Ultrashort Laser Pulse Irradiation Research Articles

Morphological study of depth-controlled high quality holes and lines fabricated on a metal substrate with a thin metal film by picosecond laser pulses

Laser micromachining is a convenient technique to structure and/or functionalize the surfaces of various materials. The use of ultrashort laser pulses has an obvious advantage to minimize the heat effect, which results in the formation of a hole without rims. Nevertheless, it is still difficult to fabricate a line without rims even with ultrashort laser pulses, because multiple irradiation of ultrashort laser pulses with a sufficient spatial overlap promotes the formation of rims and undesired structures. In this work we employ a metal substrate with a thin metal film, and demonstrate the fabrication of depth-controlled high quality lines, not to mention holes, through the selective removal of the film by picosecond laser pulses without forming rims and undesired structures. Morphological study of the target surface reveals that the fabricated structures have unprecedentedly flat bottoms with nearly vertical sidewalls to the surface.

Influence of the material properties and the process parameters on the ablation behavior for the laser structuring of the diffusion media for fuel cells

Hydrogen-powered polymer electrolyte membrane fuel cells (PEMFCs) show promising potential to power a wide range of mobile and stationary applications and to reduce greenhouse gas emissions significantly. In PEMFCs, the oxygen transport and the water transport are essential for a long lifetime and high-performance characteristics. The diffusion media (DM), located between the bipolar plate and the catalyst-coated membrane, is a crucial component of the fuel cell that significantly affects the cell-internal processes. Usually, the DM is a two-layer material system consisting of a microporous layer based on carbon black particles coated onto a porous gas diffusion layer (e.g., carbon paper). The properties of the microporous layer regarding the water transport at high current densities and, consequently, the fuel cell’s performance and lifetime can be improved by laser structuring. Within this work, different microporous layers with varying binder content and porosities were structured by locally ablating the material using ultrashort-pulsed laser radiation in the infrared wavelength range. The effect of varying process parameters was additionally investigated. Furthermore, the ablation efficiencies were calculated for increasing pulse repetition rates to qualify a process window for an industrial structuring process. The size of the micro-drillings and the heat-affected zone surrounding the hole were evaluated through topographic and microstructure analyses using a laser scanning microscope and a scanning electron microscope with energy-dispersive x-ray spectroscopy. The results showed a rather small influence of the porosity and composition of the microporous layer on the ablation behavior. In contrast, the laser structuring parameters influenced the micro-drilling geometry significantly.

Laser Engineering Nanocarbon Phases within Diamond for Science and Electronics.

Diamond, as the densest allotrope of carbon, displays a range of exemplary material properties that are attractive from a device perspective. Despite diamond displaying high carbon-carbon bond strength, ultrashort (femtosecond) pulse laser radiation can provide sufficient energy for highly localized internal breakdown of the diamond lattice. The less-dense carbon structures generated on lattice breakdown are subject to significant pressure from the surrounding diamond matrix, leading to highly unusual formation conditions. By tailoring the laser dose delivered to the diamond, it is shown that it is possible to create continuously modified internal tracks with varying electrical conduction properties. In addition to the widely reported conducting tracks, conditions leading to semiconducting and insulating written tracks have been identified. High-resolution transmission electron microscopy (HRTEM) is used to visualize the structural transformations taking place and provide insight into the different conduction regimes. The HRTEM reveals a highly diverse range of nanocarbon structures are generated by the laser irradiation, including many signatures for different so-called diaphite complexes, which have been seen in meteorite samples and seem to mediate the laser-induced breakdown of the diamond. This work offers insight into possible formation methods for the diamond and related nanocarbon phases found in meteorites.

Transient ablation topography of a thin chromium film after ultrashort pulsed laser irradiation in the spallation and phase explosion regime

Time-resolved imaging pump–probe reflectometry is an established technique to characterize the dynamic processes occurring during the material ablation upon ultrashort pulsed laser irradiation. In the phase explosion regime, however, the presence of an expanding liquid–vapor mixture on top of the ablated material makes an unambiguous determination of the transient ablation topography almost impossible, especially when only front-side pump–probe reflectometry is used. To circumvent this limitation, the front-side reflectometry was complemented by rear-side reflectometry, where the probe radiation does not interact with the liquid–vapor mixture, and the underneath ablation kinetics of the material becomes observable. To interpret the measured front- and rear-side reflectances, an optical multilayer model of the ablation process was developed. Its applicability is demonstrated on a thin chromium film that was ablated by a single pump pulse. The optical model replicates the Newton rings measured during spallation and phase explosion very well, which allows the reconstruction of the transient ablation topography. The combination of the front- and rear-side reflectometry with modeling offers a valuable tool for studying the ablation processes in thin metal films, because it overcomes the challenges of the phase explosion regime, and thus enhances the understanding of material dynamics upon laser irradiation.

Transient responses of dielectrics during ultrashort pulsed laser irradiation using a generalized thermoelastic model considering temperature-dependent thermophysical properties

A novel model involving electron dynamics, electron-lattice interactions, and thermoelastic excitation for dielectrics exposed to ultrashort pulsed laser is proposed. The thermoelastic equations are modified based on the G-L generalized thermoelasticity and consider the temperature-dependent thermophysical properties. An axisymmetric problem of ultrashort pulsed laser irradiation on fused silica is presented. The fully-coupled equations are solved simultaneously using the finite element method to obtain the transient electron density, electron temperature, lattice temperature, and stresses. The evolution of transient responses and the effects of laser intensity, pulse duration, and temperature-dependent thermophysical properties are analyzed in detail. It was found that there are kinks in the distribution of transient responses, which is attributed to the rapid change of reflectivity. The axial stress has wave-like characteristics. The electron temperature is slightly lower than the lattice temperature after the thermal equilibrium is achieved. Maybe the temperature-dependence of lattice thermal conductivity could be neglected to reduce computational expenses.

Thermal Lattice Field during Ultra-Short Laser Pulse Irradiation of Metal Targets: A Fokker–Planck Analytical Model

The ultrafast fs laser pulse heating of thin metal films is studied for the first time using the two-temperature model on the basis of the Fokker–Planck formalism. The incident laser radiation is multi-modal, while the electron temperature is described during the first 2 fs. The predictions are intended for use by experimentalists in optoelectronics, photonics, laser processing, electronics, and bio- and nanomedicine. The crucial role of the nano-sized spatial dimensions of the metal sample is highlighted. A significant result of this study is the interdependence between the target’s size, the phonon/lattice characteristics, and the coefficient β (the quotient of non-diffusive phenomena), which varies between zero (pure diffusive case) and one (pure non-diffusive case).

Scaling up picosecond laser ablation of a LATGP-type glass-ceramic solid electrolyte for all-solid-state battery production

All-solid-state batteries are an intensively studied technology expected to complement conventional lithium-ion batteries in the near future. Research into fundamental production processes is crucial for enabling economic, industrial-scale all-solid-state battery production. Laser radiation is a production tool suited for contour cutting and surface structuring of inorganic solid electrolyte separators. In the scope of this work, the laser ablation of the commercial glass-ceramic solid electrolyte lithium aluminum germanium titanium phosphate (LAGTP) using ultrashort-pulsed laser radiation at different wavelengths, fluences, and pulse repetition rates is studied. Ablation thresholds, efficiencies, and rates were determined by manufacturing percussion drillings, trenches, and cavities. Geometry dependencies, such as the ablation rate decay with advancing depth during laser cutting, necessitated an application-oriented determination of ablation metrics. The surface qualities of the solid electrolyte layers were evaluated after laser ablation by scanning electron microscopy and surface roughness measurements. A wavelength of 532 nm was found to be beneficial compared to shorter and longer wavelengths in terms of productivity and ablation quality, reaching volume ablation efficiencies up to 36 μm3∙μJ−1 and surface roughnesses Sa below 500 nm. The determined metrics are discussed for ablation cutting and structuring of solid electrolyte layers in all-solid-state battery production, considering throughput and surface quality. The results provide insight into the ablation behavior of glass-ceramic materials and support the application of ultrashort-pulsed laser ablation in next-generation battery production.

Pulse Duration Dependence of Novel Metal Alloying on Fe/Cr/Ni Thin Films by Ultra-Short Pulsed Laser Irradiation

Pulse Duration Dependence of Novel Metal Alloying on Fe/Cr/Ni Thin Films by Ultra-Short Pulsed Laser Irradiation

Dynamic correction of optical aberrations for height-independent selective laser induced etching processing strategies.

Optical aberrations are a critical issue for tight focusing and high precision manufacturing with ultrashort pulsed laser radiation in transparent media. Controlling the wave front of ultrashort laser pulses enable the correction of low order phase front distortion and significantly enhances the simplification of laser-based manufacturing of 3D-parts in glass. The influence of system-inherent, dominating aberrations such as spherical and astigmatic aberrations affect the focal area, the beam caustic and therefore the focus intensity distribution. We correct these aberrations by means of a spatial light modulator (SLM) for various processing depths in glass thickness of up to 12 mm. This flexible aberration correction significantly simplifies the process control and scanning strategies for the selective laser induced etching process. The influence on the selectivity is investigated by comparing the three different focus conditions of the intrinsic microscope objective aberration corrected, the aberrated and the SLM aberration corrected beam profile. The previously necessary pulse energy adjustment for different z positions in the glass volume is compensated via SLM aberration correction in the end. Furthermore, the spatial extend of the modified and etched area is investigated. In consequence, a simplified scan strategy and depth-independent processing parameters can be achieved for the selective laser induced etching process.

Ultra-Short Laser Pulses Propagation Through Mouse Head Tissues: Experimental and Computational Study

We present a prototype and verification of a multichannel laser system applicable to optogenetic research. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In vivo</i> photostimulation of neural cells expressing photoconvertible phytochromes or opsins requires enough light irradiation delivery to the brain that cannot be supported by continues-wave (CW) light sources. The use of ultra-short pulsed (USP) lasers operating in the second near-infrared region (II-NIR) and allowing nonlinear activation and deactivation of the photoactuators is a promising method that allows to increase the penetration depth and provide spatio-temporal localisation of radiation in tissues. This study aimed to investigate the efficiency of USP light propagation in the skin, skull, and brain of the mouse head, as well as to compare it with the corresponding CW radiation propagation in the 750–830 nm and 1086–1183 nm wavelength ranges. The experimental results and computer modelling demonstrate that about 10–12% of the initial laser radiation can reach the brain tissues. These results prove that under certain conditions, the USP laser radiation can reach a penetration depth with required power that will be sufficient for non-linear activation of opsins/phytochromes in the brain of living animals.

Microscopic mechanism of ultrashort-pulse laser ablation of metals: a molecular dynamics study incorporating electronic entropy effects

The microscopic mechanism of metal ablation induced by ultrashort laser pulse irradiation is investigated. A two-temperature model scheme combined with molecular dynamics (TTM-MD) is developed to incorporate electronic entropy effects into the simulation of metal ablation while satisfying the energy conservation law. Simulation with the TTM-MD scheme reveals that ultrashort laser pulse irradiation near the ablation threshold causes high-energy atom/ion emission and sub-nanometer depth ablation, as observed experimentally, due to the electronic entropy effect. It is also shown that the electronic entropy effect is also significant in spallation.

Effect of repetition rate and peak fluence on ablation depth with ultrashort pulse laser irradiation in silicon

Effect of repetition rate and peak fluence on ablation depth with ultrashort pulse laser irradiation in silicon

Modeling of inhomogeneous heating behavior in ultrafast laser interaction with dielectric–dielectric nanocomposite with band gap contrast

Inspired by the optical data storage and photothermal therapy applications of metal–dielectric nanocomposite materials relying on the difference in laser absorption mechanism between metal and dielectric, the ultrashort single pulse laser irradiation of zirconia–alumina nanocomposite, a dielectric–dielectric nanocomposite with band gap contrast, was investigated. A femtosecond (fs) laser of 250 fs pulse duration and 10 W maximum average power was used for the experiment. A seemingly unusual phase melting phenomenon - whereby the higher melting point zirconia phase (2715 °C) was melted, while the lower melting point alumina phase (2072 °C) remained intact - was observed, thereby suggesting a huge temperature heterogeneity in the composite under ultrashort single pulse laser irradiation. Theoretical modeling indicates that the material band gap plays a significant role in laser energy absorption under an intense laser field. A sharp increase in the total absorbed laser energy and concomitant temperature increase around the laser ablation threshold was revealed. Under appropriate laser fluence, the smaller band gap zirconia phase (5.8 eV) can absorb much more laser energy than the alumina phase (8.8 eV), resulting in a remarkable temperature difference between these two phases. This research demonstrates the capability of generating huge phase specific temperature differences in composite nanostructures composed of phases with band gap contrast by using ultrashort pulsed laser irradiation.

Volatile Lubricants Injected Through Laser Drilled Micro Holes Enable Efficiently Hydrocarbon-Free Lubrication for Deep Drawing Processes

In order to reduce the use of classic lubricants such as synthetic or mineral oils, emulsions or waxes in the deep drawing process, a new tribological system based on volatile lubricants was investigated. In this system, a volatile medium is injected under high pressure through laser drilled micro holes directly into the contact zone between the tool and the sheet metal and serves as a temporary lubricant. In order to investigate this tribological system under realistic conditions, strip drawing experiments with different volatile lubricants (air, nitrogen, carbon dioxide and argon) were performed on galvanized sheets. Therefore, a new generation of strip drawing tools was designed and numerically calculated for low elastic deformations to ensure a uniform contact pressure distribution over the entire friction contact area. To obtain a homogeneous distribution of the volatile lubricants, a number of micro holes with a depth of several millimeters were drilled into the hardened strip drawing jaws using ultrashort pulsed laser radiation. Taking into account the capabilities of this laser drilling technique in terms of size and shape of the micro holes, computational fluid dynamics simulations were performed to predict the flow behavior of the lubricant within the micro hole as well as the contact zone and were compared with observable effects in outflow tests. The chemical composition of the acting tribological layers was characterized by means of X-ray photoelectron spectroscopy and their changes during the deep drawing process were correlated with the lubricants used as well as the measured wear and friction values.

Fabrication of microstructures in the bulk and on the surface of sapphire by anisotropic selective wet etching of laser-affected volumes

In this paper a processing technique for sapphire is presented which combines laser-induced amorphization and subsequent selective wet etching of amorphized sapphire as well as anisotropic wet etching of single-crystalline sapphire (α-Al2O3). Using this technique, microstructures can be realized on the surface and in the bulk of sapphire substrates. By focusing ultra-short laser pulses inside sapphire, its structure can be transformed from crystalline into amorphous. The modified material can be selectively removed using etchants, such as hydrofluoric acid or potassium hydroxide (KOH), solely dissolving the amorphized part. In this work, however, an etchant consisting of a standard solution of sulphuric acid and phosphoric acid (96 vol% H2SO4: 85 vol% H3PO4, 3:1 vol%) at 180 °C is utilized. This method allows the realization of structures which are impossible to achieve when using conventional etchants which solely dissolve the amorphized sapphire. Ultrashort pulsed laser irradiation (230 fs) is used in this study as starting point for the subsequent anisotropic etching to form microstructures on the surface or in the bulk of sapphire that are terminated by characteristic crystal planes. In particular, the appearance of etching-induced patterns formed by stacks of rhombohedra is shown for structures below the surface, whereas triangular pits are achieved in surface processing.

Electron dynamics in fused silica after strong field laser excitation detected by spectroscopic imaging pump-probe ellipsometry

Laser ablation of dielectrics by ultrashort pulsed laser radiation is induced by nonlinear photoexcitation, such as multiphoton and tunneling processes, and the subsequent impact excitation, due to electron-electron collisions. Previous theoretical and experimental studies demonstrate that these two phenomena strongly depend on the transient optical properties of the dielectric and the electron-electron collision times. To estimate these transient optical properties and the collision time, the excited electrons are usually considered as a nearly free-electron gas, being described by the Drude theory. All previous experimental studies support using the Drude theory by time-resolved measurements of the transient reflectance during and after irradiation. However, a comprehensive characterization of the transient complex refractive index, and, thus, a more complex view of the optical properties upon irradiation above the ablation threshold fluence was still missing. Therefore, this paper presents the time-, spectral-, and fluence-resolved complex refractive index of fused silica during the interaction with ultrashort pulsed laser radiation (800 nm, 40 fs, and 4.4 J/${\mathrm{cm}}^{2}$) measured by spectroscopic imaging pump-probe ellipsometry. Comparing the state-of-the-art Drude model to the experimental data reveals that, although the Drude model replicates the transient reflectance of the material, it deviates significantly from the transient refractive index in the considered temporal range up to 200 fs after irradiation. Thus, the Drude model cannot correctly describe the transient optical properties, which leads to the assumption that the electrons cannot be considered as a nearly free-electron gas at this time. Alternatively, a Lorentz model, considering the excited electrons as bound, can replicate the measured data much better, which indicates that the electrons might be excited into localized states.

Impact on surface topography of interference patterned steel and brass using an ultrashort pulsed laser source

According to the Organization for Economic Cooperation and Development in Paris, the overall economic damage caused by counterfeiting worldwide is estimated at around 250 billion euros per year. In addition to the economic damage, counterfeit drugs or safety-relevant components endanger costumers. Individual forgery-proof laser marking of each product in a batch enables a digital product identification for all consumers in the future. Direct laser interference patterning, also known as laser interference structuring, can be used to apply a unique pattern matrix to surfaces, in particular, products. Each element of this matrix with diameters of an ablation spot could be provided with a defined periodic micropattern before shipment, resulting in a pattern-dependent interaction with incident light. The application on different surfaces can be realized in one process step using an automated spatial period variation under ambient conditions. In the case of two-beam interference structuring using two partial beams, line-like structures can be patterned. The spectrum of incident light, the angle of view, and the spatial periodicity in the nano- and micrometer range determine the color perception of the human eye. In this study, an ultrashort pulsed laser radiation of 10 ps and a wavelength of 532 nm are used for ablation of line-like spatial periods of 1.00, 1.25, and 1.50 μm under ambient conditions. The texture quality is analyzed by atomic force microscopy and scanning electron microscopy. Two materials with similar densities but different thermal conductivities and thermal diffusivities, stainless steel and brass, are structured and analyzed. Scanning electron microscopy images show the influence of heat-affected zones on texture. Electron backscatter diffraction analyses characterize the influence on the microstructure in the marginal regions.

Mathematical modeling of ultrashort pulsed laser irradiation in the cornea

A simple mathematical model for the temperature evolution in the cornea exposed to short-pulsed Ho: YAG laser under Laser Thermo Keratoplasty (LTK) treatment is developed by incorporating the heat flux phase-lag in the Fourier’s heat transfer model and laser source term is described by Lambert Beer’s law. An analytical solution to the mathematical model is obtained using the Laplace transformation technique. The computational results for the temperature profile and the temperature variation with time are presented through the graphs. The effect of some typical parameters: the heat flux phase-lag, convection coefficient and thermal conductivity on the temperature distribution and temperature variations are illustrated and discussed.

Mathematical modeling of ultrashort pulsed laser irradiation in the cornea

A simple mathematical model for the temperature evolution in the cornea exposed to short-pulsed Ho: YAG laser under Laser Thermo Keratoplasty (LTK) treatment is developed by incorporating the heat flux phase-lag in the Fourier’s heat transfer model and laser source term is described by Lambert Beer’s law. An analytical solution to the mathematical model is obtained using the Laplace transformation technique. The computational results for the temperature profile and the temperature variation with time are presented through the graphs. The effect of some typical parameters: the heat flux phase-lag, convection coefficient and thermal conductivity on the temperature distribution and temperature variations are illustrated and discussed.

Laser-Induced Graphitization of Diamond Under 30 fs Laser Pulse Irradiation.

The degree of laser-induced graphitization from a sp3-bonded to a sp2-bonded carbon fraction in a single crystal chemical vapor deposited (CVD) diamond under varying fluence of an ultrashort pulsed laser (30 fs, 800 nm, 1 kHz) irradiation has been studied. The tetrahedral CVD sp3 phase is found to transition to primarily an sp2 aromatic crystalline graphitic fraction below the critical fluence of 3.9 J/cm2, above which predominantly an amorphous carbon is formed. A fractional increase of fluence from 3.3 to 3.9 J/cm2 (∼20%) results in a substantially (∼3-fold) increased depth of the sp2 graphitized areas owing to the nonlinear interactions associated with a fs laser irradiation. Additionally, formation of a C═O carbonyl group is observed below the critical threshold fluence; the C═O cleavage occurrs gradually with the increase of irradiation fluence of 30 fs laser light. The implications for these findings on enhancement of fs driven processing of diamonds are discussed.