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|Title:||Study of Ultrashort Pulse Laser Induced Surface Ripples and Investigation of Other Applications of Ultrashort Pulse Laser Micromachining and Ablation|
|Keywords:||Pulse Laser;Ultrashort;Surface Ripples;Laser Micromachining|
|Abstract:||<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>|
|Appears in Collections:||Digitized Open Access Dissertations and Theses|
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