We investigated the morphology and mechanism of laser-induced damage in the ablation cutting of thin glass sheets with
picosecond laser. Two kinds of damage morphologies observed on the cross-section of the cut channel, are caused by
high-density free-electrons and the temperature accumulation, respectively. Notches and micro-cracks can be observed
on the top surface of the sample near the cut edge. The surface micro-cracks were related to high energy free-electrons
and also the heat-affected zone. Heat-affected-zone and visible-cracks free conditions of glass cutting were achieved by
controlling the repetition rate and spatial overlap of laser pulses.
We present a numerical model of internal modification in bulk borosilicate glass by high repetition rate picosecond laser pulses. We study free-electron dynamics, nonlinear energy deposition and thermal conduction. The optical absorptivity and modification regions both have good agreements with the experimental results. The smooth outer zone is the molten region and the inner-structure formation is caused by high-density free-electrons generated by thermal ionization. Excitation, relaxation and accumulation of free-electron density in the focal volume are analyzed using different pulse shapes and a double-pulse train. The deposited energy distribution and modification zone are controlled by pulse shaping.
Antimicrobial photodynamic therapy (aPDT) is a promising method to treat local bacterial infections. The therapy is painless and does not cause bacterial resistances. However, there are gaps in understanding the dynamics of the processes, especially in periodontal treatment. This work describes the advances in fundamental physical and mathematical modeling of aPDT used for interpretation of experimental evidence. The result is a two-dimensional model of aPDT in a dental pocket phantom model. In this model, the propagation of laser light and the kinetics of the chemical reactions are described as coupled processes. The laser light induces the chemical processes depending on its intensity. As a consequence of the chemical processes, the local optical properties and distribution of laser light change as well as the reaction rates. The mathematical description of these coupled processes will help to develop treatment protocols and is the first step toward an inline feedback system for aPDT users.
Ultrafast reflection and secondary ablation have been theoretically investigated with a Fresnel-Drude model in laser processing of transparent dielectrics with picosecond pulsed laser. The time-dependent refractive index has a crucial effect on the cascade ionization rate and, thereby, on the plasma generation. The relative roles of the plasma gas and the incident angle in the reflection are discussed in the case of the oblique incidence. The angular dependence of the reflectivity on the laser-excited surface for s- and p-polarization is significantly different from the usual Fresnel reflectivity curve in the low-fluence limit. A road map to the secondary ablation induced by the reflected pulse is obtained on the angles of the first and second incidence. It indicates that the laser-induced plasma plays a major role in the secondary ablation, which could overcome the saturation of the ablation crater depth or generate microcracks underneath the crater wall.
Processing of thin and ultra-thin glass displays is becoming more important in the fast increasing market of display manufacturing. As conventional technologies such as mechanical scribing followed by manual breaking mostly lead to bad edge quality and thus to a huge amount of reject, other processes like ablation processes  with picosecond lasers are getting more and more interesting. However processing with ultrashort pulsed lasers partially leads to unwanted effects which should be understood in a better way by means of intensive basic research. Therefore the ablation mechanism of ultrashort pulses on transparent materials was investigated in this research project. On the one hand the ablation mechanism was analyzed in a simulative way by means of rate equations on the other hand by laboratory experiments.
Thin glass sheets (thickness <1 mm) have a great potential in OLED and LCD displays. While the conventional
manufacturing methods, such as mechanical scribing and breaking, result in poor edge strength, ultra-short-pulsed laser
processing could be a promising solution, offering high-quality cutting edges. However laser precision glass cutting
suffers from unwanted material modification and even severe damage (e.g. cracks and chipping). Therefore it is essential
to have a deep understanding of the ultra-short-pulsed laser ablation mechanism of transparent dielectrics in order to
remedy those drawbacks.
In this work, the ablation mechanism of transparent dielectrics irradiated by picosecond laser pulses has been studied.
Ultrafast dynamics of free-electrons is analyzed using a rate equation for free-electron density including multi-photon
ionization, avalanche ionization and loss terms. Two maps of free-electron density in parameter space are given to
discuss the dependence of ablation threshold intensity/fluence on pulse duration. The laser ablation model describing
laser beam propagation and energy deposition in transparent dielectrics is presented. Based on our model, simulations
and experiments have been performed to study the ablation dynamics. Both simulation and experimental results show
good agreement, offering great potential for optimization of laser processing in transparent dielectrics. The effects of
recombination coefficient and electron-collision time on our model are investigated.