Theoretical fundamentals of short pulse laser–metal interaction: A review

Using nanosecond laser pulses to debond the glass-EVA layer from silicon photovoltaic modules

The active silicon cell of a solar photovoltaic (PV) panel is covered by an ethylenevinylacetate (EVA) adhesive and a protective top glass layer. Separating this glass-EVA layer from the underlying silicon represents a bottleneck for recycling PV panels. Previous work has shown that the EVA-Si bond can be weakened by applying a continuous source of heat to melt the EVA. In this paper, a new method using nanosecond laser pulses is demonstrated to induce transient melting selectively at the EVA-Si interface. This impulsive heating method can cleanly separate the glass-EVA layer from the silicon in both model and commercial multicrystalline PV panels. The dependence of this debonding on parameters like laser pulse fluence (laser pulse energy per area), wavelength, applied pressure, and scan speed were characterized. For model PV panels, the single-pulse laser fluences required for spontaneous separation of the assembly under the force of gravity, were 0.23, 0.32 and 0.78 J/cm2 for 355 nm, 532 nm and 1064 nm, respectively. The use of shorter wavelengths reduces the laser fluence needed for debonding, while higher fluences can compensate for faster laser beam scanning rates. Optical and electron microscopy images of the Si surfaces before and after laser irradiation show that the textured antireflection layer is destroyed but the silver metal grid remains intact. Preliminary experiments using 532 nm pulses showed that the laser debonding method could remove the glass-EVA layer from sections of decommissioned commercial PV panels, even when the top glass layer was densely cracked.

A versatile setup for nanosecond laser polishing processes with in situ analysis capabilities

Laser polishing of material surfaces is a complex process depending on many variable parameters, such as, e.g., the properties of the used laser and optics (wavelength, pulse duration, fluence, and profile), as well as the processing (spot size, feed rate, and line or point overlap), and the thermodynamical properties of the material to be polished (heat capacity, heat conduction, etc.). For the successful laser polishing of any material, a systematic variation of all the process parameters is required to obtain satisfactorily polished surfaces with an appropriate set of parameters for the material of interest. In order to allow systematic studies of laser polishing processes, a new setup employing a highly stable nanosecond laser with an adjustable wavelength has been realized. The sample is located in a small high-vacuum chamber with the capability of introducing additional gases in a controlled manner, and the entire chamber is scanned in the beam to allow laser polishing of selected spots, lines, or larger areas. The setup is fully remote-controlled and allows in situ inspection of the initiated processes by means of a long-range microscope, electrical measurements, reflected laser light from the sample surface, and an analysis of the vacuum within the process chamber. The main properties of the setup will be presented, and some exemplary results on niobium and molybdenum metal samples will be discussed.

Sub-ps Pulsed Laser Deposition of Boron Films for Neutron Detector Applications

In view of the demand for high-quality thermal neutron detectors, boron films have recently attracted widespread research interest because of their special properties. In this work, we report on the deposition of boron films on silicon substrates by sub-picosecond pulsed laser deposition (PLD) at room temperature. Particular emphasis was placed on the investigation of the effect of the laser energy density (fluence) on the ablation process of the target material, as well as on the morphological properties of the resulting films. In addition, based on the study of the ablation and deposition rates as a function of the fluence, the ablation/deposition mechanisms are discussed. We show that well-adherent and stable boron films, with good quality surfaces revealing a good surface flatness and absence of cracks, can be obtained by means of the PLD technique, which proves to be a reliable and reproducible method for the fabrication of thick boron coatings that are suitable for neutron detection technology.

Multi-temperature modeling of femtosecond laser pulse on metallic nanoparticles accounting for the temperature dependences of the parameters

This review considers the fundamental dynamical processes of metal nanoparticles during and after the impact of a femtosecond laser pulse on a nanoparticle, including the absorption of photons. Understanding the sequence of events after photon absorption and their timescales is important for many applications of nanoparticles. Various processes are discussed, starting with optical absorption by electrons, proceeding through the relaxation of the electrons due to electron–electron scattering and electron–phonon coupling, and ending with the dissipation of the nanoparticle energy into the environment. The goal is to consider the timescales, values, and temperature dependences of the electron heat capacity and the electron–phonon coupling parameter that describe these processes and how these dependences affect the electron energy relaxation. Two- and four-temperature models for describing electron–phonon relaxation are discussed. Significant emphasis is paid to the proposed analytical approach to modeling processes during the action of a femtosecond laser pulse on a metal nanoparticle. These consider the temperature dependences of the electron heat capacity and the electron–phonon coupling factor of the metal. The entire process is divided into four stages: (1) the heating of the electron system by a pulse, (2) electron thermalization, (3) electron–phonon energy exchange and the equalization of the temperature of the electrons with the lattice, and (4) cooling of the nanoparticle. There is an appropriate analytical description of each stage. The four-temperature model can estimate the parameters of the laser and nanoparticles needed for applications of femtosecond laser pulses and nanoparticles.

One-Temperature Analytical Model for Femto-/Atto-Second Laser–Metals Drilling: A Novel Approach

Recently, ultrafast lasers have been developed and potentially become a point of interest worldwide, as their interaction with matter is yet unknown and can be mediated by new physical mechanisms. Real-time experimentation requires enormous costs, and there is therefore a need to develop computational models for this domain. By keeping in view this idea, a non-Fourier heat equation has solved the case of ultrafast laser–material interaction. Initial and boundary conditions were considered, and a one-dimensional mathematical model was presented. The simulations were compared with the experimental results for ultrashort laser–metallic sample interaction, and a close correlation was proven. It was found that the coupling of electron–phonon becomes “zero” due to short laser–material interaction time. The propagation of thermal waves was identified due to non-Fourier heat implementation. When the pulse duration increases, the variation in the thermal distribution becomes trivial due to an inverse correlation between the pulse duration and total energy within the pulse. When the laser–material interaction time decreases from fs to as, the generation of thermal waves increases and the powerful laser intensity acts as a shock wave during laser–material interaction, which causes a higher intensity of the thermal wave.

Ultrafast Time-Resolved Pump-Probe Investigation of Nanosecond Extreme Ultraviolet-Light-Induced Damage Dynamics on B4C/Ru Nano-Bilayer Film

An ultrafast time-resolved pump-probe setup with both high temporal and spatial resolution is developed to investigate the transient interaction between a nanosecond extreme ultraviolet (EUV) pulse and matter. By using a delayed femtosecond probe pulse, the pattern evolution of surface modification induced by an EUV pump at a wavelength of 13.5 nm can be imaged at different delay times, which provides deep insight into the EUV-induced damage dynamics and damage mechanisms. As a demonstration, single-shot EUV damage on a B₄C(6.0 nm)/Ru(30.4 nm)/D263 nano-bilayer optical film is studied using this pump-probe method. A recoverable phenomenon is found during the evolution process of the dome-shaped damage region. This is explained by the elastic and plastic deformations resulting from the huge compressive stress difference at the Ru-substrate interface with the help of simulations on the thermal effects and mechanical responses. This damage mechanism is further proven by the complementary experiments at a higher EUV fluence at 13.5 nm.

Picosecond Laser-Ablated Nanoparticles Loaded Filter Paper for SERS-Based Trace Detection of Thiram, 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), and Nile Blue

Recently, filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have stimulated significant attention owing to their promising advantages such as being low-cost, easy to handle, and practically suitable for real-field applications in comparison to the solid-based substrates. Herein, a simple and versatile approach of laser-ablation in liquid for the fabrication of silver (Ag)-gold (Au) alloy nanoparticles (NPs). Next, the optimization of flexible base substrate (sandpaper, printing paper, and FP) and the FP the soaking time (5−60 min) was studied. Further, the optimized FP with 30 min-soaked SERS sensors were exploited to detect minuscule concentrations of pesticide (thiram-50 nM), dye (Nile blue-5 nM), and an explosive (RDX-1,3,5-Trinitroperhydro-1,3,5-triazine-100 nM) molecule. Interestingly, a prominent SERS effect was observed from the Au NPs exhibiting satisfactory reproducibility in the SERS signals over ~1 cm2 area for all of the molecules inspected with enhancement factors of ~105 and relative standard deviation values of <15%. Furthermore, traces of pesticide residues on the surface of a banana and RDX on the glass slide were swabbed with the optimized FP substrate and successfully recorded the SERS spectra using a portable Raman spectrometer. This signifies the great potential application of such low-cost, flexible substrates in the future real-life fields.

Femtosecond Laser-Based Additive Manufacturing: Current Status and Perspectives

The ever-growing interest in additive manufacturing (AM) is evidenced by its extensive utilisation to manufacture a broad spectrum of products across a range of industries such as defence, medical, aerospace, automotive, and electronics. Today, most laser-based AM is carried out by employing continuous-wave (CW) and long-pulsed lasers. The CW and long-pulsed lasers have the downside in that the thermal energy imparted by the laser diffuses around the irradiated spot and often leads to the creation of heat-affected zones (HAZs). Heat-affected zones may degrade the material strength by producing micro-cracks, porous structures and residual stresses. To address these issues, currently, attempts are being made to employ ultrafast laser sources, such as femtosecond (fs) lasers, in AM processes. Femtosecond lasers with pulse durations in the order of 10−15 s limit the destructive laser–material interaction and, thus, minimise the probability of the HAZs. This review summarises the current advancements in the field of femtosecond laser-based AM of metals and alloys. It also reports on the comparison of CW laser, nanosecond (ns)/picosecond (ps) lasers with fs laser-based AM in the context of heat-affected zones, substrate damage, microstructural changes and thermomechanical properties. To shed light on the principal mechanisms ruling the manufacturing processes, numerical predictions are discussed and compared with the experimental results. To the best of the authors’ knowledge, this review is the first of its kind to encompass the current status, challenges and opportunities of employing fs lasers in additive manufacturing.

Prediction of the irradiation doses from ultrashort laser-solid interactions using different temperature scalings at moderate laser intensities

The ionising radiation created by high intensity and high repetition rate lasers can cause significant radiological hazard. Earlier defined electron temperature scalings are used for dose characterisation and prediction using Monte Carlo modelling. Dosimetric implications of different electron temperature scalings are investigated and the resulting equivalent doses are compared. It was found that scaling defined by Beget al(1997Phys. Plasmas4447-57) predicts the highest electron temperatures for given intensities, and subsequently the highest doses. The atomic number of the target, x-ray generation efficiency and interaction volume are the other parameters necessary for the dose evaluation. The set of these operational parameters should be sufficient to characterise radiological characteristics of ultrashort laser pulse based x-ray generators and evaluate radiological hazards of the laser processing facilities.

Femtosecond laser inscribed helical sapphire fiber Bragg gratings

This Letter reports a novel helical sapphire fiber Bragg grating (HSFBG) in a single crystal sapphire fiber with diameter of 60 µm fabricated by a 515 nm femtosecond laser. Due to the large refractive index modulation region and high structural symmetry of the HSFBGs, high-reflectivity and high-quality spectra can be prepared and additionally have good bending resistance. The spectral properties of HSFBGs with different helical diameters are studied. When the helical diameter is 30 µm, the reflectivity of HSFBG is 40%, the full width at half-maximum is 1.56 nm, and the signal-to-noise ratio is 16 dB. For the HSFBG bending test, the minimum bending radius is 5 mm, which can still maintain relatively good spectral quality. In addition, the HSFBG array with different periods has been successfully cascaded in a sapphire fiber. The experimental results of the HSFBG high-temperature test show that this HSFBG can work reliably at 1600°C, and the temperature sensitivity in the high-temperature range can reach 35.55 pm/°C. This HSFBG can be used in high-temperature and harsh environments, such as metal smelting and aeroengine structural health monitoring.

Laser Micro Polishing of Tool Steel 1.2379 (AISI D2): Influence of Intensity Distribution, Laser Beam Size, and Fluence on Surface Roughness and Area Rate

Within the scope of this study, basic research was carried out on laser micro polishing of the tool steel 1.2379 (AISI D2) using a square, top-hat shaped intensity distribution. The influence of three different quadratic laser beam sizes (100 µm, 200 µm, 400 µm side length) and fluences up to 12 J/cm2 on the resulting surface topography and roughness were investigated. Surface topography was analyzed by microscopy, white light interferometry, spectral roughness analysis, and 1D fast Fourier transformation. Scanning electron microscopy and electrical discharge analyses indicate that chromium carbides are the source of undesired surface features such as craters and dimples, which were generated inherently to the remelting process. Particularly for high laser fluences, a noticeable stripe structure was observed, which is typically a characteristic of a continuous remelting process. Although the micro-roughness was significantly reduced, often, the macro-roughness was increased. The results show that smaller laser polishing fluences are required for larger laser beam dimensions. Additionally, the same or even a lower surface roughness and less undesired surface features were created for larger laser beam dimensions. This shows a potential path for industrial applications of laser micro polishing, where area rates of up to several m2/min might be achievable with commercially available laser beam sources.