Femtosecond Laser Surface Texturing of Materials for Various Applications

lasers represent an field with field intensities approaching and even atomic binding field.

When irradiated on a target, the responses change from linear to nonlinear within a very short time. In most situations absorption dominates and can be used in micromachining 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 fluences 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:   

• Background and Introduction………………….. 1
• Unresolved issues…………………………………… 2
• Objectives of this Work………………………………… 5

References…………………….. 6
Chapter 2:
Laser physics…………………………………… 7

• Introduction……………………………… 7
• Fundamentals of laser Physics……………………………… 9
• Modeling of laser-material interactions…………………… 11

References…………………… 14
Chapter 3:

• Laser-material interactions…………………… 15
  • Introduction……………………………….. 15
  • Heating…………………………… 20
  • Melting………………………………………… 21
  • Vaporization……………………………. 22
• Theory of femtosecond laser (ablation) material ………………….. 23
  • Introduction………………………….. 23
  • Penetration of laser field into target and electron collision frequency…25

• Absorption mechanism: electrons pulled out of the target by energetic electrons……………………………. 27
• Threshold of ablation for metals…………………. 28

• Molecular dynamics simulation of femtosecond laser-material interactions (target Ti)……… 29
  • Introduction……………………………… 29
  • Computational model and simulation details………… 30
  • Results and ………… 32
    • Analysis of the three regimes: melting, spallation and phase explosion…… 40

References………………… 43
Chapter 4:
Case study…………………. 44

References………………………49
Chapter 5:
Summary and future work……………………. 50

Introduction

Background Of Study

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.

References

Y. Vorobyev and Chunlei Guo, App Phy let 92, 041914(2008)

A. Y. Vorobyev, Chunlei Guo, Hindawi Publishing Corporation ,Advances in Mechanical Engineering Volume 2010, Article ID 452749(2009)

S. Mwenifumbo, M. Li, J. Chen, A. Beye, W. Soboyejo, J Mater Sci: Mater Med (2007)
 J. Chen, S. Mwenifumbo, C. Langhammer, J.-P. McGovern, M. Li, A. Beye, W. Soboyejo, Wiley InterScience (2007).

W. O. Soboyejo, B. Nemetski, S. Allameh, N. Marcantonio, C. Mercer, J. Ricci, Wiley Periodicals, Inc (2002)

Anil Kurella, Narendra B. Dahotre, J Biomater Appl 2005 20: 5 DOI:10.1177/0885328205052974