No description available.

A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a transition from thermal diffusion-dominated transport at 200-kHz repetition rate to strong heat accumulation at 0.5 to 2 MHz was observed to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200-mW average power and ~15-mm/s scan speed was discovered across the range of 200-kHz to 2-MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at ~1uJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. The waveguides were shown to be thermally stable up to 800°C, showing promise for high temperature applications. Using a low numerical aperture (0.4) lens, the effect of spherical aberration was reduced, enabling similar low-loss waveguides over an unprecedented 520-um depth range, opening the door for multi-level, three-dimensional, optical integrated circuits. In contrast to borosilicate glass, waveguides written in pure fused silica under similar conditions showed only little evidence of heat accumulation, yielding morphology similar to waveguides fabricated with low repetition rate (1 kHz) Ti-Sapphire lasers. Despite the absence of heat accumulation in fused silica owing to its large bandgap and high melting point, optimization of the laser wavelength, power, repetition rate, polarization, pulse duration and writing speed resulted in uniform, high-index contrast waveguide structures with low insertion loss. Optimum laser exposure recipes for waveguide formation in borosilicate and fused silica glass were applied to fabricate optical devices such as wavelength-sensitive and insensitive directional couplers for passive optical networks, buried and surface microfluidic and waveguide networks for lab-on-a-chip functionality, and narrowband grating waveguides for sensing.

The interaction of laser radiation with a material can excite coherent lattice vibration. The observation of such periodic motion of the atoms in the lattice provides information on the properties of the material. In the present work a femtosecond pump-probe technique was applied for studies of acoustic vibrations in nanoparticles and nanowires, and coherent optical phonons in thin films. The elastic properties of spherical Ag nanoparticles and Ag and Bi nanowires were studied in a dual-color femtosecond pump-probe experiment. The results of the period determinations of the acoustic vibrations, obtained from the time-domain measurements with low intensity pump pulses, together with the information about the size of the particles, were used to determine the elastic constants of the materials. Also changes in the measured acoustical response of the Ag nanowires under high intensity laser excitation were studied. In addition the coherent optical phonon excitation in a Bi film was studied in a femtosecond pump-probe experiment. A red-shift of the phonon frequency at the high photoexcitation density was observed. To separate the effect of the lattice softening and the lattice anharmonicity the excitation with two pump pulses was employed. Numerical simulations, which took into account the evolution of the spatial inhomogeneity of the excitation density, were carried out and compared to the experimental results.

A variable (0.2 to 5 MHz) repetition rate femtosecond laser was applied to delineate the role of thermal diffusion and heat accumulation effects in forming low-loss optical waveguides in borosilicate glass across a broad range of laser exposure conditions. For the first time, a transition from thermal diffusion-dominated transport at 200-kHz repetition rate to strong heat accumulation at 0.5 to 2 MHz was observed to drive significant variations in waveguide morphology, with rapidly increasing waveguide diameter that accurately followed a simple thermal diffusion model over all exposure variables tested. Amongst these strong thermal trends, a common exposure window of 200-mW average power and ~15-mm/s scan speed was discovered across the range of 200-kHz to 2-MHz repetition rates for minimizing insertion loss despite a 10-fold drop in laser pulse energy. Waveguide morphology and thermal modeling indicate that strong thermal diffusion effects at 200 kHz give way to a weak heat accumulation effect at ~1uJ pulse energy for generating low loss waveguides, while stronger heat accumulation effects above 1-MHz repetition rate offered overall superior guiding. The waveguides were shown to be thermally stable up to 800°C, showing promise for high temperature applications. Using a low numerical aperture (0.4) lens, the effect of spherical aberration was reduced, enabling similar low-loss waveguides over an unprecedented 520-um depth range, opening the door for multi-level, three-dimensional, optical integrated circuits. In contrast to borosilicate glass, waveguides written in pure fused silica under similar conditions showed only little evidence of heat accumulation, yielding morphology similar to waveguides fabricated with low repetition rate (1 kHz) Ti-Sapphire lasers. Despite the absence of heat accumulation in fused silica owing to its large bandgap and high melting point, optimization of the laser wavelength, power, repetition rate, polarization, pulse duration and writing speed resulted in uniform, high-index contrast waveguide structures with low insertion loss. Optimum laser exposure recipes for waveguide formation in borosilicate and fused silica glass were applied to fabricate optical devices such as wavelength-sensitive and insensitive directional couplers for passive optical networks, buried and surface microfluidic and waveguide networks for lab-on-a-chip functionality, and narrowband grating waveguides for sensing.

Lasing from femtosecond laser filaments is a relatively new field of study that has been studied since its first observation in 2003. Such lasing effect is of interest to the scientific community due to its possible application in remote sensing. This thesis studies the lasing dynamics of the excited molecular nitrogen ion N2+ which emits primarily at 391 nm and 428 nm wavelengths. We start by studying the ellipticity dependence of the gain from filaments in ambient air. We then study the ellipticity dependence in a vacuum in a supersonic gas jet to remove the complexity of filamentation. We show that recollision doesn't play a significant role in creating a population inversion by comparing the ellipticity dependence of the gain and high harmonic generation. The rest of this thesis is devoted to shining some light on another possible mechanism. We characterize the gain by its temporal profile, jet position dependence, and density dependence.

Microstructuring of silicon is performed to alter its optical and electrical properties for use in photonic devices. Femtosecond lasers are a favourable structuring tool because they are extremely precise due to the confinement of their interaction to the focal volume. Experiments were carried out on N-type, P-type, and intrinsic silicon with a femtosecond laser operating at 800 nm, with pulse duration of 40 fs, and 1 kHz repetition rate. A single pulse produced a micro-ring structure surrounding a crater. It is caused by the motion of material according to the pressure gradient induced by the Gaussian profile of the laser. Multiple-pulse structures were similar to the single pulse except for the central protrusion of material. Two factors are responsible for multiple-pulse structures: (1) geometrical difference of the plasma compared to the single pulse (2) reflections of shockwaves produce protruding structures. Polarization dependence of all structures was observed.

No description available.

Refractive surgery is designed to minimize the need for glasses and/or contact lenses and is often used for convenience, cosmetic or occupational purposes. The two most common types of laser eye surgery are LASIK and LASEK. During LASIK, the first step is the creation of the corneal flap using either a femtosecond laser or a mechanical microkeratome. The femtosecond laser has been seen to create more uniform flaps that reduce the risk of intraoperative and postoperative flap complications compared to the mechanical microkeratome. The purpose of this study was to investigate the effect on visual outcomes of second surgery LASEK on patients following aborted LASIK surgeries due to Femtosecond laser flap complications. LASIK was performed as planned and the corneal flap was created by the femtosecond laser but could not be lifted when a surgical cut was made. The procedure was aborted and LASEK was performed within a few weeks to attain the desired vision correction. A total of 14 patients were identified over a 6-year period ranging from 2009-2015. Most patients underwent surface ablation within two weeks of the initial aborted procedure. 12 of the 14 patients had a UCVA of 20/20, while the remaining 2 patients had a UCVA of 20/25 at their last postoperative visit. None of the patients required surgical enhancements despite the flap complications and no major postoperative complications were noted in any of the patients. Provided the corneal flap was well centered and there was no evidence of microstriae or epithelial ingrowth, surface ablation LASEK can be performed within a week of the aborted LASIK procedure to minimize discomfort and trauma to the patient. LASEK is associated with a slightly longer healing time but no evidence of corneal or retinal issues were noted in these patients. None of the patients experienced any significant changes in refraction between surgeries and was thus a reliable indicator of refractive stability following the flap complication. No significant differences were noted with delaying the second surgery LASEK as several patients attained 20/20 vision when it was performed within a week. The most common flap complication was the formation of an incomplete flap that could not be lifted. The cause could not be identified.

The photophysics of pigments is influenced, to an extent depending on its structure, by the properties of the environment. Proteins represent a very specific environment at least in two aspects: i) they are native to most of the pigments in living systems; ii) they facilitate modifications of pigment configuration, leading to changes not only in its spectroscopic properties, but also in its functional abilities. In studies presented in this thesis, femtosecond pump-probe spectroscopy was used to study predominantly the photosynthetic antenna complexes of bacteria and algae. Based on spectroscopic evidence, the structural modifications of pigments imposed by the protein were deduced or hypothesized, together with their functional relevance.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16403号 / 工博第3484号 / 新制||工||1526(附属図書館) / 29034 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 三浦 清貴, 教授 平尾 一之, 教授 田中 勝久 / 学位規則第4条第1項該当