This project explores optimization of two-photon excited fluorescence (TPEF) enhancement between tunable narrowband and un-tuned broadband femtosecond (fs) laser pulse excitations for two-photon microscopy (TPM). The research is conducted preliminarily in time domain and comprehensively in frequency domain to understand the physics behind TPEF enhancement by un-tuned sub-10 fs nearly transform-limited pulse (TLP) versus tunable 140 fs pulse. The preliminary study on inverse proportionality of TPEF yield to fs-pulse duration delimits a general lower-bound to narrowband fs-pulse regime (pulse duration > 40 fs) with assumption of dye-molecule frequency invariant response. Deviations from this inverse proportionality in broadband fs-pulse regime (pulse duration < 40 fs) highlights dye-molecule frequency variant response, necessity of group delay dispersion (GDD) compensation, and broadband TLP for TPEF enhancement. The follow-up comparative study is made on un-tuned sub-10 fs TLP versus tunable 140 fs pulse excitations using three dye-phantoms (Indo-1, FITC, and TRITC) representative of fluorescent probes with similar TPEF characteristics. The integrated experimental system, with custom-designed GDD compensation, dispersion-less laser-beam expanding and focusing, and compound-lens for efficient fluorescence collection with good spectral resolution, ensures accurate TPEF measurements. Differentiated TPEF enhancements of Indo-1 (1.6), FITC (6.7), and TRITC (5.2) proportionally agree with calculated ones due to the overlap of fs-pulse second harmonic (SH) power spectrum with dye-molecule two-photon excitation (TPE) spectrum. Physically speaking, with broadband sub-10 fs TLP readily involved in both degenerate (v1 = v2) and non-degenerate (v1 ≠ v2) two-photon absorption (TPA), this un-tuned ultrashort fs-pulse excitation simultaneously allows for more accessibility to TPA-associated final states and diversely promotes population of thus excited dye-molecules with the three dye-phantoms. Under environmental influences (mutual quenching through one-photon absorption(s) and solvent effect), multicolor TPEF enhancement observed from a mixture of the three dyes shows promise of sub-10 fs TLP as simultaneous excitation for multiple-dye labeled samples in contrast to compromised excitation with narrowband fs-pulse tuning. Both single- and multicolor TPEF enhancements clarify tradeoff between tunability of narrowband fs-pulse and un-tuned broadband fs-pulse excitations, being instructive to further considerations on optimization of TPEF enhancement by strategic utilization of broadband fs-pulse for better performance of TPM.

Graphene is a single hexagonal atomic carbon layer. Since its discovery, graphene is emerging as an exciting and promising new material to impact various areas of fundamental research and technology. It has potentially useful electrical properties for device applications such as graphene photodetectors and graphene-based sensors. This thesis focuses on the femtosecond laser processing of graphene from both scientific and industrial points of view. Started from the manufacturing process, a new manufacturing route for graphene devices based on a femtosecond laser system is explored. In this thesis, the graphene ablation threshold was determined in the range of 100 mJ/cm2. In this deposited fluence range, selective removal of graphene was achieved using femtosecond laser processing with little damage to the SiO2 /Si substrate. This finding supports the feasibility of direct patterning of graphene for silicon-substrate field effect transistors (FETs) as the gate dielectric, silicon dioxide is only negligibly removed (2~10 nm) and no damage occurs to the silicon. Beyond the selective removal of graphene, the effects of exposing femtosecond laser pulses on a monolayer of graphene deposited on a SiO2/Si substrate is also studied under subthreshold irradiation conditions. It has been demonstrated that a femtosecond laser can induce defects on exposure. The dependence of the D, G, and 2D Raman spectrum lines on various laser pulse energies was evaluated using Raman Spectroscopy. The I (D)/I (G) ratio was seen to increase with increasing laser energy. The increase in the D’ (intravalley phonon and defect scattering) peak at 1620 cm-1 appeared as defective graphene. These findings provide an opportunity for tuning graphene properties locally by applying femtosecond laser pulses. Applications might include p-n junctions, and the graphene doping process. To explore the power absorption process in graphene and the SiO2/Si substrate, a theoretical model was developed based on the transfer-matrix method. The results revealed that the most significant absorption was in the silicon substrate. The light reflection form each layer was considered. The model shows the temperature oscillations are more significant in the silicon layer compared to the silicon dioxide which can provide a theoretical rationale for the swelling effect observed in the experiments. This model can assist in the choice of laser parameters chosen for future laser systems used in the production of graphene devices.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第15385号 / 工博第3264号 / 新制||工||1491(附属図書館) / 27863 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久 / 学位規則第4条第1項該当

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12459号 / 論工博第4041号 / 新制||工||1497(附属図書館) / 28069 / (主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久 / 学位規則第4条第2項該当

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16865号 / 工博第3586号 / 新制||工||1542(附属図書館) / 29540 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 平尾 一之, 教授 三浦 清貴, 教授 田中 勝久 / 学位規則第4条第1項該当

>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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