Effect of ionization on laser-induced plume self-similar expansion

The dynamics of a laser ablation plume during the first stage of its expansion, just after the termination of the laser pulse is modelled. The one-dimensional expansion of the evaporated material, considered as an ideal fluid, is governed by one-fluid Euler equations. For high energetic ions, the charge separation can be neglected and the hydrodynamics equations solved using self-similar formulation. Numerical solution is obtained, first when the laser fluence range is low enough to deal with a neutral vapor, and in a second stage, when ionization effects on the expansion are taken into account, for different material targets. As a main result, we found that the presence of ions in the evaporated gas enhances the self-similar expansion.     

Ablation of polymer foils for backside opening of thin film layered flexible polymer substrates

For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside.     

Influence of photoexcitation pathways on the initiation of ablation in poly (methyl methacrylate)

Experimental and theoretical studies of laser ablation of polymers, under various processing conditions, have identified many possible photoexcitation pathways and consequently many likely processes responsible for the onset of ablation. We investigate the role of these processes—namely the thermal, mechanical and chemical processes—occurring in a polymeric substrate during UV irradiation. Molecular dynamics simulations with an embedded Monte Carlo-based reaction scheme were used to study ablation of Poly (methyl methacrylate) at 157 nm. Laser-induced heating and chemical decomposition of the polymeric substrate are considered as ablation pathways. For the heating case, the mechanism of ejection is thermally driven limited by the critical number of bonds broken. This fragmentation process is well reproduced by the existing bulk photothermal ablation model. Alternatively, if the photon energy goes toward direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products leading to near complete decomposition, loss of strength and cohesiveness of the top layers of the polymeric substrate. The ejection of small gaseous molecules weakens and hollows out the substrate, facilitating liftoff of larger fragments of material. These larger clusters are thermally ejected and the photochemical ablation process can be described by the two-step model proposed by Kalontarov.     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

Synchronization and control of chaos in coupled chaotic multimode Nd:YAG lasers

In this Letter we numerically investigate the dynamics of a system of two coupled chaotic multimode Nd:YAG lasers with two mode and three mode outputs. Unidirectional and bidirectional coupling schemes are adopted; intensity time series plots, phase space plots and synchronization plots are used for studying the dynamics. Quality of synchronization is measured using correlation index plots. It is found that for laser with two mode output bidirectional direct coupling scheme is found to be effective in achieving complete synchronization, control of chaos and amplification in output intensity. For laser with three mode output, bidirectional difference coupling scheme gives much better chaotic synchronization as compared to unidirectional difference coupling but at the cost of higher coupling strength. We also conclude that the coupling scheme and system properties play an important role in determining the type of synchronization exhibited by the system.     

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

Characteristics of thermionic electron emission during femtosecond laser ablation of gold film are studied numerically. For the rigorous calculation of electron and lattice temperatures, an enhanced two-temperature model with transient thermal and optical properties is developed and it is demonstrated that the model predicts the damage threshold fluences closely matching experimental data. From the calculated electron emission characteristics, quantum efficiency and electron current density are estimated.     

Theoretical investigations of material modification using temporally shaped femtosecond laser pulses

We present a two-dimensional model, based on a drift–diffusion approach, developed to describe the dynamics of electronic excitation and lattice heating in several dielectric materials with different electron–phonon coupling properties (e.g. fused silica and sapphire) under the action of femtosecond near-infrared laser pulse trains with variable separation time between pulses. The modeling approach was aimed to describe the mechanisms that enable the spatial modulation of the structures induced by temporally modulated laser excitation and ablation of wide-band-gap dielectric materials. The possible geometric contours of the laser-induced craters on the target surfaces are discussed on the basis of the lattice-temperature profiles obtained by modeling. It was found that the observed difference in the crater shapes generated in fused silica and sapphire is conditioned by the difference in dynamics of electron excitation and recombination channels characteristic of these two materials. This effect can be used to convert a given temporal pulse modulation into spatial modulation, opening up new perspectives for material processing in order to obtain desired structure profiles. PACS 79.20.Ds; 42.62.-b     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Theoretical Study of the Influence of Femtosecond Laser Wavelength on the Evolution of a Double-Minimum Electronic Excited State Wave Packet for NaRb

Employing the two-state model and the time-dependent wave packet method, the influence of femtosecond laser wavelength on the evolution of the double-minimum electronic excited state wave packet is numerically investigated. For different laser wavelengths, evolutions of the double-minimum electronic excited state wave packet with time and internuclear distance are different. One can control the evolution of the wave packet by varying the laser wavelength appropriately, which will benefit the light manipulation of atomic and molecular processes. Furthermore, study of the dynamics of the NaRb molecule may yield clues to creating an ultracold molecule.     

热物性参量对飞秒激光烧蚀金属影响的分子动力学模拟

利用结合双温模型的分子动力学模拟方法,研究了飞秒激光与金属相互作用的烧蚀机制.采用中心波长为800 nm,能量密度从0.043 J·cm~(-2)到0.40 J·cm~(-2)不等,脉宽分别为70 fs和200 fs的激光烧蚀金属镍和铝材料.靶材的温度、原子位型以及内部压力随时间的演化展示了材料热物性参量特性和激光参量对烧蚀结果的影响.结果显示材料电子热传导率对飞秒脉宽激光下的影响仍然较大;对比铝和镍的结果可知,铝的电子晶格耦合系数比镍的小,故电子晶格间的温度梯度持续时间较长;铝的电子热传导系数比镍的大,所以材料上下表面电子温度耦合的时间缩短.铝薄膜表面在能量密度为0.40 J·cm~(-2)激光烧蚀下呈现纳米尺寸的晶体结构.     

Ablation and ultrafast dynamics of zinc selenide under femtosecond laser irradiation

The ablation in zinc selenide (ZnSe) crystal is studied by using 150-fs, 800-nm laser system. The images of the ablation pit measured by scanning electronic microscope (SEM) show no thermal stress and melting dynamics. The threshold fluence is measured to be 0.7 J/cm2. The ultrafast ablation dynamics is studied by using pump and probe method. The result suggests that optical breakdown and ultrafast melting take place in ZnSe irradiated under femtosecond laser pulses.     

Drilling enhancement by nanosecond-nanosecond collinear dual-pulse laser ablation of titanium in vacuum

Laser ablation of titanium in vacuum was performed using single- and dual-pulse regime in order to study crater formation. Crater profiles were analyzed by optical microscopy. It was found that the repetition-rate plays an important role in a process of laser ablation. The drilling is most effective for the highest repetition-rate. For the same total number of laser pulses clear drilling enhancement was achieved by dual-pulse regime of ablation in comparison to single-pulse regime. The strongest ablation rate in dual-pulse regime was achieved for the delay time between the pulses τ = 370 ns. Results are discussed in terms of decreased ablation threshold due to continuous heating of the target during the experiment.     

Lattice dynamics in two-photon-excited CdS studied by picosecond time-resolved X-ray diffraction

Lattice dynamics and radiative processes in single-crystal cadmium sulfide induced by two-photon excitation with a femtosecond laser are investigated. The development of lattice expansion is directly observed by picosecond time-resolved X-ray diffraction. The obtained lattice dynamics are explained on the basis of a thermally induced impulsive-strain model. The model calculation indicates that two- and more-photon absorption processes occur and that reflectivity rapidly increases under laser irradiation. In photoluminescence spectroscopy, the spectra for TW cm⁻² excitation are shifted to lower energy and show an additional shoulder at 2.35 eV. Furthermore, emission due to Fabry-Perot laser modes with self-formed cavities was observed under 11 TW cm⁻² excitation. The discrepancy between carrier densities deduced from the lattice expansion and the PL spectra indicates that the predominant process at a higher carrier density is not radiative recombination, but Auger recombination followed by lattice heating.     

Ablation efficiency of α-Al2O3 in liquid phase and ambient air by nanosecond laser irradiation

Ablation efficiency and influences of laser parameters on a material removal rate by a nanosecond laser irradiation of α-Al₂O₃ are studied in gas and liquid phases. The laser ablation in the air yields maximum material removal rate of 12 ng/pulse using a 4.6-mJ pulse energy at 4-kHz repetition rate, compared to 88 ng/pulse in the water flow. Using a specific interpulse distance and a laser repetition rate further increase material removal rate by factor of 3 and 65, respectively, owing to an optimized lattice temperature and laser pulse interactions with the generated cavitation bubble. For the ablation in the air, these parameters do not significantly affect the ablation efficiency.     

Modeling of plume dynamics with shielding in laser ablation of carbon

The process of laser ablation of carbon in presence of background gas is simulated numerically. The plume dynamics in laser ablation is important to study for many reasons including temperature of plume particles and shielding of target by previously ablated plumes. Shielding leads directly to the change in energy deposition of incident laser pulse at the target surface and in turn influences the ablation dynamics and amount of material removed. Carbon ablation is studied for single and multiple laser hits typical for synthesis of nanotubes. Two models of correction of ablated velocity and pressure resulting from shielding effect are proposed and investigated. Numerical modeling of this plume dynamics and its integral effect of shielding is challenging due to inherent high nonlinearity of the problem. Some of available numerical techniques handles nonlinearity but are dissipative, e.g. Godunov type schemes. Other techniques are less dissipative but fail to account for strong nonlinearity typical for initial stages of ablation, e.g. the ENO-Roe. To effectively model this highly nonlinear plume dynamics a combination of two of above mentioned schemes is developed so as the numerical evaluation of fluxes is close to their physical values and the scheme has minimum dissipation. The non-monotonic behavior of ablated mass as a function of time duration between two laser pulses is studied.     

Expansion dynamics of an array of high density Bose-Einstein condensates produced in a 1D CO<Subscript>2</Subscript> laser optical lattice

We discuss the expansion dynamics under mean-field repulsion of an array of ⁸⁷Rb Bose-Einstein condensates produced in an all-optical scheme involving 1D lattice with nearly 10⁵ atoms, after fast evaporative cooling of just about 1 s. Single site occupation exceeds 2 × 10⁴ in our experiments. The possibility of transition to two-dimensionality was also investigated. The expansion behavior of the high density multiple micro-condensates produced directly in the CO₂ laser 1D optical lattice, with a lattice spacing of 5.3 μm, agrees well with a numerical simulation based on the mean-field theory.     

Thresholds for front-side ablation and rear-side spallation of metal foil irradiated by femtosecond laser pulse

The mechanical action of laser exposure on a foil may result in the ablation of irradiated front layer and the rear-side spallation. The dynamics of an Al foil is studied by means of two-temperature (2T) hydrodynamics and molecular dynamics (MD). It is found that the rear-side spallation threshold F _s exceeds the front-side ablation threshold F _a. We propose to extend the common approach in laser-matter experiments by pump–probe measuring of the rear-side displacement.     

The potential of UV femtosecond laser ablation for varnish removal in the restoration of painted works of art

Despite significant advances, laser ablation with nanosecond pulses presents limitations in dealing with the restoration of classes of painted works of art, such as paintings with a very thin layer of varnish. Femtosecond laser processing promises the means for overcoming such limitations. To this end, femtosecond ablation of two typical varnishes, dammar and mastic, is examined. For these varnishes, processing by Ti:Sapphire irradiation (800 nm) turns out to be ineffective. In contrast, irradiation with 248 nm ∼500 fs laser pulses results in a higher etching resolution (etching rates of ∼1 μm/pulse or less). For irradiation with few laser pulses at moderate laser fluences, etched morphology is far smoother than in the processing with nanosecond laser pulses. Furthermore, chemical modifications are considerably reduced (by nearly an order of magnitude), and exhibit a number of additional novel differences. Both etching rates and extent of chemical modifications are largely independent of varnish absorptivity. In all, femtosecond UV laser irradiation is indicated to hold a high potential, offering new perspectives for the restoration of painted works of art. Finally, a tentative model is advanced accounting in a consistent way for the observations.