Nanoscale hydrodynamic instability in a molten thin gold film induced by femtosecond laser ablation

A mechanism of the formation of a nanotip with a nanoparticle at its top that appears in a thin metal film irradiated by a single femtosecond laser pulse has been studied experimentally and theoretically. It has been found that the nanotip appears owing to a melt flow and a nanojet formation, which is cooled and solidified. Within a proposed hydrodynamic model, the development of thermocapillary instability in the melted film is treated with the use of the Kuramoto-Sivashinsky-type hydrodynamic equation. The simulation shows that the nanojet nucleates in the form of a nanopeak in a pit on the top of a microbump (linear stage) and, then, grows in a nonlinear (explosive) regime of an increase in thermocapillary instability in good agreement with experimental data.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

Numerical simulation of thermocapillary convection at surface modification by impulse laser radiation

Numerical simulation of metal surface alloying with impulse laser radiation has been performed. Impulse intensity influence on melt hydrodynamics and alloying substance distribution has been evaluated. For substance material, the authors used data on iron including dependence of surface tension on melt temperature and admixture concentration.     

CO2 laser heating of surfaces: Melt pool formation at surface

The melt pool formation during the heating of titanium and steel surfaces by a moving CO₂ laser beam is examined. The repetitive pulses are introduced in the simulations and the Marangoni effect in the melt pool is incorporated in the model study. The influence of laser scanning speed and the laser intensity parameter on the melt pool size is also considered. The enthalpy–porosity method is adopted to account for the phase change in the irradiated spot. It is found that the influence of laser scanning speed on the melt pool size is considerable, which is more pronounced for laser beam parameter β=1. The melt pool size is smaller for stainless steel as compared to that corresponding to titanium.     

Simulation of phase transitions induced in gallium arsenide by combined laser radiation

We performed a numerical simulation of phase transitions in gallium arsenide that are induced by the combined action of nanosecond laser pulses initiating melting and an additional neodymium-glass laser irradiation enabling the control of the interface velocity. In the case of counterpropagating laser beams, a strong temperature dependence of the absorption factor at 1.06 μm occurs. It causes a thermal wave, which separates from the melting front and, propagating towards the neodymium-glass laser beam, screens the melt. For copropagating laser beams, regimes with a nonmonotonic time dependence of the melt depth may exist.     

Simulation of dynamics of phase transitions in CdTe by pulsed laser irradiation

Numerical simulation of melting and solidification processes induced in CdTe by nanosecond radiation of ruby laser (λ = 694 nm, τ = 20 and 80 ns) and KrF excimer laser (λ = 248 nm, τ = 20 ns) taking into account components diffusion in melt and their evaporation from the surface has been carried out. Cd atoms evaporation has shown to essentially affect the dynamics of phase transitions in the near-surface region. Thus, in the case of the influence of ruby laser irradiation intensive surface cooling results in the formation of nonmonotone temperature profile with maximum temperature in semiconductor volume at the distance of ∼20 nm from the surface. The melt formed under the surface extends both to the surface and to the semiconductor volume as well. As a result of cadmium telluride components evaporation and diffusion in the melt the near-surface region is enriched with tellurium. The obtained melting threshold value of irradiation energy density is in a reasonable agreement with experimental data.     

Ultrasound diagnostics of optical breakdown and subcritical microplasma in the laser plume

Optical breakdown and the formation of hot subcritical plasma in the laser plume during nanosecond laser ablation of a graphite target are studied by contact and contactless methods of ultrasound diagnostics. It is shown that optical breakdown of vapors during plasma formation is associated with threshold near-critical explosive boiling of an overheated material melt; further heating of subcritical plasma is described by known dimensional relations.     

强激光与等离子体相互作用中低频电磁场孤子波的产生及其捕获

用二维粒子模拟程序研究了超短脉冲强激光与等离子体相互作用中局域低频电磁场的产生现象.这种低频电磁场在超短脉冲激光激发尾波场、脉冲后沿产生频率下移的过程中形成.通常它们的振荡频率接近于或低于电子等离子体振荡频率，因而被捕获在等离子体中(即传播速度接近于零).在演化过程中，通常它们以孤子场的形式出现.这种孤子波的形成及其演化与离子运动有极大关系.用相对论强激光脉冲可以产生达到相对论振幅的电磁场孤子波，后者可以把离子加速到非常高的能量.研究还表明，在二维几何位形下，孤子波产生与入射光的偏振态有很大关系.     

Broadband blooming of a medium modified by an incorporated layer of nanocavities

The optical properties of a dielectric medium with the ordered distribution of nanopores in the surface layer have been investigated. It has been shown that the presence of a single layer of nanocavities near the medium-vacuum interface can increase the transparency of the medium to values close to 100% in a wide wavelength range.     

Femtosecond laser nanostructuring of silver film

In this paper, we report an evolution of surface morphology of silver film irradiated by a 1 kHz femtosecond laser. By SEM observations, it is noted that different nanostructures with respective surface features depend highly on the number of pulses and the laser fluence. Especially when the laser fluence is below the threshold fluence of film breakdown, a textured nanostructure including many nanobumps and nanocavities will appear on the surface of silver film. In order to determine an optimal regime for nanostructuring silver film and to further study the underlying mechanism, we perform a quantitative analysis of laser fluence and pulse number. The results show that this nanostructure formation should be due to a sequential process of laser melting, vapor bubbles bursting, heat stress confinement, and subsequent material redistribution. As a potential application, we find this nanostructured silver film can be used as the active substrate for surface enhanced Raman scattering effect.     

Effect of Gaussian acoustic nanocavities in a narrow constriction on ballistic phonon transmission

We investigate the effect of Gaussian acoustic nanocavities in a narrow constriction on ballistic phonon transport through a semiconductor nanowire at low temperatures. When the transverse width of acoustic nanocavities takes a Gaussian function, it is found that the resonant peaks and band gaps in transmission spectra are obvious, indicating that the system has selective transmission and filters actions for ballistic phonons. The number and length of nanocavities have significant effects on the phonon transmission and thermal conductance. The results are compared with those in uniform width acoustic nanocavities. The Gaussian acoustic nanocavities are therefore a promising phononic device to manipulate ballistic phonons in nanophononics.     

An investigation into melt flow dynamics during repetitive pulsed laser drilling of transparent media

This paper presents an investigation into the dynamics of repetitive pulsed laser drilling of a visually transparent media using a CO₂ laser source. This enabled the use of a high-speed imaging system for observing, in real time, the behaviour of the drilling process in the laser drilled cavity of 1.5 mm diameter holes of up to 18.5 mm in depth. The work revealed that the instantaneous drilling velocity within each laser pulse can vary considerably from the average drilling velocity as a result of the non-uniform temporal pulse shape and the oscillation of the melt ejection rate. During beam breakthrough, both upward and downward melt ejections were observed to occur inside the drilled hole for a short period of time, after which the material was ejected through the exit end of the holes. It has been shown in this work that the downward melt flow velocity increases with hole depth for a positively tapered hole (from 0.09 to 1.43 m/s) and decreases with hole depth for a negatively tapered hole geometry (from 0.4 to 0.1 m/s), as a result of the change in the assist gas velocity inside the drilled hole with respect to the hole taper geometry. The mechanisms of forming the positively and negatively tapered holes in the transparent media have been correlated with the hole geometry and melt flow velocity. The work has demonstrated a new method of studying the melt dynamics in laser drilling.     

Microstructure analysis of magnesium alloy melted by laser irradiation

The effects of laser surface melting (LSM) on microstructure of magnesium alloy containing Al8.57%, Zn 0.68%, Mn0.15%, Ce0.52% were investigated. In the present work, a pulsed Nd:YAG laser was used to melt and rapidly solidify the surface of the magnesium alloy with the objective of changing microstructure and improving the corrosion resistance. The results indicate that laser-melted layer contains the finer dendrites and behaviors good resistance corrosion compared with the untreated layer. Furthermore, the absorption coefficient of the magnesium alloy has been estimated according to the numeral simulation of the thermal conditions. The formation process of fine microstructure in melted layers was investigated based on the experimental observation and the theoretical analysis. Some simulation results such as the re-solidification velocities are obtained. The phase constitutions of the melted layers determined by X-ray diffraction were β-Mg₁₇Al₁₂ and α-Mg as well as some phases unidentified.     

Theoretical and experimental investigation into laser melting of steel samples

Among the available laser applications, laser melting has turned out to be a powerful technique for the production of mechanically improved surfaces. To enhance the understanding of the laser melting process investigations into modeling of the heating mechanism initiating the laser melting are necessary. In the present study, a mathematical modeling of the laser melting process is introduced and power require ments for the laser melting are predicted as functions of workpiece properties and velocity. Maximum melt width is predicted analytically and compared with the experimental results. In this regard, an experiment is conducted to melt the mild steel samples with a cw CO₂ laser at different power settings and workpiece velocities. It is found that the melt variables predicted from theory are in agreement with the experimental results.     

Radiation-induced defects in Pr3+-activated Liyf4 laser host

Rare earth doped fluorides have been used in laser applications. Not much is known about the effect of ionizing radiation on the lasing and other properties of fluorides. Therefore, in recent years much attention has been paid to the study of radiation-induced defects in laser materials, as they affect the optical and stimulated emission properties. The defect formation by γ-ray irradiation in Pr³⁺ activated LiYF₄, powder prepared by melt method, have been studied by thermoluminescence and electron spin resonance techniques and are reported in this paper. It is shown that LiYF₄:Pr³⁺ is sensitive to γ-ray radiation. Characterization of this laser material using ESR and photoluminescence techniques is also described.     

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.     

Total light absorption in a wide range of incidence by nanostructured metals without plasmons

Metals structured by nanocavities have recently been demonstrated to efficiently absorb light in a wide range of angles of incidence. It has been assumed that nanovoid plasmons are at the origin of the strong absorption. It is shown that it is possible to totally absorb incident light without plasmons. To avoid their excitation, a diffraction grating consisting of cylindrical cavities in a metallic substrate is illuminated in transverse electric polarization. It is found that cylindrical cavities can sustain cavity resonances with a high enhancement of the light intensity, provoking a total absorption of light in a wide range of incidence.     

Laser melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface (β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.     

Fast simulation code for heating, phase changes and dopant diffusion in silicon laser processing using the alternating direction explicit (ADE) method

Enhancements to our existing finite-differences code for the simulation of laser heating, melting and evaporation of silicon are presented. The Knudsen evaporation model has been added to the previously used enthalpy-based model in order to simulate laser pulses with pulse lengths down to a few nanoseconds. Fick’s diffusion law has also been incorporated allowing laser doping by dopant diffusion in silicon melt to be described. Finally, the basic equations for the alternating direction explicit method (ADE) have been adapted to consider nonlinear temperature-enthalpy relations, thus including affects due to phase changes. This improved the simulation speed by up to factor of 100 compared to standard explicit and implicit time integration methods. Details of the ADE algorithm and numerical stability issues are presented in this paper. Validation of the code is presented by comparing to different integration methods and to experimental results. The final code successfully simulates melting, evaporation and dopant diffusion by multiple laser pulses in three dimensions in an acceptable computing time.