Dynamics of mid-infrared femtosecond laser resonant ablation

Resonant ablation is beneficial to avoiding uncontrollable subsurface damages in the laser ablation of polymers. In this paper the dynamics of mid-infrared laser resonant ablation of polylactic acid and toluene was calculated by using fluid dynamic equations. The merits and drawbacks of mid-infrared femtosecond laser resonant ablation of high molecular weight polymers have been discussed.     

Energy efficiency of femtosecond laser ablation of refractory metals

We present the results of an experimental study of the ablation energy thresholds and ablated mass for a number of refractory metals (Ti, Zr, Nb, Mo) by femtosecond (τ _0.5 = 45–70 fs) exposed to laser pulses in the ultraviolet — near infrared range (λ = 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10^–2 Pa). We have analyzed the ablation efficiency (mass yield per unit energy of the acting coherent radiation) and ablation energy thresholds vs. the laser pulse duration and photon energy.     

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.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

Spot-size dependence of the ablation threshold in dielectrics for femtosecond laser pulses

The multi-pulse ablation threshold of barium borosilicate glass was measured using 30-fs pulses of a high repetition rate (1 kHz) laser system. The threshold fluence was found to decrease with increasing beam radius ranging from 20 to 400 m. Two existing models are applied by considering thermal accumulation and point defects, respectively . PACS 42.62.C; 44.10.+i; 77.22.Jp; 77.84.Bw; 79.20.D     

Langmuir probe investigation of surface contamination effects on metals during femtosecond laser ablation

Characterisation of the plasma plume induced by femtosecond laser-metal interactions has been carried out using a Langmuir probe. A double peak distribution of ablated ions and electrons has been recorded during time of flight (TOF) experiments for three metals studied (Ag, Cu and Ni). The first peak which occurs earliest in time is attributed to a surface layer of contaminants on the metal surface as it is shown to disappear after several laser shots. The re-growth of this peak, thought to be due to a recontamination process on the surface of the metal, is the subject of this paper. Two re-contamination mechanisms were considered; adsorption of contaminants from the ambient gas, and surface diffusion effects from the surrounding contaminants. Re-contamination rates for Ag, Cu and Ni were studied under two distinct gas pressures to investigate the contamination effects from the ambient. Effects arising from surface diffusion were investigated by raising the temperature of the metal sample to increase the surface mobility of the contaminants. The total contribution of contamination species present in the ablation plume was estimated by conducting angular distribution measurements of the plume. Surface diffusion of the surrounding contaminants was found to be the dominant recontamination process.     

Hydrodynamic modeling of femtosecond laser ablation of metals in vacuum and in liquid

We numerically examine the mechanisms involved in nanoparticle formation by laser ablation of metallic targets in vacuum and in liquid. We consider the very early ablation stage providing initial conditions for much longer plume expansion processes. In the case of ultrashort laser ablation, the initial population of primary nanoparticles is formed at this stage. When a liquid is present, the dynamics of the laser plume expansion differs from that in vacuum. Low compressibility of the ambient liquid results in strong confinement conditions. As a result, ablation threshold rises drastically, the ablated material is compressed, part of it becomes supersaturated and the backscattered material additionally heats the target. The extension of a molten layer leads to the additional ablation at a later stage also favoring nanoparticle formation. The obtained results thus explain recent experimental findings and help to predict the role of the experimental parameters. The performed analysis indicates ways of a control over nanoparticle synthesis.     

Dynamics of the ejected material in ultra-short laser ablation of metals

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm². The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking. PACS 79.20.Ds; 52.38.Mf; 02.70.Ns; 81.05.Bx     

Ultra-short pulse laser ablation of metals: threshold fluence, incubation coefficient and ablation rates

In this paper we present femtosecond laser ablation studies of the metals copper, silver and tungsten. Measurements of the threshold fluence determined from the hole diameters versus fluence provide incubation coefficients of the three materials, which are found to be equal within one standard deviation. Furthermore, we have determined the single-shot threshold fluences to be 1.7±0.3 J/cm², 1.5±0.4 J/cm² and 0.44±0.02 J/cm² for copper, silver and tungsten, respectively. These are in good agreement with theoretical values calculated neglecting heat diffusion.     

Dynamics of the spallative ablation of a GaAs surface irradiated by femtosecond laser pulses

The spallation of a nanometer-thick melt layer on a GaAs surface during its ablation by femtosecond laser pulses occurs with subnanosecond delays and lift-off velocities that depend on the laser fluence after its complete thermal (hydrodynamic) expansion/acoustic relaxation. The position of the spall interface in the melt is determined by the depth of the formation of a two-dimensional subsurface layer of nanobubbles (nanofoam), whereas the strongly heated surface layer of the melt above the nanofoam is partially removed in the form of a vapor-drop mixture. At the thermal expansion stage, acoustic reverberations are observed in the melt layer and characterize both the dynamics of an increase in its thickness and the shift of the cavitation region (nanofoam) inside the melt. Moreover, these reverberations can additionally stimulate spallation, promoting cavitation in the completely unloaded melt in the case of passage of a weak rarefaction wave.     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

飞秒激光烧蚀硅表面产生微纳结构过程中激光脉冲数目的影响

为了研究飞秒激光脉冲数目与硅表面形貌之间的关系,在相同的SF6气体氛围下,改变照射硅表面的飞秒激光脉冲数,发现在飞秒激光照射下由硅表面形成的微型锥状尖峰的高度与飞秒激光脉冲数呈现一种非线性关系.通过对该关系的研究有利于找出在制造具有较高吸收效率的高微型锥状尖峰的"黑硅"的实验条件,有利于基于"黑硅"材料的光电器件转化效...     

Dynamics of processes occurring in laser ablation of metals in a liquid

We present the results of investigations undertaken to establish relationships between certain characteristics of the disintegration of metals in air and liquid under laser radiation and their thermophysical properties, as well as between the basic parameters of a gas cavity formed in a liquid and the energy of the laser ablation products of metals. Consideration is given to the possibilities of applying the dynamic properties of this cavity to control the time behavior of the radiation of solid-state lasers and to solve spectroanalytical problems. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave, Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 668–673, September–October, 1997.     

Experimental investigation of recoil-momentum-generation efficiency under near-IR femtosecond laser ablation of refractory metals in vacuum

The use of ultrashort laser pulses is a way to increase recoil momentum under laser ablation of materials, because, in this case, the energy deposition per unit volume of the target material is substantially higher due to reduced heat dissipation. By using methods of combined interferometry, we estimated the specific impulse (～200–900 s), momentum coupling coefficient (～2 × 10^?5?3 × 10^?4 Ns/J), laser-energy conversion efficiency to kinetic energy of the gas-plasma flow (～0.05–0.82), and degree of the gas-plasma flow monochromaticity (～0.72–0.92) under femtosecond (τ ～ 45 fs, λ ～ 800 nm) ablation of refractory metals (Ti, Zr, Mo, and Nb) in vacuum.     

Modeling of femtosecond laser damage threshold on the two-layer metal films

The heating processes of the single-layer gold thin film and the two-layer film assembly of gold padded with other metal (silver, copper and nickel) irradiated by femtosecond laser pulse are studied by the two-temperature model. It is found that the substrate metal can change energy transport, which is corresponding to the temperature changing process, and the thermal equilibrium time. Compared with the single-layer gold film at the same laser fluence, the two-layer film structure can change the damage threshold of the gold surface. Our results indicate that we can maximize the damage threshold of the gold film surface by altering the thickness ratio of the gold layer and the substrate layer in the two-layer film assembly.     

Quantifying the quality of femtosecond laser ablation of graphene

The influence of beam intensity on laser ablation quality and ablation size is experimentally studied on graphene-coated silicon/silicon dioxide substrates. With an amplified femtosecond-pulsed laser system, by systematically decreasing the average power, periodic stripes with decreasing widths are ablated. Histogram analyses of the untouched and ablated regions of scanning electron microscope images of the fabricated structures make it possible to quantify the ablation quality. These analyses reveal that submicron ablation can be achieved while maintaining 75 % ablation accuracy by adjusting the beam intensity around the ablation threshold.     

Sub–damage–threshold femtosecond laser ablation from crystalline Si: surface nanostructures and phase transformation

Surface structures and structural transformations are investigated upon femtosecond laser ablation (800 nm, 120 fs) from crystalline silicon (100) targets placed under ultra-high vacuum. After repetitive illumination with several thousand laser pulses at intensities below the single shot damage threshold, at normal incidence, the crater morphology indicates the development of periodic structures at the crater bottom, with the orientation depending on the laser beam polarization. Periods of 200 nm and 600–700 nm, respectively, are shorter than the laser wavelength and appear as a result of surface instability. The ablation dynamics monitored by time-of-flight mass spectrometry shows the emission of positive silicon ions and clusters with kinetic energies of about 7 eV. Raman spectroscopy reveals phase transformations in the irradiated spot from Si-I to the polymorphs Si-III, Si-IV, Si-XII, and amorphous silicon as well as a stable, uncommon phase of hexagonal Si-wurzite. PACS 61.80 Ba; 81.05 Cy; 82.80.Rt; 81.65.Cf; 78.30 Am     

Nanosecond and femtosecond excimer laser ablation of fused silica

Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 m/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm²) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.The results lead to the conclusion that two different ablation mechanisms are involved: the first is based on two photon bulk absorption, the second on controlled surface damage in relation with (partially laser induced) singularity conditions at the surface.Presented at LASERION '91, June 12–14, 1991, München (Germany)