>Magister Scientiae - MSc / Solid-state reactions of bi-metallic systems can be driven or activated by various external stimuli like pressure, energetic photons, energetic charged particles or heat. For an example, high pressure torsion can be applied to aluminium-copper (Al-Cu) to drive solid-state reaction [1.1]. Oh-ishi et al. [1.1] applied a pressure of 6 GPa to Al and Cu half discs. Following this, x-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) were used to confirm the formation of different intermetallic phases such as Al2Cu and Al4Cu9. One of the first reported case where photons were used to drive solid phase diffusion was reported in 1998 by Ditchfield et al. [1.2]. The study was carried out to study the non-thermal effects of photons illumination on surface diffusion, an important process in microelectronics fabrication. Surface diffusion governs several important steps in microelectronics fabrication including the formation of hemispherical grained silicon used in memory devices [1.2], filling of channels with metals for devices interconnection purposes among others [1.2]. In this study, germanium-indium (Ge-In) on silicon was used because the thermal diffusion of this system was well understood [1.3]. Surface diffusion was measured in ultrahigh vacuum via second harmonic microscopy when the sample was illuminated with pulsed Nd: YAG laser at a wavelength of 1064 nm [1.3]. This study showed conclusively that photons could be used to drive solid-state reactions.

Ti: sapphire femtosecond laser ablation of dielectrics (fused silica and BK7 glass) and metals (Cu, Fe, Al) is presented. Results of laser -induced breakdown experiments in fused silica and BK7 glass employing 130 fs -1.7 ps, 790 nm laser pulses are reported. The fluence ablation threshold does not follow the scaling of ~ when pulses are shorter than 1 ps. Single-shot and multi-shot (130 fs pulse) ablation of selected materials are investigated with laser wavelengths of 395 nm, 790 nm, and 1300 nm. The ablation threshold is almost independent of the laser wavelength. The surface morphologies in metals after ultrashort pulse ablation are very different from dielectrics and semiconductors. The roughness of the ablated surface depends on the thermal properties of the metal target. The preliminary TEM result from Cu single crystal that was irradiated by single laser pulses shows few defects in the center region of the ablated crater. Single-shot ablation of single-crystal Fe induces much different surface features than on selected samples of poly-crystal Fe metal. / Thesis / Master of Engineering (ME)

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The SPECIFIC method a fast and accurate method for generating shaped femtosecond laser pulses. The femtosecond pulses are user specified from pulse parameters in the temporal domain. The measured spectral and recovered temporal phase and amplitudes from SEA TADPOLE are compared with the theoretical pulse profile from the user specified input. The SPECIFIC method has been shown to be a technique that can generate a diverse array of spectral/temporal phase and amplitude as well as polarization pulse shapes for numerous scientific applications. The spatio -temporal -spectral properties of focusing femtosecond laser pulses are studied for several pulse shapes that are important for non-linear spectroscopic studies. We have shown with scanning SEA TADPOLE that the spatio-spectral phase of focusing double pulse profile changes across the laterally across the beam profile. The spectral features of the sinusoidal spectral phase shaped pulse has been shown to tilt at with a changing angle away from the focus of the lens. Using spatio-spectral coupling, we have shown that multiple spatio-temporal foci can be generated along and perpendicular to the focusing direction of a femtosecond laser pulse. The spatial position of the spatio-temporal foci is controlled optically. Using sinusoidal spectral phase modulated pulse trains fragment ion production from Benzonitrile parent molecule can be controlled. A spectral transmission window perturbed the temporal pulse amplitudes resulting in fragment ion production dependant on spectral window position. The spectral window ion production was shown to also be dependant on temporal phase sequence. / Chemistry

Laser filamentation is a phenomenon currently being widely studied in which an ultrashort laser pulse self focuses as a result of the nonliner Kerr effect. Lim- ited data is available in terms of spatial characterization of the filament. We study the spatial distribution of molecular and atomic species generated by the filament in order to infer the relevant dynamics. We find evidence for a new impulsive vibrational excitation scheme which we introduce in this dissertation. Insight into the mechanisms of ionization is gained by consideration of the details of this process. In addition, the suitability of filaments to stimulate impulsive Raman scattering for spectroscopic purposes is evaluated. The data presented show the first measurements of ions by impulsive Raman spectroscopy. This method has advantages over previous techniques. Signal is directional and so it can be more completely collected and can be measured stand off. The energy required for detection is also provided by a probe beam rather than from the analyte molecules themselves and so there is no limit to its intensity as in the case of fluorescence spectroscopy. / Physics

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. / Thesis / Master of Applied Science (MASc)

This dissertation is on the development of a laser system for the generation of femtosecond frequency combs in the vacuum-ultraviolet (VUV) via intracavity high-harmonic generation (HHG). The HHG process yields coherent vacuum ultraviolet (VUV) light resulting from the ionization of noble gases driven by intense near-IR femtosecond frequency combs in an optical enhancement cavity. An injection locked amplification cavity (fsAC) was developed in order to generate a high power femtosecond frequency combs based on a Ti:Sapphire oscillator. Detailed amplifier performance was investigated in order to evaluate the coherence of the pulse amplification process. A passive power enhancement cavity for fs pulses (fsEC) was designed for intracavity high harmonic generation. For maximum power enhancement and conversion efficiency, the intracavity dispersion was compensated and various design layouts tested. A careful analysis of the phase matching conditions was performed, taking into account the effect of reabsorption of the generated high harmonic light, to compare different cavity geometries and determine which would produce the most efficient harmonic yield. Numerical simulations were also performed to determine the level of intra-cavity ionization that could be sustained before disrupting the pulse enhancement process. Based on the results of these simulations and calculations, it was determined that for a xenon gas target, a moderate peak intensity of the order of ~ 5×10¹³W/cm² produces harmonics most efficiently.