Single-pulse Ablation Threshold Research Articles

Influence of working parameters on multi-shot femtosecond laser surface ablation of lithium niobate

This paper presents an experimental investigation on the ultrashort pulsed laser ablation of 128° Y-cut Lithium Niobate (LiNbO3) using multi-shot fs-laser pulses at different pulse repetition frequencies (1, 10 and 100kHz). The ablation threshold fluence was observed to rapidly decrease as the number N of incident laser pulses increased, regardless of the repetition frequency. This behavior, compatible with the incubation effect, was accurately modeled by a power law. The calculated single pulse ablation threshold Fth,1=1.98±0.15J/cm2 is consistent with values reported in existing literature. The incubation coefficient S∗ appears to be independent of the repetition frequencies. In contrast, the asymptotic ablation threshold Fth,∞ decreased as the repetition frequency was increased. The study delves deeper into the impact of laser operational variables, including pulse energy, repetition frequency, total pulse count, and scanning speed, on the surface roughness and milled depth of fs-laser micromilled zones on LiNbO3 substrates. A discernible trade-off between achieving control over the obtained depth and the surface finish of the process was identified, providing valuable insights for achieving precise control over fs-laser processing of LiNbO3 surfaces.

Direct Femtosecond Laser Processing for Generating High Spatial Frequency LIPSS (HSFL) on Borosilicate Glasses with Large-Area Coverage

Large-area nanostructuring of glasses using intense laser beams is a challenging task due to the material’s extreme non-linear absorption of laser energy. Precise optimization of the process parameters is essential for fabricating nanostructures with large-area coverage. In this study, we report the findings on creating high-spatial-frequency LIPSS (HSFL) on borosilicate glass through direct laser writing, using a femtosecond laser with a wavelength λ = 800 nm, pulse duration τ = 35 fs, and repetition frequency frep = 1 kHz. We measured the single-pulse ablation threshold and incubation factor of Borosilicate glasses to achieve high-precision control of the large-area surface structuring. Single-spot experiments indicated that, when there was higher fluence and a larger number of irradiated laser pulses, a melt formation inside the irradiated area limited the uniformity of LIPSS formation. Additionally, the orientation of the scan axis with the laser beam polarization was found to significantly influence the uniformity of LIPSS generated along the scan line, with more redeposition and melt formation when the scan axis was perpendicular to the laser beam polarization. For large-area processing, the borosilicate glass surface was scanned line-by-line by the laser beam, with a scan orientation parallel to the polarization of the laser. The optical characterization revealed that the transmittance and reflectance of the borosilicate glass decreased significantly after processing. Additionally, the surface’s wettability changed from hydrophilic to super-hydrophilic after processing. These chemical contamination-free and uniformly distributed structures have potential applications in optics, microfluidics, photovoltaics, and biomaterials.

Parameter Optimization of RB-SiC Polishing by Femtosecond Laser

Reaction-boned silicon carbide (RB-SiC) is considered a new material for large lightweight ground-based space telescopes due to its high specific stiffness, low thermal deformation, and excellent optical quality. The excellent mechanical properties of RB-SiC result in the low efficiency of traditional polishing and mechanical polishing. In this paper, a polishing method for RB-SiC based on a femtosecond laser is proposed to improve surface quality. A theoretical heat conduction model was established in the process of femtosecond laser irradiation of SiC. We analyzed the ablation type and calculated the single-pulse ablation threshold of SiC, which verified the feasibility of femtosecond laser polishing. Further, the effects of polishing parameters on the polished surface quality were analyzed by a series of experiments, and the optimal parameters were selected. It was observed to improve polishing efficiency and can replace the intermediate steps of traditional mechanical polishing.

Threshold fluence and incubation during multi-pulse ultrafast laser ablation of quartz.

In this work, the incubation effect on the laser ablation threshold of quartz, after multi-shot irradiation with femtosecond pulses at 1030-nm-wavelength with different repetition rates, was investigated. A strong decrease of the multi-pulse ablation threshold with the number of pulses N was found due to incubation. Moreover, the influence of the repetition rate was negligible in the investigated frequency range which went from 0.06 to 200 kHz. A saturation of the threshold fluence value was observed at number of pulses N > 100 which has been found to be well fitted by an exponential incubation model. Using such model, we estimated the single-pulse ablation threshold value and the incubation coefficient for quartz, which were found equal to Fth,1 = 6.23 ± 0.23 J/cm2 and k = 0.058 ± 0.004.

Theoretical and Experimental Studies on Ultraviolet Nanosecond Laser Micromachining of Sapphire

The micromachining of sapphire by laser has dominated research in resent years. This paper provided theoretical and experimental studies of the micromachining of sapphire based on the UV nanosecond laser. The results indicate that the ablation is dominated by the photochemical and photo-thermal through a finite element analysis. The single-pulse ablation threshold of UV nanosecond on sapphire is 15.58 J/cm2, and the ablation threshold gradually decreases and tends to stable finally with the increase of pulse number. Furthermore, the experiment proves that the ablation morphology with a smooth surface and little molten sediment can be realized by optimizing laser processing parameters.

Ultrashort pulsed laser ablation of zirconia-alumina composites for implant applications

• Influence of alumina content on fs-laser machining of zirconia-alumina composites was studied. • Significant influence of material composition on material removal behavior was demonstrated. • Single phase materials have larger laser ablation thresholds and smaller material removal rate than their composites. • A lower scan speed is preferred for single phase materials while a higher scan speed is beneficial for the composites. Laser texturing of zirconia-alumina ceramics is a promising surface modification method for many applications, such as enhancing osseointegration of alumina toughened zirconia (ATZ) dental implants or improving friction behavior of zirconia toughened alumina (ZTA) hip replacement bearing components. The common problems in laser texturing of ATZ/ZTA ceramics are thermal cracking, low material removal rate (MRR), and laser-induced phase transformation (LIPT). Furthermore, the compositional variation of those ceramics will complicate these problems. In order to improve the manufacturing processability, ultrashort pulsed laser ablation behavior of ATZ/ZTA composites was investigated in this research. Single phase zirconia and alumina were found to have smaller MRRs than their composites, and this behavior was negatively correlated to the materials single-pulse laser ablation threshold. However, under multi-pulse laser irradiation, the ablation thresholds of all materials saturated to the same level, indicating that single phase materials were more sensitive to the incubation effect than their composites. This led to the MRRs of the single phase materials being reduced at larger scan speeds, while the MRRs of their composites remained independent of scan speed. It was also shown that single phase materials were less susceptible to thermal cracking than their composites under excessive heat accumulation. These results suggest that a lower scan speed is preferred for single phase materials in order to achieve a larger MRR, while a higher scan speed is beneficial for the composites for suppressing thermal cracking without compromising ablation efficiency. In addition, no LIPT was detected for zirconia dominated materials after laser ablation.

Sub-micrometer pillar formation in fused silica using double-pulse back-surface laser ablation

This work presents an investigation into the characteristics of double-pulse, back-surface ablation of fused silica via the use of a picosecond pulse laser with a wavelength of 532 nm.The processing depth of back-surface ablation was found to be greater than that of front-surface ablation for a given laser power. The processing depth of back-surface ablation was further increased via the use of double-pulse ablation using an appropriate laser fluence combination for the first and second pulse relative to the single-pulse ablation threshold, Fth. Setting the laser fluence of pulses in ascending order (where the first pulse is below Fth and the second pulse is greater than Fth) is found to increase the processing depth. Furthermore, non-trivial features of the processing trace were found; an axisymmetric circular trench with a sub-micrometer scale protrusion at its center was observed in the case of double-pulse, back-surface ablation using laser pulses in an ascending order of laser fluence under certain conditions. The protrusion has a pillar-like shape with a diameter of several hundred nanometers; the pillar-like structure has a height of about 1.3 µm from the trench bottom, and protruded 0.7 µm from the back-surface plane. When the first pulse had a laser fluence of greater than Fth, the protrusion was not present and the shape was similar to that obtained when using a single-pulse ablation.The results of this work show the superiority of double-pulse, back-surface ablation of fused silica compared with the equivalent single-pulse technique in terms of enhanced throughput and the capability to create unique sub-micrometer structures.

Subthreshold Erosion of an Organic Polymer Induced by Multiple Shots of an X-Ray Free-Electron Laser

Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the $\mathrm{C}---\mathrm{C}$ bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

Surface ablation thresholds of femtosecond laser micropatterning silver nanowires network on flexible substrate

In order to achieve high precision laser patterning of silver nanowires (AgNWs) network on polyethylene terephthalate (PET) substrate, the ablation morphology of AgNWs on PET flexible substrate by femtosecond (fs) laser is analyzed. The ablation threshold of AgNWs and PET are evaluated by investigating the dependency of the diameters versus laser pulse energy. The single-pulse ablation threshold of PET and AgNWs are 1.50 J/cm2 and 0.55 J/cm2, respectively. The ablation mechanism of AgNWs is discussed in terms of laser-induced surface plasmon polariton. By controlling the energy of laser irradiation, AgNWs can be effectively removed with slightly damage to the thermally sensitive PET substrate. Furthermore, The AgNWs can be welded in an energy range below the ablation threshold of AgNWs. This study provides a deeper understanding for fs patterning of AgNWs network and represents a promising strategy for fabricating high performance flexible AgNWs-based electronics.

Femtosecond laser micro-structuring of amorphous polyether(ether)ketone at 775 nm and 387 nm

Laser micro-structuring of amorphous polyether(ether)ketone (PEEK) has been demonstrated with 180 fs/1 kHz NIR (775 nm) and NUV (387 nm) laser pulses. Significant differences in laser-material interaction is observed between the NIR and NUV wavelengths with single pulse ablation threshold found to be 2.01 ± 0.05 J cm−2 and 0.23 ± 0.02 J cm−2 at 775 nm and 387 nm respectively. This is connected to the requirement for multi-photon absorption at 775 nm, where PEEK is transparent, while at 387 nm, significant linear absorption within the material bandgap occurs, favouring sequential 2-photon absorption via reverse saturated absorption (RSA). Open aperture Z scan data yields a 2-photon absorption coefficient β387(I) which is intensity dependent, consistent with RSA. Multi-pulse exposure yields incubation coefficients of S775 = 0.72 ± 0.01 and S387 = 0.85 ± 0.02. This significant reduction in NUV incubation is consistent with a much higher level of electron excitation to the conduction band. Consequently, ablation of PEEK with NUV fs pulses is superior, exhibiting much reduced melting and re-deposition. Precision NUV polymer surface micro-structuring is accomplished while laser induced periodic surface structures (LIPSS) with pitch Λ ∼ 0.4 μm are observed at the base of ablated regions, aligned parallel to the incident polarisation. The ease of NUV LIPSS generation allowed surface patterning of large scanned areas which exhibit white light diffraction due to this sub-micron periodic surface modulation.

A study on ultrafast laser micromachining and optical properties of amorphous polyether(ether)ketone (PEEK) films

Polyether(ether)ketone (PEEK) has been widely used in the electronics and biomedical industries due to excellent electrical, chemical and thermal stability. The ablation and optical properties of amorphous PEEK have been investigated using ultrahigh intensity fs laser pulses (180 fs/ 1 kHz) for precision manufacture. The single pulse ablation threshold was found to be 2.01±0.05 J cm-2 and 0.23±0.02 J cm-2 at 775 nm and 387 nm respectively. The significant difference has been attributed to the change in wavelength dependent nonlinear absorption order from multi-photon at 775 nm (via virtual states) to sequential 2-photon at 387 nm with photon energy within the material band edge. The 2-photon absorption coefficient at 387 nm was measured to be β ∼ 38 cm GW-1 at I ∼3 GW cm-2 leading to a strong light-matter coupling. Fs ablation quality is much superior at 387 nm with almost no melting and debris, allowing the observation of laser induced periodic surface structures (LIPSS) with pitch Λ < 0.4 µm. Large area LIPSS have been machined on 200 µm thick amorphous PEEK film with the direction of LIPSS parallel to the direction of linear polarisation.

Friction-reducing effect and damage mechanism of laser micro-texture on diamond films during friction

Diamond films are often applied in places where there is intense friction. The use of lasers to fabricate textured arrays on diamond films can effectively reduce the friction coefficient. In this paper, we use a UV nanosecond laser to produce periodic linear and pit micro-textures over the areas of [Formula: see text]. Laser irradiation was below the single-pulse ablation threshold corresponding to the conditions of surface graphitization during the multi-pulse irradiation. Irradiated graphitization can be detected by Raman spectroscopy, and at the same time, lattice distortion and crack initiation that accompany graphitization can be obtained by XRD and SEM, respectively. The shape of the texture also has a great influence on the residual stress. Grooved texture released the residual stress, but pit texture gave rise to uneven stress, as shown in the test result. SEM was used for observing morphological change at the edge of texture and the mechanism of internal stress changes is analyzed. Detection of texture antifriction performance by reciprocating friction and wear tester. After rubbing, the layered shedding marks of the edge of the texture were observed, and the influence of the superposition of residual stress and external stress on the texture damage was proved based on this.

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.

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.

Ablation of silicon and ultrathin fibers using single femtosecond pulse

We exploit the nonlinear multiphoton interaction of a few-cycle femtosecond (fs) pulse with viscoelastic microfibers in order to produce nanoscale grooves on its surface. The single fs pulse has been extracted from 1 kHz pulse train by double-shutter gating technique by placing two mechanical shutters in the beamline and simultaneously triggering them with a controlled delay. With adjustment of the time delay between two shutters, a small transmission window has been created to cleanly extract a single or desired number of pulses. We found that the single-pulse ablation threshold for microfiber is $$1 \hbox { J}/\hbox {cm }^2$$ while for absorbing surfaces like crystalline Si is $$0.01 \hbox { J}/\hbox {cm}^2$$. Precise diffraction unlimited ablation of materials opens a route to process nanoscale waveguides, microfluidic devices to isolate cells or macro-molecules.

Investigation of the ultrashort pulsed laser processing of zinc at 515 nm: Morphology, crystallography and ablation threshold

Bulk polycrystalline pure zinc is ablated in air using a picosecond laser source to perform single pulse and multi-pulse processing at a wavelength of 515 nm. The geometries and surface morphologies of the resulting craters are determined by confocal laser scanning microscopy and scanning electron microscopy. Further, the ablation threshold and its corresponding incubation coefficient is determined from the ablated volume for multiple laser pulses. Two different thermal ablation regimes are identified. The single pulse ablation thresholds are found to equal 0.1 J/cm2 and 0.68 J/cm2 respectively for the two regimes. It is found that the incubation coefficients are larger than unity, indicating material removal becomes energy expensive for multiple pulse laser irradiation. It is also found that an irradiated area undergoes laser induced preferred crystal orientation.

High repetition rate femtosecond laser heat accumulation and ablation thresholds in cobalt-binder and binderless tungsten carbides

Femtosecond (fs) laser ablation has been studied for the potential of fast, high precision machining of difficult-to-machine materials like binderless tungsten carbide. Obstacles that have limited its efficiency include melting from heat accumulation (HA), particle shielding, and plasma shielding. To address HA without shielding effects, high repetition rate (57.4 MHz), ultra-low fluence fs laser irradiation is performed to study the incubation effect and subsequent HA-ablation threshold of fine-grained tungsten carbides. Exposure times on the order of 100 ms were conducted in air with fluences (1.82 to 9.09 mJ/cm2) two orders of magnitude below the single fs pulse ablation thresholds reported in literature (0.4 J/cm2). Heat accumulation at high repetition rate explains the ultra-low fluence melt threshold behavior resulting in melt crowns around ablated holes and grooves. The results of this study aid in predicting heat buildup in high repetition rate laser irradiation for applications that wish to achieve high ablation rates of difficult-to-machine, ultrahard materials and help enable shaping of binderless tungsten carbide for use in applications too extreme for bindered tungsten carbide.

Picosecond-pulsed laser ablation of zinc: crater morphology and comparison of methods to determine ablation threshold

Ablation of bulk polycrystalline zinc in air is performed with single and multiple picosecond laser pulses at a wavelength of 1030 nm. The relationships between the characteristics of the ablated craters and the processing parameters are analyzed. Morphological changes of the ablated craters are characterized by means of scanning electron microscopy and confocal laser scanning microscopy. Chemical compositions of both the treated and untreated surfaces are quantified with X-ray photoelectron spectroscopy. A comparative analysis on the determination of the ablation threshold using three methods, based on ablated diameter, depth and volume is presented along with associated incubation coefficients. The single pulse ablation threshold value is found to equal 0.21 J/cm2. Using the calculated incubation coefficients, it is found that both the fluence threshold and energy penetration depth show lesser degree of incubation for multiple laser pulses.

Ablation morphology and ablation threshold of Ti-6Al-4V alloy during femtosecond laser processing

The research explores the use of femtosecond laser to clean the surface of Ti-6Al-4V alloy. The laser ablation threshold was determined from either depths or diameters of laser-induced craters. Both methods are expected to give the same results; however, it was found that ablation threshold can be determined more reliably from the diameters than from the depths. This can be understood from the study of the surface morphology. As the laser fluence increased, the ablated surface became increasingly rougher, making it difficult to measure the depths accurately. In contrast, diameters of laser-induced craters could be measured with much better certainty and thus enabled the ablation threshold to be determined more accurately. The ablation threshold was found to be 0.142 ± 0.010 J/cm2 for 100 laser pulses at 130 fs pulse duration and 790 nm wavelength and it tends to decrease with increase in number of laser pulses. The single-pulse ablation threshold was determined to be 0.272 ± 0.021 J/cm2.

Ablation enhancement of metal in ultrashort double-pulse experiments

We report an ablation enhancement (both deepening and widening in ablation profiles) on nickel by ultrashort double-pulse laser irradiation, an exception of the typical reduction trend observed in ablation rates for metals irradiated with double-pulse excitations. Maximized at a pulse-separation (τs) of approximately 4 ps, the enhancement was observed before τs was increased to approximately the electron-lattice energy relaxation time. An achievement of ablation, by temporal shaping a weak pulse with energy well below the single-pulse ablation threshold into two considerably weaker pulses, was demonstrated. It is proposed that the enhanced electron-phonon coupling and drop of electronic heat conductivity, both of which result from the reductions in electron temperature (by pulse shaping), co-contribute to the widening in ablation profiles, while the former one is responsible for the deepening. This letter proposed an ablation enhancement approach to ultrashort laser processing of nickel.