Femtosecond Laser Surface Texturing of Materials for Various Applications

Femtosecond Laser Surface Texturing of Materials for Various Applications

ABSTRACT

Femtosecond lasers represent an electromagnetic field with field intensities approaching and even exceeding atomic binding field. When irradiated on a target, the material responses change from linear to nonlinear within a very short time. In most situations nonlinear absorption dominates and can be used in micro-machining of materials. In this work, analytical formulae are outlined relating laser and target parameters. This permits prediction of ablation conditions of materials. Ions are pulled out of a target due to charge separation caused by escaping electrons in the ablation layer which have acquired sufficient laser energy. In most cases the escaping electrons have energies equal or greater than the sum of the work function and the binding energy of the lattice. Additionally, the mechanisms of femtosecond laser melting, spallation and phase explosion of a titanium target are investigated using cascade simulations where the radiation event is modeled using molecular dynamics (MD) simulation combine with two temperature model (TTM). The model accounts for the electron heat conduction in the metal target and provide an adequate representation of the fast heating and cooling of the surface regions of the target. It uses the well-known TTM to represent heat transfer through and between electronic and atomic subsystems.

The ablation yield is established for different laser influences and the temperature evolution of the system identified. We conclude with a chapter that looks at two applications of femtosecond laser textured surfaces precisely in the photo-optics industry and in medicine.

TABLE OF CONTENTS

Abstract ………………………………………………………………………………………..iii
Dedication.……………………………………………………………………………………..iv
Table of contents ……………………………………………………………………………..v
List of figures…………………………………………………………………………………vii
Acknowledgement …………………………………………………………………………….viii
Chapter1:
Introduction……………………………………………………………………………………………………..1
1.1 Background and Introduction……………………………………….……………..1
1.2 Unresolved issues………………………………………………………………………2
1.3 Objectives of this Work…………………………………………………………….5
References…………………………………………………………………………..6
Chapter 2:
Laser physics…..…………………..….…………………………………….….……………..7
2.1 Introduction……………………………………………………………………..……7
2.2 Fundamentals of laser Physics……………………………………………………9
2.3 Modeling of laser-material interactions….………………………………………..11
References……………………………………………………………………………..14
Chapter 3:
3.1 Laser-material interactions……………………………………………………….15
3.1.1 Introduction……………………………………………………………………15
3.1.2 Heating………………………………………………………………………..20
3.1.3 Melting…………………………………………………………………….21
3.1.4 Vaporization………………………………………………………………22
3.2 Theory of femtosecond laser (ablation) material interactions.…………………23
3.2.0 Introduction………………………………………….. ……………………23
3.2.1 Penetration of laser field into target and electron collision frequency
……………………………………………………………………………25
3.2.2 Absorption mechanism: electrons pulled out of the target by
energetic electrons……………………………………………………..27
3.2.3 Threshold of ablation for metals…..…………………………….……28
vi
3.3 Molecular dynamics simulation of femtosecond laser-material interactions
(target Ti)…………………………………………………………………………..29
3.3.1 Introduction……………………………………………………………..29
3.3.2 Computational model and simulation details………………………….30
3.3.3 Results and discussions.………………………………………………32
3.3.3.1 Analysis of the three regimes: melting, spallation and
phase explosion………………………………………….…40
3.3.4 Conclusions.…………………………………………………………….41
References………………………………………………………………43
Chapter 4:
Case study…………………………………………………………………….………………44
4.1 Introduction…………………………………………………………….…………….44
4.2 Surface texturing for enhanced optical properties………………………….……44
4.2.1 Light trapping due to grooves in solar cells………………….………………44
4.3 Surface texturing for enhanced biological interaction……………………………47
References………………………………………………………………………………..49
Chapter 5:
Summary and future work……………………………………….…………………………50

CHAPTER ONE

1.1 Background and Introduction

Modification of surface properties over multiple length scale plays an important role in optimizing a material’s performance for a given application. A materials susceptibility to wear and surface damage can be reduced by altering its surface chemistry, morphology, and crystal structure. Also, one can consider the optical properties as well as the frictional, adhesive, and wetting forces acting at a materials interface as being strongly influenced by the size and shape of the micro and nano-scale features present.

It has been established that the interaction of laser light with a material can lead to permanent changes in the material’s properties not easily achievable through other means. Laser irradiation induces changes to the local chemistry, the local crystal structure, and the local morphology, all of which affect how the material behaves in a
given application.

This concept of texturing a material’s surface has a wide range of applications ranging from light trapping devices[9,10] to biomedical applications such as implants [1,6,7].

However a variety of techniques have been utilized to texture a material’s surface such as etching [13], lithographic techniques combined with isotropic etching [14], mechanical scribing [15] and solution based pattern deposition [16]. In contrast, laser texturing is a non-contact technique which can be utilized on materials. Laser is a monochromatic light source. One fundamental advantage of lasers as a tool for material processing is the ability to precisely control where in the material and what rate of energy is deposited. When the material absorbs the laser energy, particles from the material surface can be removed and this is known as laser ablation.

Many people have worked on laser material interactions predicting various mechanisms responsible for the breakdown of the target. Zhigilei et. al, Perez et. al, and a host of others, [2-5], have all identified melting and re-solidification as the chief cause of material breakdown when irradiated by a laser. Soboyejo et. al, Jianbo et. al, Mwenifumbo et. al, and Anil et. al [1, 6-8] have all shown that micro/nano-scale features created on material surfaces when irradiated by ultra-short laser pulses enhances cell proliferation. With all these advances, laser-material interactions still remains a complex topic as it embodies various branches of physics and is not without shortcomings as there are many challenges encountered when a laser is used to modify the morphology of a substance.

Some of the shortcomings range from changes in the material’s properties to the interaction time (longevity) of the laser pulse with the material.

1.2 Unresolved Issues

Significant work has been done in the field of laser-material interactions, [2-5]. However, most of the work focused on explaining the basic physical principles responsible for the material breakdown associated with laser-material interactions. Moreover, the current understanding of material breakdown associated with laser-material interactions is less well understood. This will therefore be explored in the current work.

The main issue here is the ability to precisely deposit a large amount of energy into a material over a short time scale and in a spatially confined region near the surface. This allows control of local surface properties relative to bulk and relative to other regions on the surface. However, more importantly the effect of this incident energy, the interaction time scale and other laser parameters can lead to material responses and changes that span multiple length scales. Another challenge is that when the laser pulse falls on a material, it triggers complex multi-scale features and a cascade of interrelated processes. This renders the understanding of laser-material ablation complex.

The time it takes for excited electronic states to transfer their energy to phonons and thermalize depends on the specific material and specific mechanism within the material.

When the laser-introduced excitation rate is low, in comparison with the thermalization rate, the details of the transient electronic states are not significant. Furthermore, one can consider the absorbed laser energy as being directly transformed into heat. Such processes are called photo-thermal and the material response can be treated in a purely thermal way. The resulting melting and re-solidification processes can greatly alter the material’s microstructure and properties [9-11].

When the laser introduced rate is high, in comparison with the thermalization rate, large excitations can build up at the intermediary states. Such excitation energies can be significant enough to directly break bonds. This is called photo-decomposition. It can be used for material texturing via what is often referred to as the photochemical processing of materials.

Biological implants are often used to reinforce or replace diseased or damaged tissues in human bodies. However, there is still a major challenge with these implants as their life-time is limited requiring continuous and costly retrieval and revision surgery to reattach the implant. Recent advances in biomaterial engineering have limited the number of failures due to wear or fracture of the implant itself but loosening of the load bearing surfaces of the implants from the supporting hard tissue can still lead to malfunction. Abrasion between the loose implant and the bone surface can cause pain and further wear. Accumulation of debris particles can trigger a macrophage-induced inflammation response that can lead to bone loss (osteolysis) and further implant loosening.

Much of current implant research has focused on engineering biomaterials that allow for rapid integration with the supporting hard tissue, resist loosening, and shorten the recovery period. Biological cells and tissues mainly interact with the outermost atomic layers of an implant. Therefore, modifying only the surface morphology and chemistry is sufficient to elicit novel biological responses from existing materials. Laser processing is ideally suited for such an endeavor. The nano-scale and micro-scale surface features necessary for optimal adhesion to laser micro-grooved surfaces need to be established.

Lasers provide excellent controllability, agility, and efficiency at removing small amount of metal from a substance. However, local heating during laser machining results in more subtle physical and chemical changes that may influence cell behavior. For example, contact guidance may be induced by microgrooves with spacings and depths that are comparable to the cell [7, 8, 12]. The spreading and proliferation of cells within microgrooves may also be influenced by the distribution of nano-scale features within the grooves [7, 8]. These are features that can be controlled by the use femtosecond lasers. However, there is still no fully accepted view on how femtosecond lasers
influence micro-groove and nano-scale structure formation.

1.3 Objectives of this Work

This work will explore femtosecond laser-material interactions. With so many divergent views on the physics responsible for the material breakdown, the objective of this work will be to identify and provide alternative insights into the physical processes that occur during the interactions of femtosecond laser beams with materials. A physics-based model will be developed for the prediction of material removal during laser-material
interactions.

In this work, an attempt will be made to prove that when femtosecond lasers are used for surface modifications, the physical and chemical properties of the material are retained. It will be shown that ionization and free electron heating completes in such a short time that the lattice temperature remains unchanged during the absorption of femtosecond pulses. Consequently, the underlying microstructure and properties of the surrounding materials should be unchanged.

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