Characterization of Femtosecond Laser Machining on Dielectric Materials

Abstract

This thesis presents the investigations of femtosecond laser machining on three different dielectric materials, namely quartz, sapphire and diamond. The laser micromachining experiments were performed with a Titanium:Sapphire solid state laser with a repetition rate of 1 kHz, centered at a wavelength of 800 nm and pulse duration of 150-200 femtoseconds (fs). A 5x microscope objective for surface micromachining and a 50x microscope objective for subsurface micromachining. The 50x microscope objective was used to obtain a smaller spot size and a shorter confocal parameter. The purpose of this research was to study the interaction between the femtosecond laser pulses and quartz, sapphire and diamond which have bandgap energies of 8.4 eV (λ=148 nm), 9.9 eV (125 nm), and c)· diamond 5.5 eV (225 nm) respectively. Since the photon energy of the laser was below the wide bandgap energies of the aforementioned dielectrics, the materials were essentially transparent to the incident laser. In order to study the behavior of the dielectric materials under femtosecond laser irradiation, several experiments with varying type and number of pulses (N) were performed, such as single pulse ablation, plural pulse ablation (N ≤ 100 pulses), multiple pulse ablation (N ≤ 100 pulses), and continuous lines micromachining on the surface and in the sub-surface of materials were performed. The features, damage, and structural changes introduced by femtosecond laser irradiation on the materials studied were characterized through examination of both the plan and cross-section views. The characterization process was carried out using optical microscopy (operated in the Nomarski mode), scanning electron microscopy, focused ion beam, atomic force microscopy, and transmission electron microscopy. The laser micromachining demonstrated distinct behaviors of the three wide bandgap materials. Quartz was very prone to cracking and showed nearwavelength alternating crystalline and amorphous sub-structure with the orientation parallel with respect to the electric field direction. Sapphire showed sub-wavelength ripples formation in lower fluences. Finally, diamond showed a strong tendency for ripples formation from near- to sub-wavelength spacing with the orientation of the ripples perpendicular and parallel with respect to electric field polarization.