Nadezhda M. Bulgakova

Continuum Models of Ultrashort Laser–Matter Interaction in Application to Wide-Bandgap Dielectrics

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.

Insights into Laser-Materials Interaction Through Modeling on Atomic and Macroscopic Scales

Computer simulations and theoretical analysis of laser-materials interactions are playing an increasingly important role in the advancement of modern laser technologies and broadening the range of laser applications. In this chapter, we first provide an overview of the current understanding of the laser coupling and transient variation of optical properties in metals, semiconductors and dielectrics, with the focus on the practical implications on the energy deposition and distribution in the irradiated targets. The continuum-level modeling of the dynamic evolution of laser-induced stresses, nonequilibrium phase transformations, and material redistribution within the laser spot are then discussed, and the need for the physical insights into the mechanisms and kinetics of highly nonequilibrium processes triggered by the laser excitation is highlighted. The physical insights can be provided by atomistic modeling, and several examples are discussed where large-scale molecular dynamics simulations are used for investigation of the mechanisms of the generation of crystal defects (vacancies, interstitials, dislocations, and twin boundaries) and the material redistribution responsible for the formation of laser-induced periodic surface structures in the single-pulse ablative regime. The need for the integrated computational approach fully accounting for the strong coupling between processes occurring at different time- and length-scales is highlighted.

Ultrashort laser modification of transparent materials: Synergy of excitation/relaxation kinetics, thermodynamics, and mechanics

M trioxide aggregate (MTA) and Biodentine have been shown to be bioactive because of its ability to produce biologically compatible carbonated apatite. These cements release some of their components in phosphate-containing fluid, triggering the initial precipitation of amorphous calcium phosphates, which act as precursors for the formation of carbonated apatite. This spontaneous precipitation promotes a biomineralization process that leads to the formation of an interfacial layer with Tag-like-structures at the cement-dentin interface. The ability to induce the formation of apatite allows the integration of the biomaterial into the environment. However, host responses to biomaterials are dependent on the innate and nonspecific immune responses that occur in the surrounding tissues. Our studies provide compelling evidence of the in vivo biomineralization process promoted by MTA. SEM analysis showed the deposition of apatite-like clusters on collagen fibrils at 12h after implantation. SEM-EDAX indicated that the precipitates were mainly composed of calcium and phosphorus. To our knowledge, our studies are the first to provide evidence that the biomineralization process occurs simultaneously with the initial acute inflammatory response. MTA induced the activation of NF-kB signaling system at the early stage of inflammation. This finding can be associated with a proinflammatory and pro–wound-healing environment. Therefore, we hypothesize that together with the alkalinity of the material, the precipitation of apatite by biomaterials during the acute phase of inflammation may induce changes in gene expression and subsequently in cell functional activity. These changes are likely to stimulate repair, wound healing and dentinogenesis or cementogenesis. Inflammatory molecular signaling and biomineralization ability of bioactive dental materialsC nanocrystals (NC) are very promising building blocks for materials with designed functions due to their wide and easy tunability of catalytic, electronic, and optical properties by adjusting the NC composition, shape and size1. The chemistry of organic/inorganic interface in colloidal NC plays an essential role in the synthesis and growth, and strongly affects physical and chemical properties of NC. In spite of advances in the synthesis of NC and significant efforts in comprehension of chemical reactions, reaction products and surface chemistry, an understanding of chemical reactions and how organic molecules bind and pack on nanocrystal surfaces is still ambiguous.W report on the layer-by-layer (LBL) supramolecular assembly of redox responsive, organometallic poly(ferrocenylsilanes) (PFS) films on planar and porous substrates. Positively or negatively charged side groups render PFS water soluble and these polyelectrolytes allow the use of electrostatic self-assembly process for the fabrication of novel functional supramolecular nanostructures. PFS polyanions and polycations were first used to assemble multilayers on planar ITO electrode. UV/Vis spectroscopy and ellipsometry showed a linear increase of UV absorbance, and film thickness. Electrochemical behaviour of the redox multilayers with different thickness was recorded by capturing cyclic voltammograms (CVs). Film disassembly (multilayer re-dispersed in water) was performed by exposing the multilayers to the different values of holding potentials, corresponding to partial or full oxidation of PFS. Disassembly kinetics was quantitatively monitored by determining the amount of polymer released from CV experiments, as well as from UV/Vis spectroscopy. The release of encapsulated guest molecules such as labelled Dextran, Alexa Fluor®488 at different depths in the multilayer films was studied by fluorescence spectroscopy. For the preparation of multilayer films on Al2O3 membrane template, relatively high concentration of PFS solution was employed. PFS nanotubes were subsequently obtained by template removal using sodium hydroxide. The wall structure and characteristics of PFS nanotube received particular attention. The tubular structures were verified by the confocal laser scanning microscope (CLSM) using labeled Dextran, Alexa Fluor®488 as probe, as well as the measurements of SEM and TEM. These assembled nanotubes are excellent candidates for the investigation of guest payload release control triggered by redox stimuli. Smart nanostructured systems for controlled delivery of molecular payloads

Adaptive spatio-temporal techniques for smart ultrafast laser processing of optical glasses

Ultrafast lasers emerged as efficient tools to process transparent materials on minimal scales. Localized refractive index changes can serve as building blocks for embedded optical functions. The requirements of a desired photonic response involve precise adjustments of the refractive index which usually depends on the material relaxation paths. Advanced strategies are then required to improve the irradiation results. Recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Considering the potential of optical functionalization, we discuss the possibility of controlling laser-induced modifications of transparent materials employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in glasses will be shown, accompanied by concepts of efficient processing approaches. This involves an engineering aspect related to simultaneous processing of structural modifications in 3D arrangements where a feasible solution is represented by dynamic spatial beam shaping techniques. The approach has a dual aspect and includes corrections for beam propagation errors and spatial intensity distributions in desired forms for parallel processing. Adding the possibility of laser-induced birefringence, photowritten structures can be arranged in patterns generating complex propagation and polarization effects.Ultrafast lasers emerged as efficient tools to process transparent materials on minimal scales. Localized refractive index changes can serve as building blocks for embedded optical functions. The requirements of a desired photonic response involve precise adjustments of the refractive index which usually depends on the material relaxation paths. Advanced strategies are then required to improve the irradiation results. Recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Considering the potential of optical functionalization, we discuss the possibility of controlling laser-induced modifications of transparent materials employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in glasses will be shown, accompanied by concepts of efficient processing approaches. This invol...