<p> This thesis reports findings from three series of experiments related to ultrashort laser pulse interactions with materials. The first series investigates the formation of laser induced ripples that have spatial periods much shorter than the irradiation wavelength after laser irradiation. The second series of experiments explores the capabilities of ultrashort pulse laser micromachining on optical fiber modifications for niche applications. Lastly, preliminary work in establishing a double-pulse ablation technique is reported. </p> <p> The first set of experiments reported in this thesis investigates the morphology of surface ripples that are generated when irradiated with multiple ultrashort laser pulses. Two types of surface ripples can form after irradiation. The fust type has spatial periods near the wavelength of the irradiation pulses and the second has spatial periods substantially below the irradiation wavelength (typically 114 to 115 of the free-space irradiation wavelength are observed in our studies). These substantially subwavelength ripples form when the irradiation wavelength corresponds to a photon energy that is below the bandgap of the target material. The Ti:Sapphire laser systems used in this series of experiments provides pulses centered around 800 nm. Gallium phosphide (GaP) was chosen to be the main material for investigation since 800 nm corresponds to a photon energy that is below the bandgap of this material; no frequency conversion needs to be carried out when GaP is the material of choice for subwavelength ripples studies. In this series of experiments substantially different irradiation conditions were investigated: pulse durations varied from 150 fs to 7 ns, laser energies ranges from well above the ablation and modification threshold to well below, both 800 nm and 400 nm wavelengths, and "scrambled" (where polarization was rotated between each successive pulse) polarization as well as circular polarization were used. Microscopy techniques employed to study these ripples include optical microscopy, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Cross-sectional studies with transmission electron microscopy were also carried out by using focused ion beam milling to prepare thin specimens across irradiated regions. Sapphire was also used as the irradiation target for 800 nm and 400 nm pulses since it has a large bandgap and even 400 nm corresponds to an energy that is below its bandgap. Irradiation conditions where the two types of ripples are observed are determined. Also, microscopy of the ripple features provided insights in to the formation mechanism of the subwavelength ripples. </p> <p> In the second series of experiments, preliminary work was performed to investigate the capabilities of ultrashort laser micromachining in fiber optic applications. This series of experiments can be subdivided in to two categories. </p> <p> The goal of the first fiber investigation was to create a slit in a metallic coating deposited on a fiber facet. Such a feature might eliminate the use of external slits (e.g. for spectrometers), especially ifthe output of the fiber depends on its geometry (e.g. polarization-maintaining fiber). The first experiment carried out was micromachining of a ~ 180 nm layer of gold that was deposited on a glass substrate, in order to determine irradiation conditions where the gold layer can be removed while the glass is not damaged. Once the irradiation condition was established by studying the micromachined gold layer on glass substrate, gold layers were deposited on fiber facets for micromachining experiments. The results showed promising potential, but fme tuning of the irradiation parameters, and processing as well as microscopy techniques are needed before useful applications can be realized. </p> <p> The second set of fiber experiments investigates irradiation conditions that are appropriate to micromachine features into fibers such as v-grooves and beveled ends. Preliminary work was carried out to determine a suitable focusing scheme for this application. Different pulse durations and a pulse train were also employed in hope of minimize chipping and cracking. This investigation did not reach a conclusion on whether micromachining with ultrashort laser pulses are in fact suitable for processing of optical fibers, where high quality facets are required. Future investigation could provide further information on the feasibility of laser micromachining on fabricating features in optical fibers. </p> <p> Lastly, a double-pulse ablation scheme was established and explored. Double-pulse ablation had been reported in the literature to improve material removal rate and the appearance of the fmal morphology. However, this setup can be adapted to investigate the ablation mechanisms and provide insight into the state of the material at different time frames of ablation. While the experimental results are preliminary, this technique showed potential, along with possible extensions of this technique, to further investigate the ablation mechanisms. </p> / Thesis / Master of Applied Science (MASc)

This PhD thesis reports an investigation into medical coronary stent cutting using three different types of lasers and associated physical phenomena. This study is motivated by a gap in the current knowledge in stent cutting identified in an extensive literature review. Although lasers are widely used for stent cutting, in general the laser technology employed is still traditionally based on millisecond pulsed Nd:YAG lasers. Although recent studies have demonstrated the use of fibre lasers, picosecond and femtosecond lasers for stent cutting, it has been preliminary studies.To further understand the role of new types of lasers such as pulsed fibre lasers, picosecond and femtosecond pulsed lasers in stent cutting, these three lasers based stent cutting were investigated in this project. The first investigation was on a new cutting method using water assisted pulsed (millisecond) fibre laser cutting of stainless steel 316L tubes to explore the advantages of the presence of water compared to the dry cutting condition. Significant improvements were observed with the presence of water; narrower kerf width, lower surface roughness, less dross attachment, absence of backwall damage and smaller heat affected zone (HAZ). This technique is now fully commercialised by Swisstec, an industrial project partner that manufactures stent cutting machines.The second investigation used the picosecond laser (with 6 ps pulse duration in the UV wavelength range) for cutting nickel titanium alloy (nitinol) and platinum iridium alloy. The main achievement in this study was obtaining dross-free cut as well as clean backwall, which may eliminate the need for extensive post-processing. Picosecond laser cutting of stents is investigated and reported for the first time. The third area of investigation was on the use of a femtosecond laser at 100 fs pulse duration for cutting nickel titanium alloy tubes. It was found that dry cutting degraded the cut quality due to debris and recast formation. For improvement, a water assisted cutting technique was undertaken, for the first time, by submerging the workpiece in a thin layer of water for comparison with the dry cutting condition. The final part of the thesis presents a three dimensional numerical model of the laser micromachining process using smoothed particle hydrodynamics (SPH). The model was used to provide better understanding of the laser beam and material interaction (with static beam) including the penetration depth achieved, phase changes, melt ejection velocity, also recast and spatter formation. Importantly, the model also simulated the wet machining condition by understanding the role of water removing the melt ejected during the process which avoided backwall damages. Results with the fibre laser in millisecond pulse duration were used for the validation purposes. The conclusions reached in this project and recommendations for future work are enclosed.The work has resulted in the publication of 3 journal papers and 2 additional journal paper submissions.

<p>This thesis presents investigations on ultrafast laser irradiation of silicon towards the goal of hybridizing ultrafast laser processing and conventional semiconductor fabrication techniques to improve device applications. The fundamental sub-threshold damage accumulation mechanisms for potential defect engineering applications were studied through the use of positron annihilation spectroscopy, in situ sample heating during laser irradiation, varying the laser repetition rate, and samples implanted with various ion species at different conditions. Positron annihilation spectroscopy results suggest an increase in the divacancy density at the surface region of silicon following near- and slightly sub-threshold ultrafast laser irradiations. Laser irradiations at increasing sample temperature up to 600°C show a general decreasing trend of single-shot thresholds, and an increase in the suppression of sub-threshold damage accumulation. There is also a temperature dependence on the surface morphology resulting from ultrafast laser irradiation. Ion implantation modified the ablation threshold fluence, and a dependence on the ion implantation conditions was observed. Surface microstructuring of silicon was shown to improve absorption of light with a sub-bandgap wavelength of 1550 nm. An initial attempt with sulfur implantation did not exhibit further improvement in the optical absorption, and first attempts in device fabrication did not provide photoresponsivity at sub-bandgap wavelengths. Ultrafast laser irradiation of SiO₂-on-Si structures yielded different modification thresholds for different thicknesses of the oxide layer. Surface morphologies obtained in the irradiation of these structures can affect potential applications. Selected studies of ultrafast laser irradiation of GaP, metal-SiO₂-Si structures, quartz, diamond, and porcine bone demonstrated similarities in ablation behavior and morphologies, and the potential for a broad range of applications. The results in combination with the proposed future work in this thesis can contribute to potential device applications while providing valuable insights into the ultrafast laser ablation mechanisms.</p> / Doctor of Philosophy (PhD)

<p> Lithium niobate is an important photonic material that has potential applications in MEMS. Unfortunately, it is difficult to process using conventional methods. This thesis is an exploratory study to determine the viability of using a femtosecond laser as a fabrication tool for lithium niobate. Unexpectedly, a rich range of behaviour, likely arising from the complex material structure and composition, was discovered. Depending on the processing conditions, it was demonstrated that machining can either result in deep, high-aspect ratio grooves with minimal surrounding damage or dramatic modification of the lithium niobate to great depths with very little material removal.</p> <p> When machining grooves, increasing the effective number of pulses Neff (i.e. decreasing cutting speed) gave rapidly increasing ablation depths until a threshold was reached, after which the grooves were nearly filled with amorphous material. The depth of these amorphous channels rapidly saturates and becomes nearly independent of Neff. The ablation depth dependence on fluence showed gentle and strong ablation regimes. The amorphous channel depth depended almost linearly on fluence. Subsequent laser passes over amorphous channels eventually removed the amorphous material from the groove, indicating a dependence on the time between laser pulses. Crystal orientation was not a factor.</p> <p> The results are understood in terms of incubation and wave guiding. The first pulses ablate some material and incubate a channel of material below the surface. With further pulses, increasing incubation accelerates ablation. At the threshold Neff, the absorption coefficient has increased enough that the next pulse is able to melt a significant amount of material, which expands to fill the groove. It is suggested that, initially, the amorphous material is able to guide subsequent pulses to the bottom of the channel, resulting in a very slowly increasing depth with Neff. Subsequent passes cause ablation once again since compositional changes in the amorphous material have relaxed. Irradiated samples appear thermally reduced, which would create colour centres leading to increased absorption and thus incubation.</p> <p> Femtosecond lasers are indeed able to create MEMS structures. Multiple passes in the ablation regime yielded deep grooves, with laser polarization perpendicular to the groove giving the best results. Fabrication of micro-cantilevers and bridges was demonstrated.<p> / Thesis / Master of Applied Science (MASc)

This dissertation presents the exploration of two fiber optics techniques involving high power pulse laser delivery. The first research topic is "Embedded Active Fiber Optic Sensing Network for Structural Health Monitoring in Harsh Environments", which uses the fiber delivered pulse laser for acoustic generation. The second research topic is "Fiber Optics Guided Laser Micromachining", which uses the fiber delivered pulse laser for material ablation. The objective of the first research topic is to develop a first-of-a-kind technology for remote fiber optic generation and detection of acoustic waves for structural health monitoring in harsh environments. Three different acoustic generation mechanisms were studied in detail, including laser induced plasma breakdown (LIB), Erbium-doped fiber laser absorption, and metal laser absorption. By comparing the performance of the acoustic generation units built based on these three mechanisms, the metal laser absorption method was selected to build a complete fiber optic structure health monitoring (FO-SHM) system. Based on the simulation results of elastic wave propagation and fiber Bragg grating acoustic pulse detection, an FO-SHM sensing system was designed and built. This system was first tested on an aluminum piece in the room temperature range and successfully demonstrated its capability of multi-parameter monitoring and multi-point sensing. With additional studies, the upgraded FO-SHM element was successfully demonstrated at high temperatures up to 600oC on P-91 high temperature steels. During the studies of high power pulse laser delivery, it was discovered that with proper laser-to-fiber coupling, the output laser from a multimode fiber can directly ablate materials around the fiber tip. Therefore, it is possible to use a fiber-guided laser beam instead of free space laser beams for micromachining, and this solves the aspect ratio limitation rooted in a traditional laser beam micromachining method. In this dissertation, this Guided Laser MicroMachining (GLMM) concept was developed and experimentally demonstrated by applying it to high aspect ratio micro-drilling. It was achieved that an aspect ratio of 40 on aluminum and an aspect ratio of 100 on PET, with a hole diameter less than 200 um. / PHD / This dissertation presents two research topics both related to high power laser and fiber optic. The first topic studies the application of using optical fiber and high power laser for ultrasonic non-destructive evaluation. The general idea is to use fiber optic to remotely generate and monitor ultrasonic waves on a workpiece. Due to the fact that there are no electronic components involved in the sensing part of the system, this system can work at high temperature and is unsusceptible to EMI. The second topic studies the usage of optical fiber in high aspect ratio micromachining. The key concept is to use a fiber tip and the output high power laser as a "drilling tip", which eliminate the aspect ratio limitation rooted in the traditional free-space laser micromachining method. With this concept and a demonstrative micromachining system, we achieved record-breaking aspect ratio on both aluminum and plastic.

Recent advances in microelectronics and packaging industry are characterized by a progressive miniaturization in response to a general trend toward higher integration and package density. Corresponding to this are the challenges to traditional manufacturing processes. Some of these challenges can be satisfied by laser micromachining, because of its inherent advantages. In laser micromachining, there is no tool wear, the heat affected zone can be localized into a very small area, and the laser micromachining systems can be operated at a very wide range of speeds. Some applications of laser micromachining include pulsed Nd:YAG laser spot welding for the photonic devices and laser microdrilling in the computer printed circuit board market. Although laser micromachining has become widely used in microelectronics and packaging industry, it still produces results having a variability in properties and quality due to very complex phenomena involved in the process, including, but not limited to, heat transfer, fluid flow, plasma effects, and metallurgical problems. Therefore, in order to utilize the advantages of laser micromachining and to achieve anticipated results, it is necessary to develop a thorough understanding of the involved physical processes, especially those relating to microelectronics and packaging applications. The objective of this Dissertation was to study laser micromachining processes, especially laser drilling and welding of metals or their alloys, for the microscale applications. The investigations performed in this Dissertation were based on analytical, computational, and experimental solutions (ACES) methodology. More specifically, the studies were focused on development of a consistent set of equations representing interaction of the laser beam with materials of interest in this Dissertation, solution of these equations by finite difference method (FDM) and finite element method (FEM), experimental demonstration of laser micromachining, and correlation of the results. The contributions of this Dissertation include: 1)development of a finite difference method (FDM) program with color graphic interface, which has the capability of adjusting the laser power distributions, coefficient of energy absorption, and nonlinear material properties of the workpiece as functions of temperature, and can be extended to calculate the fluid dynamic phenomena and the profiles of laser micromachined workpieces, 2)detailed investigations of the effect of laser operating parameters on the results of the profiles and dimensions of the laser microdrilled or microwelded workpiece, which provide the guideline and advance currently existing laser micromachining processes, 3)use, for the first time, of a novel optoelectronic holography (OEH) system, which provides non-contact full-field deformation measurements with sub-micrometer accuracy, for quantitative characterization of thermal deformations of the laser micromachined parts, 4)experimental evaluations of strength of laser microwelds as the function of laser power levels and number of microwelds, which showed the lower values than the strength of the base material due to the increase of hardness at the heat affected zone (HAZ) of the microwelds, 5)measurements of temperature profiles during laser microwelding, which showed good correlations with computational results, 6)detailed considerations of absorption of laser beam energy, effect of thermal and aerodynamic conditions due to shielding gas, and the formation of plasma and its effect on laser micromachining processes. The investigations presented in this Dissertation show viability of the laser micromachining processes, account for the considerations required for a better understanding of laser micromachining processes, and provide guideline which can help explaining and advancing the currently existing laser micromachining processes. Results of this Dissertation will facilitate improvements and optimizations of the state-of-the-art laser micromachining techniques and enable the emerging technologies related to the multi-disciplinary field of microelectronics and packaging for the future.

The three-dimensional (3-D) writing capability of a high repetition rate (1 MHz) fiber-amplified femtosecond laser with a wavelength of 522 nm was harnessed together with wet-chemical etching for laser-patterning of 3-D optofluidic microsystems in fused silica glass, by the method of Femtosecond Laser Irradiation followed by Chemical Etching (FLICE). Selective chemical etching of laser irradiated glass with dilute hydrofluoric acid (HF) enabled micro-fabrication of high aspect-ratio embedded micro-channels and fine-period 3-D glass meshes in a 3-D inverted woodpile (IWP) arrangement that permitted high density lab-on-a-chip (LOC) integration of flow channels, reservoirs, glass chromatography columns, and optical circuit devices. Optical waveguides, reservoirs, micro-channels, and IWP structures were first laser patterned and followed by selective wet etching controlled by the polarization orientation of the writing laser. With the laser polarization perpendicular to the scanning direction, the volume nanogratings were aligned perpendicular to glass surfaces to facilitate HF etching and thus created designer shaped micro-channels with the smoothest sidewall surfaces measured at present and terminated with open reservoirs. An array of vertical access holes spaced periodically apart facilitated etching of continuous and highly uniform buried channels of unrestricted length in the glass to interconnect flow channels and reservoirs. Alternatively, laser polarization parallel to the scan direction provided low-loss optical waveguides with nanograting walls resisting the acid etching, providing a convenient one-step laser scanning process of optofluidic microsystems prior to wet etching. For the first time, dual-channel capillary electrophoresis was demonstrated by simultaneous fluorescent detection of separating dyes in a 3-D microsystem having over- and under-passing crossed channels in fused silica. In addition, an on-chip particle counting device based on capillary force to drive analytes through an embedded micro-channel into a calibrated reservoir for particle counting was designed and demonstrated. Further, a new type of glass mesh structure is presented where a 3-D IWP micro-channel array with diamond-like symmetry was integrated inside a micro-channel for capillary electrophoretic chromatography. The FLICE technique thus enables rapid prototyping of fully integrated 3-D optofluidic systems in bulk fused silica glasses for numerous applications, and these provide the groundwork and open new 3-D design approaches for advanced microsystems in the future.

The three-dimensional (3-D) writing capability of a high repetition rate (1 MHz) fiber-amplified femtosecond laser with a wavelength of 522 nm was harnessed together with wet-chemical etching for laser-patterning of 3-D optofluidic microsystems in fused silica glass, by the method of Femtosecond Laser Irradiation followed by Chemical Etching (FLICE). Selective chemical etching of laser irradiated glass with dilute hydrofluoric acid (HF) enabled micro-fabrication of high aspect-ratio embedded micro-channels and fine-period 3-D glass meshes in a 3-D inverted woodpile (IWP) arrangement that permitted high density lab-on-a-chip (LOC) integration of flow channels, reservoirs, glass chromatography columns, and optical circuit devices. Optical waveguides, reservoirs, micro-channels, and IWP structures were first laser patterned and followed by selective wet etching controlled by the polarization orientation of the writing laser. With the laser polarization perpendicular to the scanning direction, the volume nanogratings were aligned perpendicular to glass surfaces to facilitate HF etching and thus created designer shaped micro-channels with the smoothest sidewall surfaces measured at present and terminated with open reservoirs. An array of vertical access holes spaced periodically apart facilitated etching of continuous and highly uniform buried channels of unrestricted length in the glass to interconnect flow channels and reservoirs. Alternatively, laser polarization parallel to the scan direction provided low-loss optical waveguides with nanograting walls resisting the acid etching, providing a convenient one-step laser scanning process of optofluidic microsystems prior to wet etching. For the first time, dual-channel capillary electrophoresis was demonstrated by simultaneous fluorescent detection of separating dyes in a 3-D microsystem having over- and under-passing crossed channels in fused silica. In addition, an on-chip particle counting device based on capillary force to drive analytes through an embedded micro-channel into a calibrated reservoir for particle counting was designed and demonstrated. Further, a new type of glass mesh structure is presented where a 3-D IWP micro-channel array with diamond-like symmetry was integrated inside a micro-channel for capillary electrophoretic chromatography. The FLICE technique thus enables rapid prototyping of fully integrated 3-D optofluidic systems in bulk fused silica glasses for numerous applications, and these provide the groundwork and open new 3-D design approaches for advanced microsystems in the future.

This report covers the investigation of the next generation of sensors to be used in the sensor based tube system known as SentusysTM. One essential feature of the next generation of sensors is high-temperature endurance. The sensors (strain gauges and thermocouples) have been produced by means of thermal spraying techniques, short-pulse laser ablation and laser cladding. It has been found that the sensors seem to work in general, however, much research and development remain.

Microfluidics is a field of research that enables the manipulation of fluids in the submillimetre length scale. The technology allows the development of lab-on-a-chip devices, which are miniaturized systems for chemical and biological analysis. Currently, the conventional manufacturing methods for these systems require multiple time-consuming steps. Therefore, focus has shifted towards laser micromachining as an alternative method. Direct laser writing would circumvent many of the steps required for the conventional methods, drastically reducing the process time. In this Master thesis project, it was shown that microfluidic chips can be manufactured using a Nd:YVO4 (532 nm) nanosecond laser system. The process was optimized for silicon and borosilicate glass substrates. Acoustic focusing of polystyrene beads was demonstrated for a system etched in silicon. The optimized process used a power of 50%, a frequency of 10 kHz, a scan speed of 60 mm/s with triple lines as fill type and it had an etch rate of 4.3 μm/pass. Processed wafers were cleaned in buffered HF and bonded using anodic bonding as well as adhesive bonding. Processing of glass proved unpredictable, resulting in cracks and chippings. However, in- and outlets were successfully etched through thin glass wafers. It was found that crucial factors for the process were to control the focus, positioning of structures, structure orientation and the pulse separation for a uniform distribution of pulses. Based on the results, it is estimated that the manufacturing process could be done in two to three days using the laser micromachining process.