Lasers in Materials Science

Laser processing of materials has achieved significant successes in pulsed laser deposition, micro- and nanostructuring and surface modification and analysis. However, materials scientists often do not think about the physics of those lasers, which determines their properties and therefore also the way in which these lasers can be employed in laser processing. This chapter discusses the essential theory of laser gain, oscillation and amplification, and provides examples drawn from lasers now frequently used in materials processing. The implications for the design of new lasers and new materials-processing strategies are considered, using the example of a picosecond laser system for polymer processing.

We consider the laser absorption mechanisms, the energy dissipation processes, and the consequences for the target material after pulsed irradiation of—mainly—solid targets. The strong dependence on laser pulse duration is emphasized. Both, the classical thermodynamic equilibrium route (heating, melting, and evaporation) and hyperthermal processes beyond thermodynamic equilibrium (superheating, phase explosion, surface/bulk instability) are comprehensively discussed, under particular consideration of relevant time scales. The implication of different processes for laser ablation and material structuring is addressed.

The absorption of sub-picosecond pulse laser radiation by the electronic system of dielectrics and metals leads to non-thermal processes such as ballistic transport, electron–electron collision, and electron emission across interfaces. Multi-photon excitation and impact ionization with subsequent avalanche ionization occur in dielectrics and single photon absorption in metals. Finally, electron–phonon-scattering sets in, electrons and lattice equilibrate, and thermal phenomena take over. The state of the current understanding of non-thermal phenomena is reviewed.

Molecular dynamics (MD) simulations of laser-materials interactions are playing an important role in investigation of complex and highly non-equilibrium processes involved in short pulse laser processing and surface modification. This role is defined by the ability of MD simulations to reveal in-depth information on the structural and phase transformations induced by the laser excitation and, at the same time, to provide clear visual representations, or “atomic movies,” of laser-induced dynamic processes. This chapter provides a brief overview of recent progress in the description of laser coupling and relaxation of photo-excited states in metals, semiconductors, insulators and molecular systems within the general framework of the classical MD technique and presents several examples of MD simulations of laser melting, generation of crystal defects, photomechanical spallation, explosive boiling and molecular entrainment in laser ablation. Possible directions of future progress in atomistic modeling of laser-materials interactions and the potential role of MD simulations in the design of an integrated multiscale computational model capable of accounting for interrelations between processes occurring at different time- and length-scales are discussed.

This chapter is aimed to provide a basic introduction into the principles of modeling approaches which have been developed for getting insight into various interconnected processes initiated inside transparent materials under the action of ultrashort laser pulses with consequences in volumetric modification of material structure. In view of extreme complexity of the problem, modification mechanisms and their driving processes are still far from complete understanding and require further considerable research efforts. Here we focus our consideration on established approaches that treat matter as a continuum medium. They include models describing laser beam propagation through a non-linear transparent glass or crystal with kinetics of electron plasma generation upon beam focusing and attempts to consider further material evolution with insights into thermodynamic state, stress dynamics, and plastic deformations. We underline that the quality of the final structures is determined by the synergetic action of laser excitation/relaxation kinetics, thermodynamics, and mechanics. The chapter does not pretend to completeness and aims to outline main ideas, achievements, and most intriguing findings which are still waiting for explanations and theoretical treatments.

The natural temporal scale of ultrafast electronic processes in atoms, molecules, nanostructures and solids is in the attosecond range. In this work we briefly review advances in attosecond science. Particular attention is devoted to the generation, temporal characterization and application of isolated attosecond pulses.

Laser interactions have traditionally been at the center of nanomaterials science, providing highly nonequilibrium growth conditions to enable the synthesis of novel new nanoparticles, nanotubes, and nanowires with metastable phases. Simultaneously, lasers provide unique opportunities for the remote characterization of nanomaterial size, structure, and composition through tunable laser spectroscopy, scattering, and imaging. Pulsed lasers offer the opportunity, therefore, to supply the required energy and excitation to both control and understand the growth processes of nanomaterials, providing valuable views of the typically nonequilibrium growth kinetics and intermediates involved. Here we illustrate the key challenges and progress in laser interactions for the synthesis and in situ diagnostics of nanomaterials through recent examples involving primarily carbon nanomaterials, including the pulsed growth of carbon nanotubes and graphene.

Elemental and compound nanoparticles (NPs) are increasingly attractive due to their peculiar physico-chemical properties. Any large scale application of NPs requires a strict control on their synthesis and self-assembling. Inherent to the synthesis stage is the control of size, shape, composition, structure of the single NP. When NPs self-assemble on a suitable substrate the morphology and nanostructure of the NP architecture are the key parameters driving the performance of the resulting artificial surface. Pulsed laser ablation allows to pursue the above goals under different conditions including nanosecond and ultra-short femtosecond laser pulses, as well as an ambient fluid, either a gas at high pressure, or a radiation transparent liquid, besides vacuum. In this chapter we offer an outline of the mechanisms underlying NP synthesis in the above environments and of the most popular models currently recognized in the literature to interpret observed experimental trends. Concerning plasma plume propagation through an ambient gas attention is focused on the prediction versus observation of the size of isolated NPs and on a critical discussion of the morphology—properties relationship of noble metal NP arrays, considering their optical properties in the frame of enhanced vibrational spectroscopies (SERS). Ablation in a liquid of a solid target leads to a chemically stable suspension of different nanostructures in a one-step, environment friendly, clean process. For noble metal NPs the effect of liquid layer thickness and laser spot diameter on the concentration, size distribution and mutual aggregation of the produced NPs is discussed in relation to a more general picture of the process. Irradiation under vacuum with ultra-short fs laser pulses is a clean physical method to synthesize NPs; indeed in the majority of materials, random stackings of NPs, whose size ranges between 10 and 100 nm constitute the deposited film. Selected experiments on NP synthesis upon fs ablation of mainly elemental targets are reviewed focusing mainly on the features of the expanding plasma and on established mechanisms of NP synthesis. Possible lines of future development in the field are envisaged.

Catalyst architected in form of coating with nano-particles (NPs) are under intense investigation in the catalysis community due to their exceptional activity and selective nature in catalytic processes as compared to the corresponding bulk counterpart, especially because of their large surface-to-volume atomic ratio, size- and shape-dependent properties, and high concentration of low-coordinated active surface sites. Here we report on selected examples to demonstrate how Pulsed Laser Deposition (PLD) technique is able to synthesize NPs in a single step with the required relevant features for catalysis application. Co NPs embedded in B matrix films have been synthesized by PLD technique by taking advantage of the phase explosion process of superheated liquid where a mixture of vapor and liquid droplets leave the irradiated target surface and get deposited on the substrate. Just these NPs of low cost materials (Co–B) exhibit catalytic properties comparable to that of precious metals in hydrogen production by hydrolysis of NaBH₄ and NH₃BH₃. The catalytic activity increases further when the Co–B NPs are supported over a porous C films with high surface area synthesized by PLD. PLD was also utilized to produce Co₃O₄ NPs assembled coating by reactive ablation of Co metal in oxygen atmosphere at various substrate temperatures from room temperature to 250 °C. The important characteristics for catalysis such as shape and the size of NPs with narrow size distribution and mixed disordered-nanocrystalline phase were obtained in a single step in Co₃O₄ NPs synthesized by PLD technique. The Co₃O₄ NPs assembled coatings on glass have been tested in degradation of methylene blue dye solution, considered as a water pollutant, via photo-Fenton reaction in presence of H₂O₂. It was observed that the present Co₃O₄ NPs heterogeneous catalyst exhibits significantly better degradation activity for methylene blue solution than that obtained with homogeneous Co²⁺ ions.

In this chapter we provide an overview of the results obtained on both lead based and lead free ferroelectric thin films deposited by PLD, in relation with the actual scientific and economic tendencies. There is an increasing trend to replace or to reduce the use of toxic elements such as lead, but there is no obvious complete solution for this problem, both types of multifunctional oxides having attractive properties. The perovskite materials, ABO₃ oxides, environmental-friendly or not, will continue to be studied and their properties enhanced through different methods. Moreover, based on these properties (piezoeletric, ferroelectric, transport, optical or magnetic), new functionalities of the material can be added or modified with the help of material nanostructuring techniques. Properties of lead based oxides thin films such as PLZT and PMN-PT were investigated. PLZT thin films with different compositions have interesting dielectric and electro-optic behaviour. The effect of self-polarization in thin PMN-PT relaxor films could be of interest for pyrosensors and other applications based on the pyroelectric effect. As regarding the lead-free oxide materials, the obtaining and the characterisation of SBN and NBT-BT thin films will be presented. For the SBN thin films, the resulting value of the electro-optic coefficient r _eff was calculated to be higher than the values reported for LiNbO₃. Having a rather high Curie temperature, of 128 °C, compared to the higher Sr content compositions, the SBN:50 thin films can be potentially used in electro-optic devices operating near room temperature. Solid-solution systems (1−x)NBT-xBT based on Na_0.5Bi_0.5TiO₃ (NBT) and BaTiO₃ (BT) were investigated: for compositions situated at the morphotropic phase boundary between rhombohedral and tetragonal phase, high piezoelectric coefficient values and huge electric field-induced strain have been obtained.

We review recent results on biomaterial nanostructured layers transferred by matrix-assisted pulsed laser evaporation (MAPLE). The chapter is organized according to three main applications of these nanostructures: drug delivery systems, biosensing and biomimetic coating of metallic implants. The synthesized layers were optimized based upon the results of investigations performed by physical–chemical methods. Biocompatibility and bioactivity were assessed by dedicated in vitro tests. From the first category we chose the composite alendronate–hydroxyapatite (HA). The coating of metallic implants with these layers demonstrated to enhance human osteoblasts proliferation and differentiation, while inhibiting osteoclasts growth, with benefic effects for the treatment of osteoporosis. Enzyme ribonuclease A (RNase A) immobilized on solid supports has applications in control of the enzymatic reaction, and improved stability as compared to the free enzyme. The results by reversed-phase high-performance liquid chromatography showed that immobilization process does not affect the RNase A behavior. The transfer of pure levan and oxidized levan was obtained by MAPLE without any addition of plasticizers or pigments. The nanostructures exhibited high specific surface areas fully compatible with their potential use in drug delivery systems. For the second application, we refer to the transfer and immobilization of IgG molecules. We investigated the effect of the lipid addition in the initial solution upon the protein thin films adhesion to substrates. From the third class, we selected magnesium substituted octocalcium phosphate (OCP) and strontium substituted OCP deposited by MAPLE on Ti substrates which proved to enhance osteoblast activity and differentiation. We conclude that under optimized conditions, the thin films obtained by MAPLE were similar in composition, morphology and structure with the base material, and most likely preserved their functionality and biological performances.

Laser induced breakdown spectroscopy (LIBS) and pulsed laser deposition (PLD) are important techniques for the analysis of materials and the fabrication of thin films (metals, alloys and inorganic compounds). These techniques are not applicable to polymers, organic and biomaterials, mostly destroyed by the energetic laser pulses. To overcome this drawback, matrix assisted laser techniques were introduced: matrix assisted laser desorption ionization (MALDI) and matrix assisted pulsed laser evaporation (MAPLE), for mass spectroscopy and thin film deposition, respectively. They offer an efficient mechanism to transfer easy-to-be-decomposed materials from the condensed phase into the vapor phase. The material of interest (polymers, biological cells, proteins…) is diluted in a volatile solvent, with a typical concentration of a few wt%, to form the target to be irradiated with a pulsed laser beam. The laser energy is principally absorbed by the solvent and converted to thermal energy, allowing the solvent to vaporize. The molecules of the material of interest receive enough kinetic energy through collective collisions with the evaporating solvent to be transferred in the gas phase and finally analyzed or deposited on a suitable substrate. Here, important results of MALDI and MAPLE are reported and their working mechanisms are discussed.

Laser based techniques constitute an advantageous versatile approach for the assembly and control at nanometer scale of polymers and biopolymers, fundamental components of soft matter. In this chapter, laser nanostructuring of thin films of these materials will be illustrated by studies on laser induced periodic surface structures (LIPSS) and on laser foaming and on their respective application for surface enhanced Raman spectroscopy based sensors and for scaffolds in tissue engineering.

Laser Synthesis of TiN coatings on top of Ti pieces is performed by means of a free electron laser and also conventional lasers in reactive atmospheres. The produced coatings were investigated by various techniques. The results and properties of the resulting coatings are presented and discussed in connection with the different laser specialties. For the free electron laser treatment it was found that its ability to tune the pulse timing can be used to tailor the coating structure and properties (phases, hardness, strain, grain-size, etc.). This is discussed in connection with results of modeling the temperature, the plasma evolution, the mass transport, and the solidification behavior during and after the laser irradiation.

Ultrafast lasers can perform high-quality, high-precision surface micromachining of glasses through multiphoton absorption. When an ultrafast laser beam with a moderate pulse energy is focused into glass, multiphoton absorption is confined to a region near the focal point inside the glass. Ultrafast lasers can thus perform internal modification of glass as well as surface processing. Internal modification is widely used to write 3D optical waveguides and to fabricate micro-optical components and microfluidic channels buried inside glass, enabling functional microdevices such as 3D photonic, microfluidic, and optofluidic devices to be fabricated. Glass bonding based on internal melting is another interesting application of ultrafast lasers. Tailoring the temporal profiles of ultrafast laser pulses can improve the quality and efficiency of ultrafast laser processing and enhance the fabrication resolution. This chapter comprehensively reviews several applications of surface and volume processing of glass, including surface micromachining and the fabrication of photonic, microfluidic, and optofluidic devices. It also discusses pulse-shaping techniques for achieving high-quality, high-efficiency, and high-resolution processing.