena observed during laser treatment of carbon fiber-reinforced plastics (CFRPs) with pulsed lasers observed employing laser sources with wavelengths of 355 nm, 1064 nm and 10.6 µm and pulse durations from picoseconds (11 ps) to microseconds (14 µs) are analyzed and discussed. In particular, the threshold fluence of the matrix material epoxy (EP) and the damage threshold of CFRP were calculated. Moreover, two general surface pretreatment strategies are investigated, including selective matrix removal and structure generation through indentation (ablation of both, matrix material and fibers) with a cross-like morphology. The surfaces obtained after the laser treatment are characterized by means of optical and scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy is employed for the analysis of composite and constituent materials epoxy and carbon fibers. As a result, different ablation mechanisms, including evaporation and delamination are observed, depending on the employed laser wavelength and pulse duration. For both 355 nm and 1064 nm wavelength, the laser radiation produces only partial ablation of the carbon fibers due to their higher absorption coefficient compared to the epoxy matrix. Although a selective matrix removal without residues is achieved using the pulsed CO₂ laser. Differently, both constituent materials are ablated with the nanosecond pulsed UV laser, producing indentations. The sum of the investigations has shown that existing theories of laser technology, such as the ablation threshold according to Liu et al., can be applied to composite materials only to a limited extent. Furthermore, it has been found that the pronounced heterogeneity of CFRP mostly leads to an inhomogeneous ablation result, both when creating grooves and during selective matrix removal, where the carbon fibers influence the ablation result by their thermal conductivity, depending on fiber direction. Finally, despite the material inhomogeneity, a scanning strategy has been developed to compensate the heterogeneous ablation results regarding structure depth, width and heat affected zone.

Effect of Laser Pulse Overlap and Scanning Line Overlap on Femtosecond Laser-Structured Ti6Al4V Surfaces

Surface structuring is a key factor for the tailoring of proper cell attachment and the improvement of the bone-implant interface anchorage. Femtosecond laser machining is especially suited to the structuring of implants due to the possibility of creating surfaces with a wide variety of nano- and microstructures. To achieve a desired surface topography, different laser structuring parameters can be adjusted. The scanning strategy, or rather the laser pulse overlap and scanning line overlap, affect the surface topography in an essential way, which is demonstrated in this study. Ti6Al4V samples were structured using a 300 fs laser source with a wavelength of 1030 nm. Laser pulse overlap and scanning line overlap were varied between 40% and 90% over a wide range of fluences (F from 0.49 to 12.28 J/cm²), respectively. Four different main types of surface structures were obtained depending on the applied laser parameters: femtosecond laser-induced periodic surface structures (FLIPSS), micrometric ripples (MR), micro-craters, and pillared microstructures. It could also be demonstrated that the exceedance of the strong ablation threshold of Ti6Al4V strongly depends on the scanning strategy. The formation of microstructures can be achieved at lower levels of laser pulse overlap compared to the corresponding value of scanning line overlap due to higher heat accumulation in the irradiated area during laser machining.

Incubation Effect of Pre-Irradiation on Bubble Formation and Ablation in Laser Ablation in Liquids

Pulsed laser ablation in liquids (PLAL) is a multi-scale process, which is widely studied either in batch ablation with prolonged target irradiation as well as mechanistic investigations, in a defined (single-shot) process. However, fundamental studies on defined pulse series are rare. We have investigated the effect of a developing rough morphology of the target surface on the PLAL process with nanosecond pulses and, partially, picosecond pulses. At low fluence the cavitation bubble growth as well as the ablation yield depend on the irradiation history of the target. The bubble size increases with repeated irradiation on one spot for the first 2-30 pulses as well as with the applied dose. This is discussed within the framework of incubation effects. Incubation is found to be important, resulting in a bubble volume increase by a factor of six or more between pristine and corrugated targets. The target surface, changing from smooth to corrugated, induces a more efficient localization of laser energy at the solid-liquid interface. This is accompanied by a suppressed reflectivity and more efficient coupling of energy into the laser-induced plasma. Thus, the cavitation bubble size increases as well as ablation being enhanced. At high fluence, such incubation is masked by the rapid development of surface damage within the first shots, which eventually would lead to a reduction of bubble sizes.

OLED Luminaires: Device Arrays with 99.6% Geometric Fill Factor Structured by Femtosecond Laser Ablation

The plethora of design opportunities renders organic light emitting diodes (OLEDs) ideal luminaires for general lighting applications. Progressing from lab-scale device concepts to large-area applications calls for smart device designs that are scalable and, at the same time, unsusceptible to resistive losses within the electrodes. By employing direct pulsed femtosecond laser structuring, we fabricate OLED luminaires comprising monolithically interconnected OLED arrays. We determine the laser ablation thresholds and the optimized process parameters for all functional layers. The clean laser cuts with precise ablation depths show no detectable damage to adjacent layers or any ridges, hence avoiding unwanted short-cuts or device isolation. All processes are scalable. The 3-fold structuring of the OLED luminaire is confined within 45 μm and hence below the resolution limit of the human eye, yielding a geometric fill factor beyond 99% and therefore a very homogeneous device perception.