Gentle Ablation Research Articles

High aspect ratio diamond nanosecond laser machining

Laser processing of diamond has become an important technique for fabricating next generation microelectronic and quantum devices. However, the realization of low taper, high aspect ratio structures in diamond remains a challenge. We demonstrate the effects of pulse energy, pulse number and irradiation profile on the achievable aspect ratio with 532 nm nanosecond laser machining. Strong and gentle ablation regimes were observed using percussion hole drilling of type Ib HPHT diamond. Under percussion hole drilling a maximum aspect ratio of 22:1 was achieved with 10,000 pulses. To reach aspect ratios on average 40:1 and up to 66:1, rotary assisted drilling was employed using > 2 M pulse accumulations. We additionally demonstrate methods of obtaining 0.1° taper angles via ramped pulse energy machining in 10:1 aspect ratio tubes. Finally, effects of laser induced damage are studied using confocal Raman spectroscopy with observation of up to 36% increase in tensile strain following strong laser irradiation. However, we report that upon application of 600 °C heat treatment, induced strain is reduced by up to ~ 50% with considerable homogenization of observed strain.

Effect of laser surface modification on the tensile properties of ultra-thin titanium sheet

Laser scanning irradiation (LSI) was a promising method to improve the surface hemo-compatibility of vascular stent (VS). This paper firstly studied the effect of LSI on the tensile properties of ultra-thin Ti sheet, the thickness of which was close to that of VS’s wall. Meanwhile, the influence of plastic deformation on the hydrophilicity of LSI sheets and the hydrophobicity of the ones modified with fluorosilane was evaluated. The results indicated that the tensile properties of Ti sheets gradually declined with increasing laser fluence (Fp) and decreasing scanning speed (v). As using laser gentle ablation (LGA) with Fp = 1.1 ∼ 2.9 J/cm2, the declination degree of tensile properties was relative slight, but it was obviously sharper as using laser strong ablation (LSA) with Fp = 4.0 ∼ 6.6 J/cm2. In addition, the tensile and yield strengths of LSA samples using v = 42 mm/s were much smaller than those using higher v of 126 ∼ 630 mm/s, owning to the much larger laser-ablated depth in the surfaces. All of the LSA surfaces exhibited superhydrophilicity, and after fluorination they rapidly turned to be superhydrophobic with low adhesion to water. Interestingly, plastic deformation within the tensile fracture point slightly improved the super-hydrophilicity or superhydrophobicity of these LSA surfaces rather than destroyed it. Therefore, LSA with v = 126 ∼ 630 mm/s was more suitable to be applied in the stent, which could not only obtain super-hydrophilicity/hydrophobic surfaces with hierarchical microtextures but just slightly affect the stent’s tensile properties.

Competitive mechanism of laser energy and pulses on holes ablation by femtosecond laser percussion drilling on AlN ceramics

In laser microstructure machining, results are derived through the action of multiple laser processing parameters (LPPs); therefore, multiple studies on the effect of LPPs on machining results are available. Further, few studies observed how competitive mechanism of each LPP affects machining results. As the most important LPPs, single-pulse energy and the number of pulses were varied to assess their effect on hole dimension and morphology. Additionally, competitive mechanism of LPPs on hole ablation was further explored. The study was based on femtosecond laser percussion drilling on aluminum nitride (AlN) ceramics surface. It was found that laser energy has a dominant role in initial ablation, determining both ablation and ablation mechanism. Additionally, it also determined hole inlet surface morphology and ablation mechanism remained the same regardless of increase in the number of pulses. Laser pulses assumed a dominant role after intersection point of LPPs acceleration rate and hole dimension acceleration rate. Hence, final hole dimensions and its sidewall morphology were defined. It was found that deep holes can be processed both in gentle ablation and strong ablation with lots of pulses. However, there are opposite mechanisms for inducing hole sidewall morphology; for example, in strong ablation, solidification effect is enhanced with increase of hole depth, while it is decreased in gentle ablation.

Time dependency of superhydrophilic and superhydrophobic surfaces produced by nanosecond laser irradiation assisted by post-annealing and silanization

Modifying implant surfaces to be superhydrophilic or superhydrophobic is a promising approach to improve their biocompatibility. In this paper, nanosecond laser irradiation assisted by post-annealing and silanization was employed to control the wettability of Ti6Al4V surfaces. Through adjusting the scanning interval (h) and speed (v), smooth surfaces with gradient hydrophilicity were achieved by laser gentle ablation, and superhydrophilic surfaces with gradient roughness were achieved by laser strong ablation. Assisted by subsequent post-annealing treatment or silanization, various superhydrophobic surfaces were produced within only several hours. The wettability stability of these superhydrophilic and superhydrophobic surfaces in the atmosphere and different liquid solutions was evaluated, and the related mechanism was discussed in detail. Finally, long-term superhydrophilic surfaces in both atmosphere and normal saline were obtained by applying small v and h values. Meanwhile, various long-term (in the atmosphere) superhydrophobic surfaces in a metastable state (like a rose leaf) were obtained via post-annealing treatment, and those in a Cassie state (like a lotus leaf) were produced by silanization. The results in this paper could also be applied in several potential applications such as self-cleaning, micro-fluidic chips, and micro-operation of droplets.

Comprehensive ablation study of near-IR femtosecond laser action on the titanium-based alloy Ti6Al4V: morphological effects and surface structures at low and high fluences

The surface of a titanium-based alloy Ti6Al4V was subjected to modifications by a near-IR femtosecond Ti:Sapphire laser, emitting at 775 nm pulses of 200 fs duration, in single-pulse and multi-pulse regimes, with up to 400 accumulated pulses, and pulse energies ranging from 2.5 to 250 upmu J. The whole range of induced effects is presented, from gentle ablation and pattern occurrence to substantial crater formation. Very observable laser-induced parallel periodic surface structures are reported, appearing both within the damage spot area, with low fluences, and at the peripheries of the craters, with higher fluences—but also on crater walls, and inside the crater structures. Damage threshold fluences ({F}_{mathrm{th}}) and the incubation factor (zeta ) were also determined.Graphic abstract

Ablation Threshold and Interaction of Cemented Carbide Ablated by Ultrafast Laser

Abstract In this study, we used an ultrafast laser with a wavelength of 1026 nm with the aim of analyzing the ablation threshold and morphologies of the irradiated surfaces of cemented carbide by varying the pulse duration and number of laser pulses. Specifically, we used pulse durations of 0.2, 2, and 10 ps and performed both gentle and strong ablations. For the same wavelength, laser pulse energy, and number of laser pulses, the lowest ablation threshold was 0.2 ps. When we performed a gentle ablation, we observed laser-induced periodic surface structures (LIPSSs) on the entire irradiated surface for all pulse durations. Thus, the pulse duration did not appear to affect the formation of LIPSSs. Strong ablation caused ridges to form at irradiated area outside. When the pulse duration increased, larger ridges were formed, whereas when the pulse duration decreased, coarser ridges were formed. The results obtained by using the ultrafast laser is expected to be helpful in the machining of cemented carbide.

Enhanced ablation efficiency using GHz bursts in micromachining fused silica.

We report on micromilling cavities into fused silica by a 1030 nm femtosecond laser using 2.17 GHz bursts. The milled cavities show an increased depth per layer for a higher number of pulses per burst while the ablation efficiency is also increased. The maximum ablation efficiency for the optimum fluence achieved in our experiments is 3.05mm3/min/W for a burst number of 10, which is 7.4 times higher than for the non-burst condition (0.41mm3/min/W). Furthermore, the ablation threshold for each sub-pulse is significantly reduced from 0.64J/cm2 for the non-burst condition to 0.15J/cm2 for 10 bursts. Beside the ablation efficiency, the surface roughness is also increased with the increasing burst number, while two ablation behaviors can be distinguished, namely, a gentle ablation regime for lower burst numbers and a coarse ablation regime, dominated by breaking out the surface rather than ablating it.

Fabrication of Superhydrophobic Ti-6Al-4V Surfaces with Single-Scale Micotextures by using Two-Step Laser Irradiation and Silanization.

Laser irradiation is a popular method to produce microtextures on metal surfaces. However, the common laser-produced microtextures were hierarchical (multiscale), which may limit their applicability. In this paper, a method of two-step laser irradiation, combining first-step strong ablation and sequentially second-step gentle ablation, was presented to produce micron-rough surface with single-scale microtextures. The effect of laser fluence on the Ti–6Al–4V surface morphology and wettability were investigated in detail. The morphology results revealed that the microtextures produced using this method gradually evolved from multiscale to single-scale meanwhile from microprotrusions to microholes with increasing the second-step laser fluence from 0.0 to 2.4 J/cm2. The wettability and EDS/XPS results indicated that attributing to the rich TiO2 content and micron roughness produced by laser irradiation, all the two-step laser-irradiated surfaces exhibited superhydrophilicity. In addition, after silanization, all these superhydrophilic surfaces immediately turned to be superhydrophobic with close water contact angles of 155–162°. However, due to the absence of nanotextures, the water-rolling angle on the superhydrophobic surfaces with single-scale microtextures distinctly larger than those with multiscale ones. Finally, using the two-step laser-irradiation method and assisted with silanization, multifunctional superhydrophobic Ti–6Al–4V surfaces were achieved, including self-cleaning, guiding of the water-rolling direction and anisotropic water-rolling angles (like the rice-leaf), etc.

Large-area commercial-grating-quality subwavelength periodic ripples on silicon efficiently fabricated by gentle ablation with femtosecond laser interference via two cylindrical lenses

This paper presents a new strategy for efficiently fabricating large-area subwavelength periodic ripples with commercial grating quality on a silicon surface. Two laser beams of 800 nm, 50 fs at a repeat frequency of 1 kHz were focused on a silicon wafer via two cylindrical lenses. The spacing of the interference fringes was adjusted to excite a coherent resonance-enhanced (CRE) surface plasmon polariton (SPP). The CRE SPP efficiently induced homogeneous and extremely straight periodic ripples through gentle ablation, which avoids substantial debris and reduces the roughness of the periodic ripples. At the two edges of the ablation band after single scanning, the ripples were shallow, spaced, and exceptionally straight, which is greatly advantageous in the preparation of large-area gratings line by line. The subwavelength periodic ripples were fabricated on a silicon wafer with a diameter of 100 mm by scanning at a speed of 2 mm/s and a spacing of 1.55 mm. Compared with the alternating amorphous-crystalline ripples induced by a single laser beam, the fabrication efficiency was enhanced by 2–3 orders. Measurements with a confocal optical microscope and white light diffraction demonstrate that the periodic ripples were extremely straight, smooth, and homogeneous, similar to the gold gratings. The diffraction resolution of the nanogratings of subwavelength periodic ripples was nearly as narrow as that of commercial gratings.

UV post-ionization laser ablation ionization mass spectrometry for improved nm-depth profiling resolution on Cr/Ni reference standard.

Laser ablation combined with mass spectrometry forms a promising tool for chemical depth profiling of solids. At irradiations near the ablation threshold, high depth resolutions are achieved. However, at these conditions, a large fraction of ablated species is neutral and therefore invisible to the instrument. To compensate for this effect, an additional ionization step can be introduced. Double-pulse laser ablation is frequently used in material sciences to produce shallow craters. We apply double-pulse UV femtosecond (fs) Laser Ablation Ionization Mass Spectrometry to investigate the depth profiling performance. The first pulse energy is set to gentle ablation conditions, whereas the second pulse is applied at a delay and a pulse energy promoting the highest possible ion yield. The experiments were performed on a Cr/Ni multi-layered standard. For a mean ablation rate of ~3 nm/pulse (~72 nJ/pulse), a delay of ~73 ps provided optimal results. By further increasing the energy of the second pulse (5-30% higher with respect to the first pulse) an enhancement of up to 15 times the single pulse intensity was achieved. These conditions resulted in mean depth resolutions of ~37 and ~30 nm for the Cr and Ni layers, respectively. It is demonstrated on the thin-film standard that the double-pulse excitation scheme substantially enhances the chemical depth profiling resolution of LIMS with respect to the single-pulse scheme. The post-ionization allows for extraordinarily low ablation rates and for quantitative and stoichiometric analysis of nm-thick films/coatings.

Resonant-Plasmon-Assisted Subwavelength Ablation by a Femtosecond Oscillator

``Bowtie'' nanoantennas are not unusual in the field of plasmonics, yet they still can be used in unusual ways. Harnessing the near-field enhancement due to locally resonant surface plasmons, the authors demonstrate material ablation and concurrent deposition of high-quality film at the subwavelength scale, all driven by a bowtie-based oscillator, rather than a laser. This gentle ablation is mediated by hot-electron emission and electrostatic acceleration---a different mechanism than for thermal ablation or optical breakdown. This mechanism can be made to work for either metallic or dielectric materials, rendering this approach even more appealing for nanotechnology.

Correction to: Case study on the dynamics of ultrafast laser heating and ablation of gold thin films by ultrafast pump‑probe reflectometry and ellipsometry

To increase the comprehension of ultrafast laser ablation, the ablation process has to be portrayed with sufficient temporal resolution. For example, the temporal modification of the complex refractive index $${\tilde{n}}$$ and the relative reflectance of a sample material after irradiation with ultrafast single-pulsed laser radiation can be measured with a pump-probe setup. This work describes the construction and validation of a pump-probe setup enabling spatially, temporally, and spectroscopically resolved Brewster angle microscopy, reflectometry, ellipsometry, and shadow photography. First pump-probe reflectometry and ellipsometry measurements are performed on gold at $$\lambda _{\mathrm{probe}}= 440\, \hbox {nm}$$ and three fluences of the single-pulsed pump radiation at $$\lambda _{\mathrm{pump}}= 800\,\hbox {nm}$$ generating no, gentle, and strong ablation. The relative reflectance overall increases at no and gentle ablation. At strong ablation, the relative reflectance locally decreases, presumable caused by emitted thermal electrons, ballistic electrons, and ablating material. The refractive index n is slightly decreasing after excitation, while the extinction coefficient k is increasing.

Case study on the dynamics of ultrafast laser heating and ablation of gold thin films by ultrafast pump-probe reflectometry and ellipsometry

To increase the comprehension of ultrafast laser ablation, the ablation process has to be portrayed with sufficient temporal resolution. For example, the temporal modification of the complex refractive index $${\tilde{n}}$$ and the relative reflectance of a sample material after irradiation with ultrafast single-pulsed laser radiation can be measured with a pump-probe setup. This work describes the construction and validation of a pump-probe setup enabling spatially, temporally, and spectroscopically resolved Brewster angle microscopy, reflectometry, ellipsometry, and shadow photography. First pump-probe reflectometry and ellipsometry measurements are performed on gold at $$\lambda _{\mathrm{probe}}= 440\, \hbox {nm}$$ and three fluences of the single-pulsed pump radiation at $$\lambda _{\mathrm{pump}}= 800\,\hbox {nm}$$ generating no, gentle, and strong ablation. The relative reflectance overall increases at no and gentle ablation. At strong ablation, the relative reflectance locally decreases, presumable caused by emitted thermal electrons, ballistic electrons, and ablating material. The refractive index n is slightly decreasing after excitation, while the extinction coefficient k is increasing.

Case study on the ultrafast laser ablation of thin aluminum films: dependence on laser parameters and film thickness

Laser ablation using ultra-short pulsed laser radiation allows the removing of thin films with very high spatial resolution, and working with high repetition rate as well with high through-put. The ultrafast ablation of thin films of aluminum on float glass is investigated using focused femtosecond laser radiation (λ = 1028 nm, t H = 200 fs, sech2, p f = 1 MHz) as function of the number of pulses N p per point (1–10) and the film thickness d (30–300 nm). It is observed that two thresholds are derived simultaneously for thin films with a thickness thicker than 100 nm by irradiating the metal with single pulsed laser radiation exhibiting a Gaussian intensity distribution: one threshold for gentle ablation H thr,gentle and the other for strong ablation H thr,strong. Multi-pulse irradiation varying the number of pulses per point identifies the incubation effect described by Jee et al. (J Opt Soc Am B 5(3):648, 1968). This model was applied on the thresholds for gentle and strong ablation. Also, varying the layer thickness reducing the thresholds for thin films due heat accumulation. To quantify the experimental data, numerical simulations solving the coupled heat transfer equation of the two-temperature model were performed. A new approach including the temperature dependence of the reflectivity is presented based on the model proposed by Bruckner et al. (J Appl Phys 66:1326, 1989). The results of the simulation fit qualitative well to the experimental data of gentle ablation. Theoretical investigation for double pulses with a variable pulse separation time of 1–300 ps were performed in comparison with a single pulse.

Controlling ablation mechanisms in sapphire by tuning the temporal shape of femtosecond laser pulses

We have analyzed the influence of the temporal pulse shape on femtosecond (fs) laser-induced surface ablation processes in sapphire. To this end, single transform-limited (TL), stretched, and third-order-dispersion (TOD) shaped fs pulses have been used, while the dynamics of the interaction were analyzed by fs-resolved microscopy and correlated with plasma emission intensity and crater morphology. The modification of the pulse shape enables changing the ablation mechanism from a strong, thermally mediated ablation process to a gentle ablation process mediated by Coulomb explosion (CE), with respective ablation depths of 100–200 nm and 5–10 nm. Analysis of the transient optical response allows direct comparison of the transient plasma carrier densities involved, observing comparable peak values for both processes. For strong ablation induced by TL pulses, a direct relation between plasma density and local ablation depth is found, but this does not hold for the CE-mediated process observed for TOD-shaped pulses. For TOD-shaped pulses at very high fluence, a different ablation mechanism involving explosive boiling is identified. This mechanism leads to the formation of deep craters with reduced lateral extension and steep walls. This amount of control over the ablation mechanisms by a simple selection of the pulse shape should be of interest for new surface structuring approaches.

Le traitement numérique du son pour les téléopérateurs

Modifying implant surfaces to be superhydrophilic or superhydrophobic is a promising approach to improve their biocompatibility. In this paper, nanosecond laser irradiation assisted by post-annealing and silanization was employed to control the wettability of Ti6Al4V surfaces. Through adjusting the scanning interval (h) and speed (v), smooth surfaces with gradient hydrophilicity were achieved by laser gentle ablation, and superhydrophilic surfaces with gradient roughness were achieved by laser strong ablation. Assisted by subsequent post-annealing treatment or silanization, various superhydrophobic surfaces were produced within only several hours. The wettability stability of these superhydrophilic and superhydrophobic surfaces in the atmosphere and different liquid solutions was evaluated, and the related mechanism was discussed in detail. Finally, long-term superhydrophilic surfaces in both atmosphere and normal saline were obtained by applying small v and h values. Meanwhile, various long-term (in the atmosphere) superhydrophobic surfaces in a metastable state (like a rose leaf) were obtained via post-annealing treatment, and those in a Cassie state (like a lotus leaf) were produced by silanization. The results in this paper could also be applied in several potential applications such as self-cleaning, micro-fluidic chips, and micro-operation of droplets.

Identification of non-thermal and thermal processes in femtosecond laser-ablated aluminum

Non-thermal and thermal processes due to femtosecond laser ablation of aluminum (Al) at low, moderate, and high-fluence regimes are identified by Atomic Force Microscope (AFM) surface topography investigations. For this purpose, surface modifications of Al by employing 25 fs Ti: sapphire laser pulses at the central wavelength of 800 nm have been performed to explore different nano- and microscale features such as hillocks, bumps, pores, and craters. The mechanism for the formation of these diverse kinds of structures is discussed in the scenario of three ablation regimes. Ultrafast electronic and non-thermal processes are dominant in the lower fluence regime, whereas slow thermal processes are dominant at the higher fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower fluence regime (0.06–0.5 J cm−2 single-shot irradiation under ultrahigh vacuum condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a high-fluence regime 2.5–3.5 J cm−2 multiple-shot irradiation. For the lower fluence (gentle ablation) regime, around the ablation threshold, the unique appearance of individual, localized Nano hillocks typically a few nanometers in height and less than 100 nm in diameter are identified. These Nano hillock-like features can be regarded as a nonthermal, electronically induced phase transition process due to localized energy deposition as a result of Coulomb explosion or field ion emission by surface optical rectification. At a moderate-fluence regime, slightly higher than ablation threshold multiple-pulse irradiation produces bump-formation and is attributed to ultrafast melting (plasma formation). The high-fluence regime results in greater rates of material removal with highly disturbed and chaotic surface of Al with an appearance of larger protrusions at laser fluence well above the ablation threshold. These nonsymmetrical shapes due to inhomogeneous nucleation, cluster formation, and resolidification of a metallic surface after melting are attributable to slow thermal processes (ps time scale).

Morphology and crystalline phase characteristics of α-GST films irradiated by a picosecond laser

The morphology and crystalline phase characteristics of amorphous Ge2Sb2Te5 films irradiated by a picosecond laser were investigated by 3D surface profiler, atomic force microscopy (AFM) and transmission electron microscopy (TEM) integrated with selected area electron diffraction (SAED). The laser irradiated spot was divided into strong ablation area, gentle ablation area, melting area and irradiation area. By theoretical calculation, the ablation and melting thresholds were determined to be 173.05mJcm−2 and 99.19mJcm−2 respectively. Meantime, the local fine morphologies of the ablation and melting areas were shown and analyzed. We also studied the irradiation area which was made up by the non-phase-change area and phase-change area. In the phase-change area, crystalline phase was determined to be face-centered cubic structure and crystalline phase characteristics for films with different thicknesses were discussed.

Micromachining Characteristics of NiTi Based Shape Memory Alloy Using Femtosecond Laser

Femtosecond laser micromachining (FLM) is a relatively new and promising technology for the micromachining of a wide spectrum of engineering materials with micron and submicron size features. The interaction mechanism of femtosecond laser pulses with matter is not the same as that found in traditional lasers. This manuscript presents a detailed study of the ablation characteristics of a nickel-titanium (NiTi) shape memory alloy in air with femtosecond laser pulses. The single- and multishot ablation threshold fluence and the incubation coefficient (predicting the extent to which accumulation could take place in a material) are evaluated. In addition, morphological changes, such as the emergence of a ripple pattern, are discussed along with the identification of gentle and strong ablation phases. This study provides for the understanding and characterization of NiTi micromachining using FLM technology, which could aid in the identification of new applications for smart materials in the macro-, nano-, and microelectromechanical system domains using this technology.

Direct synthesis of sp-bonded carbon chains on graphite surface by femtosecond laser irradiation

Microscopic phase transformation from graphite to sp-bonded carbon chains (carbyne) and nanodiamond has been induced by femtosecond laser pulses on graphite surface. UV/surface enhanced Raman scattering spectra and x-ray photoelectron spectra displayed the local synthesis of carbyne in the melt zone while nanocrystalline diamond and trans-polyacetylene chains form in the edge area of gentle ablation. These results evidence possible direct “writing” of variable chemical bonded carbons by femtosecond laser pulses for carbon-based applications.