Ultrashort laser pulse-matter interaction: Fundamentals and early stage plasma dynamics

Despite extensive studies for many years, the detailed mechanisms of ultrashort laser pulse (USLP)-matter interaction are still not fully understood and further fundamental investigation is required. This study seeks to provide an improved understanding of the USLP-material interaction by both theoretical and experimental investigations and to find ways to enhance laser energy coupling with different materials.^ A two-dimensional comprehensive hydrodynamic model for USLP ablation of metals and semiconductors is developed in this study. The model comprises a two-temperature model and a hydrodynamic model supplemented with a quotidian equation of state model, considering the relevant multiphysical phenomena during the laser-matter interaction. The models are capable of simulating the ablation process and the resultant plasma evolution in a wide range of laser intensity, and are valid both in air and in vacuum.^ The developed model is applied to investigate the ablation of metals in various laser intensity ranges. The dependence of ablation rates on laser intensity in air and in vacuum is studied by the model and validated against the experimental data in literature. It is revealed that there appears to be a sudden increase of the ablation rate in the high intensity range in vacuum, due to switching of the dominant absorption mechanisms. On the other hand, much lower ablation efficiency at high laser intensity in air is caused by the strong early plasma absorption of incident laser beam energy. The evolutions of both early plasma and plume plasma are measured by a shadowgraphic technique and a direct fluorescence method, respectively, and are analyzed by the numerical simulation. It is found that the electron emission process greatly affects the surface electron temperature.^ The femtosecond laser ablation of silicon in air is also investigated by the integrated model. The numerical analysis results are validated and supplemented by the experimental measurements for the ablation rate and early plasma dynamics over a wide laser intensity range. It is found that ablation efficiency first increases with laser intensity, and then begins to drop in the high laser intensity range, because of the early plasma absorption of the laser beam energy. By investigating the ion expansion speed, electric field distribution, and velocity distribution of different ions, the occurrence of Coulomb explosion (CE) is demonstrated during the ablation of silicon at high laser intensity, which leads to a fast ion ejection from the target surface, thereby increasing the material removal rate at the early stage.^ Next, double-pulse (DP) ablation of silicon is investigated by an integrated atomistic model, combined by molecular dynamics (MD), Monte Carlo (MC), particle-in-cell (PIC), and beam propagation (BP) methods. The plasma emission spectrum is measured by a spectrometer to calculate the plasma temperature and electron number density. It is observed that the double-pulse ablation could significantly increase the ablation rate of silicon, which is totally different from the case of metals. Electronic excitation and metallic transition of melted silicon are revealed to be responsible reasons of ablation enhancement at ultrashort (below 1 ps) and long (1 ps to 10 ps) pulse delays, respectively. At even longer pulse delay (over 20 ps), the plasma temperature and electron number density can be effectively increased, accompanied by the ablation rate suppression. The spatial analysis of plasma temperature shows that the second pulse energy is mainly absorbed by the front portion of the plasma, where the temperature is increased the most. The plasma reheating leads to a faster expansion of the plasma

Laser ablation of silicon with THz bursts of femtosecond pulses

In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range

Ultrafast Laser Pulse Interaction with Dielectric Materials: Numerical and Experimental Investigations on Ablation and Micromachining

Ultrafast lasers have great capability and flexibility in micromachining of various materials. Due to the involved complicated multi-physical processes, mechanisms during laser-material interaction have not been fully understood. To improve and explore ultrafast laser processing and treatment of dielectric materials, numerical and experimental investigations have been devoted to better understanding the underlying fundamental physics during laser-material interaction and material micromachining. A combined continuum-atomistic model has been developed to investigate thermal and non-thermal (photomechanical) responses of materials to ultrafast laser pulse irradiation. Coexistence of phase explosion and spallation can be observed for a considerably wide range of laser fluences. Phase explosion becomes the primary ablation mechanism with the increase of laser fluence, and spallation can be restrained due to the weakened tensile stress by the generation of recoil pressure from ejection of hot material plume. For dielectric materials, due to the much lower temperature gradient by non-linear absorption, the generated thermal-elastic stress is much weaker than that in non-transparent materials, making spallation less important. Plasma dynamics is studied with respect to ejection directions and velocities based on fluorescence and shadowgraph measurements. The most probable direction (angle) is found insensitive to laser fluence/energy. The plasma expansion velocity is closely related to electron thermal velocity, indicating the significance of thermal ablation in dielectric material decomposition by laser irradiation. A numerical study of ultrafast laser-induced ablation of dielectric materials is presented based on a one-dimensional plasma-temperature model. Plasma dynamics including photoionization, impact ionization and relaxation are considered through a single rate equation. Material decomposition is captured by a temperature-based ablation criterion. Dynamic description of ablation process has been achieved through an improved two-temperature model. Laser-induced ablation threshold, transient optical properties and ablation depth have been investigated with respect to incident fluences and pulse durations. Good agreements are shown between numerical predictions and experimental observations. Fast increase of ablation depth, followed by saturation, can be observed with the increase of laser fluence. Reduction of ablation depth at fluences over 20 J/cm2 is resulted from plasma defocusing effect by air ionization. Thermal accumulation effect can be negligible with repetition rate lower than 1 kHz for fused silica and helps to enhance the ablation depth at 10 kHz (100 pulses) to almost double of that with single pulse. The ablation efficiency decreases with fluence after reaching the peak value at the fluence twice of the ablation threshold. The divergence of tightly focused Gaussian beam in transparent materials has been revealed to significantly affect the ablation process, particularly at high laser fluence. A comprehensive study of ultrafast laser direct drilling in fused silica is performed with a wide range of drilling speeds (20-500 μm/s) and pulse energy (60-480 μJ). Taper-free and uniform channels are drilled with the maximum length over 2000 μm, aspect ratio as high as ~40:1 and excellent sidewall quality (roughness ~0.65 μm) at 270 μJ. The impacts of pulse energy and drilling speeds on channel aspect ratio and quality are studied. Optimal drilling speeds are determined at different pulse energy. The dominating mechanisms of channel early-termination are beam shielding by material modification at excessive laser irradiation for low speed drilling and insufficient laser energy deposition for high speed drilling, respectively. An analytical model is developed to validate these mechanisms. The feasibility of direct drilling high-aspect-ratio and high-quality channels by ultrafast laser in transparent materials is demonstrated

The modelling of electronic effects in molecular dynamics simulations

This thesis describes the development and applications of the continuum-atomistic molecular dynamics (MD) model in the context of radiation damage. By extending the classical MD method to incorporate the electronic excitations represented as an electron fluid and coupled to the ions in the two-temperature (2T) formalism, we have been able to correctly capture the physics governing the atomistic dynamics under huge electronic excitations. The integrated 2T-MD model has been specifically adapted to study three types of non-equilibrium scenarios: laser excitations, swift heavy ion impacts and large-scale high energy collision cascades. Using the 2T-MD model we have estimated the impact of the electron-phonon coupling and the electronic stopping power on the primary radiation damage yield in bcc iron. We have found that the cascade dynamics and the resultant damage from 50-100 keV primary knock-on atom impacts is highly sensitive to the electronic stopping treatment at low projectile velocities, which represents the first rigorous study of this type. By examining the temporal evolution of the structure factor of laser-irradiated gold thin films, we have been able to directly compare the 2T-MD results with Bragg peaks measured by ultrafast electron diffraction and have achieved an excellent agreement between theory and experiment with no fitting parameters. This has enabled us to elucidate the melting dynamics following laser irradiation at a picosecond resolution for the first time and also to validate the two-temperature approach. To simulate semiconductors under electronic excitations, the continuum part of the 2T-MD model, which represents electrons, has been replaced by two continuum equations: one for carrier density and one for their energy, to account for the finite band-gap effects. We have applied such extended method to simulate ion tracks, which result from swift heavy ion impacts. We have achieved a very good agreement with the experimental results on the ion track radii, provided that we are free to adjust the strength of the electron-phonon coupling. We propose future studies in the field of non-equilibrium atomistic modelling. In particular, we discuss ab initio methods and further improvements to hybrid MD to study the effects of the interatomic potential changes in response to high electronic excitations

Single-shot femtosecond laser ablation of nano/polycrystalline titanium investigated using molecular dynamics and experiments

Laser ablation, a crucial technique in various scientific and industrial fields, plays a pivotal role in precision manufacturing. Industries such as aerospace rely on laser technology for tasks like drilling microscale holes in jet turbine components to enhance air-cooling efficiency. Moreover, laser-based material processing is indispensable in addressing healthcare challenges with facilitating the postprocessing of 3D-printed bespoke components like patient-specific implants as an example. Ultrashort pulsed laser ablation enables precise micro and nanofabrication, enhancing material properties like wettability, adhesion and biocompatibility. This is particularly important in medical applications like implant development, as it can help reduce the possibility of post-surgery infections. Scientifically, understanding the intricacies of ultrashort pulsed laser ablation contributes to ongoing research and development efforts in ablation technology, fostering the enhancement of new material properties related to surface modifications. Additionally, laser ablation plays a crucial role in additive manufacturing technology like 3D printing of metals by facilitating the post-processing stage. This thesis investigates the ultrashort pulsed laser ablation of titanium, utilising a combination of molecular dynamics simulations and experiments. Molecular dynamics simulations are used for their capability to model systems at the atomistic scale and ultrashort timescale (femtoseconds in this work), in contrast to the finite element method, and for their computational efficiency compared to methods employing more detailed calculations like density functional theory. The primary focus of this work is on exploring the size effect by examining variations in beam spot diameter and grain size with profound implications for ultraprecision manufacturing of titanium surfaces in sub-micron length scale, produced by casting and additive manufacturing techniques. It contributes a nuanced understanding of ultrashort pulsed laser ablation by bridging the gap between molecular dynamics simulations and experiments. It extends the boundaries by simulating the largest feasible atomistic models and measuring features at the smallest scale permitted by the available metrology devices in experiments. The key observations showed the critical importance of the beam spot diameter in determining the laser fluence necessary to achieve average plasma temperatures of around 9,000 K, as well as a direct correlation between the grain size and the response of the material to laser irradiation. Notably, the simulations indicated that the 10 nm laser beam spot diameter compared to the 25 nm requires 59% more absorbed laser energy for ablation. Furthermore, the investigation revealed that by increasing the grain size in alpha-phase titanium, when the number of grains in the volume of 500,000 nm³ were reduce from 500 grains to 10, 36% more absorbed laser fluence was necessary to achieve average plasma temperatures of approximately 9,000 K, despite the material exhibiting higher heat conductivity. Additionally, a comparative analysis of ultrashort pulsed laser ablation between atomistic models of pure titanium with single crystal and polycrystalline structures were carried out using molecular dynamics simulations. The results revealed that the nanocrystalline sample modelled in this work, which exhibited lower heat conduction, produced a relatively deeper crater compared to its single crystal counterpart. The single crystal sample had a greater resistance to ablation, leading to the formation of a recast layer with rougher edges in contrast to the nanocrystalline sample. In materials science and engineering "size effect" is attributed to a phenomenon where the mechanical, thermal, optical or electrical properties of a crystalline material changes as a function of its physical size where at least one dimension is in submicron length scale. Experimental examination of the size effect was carried out on commercially pure titanium (consisting of 99.6% titanium and the remaining 0.4% containing carbon, nitrogen, hydrogen, iron and oxygen atoms) and Ti-6Al-4V alloy where craters were formed on both materials using single-shots with identical fluence while varying the diameter of the laser beam. It was observed that reducing the beam spot diameter resulted in relatively shallower craters, suggesting an increased threshold for ablation. Experiments comparing single-shot laser ablation outcomes between casted and 3D-printed Ti-6Al-4V alloy revealed that the 3D-printed surface (\u1d445\u1d44e = 32 \u1d45b\u1d45a) produced a slightly cleaner crater and smoother recast layer compared to the casted material (\u1d445\u1d44e = 45 \u1d45b\u1d45a). This observation was made after subjecting both substrates to ultrashort pulsed laser irradiation with identical laser parameters

Various damage mechanisms in carbon and silicon materials under femtosecond x-ray irradiation

We review the results of our research on damage mechanisms in materials irradiated with femtosecond free-electron-laser (FEL) pulses. They were obtained using our hybrid approach, XTANT (X-ray-induced Thermal And Nonthermal Transitions). Various damage mechanisms are discussed with respect to the pulse fluence and material properties on examples of diamond, amorphous carbon, C60 crystal, and silicon. We indicate conditions: producing thermal melting of targets as a result of electron-ion energy exchange; nonthermal phase transitions due to modification of the interatomic potential; Coulomb explosion due to accumulated net charge in finite-size systems; spallation or ablation at higher fluences due to detachment of sample fragments; and warm dense matter formation. Transient optical coefficients are compared with experimental data whenever available, proving the validity of our modeling approach. Predicted diffraction patterns can be compared with the results of ongoing or future FEL experiments. Limitations of our model and possible future directions of development are outlined.Comment: This brief review is submitted for publicatio

Multi-Dimensional Simulation and Experimental Benchmarking of Ultrashort Pulsed Laser Interactions with Metallic Targets

In this work, ultrashort pulsed laser interactions with metallic targets and laser-induced effects were theoretically investigated, and the multi-dimensional simulation package FEMTO-2D was developed based on the solution of two non-linear heat conduction equations for electron and lattice sub-systems. Inverse Bremsstrahlung absorption was considered as primary light absorption mechanism. The laser-target interaction was assumed to occur at solid-like material density since laser pulse duration is orders of magnitude shorter than the time required for material thermal expansion. A theoretical approach based on the collision theory had been implemented to define the thermal dependence of target material optical properties and thermodynamic parameters (thermal conductivity and coupling factor) for electron and lattice sub-systems. Such approach allowed elimination of several fitted parameters commonly used in TTM based computer simulations. The developed simulation package has the capability to consider different angles of laser beam incidence and polarization effects which can be important for many applications. Also, the effect of the ballistic electron heat transport in metallic targets during laser-target interaction was directly accounted for based on the collision theory. Two material removal mechanisms (evaporation and explosive boiling) were developed and implemented to simulate the laser-induced ablation in 3D. With advancing of computer technology, integrated simulation packages became a popular tool of investigation of Ultrashort Pulsed Laser (USPL) interactions with various materials that help to enhance the physics and allows minimizing the extensive experimental costs for optimization of laser and target parameters for specific applications. In this work, our developed simulation package was utilized to predict the light absorption for several metallic targets as a function of wavelength and pulse duration on wide range of the laser intensity. For the first time to our knowledge, we investigated the role of the ballistic electrons in the initial heat redistribution processes during laser-target interaction in gold without relying on experimental data. We also have used our FEMTO-2D package to predict the damage threshold of gold-coated optical components with the focus on the role of the substrate materials as a heat sink for the gold film and the effect of the mirror layer thickness. Experimental work was also conducted at the Center for Materials Under eXtreme Environment (CMUXE) in the High Energy Density Physics laboratory (HEDP Lab) to benchmark the FEMTO-2D simulation predictions for USPL-induced ablation in copper using our femtosecond terra watt laser facility. Last, we investigated the properties of the laser ablation of metallic targets with ultrashort double pulses with a focus on the role of the pulse separation time when the latter does not exceed material thermal equilibration temperature