Single-shot Ablation Threshold Research Articles

A highly automated apparatus for ultra-fast laser ablation studies.

We present a novel experimental apparatus that can be used for extensive systematic studies of (single- and multi-shot) ultra-short laser pulse ablation. It is fully automated and generates a large number of ablation sites in a short time on a small sample surface area. For each site, the apparatus takes four in situ images: an image of the incident ablation beam (to determine pulse energy), a white light reference image of the pristine sample site, an image of the reflected ablation spot, and a white light image of the ablated sample site. The setup can perform ablation experiments as a function of many parameters, including pulse energy, pulse duration, number of pulses, time between pulses, and focus size. As a proof of concept, we present example results on single-shot ablation off crystalline silicon. Using only data acquired in situ in the presented setup, we determine the single-shot ablation threshold as a function of pulse duration and verify the threshold value using optical interferometric profilometry. The values we found agree well with literature values.

Ultrafast multi-shot ablation and defect generation in monolayer transition metal dichalcogenides

Transition metal dichalcogenides are known to possess large optical nonlinearities, and driving these materials at high intensities is desirable for many applications. Understanding their optical responses under repetitive intense excitation is essential to improve the performance limit of these strong-field devices and to achieve efficient laser patterning. Here, we report the incubation study of monolayer MoS2 and WS2 induced by 160 fs, 800 nm pulses in air to examine how their ablation threshold scales with the number of admitted laser pulses. Both materials were shown to outperform graphene and most bulk materials; specifically, MoS2 is as resistant to radiation degradation as the best of the bulk thin films with a record fast saturation. Our modeling provides convincing evidence that the small reduction in threshold and fast saturation of MoS2 originate from its excellent bonding integrity against radiation-induced softening. Sub-ablation damages, in the form of vacancies, strain, lattice disorder, and nanovoids, were revealed by transmission electron microscopy, photoluminescence, Raman, and second harmonic generation studies, which were attributed to the observed incubation in 2D materials. For the first time, a sub-ablation damage threshold is identified for monolayer MoS2 to be 78% of the single-shot ablation threshold, below which MoS2 remains intact for many laser pulses. Our results firmly establish MoS2 as a robust material for strong-field devices and for high-throughput laser patterning.

Microscopic Kinetics in Poly(Methyl Methacrylate) Exposed to a Single Ultra-Short XUV/X-ray Laser Pulse.

We study the behavior of poly(methyl methacrylate) (PMMA) exposed to femtosecond pulses of extreme ultraviolet and X-ray laser radiation in the single-shot damage regime. The employed microscopic simulation traces induced electron cascades, the thermal energy exchange of electrons with atoms, nonthermal modification of the interatomic potential, and a triggered atomic response. We identify that the nonthermal hydrogen decoupling triggers ultrafast fragmentation of PMMA strains at the absorbed threshold dose of ~0.07 eV/atom. At higher doses, more hydrogen atoms detach from their parental molecules, which, at the dose of ~0.5 eV/atom, leads to a complete separation of hydrogens from carbon and oxygen atoms and fragmentation of MMA molecules. At the dose of ~0.7 eV/atom, the band gap completely collapses indicating that a metallic liquid is formed with complete atomic disorder. An estimated single-shot ablation threshold and a crater depth as functions of fluence agree well with the experimental data collected.

Femto- and nanosecond pulse duration dependence of the single-shot ablation threshold and multiple-shot morphology change of poly L-lactic acid in deep ultraviolet regime

Laser processing is a common method used to shape various structures of biodegradable polymers. In this study, we demonstrate PLLA processing with intense deep ultraviolet (DUV) pulses at 266 nm and compare the processing properties using a nanosecond DUV laser. Results allow comparison of the single-shot ablation thresholds and multiple-shot morphology of both the femtosecond and nanosecond DUV laser. For sing-shot experiments, the ablation threshold of femtosecond laser is, at least, one order of magnitude lower than that of nanosecond laser. For multiple-shot experiments, the result provides superior quality of the crater morphology in the DUV femtosecond regime.

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold

The process of damage accumulation in thin ruthenium films exposed to multiple femtosecond extreme ultraviolet (XUV) free-electron laser (FEL) pulses below the critical angle of reflectance at the FEL facility in Hamburg (FLASH) was experimentally analyzed. The multi-shot damage threshold is found to be lower than the single-shot damage threshold. Detailed analysis of the damage morphology and its dependence on irradiation conditions justifies the assumption that cavitation induced by the FEL pulse is the prime mechanism responsible for multi-shot damage in optical coatings.

Erratum to: Femtosecond pulse laser ablation of chromium: experimental results and two-temperature model simulations

In this work, the results of experimental and computational single- and multi-shot ablation threshold and the incubation effect of chromium metal sample, irradiated by ultrashort laser pulses, are presented. The experimental value of the ablation threshold is determined based on $$D^{2}$$ method by measuring the outer ablation crater diameters as a function of incident laser pulse energy using $$800 \, \mathrm{nm}, 30 \, \mathrm{fs},$$ laser pulses. The value of $$0.19 \pm 0.04 \, (\mathrm{J/cm}^{2} )$$ , is obtained for the single-shot ablation threshold fluence. The experimental results are compared with time-dependent heat flow calculations based on the two-temperature model and the effect of number and separation time of two consecutive laser pulses with the same total fluence is studied for the Cr target. Moreover, the role of pulse width and absorbed fluence in thermal equilibrium time between electrons and lattice is investigated in two-temperature model. The thermal equilibrium between electron and lattice is established after a few picoseconds for low fluences and after a few tens of picoseconds at higher fluences.

Universal threshold for femtosecond laser ablation with oblique illumination

We quantify the dependence of the single-shot ablation threshold on the angle of incidence and polarization of a femtosecond laser beam, for three dissimilar solid-state materials: a metal, a dielectric, and a semiconductor. Using the constant, linear value of the index of refraction, we calculate the laser fluence transmitted through the air-material interface at the point of ablation threshold. We show that, in spite of the highly nonlinear ionization dynamics involved in the ablation process, the so defined transmitted threshold fluence is universally independent of the angle of incidence and polarization of the laser beam for all three material types. We suggest that angular dependence of ablation threshold can be utilized for profiling fluence distributions in ultra-intense femtosecond laser beams.

Measurements of fluence profiles in femtosecond laser filaments in air.

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our approach is based on the dependence of single-shot laser ablation threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser filaments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the external beam focusing. The limiting value of the peak intensity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f-number above ∼500), is about 55 TW/cm2.

Nanopillar formation from two-shot femtosecond laser ablation of poly-methyl methacrylate

We present experimental and numerical studies on the morphological evolution and dynamics of femtosecond laser ablation of bulk poly-methyl methacrylate (PMMA) irradiated with a pair of pulses. We show that a nanopillar-like structure is formed in the middle of the ablation crater for pulse energies below single-shot ablation threshold. The nanopillar is ∼400 nm long, lies adjacent to a nanopore, and protrudes ∼150 nm above the sample surface. As the pulse energy is increased gradually, the nanopillar disappears and the nanopore inside the ablation crater becomes larger. At higher pulse energies, a volcanic eruption like structure appears in the middle of the crater whose size and height increases with energy. 2D molecular dynamics simulations reveal that a nanojet and other features observed at higher pulse energies can be formed when the reflection of a shockwave, induced by the second laser pulse, causes density pinching in the middle of the interaction region that rapidly pushes out molten material towards the surface. The shockwave is reflected from the cold boundaries of a modified region created by the first laser pulse.

Single-shot ablation threshold of chromium using UV femtosecond laser pulses

Single-shot ablation threshold for thin chromium film was studied using 266 nm, femtosecond laser pulses. Chromium is a useful material in the nanotechnology industry and information on ablation threshold using UV femtosecond pulses would help in precise micromachining of the material. The ablation threshold was determined by measuring the ablation crater diameters as a function of incident laser pulse energy. Absorption of 266 nm light on the chromium film was also measured under our experimental conditions, and the absorbed energy single-shot ablation threshold fluence was $$46 \pm 5$$ mJ/cm2. The experimental ablation threshold fluence value was compared to time-dependent heat flow calculations based on the two temperature model for ultrafast laser pulses. The model predicts a value of 31.6 mJ/cm2 which is qualitatively consistent with the experimentally obtained value, given the simplicity of the model.

Dependence of ablation threshold and LIPSS formation on copper thin films by accumulative UV picosecond laser shots

The ablation threshold and Laser-induced periodic surface structure (LIPSS) formation on copper thin film were investigated using a picosecond laser (Nd:YAG laser: 266 nm, 42 ps, 10 Hz). We show that the ablation threshold varies with respect to the number of laser shots (N) on two different substrates. The single-shot ablation threshold was estimated to be close to 170 ± 20 mJ/cm2. The incubation coefficient was estimated to be 0.68 ± 0.03 for copper thin films on silicon and glass substrates. In addition, morphology changes of the ablated regions, in the same spot area, were studied as a function of fluence and number of laser shots. An intermediate structure occurred with a mix of low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL) and regular spikes at a fluence F < 250 mJ/cm2 and 1,000 < N ≤ 10.000 shots. LSFL was observed with a spatial period close to the irradiation wavelength and an orientation perpendicular to the laser polarization, and HSFL with a spatial period of ∼120 nm and a parallel orientation. Lastly, the global relationship between the laser parameters (i.e. fluence and number of shots) and LIPSS formation was established in the form of a 2D map.

Physical evolution of nanograting inscription on the surface of fused silica

The nanograting inscription on the surface of a fused silica substrate is systematically investigated on a shot-to-shot basis. Three different evolutionary processes are observed with pulse fluence ranging from slightly below to well above the single shot ablation threshold. This dependence is explained by the interplay between local intensity distribution and incubation effect.

Saturation effects in femtosecond laser ablation of silicon-on-insulator

We report a surface morphology study on single-shot submicron features fabricated on silicon on insulator by tightly focused femtosecond laser pulses. In the regime just below single-shot ablation threshold nano-tips are formed, whereas in the regime just above single-shot ablation threshold, a saturation in the ablation depth is found. We attribute this saturation by secondary laser absorption in the laser-induced plasma. In this regime, we find excellent agreement between the measured depths and a simple numerical model. When the laser fluence is further increased, a sharp increase in ablation depth is observed accompanied by a roughening of the ablated hole.

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon.

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.

Nanomilling surfaces using near-threshold femtosecond laser pulses

We have produced crater depths of less than 10 nanometers using 100-1000 pulses of a near-infrared femtosecond laser (800 nm, 125 fs) on a copper thin film surface. By determining the single-shot ablation threshold, incubation coefficient and surface reflectivity, the femtosecond laser pulse parameters for surface nanomilling are established close to the multiple-pulse ablation threshold limit for a copper thin film. Photomultiplier measurements of a copper emission line were used as a real time monitor of the nanomilling process for which photons were detected only once every several shots. The results are consistent with a model that ablation occurs in bursts every several shots after a number of intervening incubation energy storage shots.

Ablation and Amorphization of Crystalline Si by Femtosecond and Picosecond Laser Irradiation

The ablation and amorphization of crystalline Si by femtosecond and picosecond laser irradiations are reported in this paper. Laser pulse width was varied in the range of 100 fs–200 ps. We obtained the dependences of ablation rate and fluence for amorphization on laser pulse width. At a lower fluence than a single-shot ablation threshold, femtosecond laser irradiation induced the amorphization of crystalline Si. We confirmed the thickness of an amorphous Si layer by transmission electron microscopy. The thickness of an amorphized layer, which was about 50 nm and almost uniform, did not depend on the number of irradiated laser pulses and fluence. The fluence range for amorphization decreased with increasing laser pulse width.

Single and multiple shot near-infrared femtosecond laser pulse ablation thresholds of copper

The single-shot ablation threshold and incubation coefficient of copper were investigated using an amplified near-infrared, femtosecond Ti:sapphire laser. To date, the near-infrared femtosecond ablation threshold of copper has been reported in the range of several hundred millijoules per cm2 based primarily on multiple shot ablation studies. A careful study of the single shot ablation threshold for copper was carried out yielding an incident single-shot ablation threshold of (1.06±0.12) J/cm2 for a clean copper foil surface. This was determined by measuring the diameters of the ablation spots as a function of the laser pulse energy using scanning electron microscopy for spatially Gaussian laser spots. When multiple shots were taken on the same spot, a reduction in ablation threshold was observed, consistent with a multiple shot incubation coefficient of 0.76±0.02. Similar experiments on 250 nm and 500 nm copper thin films sputtered on a silicon substrate demonstrated that scaling the threshold values with the absorbance of energy at the surface yields a consistent absorbed fluence threshold for copper of (59±10) mJ/cm2. This absorbed threshold value is consistent with the expected value from a two-temperature model for the heating of copper with an electron-lattice coupling constant of g=1017 Wm-3 K-1. Single-shot rippling of the surface in the threshold ablation intensity regime was also observed for the foil target but not for the smooth thin film target.

Femtosecond laser ablation properties of borosilicate glass

We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN&amp;gt;1) provides the multishot ablation threshold, FthN&amp;gt;1. The results depict a multishot ablation threshold of ≈1.7J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9μm, and the “effective optical penetration depth,” αeff−1≈238nm in air. The method is systematic, provides results that are consistent with the literature, and eliminates uncertainties because of instrument sensitivities. We also show that threshold measurement based on the extrapolation of volume to zero, a method used often in previous studies, is somewhat questionable. Finally, the measured dimensions of ablated craters reveal that the ablation volume per unit input energy is about 1.3–1.5μm3∕μJ at an intermediate fluence regime of 10&amp;lt;F0av&amp;lt;40J∕cm2. This value represents an order of magnitude larger ablation efficiency when compared to the ablation of glass with nanosecond ultraviolet laser pulses.

The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air

Detailed studies on the ablation of metal targets in air with femtosecond laser pulses are reported. The laser pulses used for the study were 775 nm in wavelength, 150 fs in pulse duration, and the repetition rate used was 100 Hz. Ablation thresholds of copper, titanium, niobium, and stainless steel are measured. The ablation threshold fluence depends on the number of pulses applied to the same spot. The strength of this dependence is governed by the incubation coefficient, S, which has been determined along with the single-shot ablation threshold, φ th(1), for all the metals studied. The accumulation behaviour results from the build-up of plastic deformation induced by the laser pulses. Two distinct ablation phases have been identified from ablation rate curves and an SEM investigation of the ablated morphologies. Ripples were observed on the material surface during the gentle ablation phase, whereas much rougher structures occurred during the stronger ablation phase. Morphological changes of the ablated regions were studied as a function of fluence and incident laser shot number. The observed morphologies seem to suggest that normal vaporisation and phase explosion are the most probable physical mechanisms responsible for material removal during ablation.