Ultrafast Laser Material Processing Research Articles

Ultrafast Laser Fabrication of Microchannels on Aluminium Alloy Substrate

Aluminium alloy 6061 has a versatile application within industrial heat exchangers, heat sinks, chemical equipment, and frames of aircraft and ships. Its physical and mechanical properties such as lightweight, high strength, corrosion resistance, and thermal and electrical conductivity make it a suitable material choice for these applications. Within thermal and micro-electromechanical applications, such as heat exchanges, radiators, and heat sinks used in microelectronics, the dissipation of heat plays an important role. For optimum heat dissipation, a higher surface area is required. This can be achieved by modifying the surface by fabricating microchannels. A number of processing techniques are used for fabricating microchannels on different materials. A laser is a flexible non-contact machining tool that may be used to create any profile or contour on practically any material. In recent times due to the advancement in laser technology, the use of ultrafast laser material processing is one potential route toward further extending the fabrication of high-quality microchannels without defects caused due to heat-affected zones and in a sustainable manner. In this paper, we present an experimental work of fabrication of microchannels on an aluminium alloy 6061 surfaces by using a low power (<4 W) 400 fs laser system. The dimensional accuracy of the fabricated microchannels is assessed using scanning electron microscopy and 3D profilometry. Furthermore, as processing speed and scale is of importance in industrial laser processes, the use of scanning optics is examined as a means of developing a rapid and scalable ultrafast laser process.

Pulse Burst Generation and Diffraction with Spatial Light Modulators for Dynamic Ultrafast Laser Materials Processing.

A pulse burst optical system has been developed, able to alter an energetic, ultrafast 10 ps, 5 kHz output pulse train to 323 MHz intra-burst frequency at the fundamental 5 kHz repetition rate. An optical delay line consisting of a beam-splitting polariser cube, mirrors, and waveplates transforms a high-energy pulse into a pulse burst, circulating around the delay line. Interestingly, the reflected first pulse and subsequent pulses from the delay line have orthogonal linear polarisations. This fact allows independent modulation of these pulses using two-phase-only Spatial Light Modulators (SLM) when their directors are also aligned orthogonally. With hybrid Computer Generated Holograms (CGH) addressed to the SLMs, we demonstrate simultaneous multi-spot periodic surface micro-structuring on stainless steel with orthogonal linear polarisations and cylindrical vector (CV) beams with Radial and Azimuthal polarisations. Burst processing produces a major change in resulting surface texture due to plasma absorption on the nanosecond time scale; hence the ablation rates on stainless steel with pulse bursts are always lower than 5 kHz processing. By synchronising the scan motion and CGH application, we show simultaneous independent multi-beam real-time processing with pulse bursts having orthogonal linear polarisations. This novel technique extends the flexibility of parallel beam surface micro-structuring with adaptive optics.

Single-Shot Multi-Stage Damage and Ablation of Silicon by Femtosecond Mid-infrared Laser Pulses

Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MIR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ2 scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MIR pulses (λ = 2.7–4.2 μm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and μ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.

(Invited) Manufacturing and Characterization of Advanced High Energy Silicon/Graphite Electrodes

Next generation lithium-ion batteries (LIB) with high energy density and high power density have recently become of great interest for electric vehicle and portable devices. With the further upgrade of especially electric vehicles, the next generation LIB with high power and high energy density is urgently required. For this purpose, composite electrode consisting of commercially available graphite active material mixed with silicon nanoparticles is under current development. The main objectives are a significant increase of the practical capacity and energy density of commercial anodes, an overcome of the drawbacks of pure silicon due to large volume changes during electrochemical cycling, and the development of a technology suitable for mass production. In order to reduce the intrinsic mechanical stress of silicon/graphite electrodes and to improve the lithium-ion transport kinetic, free-standing electrode structures were generated by applying ultrafast industrial capable laser material processing. This advanced laser technology is demonstrated to be a flexible and powerful tool for pushing silicon/graphite (Si/C) composite anode materials beyond state of the art electrodes towards application. The electrochemical properties of cells with unstructured and structured electrodes were systematically analyzed by means of cyclic voltammetry, galvanostatic measurements, and electrochemical impedance spectroscopy. The increased active surface enables a significant improvement of lithium-ion diffusion kinetics. Furthermore, it is expected that the increased active surface will also provide additional artificial porosity for active material expansion, which in turn will reduce the mechanical stress within the electrodes during lithiation or delithiation. In this context, in-situ scanning electron microscopy (SEM) was performed in order to analyze the active material volume changes during charging and discharging. A main engineering challenge was to optimize the electrode architecture such as the pitch distance of free-standing structures regarding an enhanced electrochemical performance and a reduced material loss. Furthermore, an alumina (Al2O3) layer with a thickness of 5 nm, which acts as an artificial SEI, was coated on structured silicon/graphite electrodes by applying Atomic Layer Deposition (ALD). Cyclic voltammetry measurements were subsequently performed in order to investigate the fundamental properties of cells with structured and Al2O3- coated silicon/graphite electrodes. Galvanostatic measurements reveal that the cells with structured electrodes exhibit excellent electrochemical properties, i.e., a significantly improved capacity retention. ALD layers can contribute to further improvement of cycle stability and cell lifetime. In addition, advanced full cells with Lithium Nickel Manganese Cobalt Oxide NMC622 as cathode and Si/C as counter electrode were assembled and the electrochemical data will be presented.

Surface morphology and straight crack generation of ultrafast laser irradiated β-Ga2O3

Single crystal (010) β-Ga2O3 was irradiated by a Ti:sapphire ultrafast laser (150 fs pulse width) with varying fluences and a number of pulses in air ambient. Femtosecond laser-induced damage threshold of β-Ga2O3 is reported. Single pulse exposure results in surface morphological changes above a threshold laser fluence of 1.11 J/cm2. Laser-induced straight cracks aligned to the [001] crystallographic direction are observed in the laser irradiated regions, which are believed to be caused by laser-induced thermal stress, due to the unique low thermal conductivity and anisotropy associated with β-Ga2O3. Multiple pulse irradiation below the single pulse damage threshold fluence exhibited the formation of high spatial frequency laser-induced periodic surface structures. Electron backscattering diffraction and Raman spectroscopy suggested that there was no apparent phase transition of the irradiated β-Ga2O3 material for either single pulse or multiple pulse irradiation. This work serves as a starting point to further understanding the material properties of β-Ga2O3 and to unlock the potential for ultrafast laser material processing of β-Ga2O3.

Thermal effects of ultrafast laser interaction with polypropylene.

Ultrafast lasers have been used for high-precision processing of a wide range of materials, including dielectrics, semiconductors, metals and polymer composites, enabling numerous applications ranging from micromachining to photonics and life sciences. To make ultrafast laser materials processing compatible with the scale and throughput needed for industrial use, it is a common practice to run the laser at a high repetition rate and hence high average power. However, heat accumulation under such processing conditions will deteriorate the processing quality, especially for polymers, which typically have a low melting temperature. In this paper, an analytical solution to a transient, two-dimensional thermal model is developed using Duhamel's theorem and the Hankel transform. This solution is used to understand the effect of laser parameters on ultrafast laser processing of polypropylene (PP). Laser cutting experiments are carried out on PP sheets to correlate with the theoretical calculation. This study shows that, in laser cutting, the total energy absorbed in the material and the intensity are two important figures of merit to predict the cutting performance. Heat accumulation is observed at low scanning speeds and high repetition rates, leading to significant heat-affected zone and even burning of the material, which is supported by experimental data and modelling results. It is found that heat accumulation can be avoided by a proper choice of the processing condition.

Multi imaging-based beam shaping for ultrafast laser-material processing using spatial light modulators

An imaging based beam shaping technique combined with parallel beam processing has been demonstrated. Binary and intensity masks have been designed and applied on phase only spatial light modulator (SLM) to shape the input laser beam in the desired profile and intensity distribution. A second SLM with a computer generated hologram(CGH) applied was used to split the incident picosecond laser beam. Hence, the laser beam was effectively converted to required target shape, intensity distribution and a separated pattern then reconstructed at the image plane of a focusing lens. The machined footprints on a polished Titanium (Ti6Al4V) substrate accurately match the desired beam in shape and depth distribution.

High Speed Pump-Probe Apparatus for Observation of Transitional Effects in Ultrafast Laser Micromachining Processes

A pump-probe experimental approach has been shown to be a very efficient tool for the observation and analysis of various laser matter interaction effects. In those setups, synchronized laser pulses are used to create an event (pump) and to simultaneously observe it (probe). In general, the physical effects that can be investigated with such an apparatus are restricted by the temporal resolution of the probe pulse and the observation window. The latter can be greatly extended by adjusting the pump-probe time delay under the assumption that the interaction process remains fairly reproducible. Unfortunately, this assumption becomes invalid in the case of high-repetition-rate ultrafast laser material processing, where the irradiation history strongly affects the ongoing interaction process. In this contribution, the authors present an extension of the pump-probe setup that allows to investigate transitional and dynamic effects present during ultrafast laser machining performed at high pulse repetition frequencies.

Tubular filamentation for laser material processing.

An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 1013 W/cm2 has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.

Ultrafast Laser Surface Structuring of Intraocular Lens Polymers

Ultrafast laser material processing is an attractive approach for surface structuring of medical polymers used for intraocular lenses. To achieve best results in terms of efficiency and quality, the wavelength and pulse energy dependence on the ablation behavior of such materials has to be studied. Ablation thresholds have been determined for a polymer with different dyes which is used for intraocular lens (IOL) manufacturing. Furthermore, the ablation efficiency, achievable precision and surface roughness have been quantified to optimize the parameters for an efficient intraocular lens manufacturing process by surface microstructuring.

A tutorial on optics for ultrafast laser materials processing: basic microprocessing system to beam shaping and advanced focusing methods

Abstract Ultrafast lasers have excellent characteristics for materials processing in terms of precision and quality. This tutorial paper introduces the basic concepts of ultrafast laser materials processing and the structure and key components of typical ultrafast laser microprocessing systems. The emphasis is on the pulse-shaping devices and systems for controlling and manipulating the spatial, temporal, and polarization properties of focused femtosecond laser pulses.

Femtosecond laser nanostructuring for femtosensitive DNA detection

In this paper, a new concept to achieve improved probe–target recognition has been devised by introducing a novel class of DNA-functionalized three-dimensional (3D), stand-free, and nanostructured electrodes. The gold nanofibrous electrodes were created using MHZ ultrafast laser material processing in air at ambient conditions. The developed nanofibrous DNA biosensor was characterized by cyclic voltammetry with the use of ferrocyanide as an electrochemical redox indicator. Currently, electrochemical signal enhancement which is of great significance for improving the sensitivity in DNA detection remains a great challenge. Through, enhanced surface area-to-volume ratio and more efficient arrangement of probe molecules on nanofibrous electrodes, our newly developed electrode system overcomes some of the sensitivity challenges of the existing systems. This nanofiber-based system realizes femtomolar (fM) sensitivity toward complementary target DNA, and demonstrates a very wide dynamic range (from 1fM to 1nM).

Nonparaxial singular beams inside the focal region of a high numerical-aperture lens

Key optical technologies, including lithography, data storage, optical tweezers, microscopy, and ultrafast laser materials processing, rely on strongly fo- cused light beams. Such beams are often used to exploit a vectorial nature of light and so detailed knowledge of the field structure inside a tight focus becomes in- creasingly important. So far, theoretical studies of intra-focal optical field compo- nents have been mainly concentrated on spatially homogeneous states of light po- larisation. In this work we present a new development in the calculations of local polarisation structure of tightly focused singular beams, including radially and azi- muthally polarised hollow beams. A rapidly increasing number of various practical applications in the last few years have attracted significant interest to studies related to three-dimensional structure of the field inside a focus of high numerical-aperture optical systems (1-5). Experimental intrafocal mapping of the squared electric field components is usually indirectly achieved with near field probes (6), point scatterers (7), fluorescent molecules and beads (8, 9), and the knife edge method (10). Recently, a new ap- proach has been developed (2) for visualising nanoscale structure of the electric field inside the focal volume of tightly focused singular beams, which is based on permanent imprinting of the field in transparent media. From the theoretical point of view, the first comprehensive studies of tightly focused linearly and circularly polarised Laguerre-Gaussian beams have been performed in Refs. (11-13). In Refs. (1, 14), the authors have attempted to analyse the focal spot of both radially and azimuthally polar- ised beams. The problem has been qualitatively solved in Ref. (14) for the case of non-apodised cylindrical vector beams, hence providing basic understanding of the focal structure of real beams. The general method allowing one to solve such problems is based on Debye vector integrals. A complexity of the problem for some types of vector beams, e.g., radially or azimuthally polarised beams, is related to finding an integrand  which would satisfy the conditions for the paraxial waves ( 2 1 ( 4 ) 0 z i          ) and simultaneously describe the vector field components. In the

Ultrafast laser ablation of metals with a pair of collinear laser pulses

We investigated the process of ultrafast laser ablation of metallic targets induced by a pair of identical laser pulses, with either p or s polarizations, temporally delayed from ≈1 ps to a few nanoseconds. We used fast ion probe diagnostics to characterize the ion plume at the moderate laser intensity (≈1012 W/cm2) typically employed in ultrafast laser deposition and material processing. We observed a consistent time-correlated enhancement of the ion yield and velocity, which lends itself to an interesting and useful method for manipulating ablation plasma characteristics. The mechanisms producing this feature are also discussed.

Effects of a gas medium on ultrafast laser beam delivery and materials processing

The propagation of a focused femtosecond laser pulse under the coupled effects of self-focusing and plasma defocusing in a gas beam-delivery medium is investigated. The results show that a focused beam profile can be dramatically distorted in air but that this distortion can be minimized by use of an inert gas, particularly helium, to deliver the beam. Model predictions are in good agreement with previous experimental results for femtosecond laser micromachining of a copper sample in four gas environments: air, nitrogen, neon, and helium at ambient pressure. The best machining quality was obtained in helium; the worst, in air.

Dynamic temporal pulse shaping in advanced ultrafast laser material processing

Phase-manipulated ultrafast laser pulses and temporally tailored pulse trains with THz repetition rates are promising new tools for quality micromachining of brittle dielectrics, allowing to adapt the laser energy delivery rate to the material properties for optimal processing. Different materials respond with specific reaction pathways to the sudden energy input depending on the efficiency of electron generation and on the ability to release the energy into the lattice. The sequential energy delivery with judiciously chosen pulse trains may induce softening of the material during the initial steps of excitation and change the energy coupling for the subsequent steps. We show that this can result in lower stress, cleaner structures, and allow for a material-dependent optimization process.

Breakdown threshold and localized electron density in water induced by ultrashort laser pulses

Optical breakdown by ultrashort laser pulses in dielectrics presents an efficient method to deposit laser energy into materials that otherwise exhibit minimal absorption at low laser intensities. During optical breakdown, a high density of free electrons is formed in the material, which dominates energy absorption, and, in turn, the material removal rate during ultrafast laser-material processing. Classical models assume a spatially uniform electron population and constant laser intensity in the focal region, which results in time-dependent expressions only, i.e., the rate equations, to predict electron evolution induced by nanosecond and picosecond pulses. For femtosecond pulses, however, the small spatial extent of the pulse requires that the pulse propagation be considered, which results in an inhomogeneous plasma and localized electron formation during optical breakdown. In this work, a femtosecond breakdown model is combined with the classical rate equations to determine both time- and position-dependent electron density during femtosecond optical breakdown in water. The model exhibits good agreement when compared with experimental results. For other transparent or moderately absorbing dielectric media, the model also shows promise for determining the time- and position-dependent electron evolution induced by ultrashort laser pulses. Another interesting result is that the maximum electron density formed during femtosecond-laser-induced optical breakdown may exceed the conventional limit imposed by the plasma frequency.

Plasma Absorption of Femtosecond Laser Pulses in Dielectrics

Dielectric (high bandgap) materials represent an important and diverse class of materials in micro and nanotechnology, including MEMS devices, biomedical and bioengineering systems, multilayer thin film coatings, fiber optics, etc. Micromachining dielectrics using ultrafast lasers is an exciting and promising new research area with many significant advantages, including precision material removal, negligible heating of the workpiece, micron and sub-micron-size feature fabrication, and high aspect ratio features. During ultrafast laser processing of dielectrics, the intense laser pulse ionizes the irradiated material and produces an optical breakdown region, or plasma, that is characterized by a high density of free electrons. These high-density electrons can efficiently absorb a large fraction of the laser irradiance energy, part of which will then be coupled into the bulk material, resulting in material removal through direct vaporization. The energy deposited into the material depends on the time and space-dependent breakdown region, the plasma rise time, and the plasma absorption coefficient. Higher coupling efficiency results in higher material removal rate; thus energy deposition is one of the most important issues for ultrafast laser materials processing, particularly for micron and sub-micron-scale laser materials processing. In the present work, a femtosecond breakdown model is developed to investigate energy deposition during ultrafast laser material interactions. One substantial contribution of the current work is that pulse propagation effects have been taken into account, which have been shown to become significant for pulse durations less than 10 ps. By accounting for the pulse propagation, the time and space-resolved plasma evolution can be characterized and used to determine the energy deposition through plasma absorption. With knowledge of the plasma absorption, changes in the pulse profile as it propagates in the focal region can be determined as well. Absorption of the laser pulse by plasma in water is compared with experimental data to validate the model, as water is a well characterized dielectric. The model, however, is also applicable to other transparent or moderately absorbing solid and liquid dielectric media during ultrafast laser-materials interactions.