Ultra short pulse laser surface modification
Surface structure plays an important role in determining the nature of interactions between materials and their surroundings. The optical, mechanical, thermal and other physical properties of surfaces can be modified and controlled through the careful design of the surface structure. Although there are several methods to modify surfaces to achieve the desired properties, each of these methods has certain limitations associated with it. In this thesis, ultra-short pulse femtosecond and picosecond lasers have been used to create surface structures on various materials to achieve desired surface properties. The advantages of using ultra-short pulse lasers for surface modification over other commonly used techniques have been highlighted.^ The first part of the thesis deals with the enhancement of the optical properties of solar cell surfaces. A picosecond laser is used to create nanostructures on the surface of silicon to modify the surface reflectance and improve the light trapping efficiency of solar cells. The effects of varying process parameters such as laser fluence, scan speed, overlapping ratio and polarization angle on the formation of surface structures are reported. The experimental results are compared with finite difference time domain (FDTD) simulations and are in good agreement, showing high predictably in reflectance values for different surface structures.^ In the next part of the thesis, the effects of surface structures on the wettability of surfaces are discussed. A femtosecond laser is used to create superhydrophobic surface structures on metal surfaces. A process to transfer surface structures from metal surfaces to polymers is demonstrated resulting in superhydrophobic polymer surfaces. Various surface micro and nanostructures are presented and their wetting properties are discussed. A fast and inexpensive method to create microfluidic devices with textured superhydrophobic inner channel walls is also presented. These channels allow a controllable fluid flow rate through microfluidic devices fabricated by taking advantage of the transferability of superhydrophobic surfaces onto polymers.^
Yung C. Shin, Purdue University.