京都大学 / 新制・課程博士 / 博士(理学) / 甲第23014号 / 理博第4691号 / 新制||理||1673(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 野上 大作, 教授 嶺重 慎, 教授 太田 耕司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Optoelectronics in the deep-ultraviolet (DUV) regime is still a growing research field that requires significant effort to understand the material properties and optimize the device structures to realize efficient DUV devices. Aluminum gallium nitride (AlGaN) is perhaps the most studied semiconductor to replace the environmentally hazardous mercury lamps; however, the external quantum efficiency of AlGaN based DUV devices is insufficient to replace the existing old-fashioned mercury UV lamps. Despite the tunability in the bandgap of AlGaN, the excessive strain accumulation associated with increased alloying of Al in AlGaN and the poor dopant activation due to the relatively large ionization energy of the donors and acceptors are not favorable for realizing efficient DUV emitters. In addition, the crossover among the light hole, heavy hole and split-off bands in the valance band for Al-rich AlGaN suppresses the transverse-electric polarization, which further worsens the external quantum efficiency. Furthermore, for DUV photodetection, commercially available Si-photodetectors suffer from poor responsivity for wavelengths shorter than 400 nm in contrast to the visible spectrum. Hence, the-state-of-art photodetectors in the DUV regime also need a significant upgrade, particularly for high-speed applications. Firstly, we utilized the high quantum confinement in plasma-assisted MBE grown ultrathin GaN QDisks to realize DUV (λ ≈ 260 nm) emission using a binary compound (GaN) in contrast to conventionally used ternary compound (AlGaN). More importantly, we experimentally demonstrated TE-dominant DUV emission, unlike Al-rich AlGaN, illustrating a unique pathway for realizing efficient DUV vertical emitters. Secondly, inspired by the light manipulation technique practiced in nature, we presented iridocytes on giant clams (Tridacna maxima), known for their symbiotic relationship with algae as a color downconverting material for DUV photodetection. Investigating the structural and optical properties of iridocytes found in Tridacna maxima, we established a robust UV communication allowing the data transfer rate of 100 Mbit/s within the forward error correction limit for modulated 375 nm-laser diode. Using a similar matrix implemented to 375 nm-laser, with high-power UV-C LED (λ ≈ 278 nm), we could establish an optical wireless communication that could allow a data-transmission rate of tens of Mbit/s within the forward error correction limit.

Les novæ naines permettent, depuis presque 50 ans maintenant, de tester les modèles d’accrétion. Ces systèmes montrent des éruptions en optique d’une durée de l’ordre de la semaine avec des temps de récurrence de l’ordre du mois. Ces éruptions sont communément attribuées à une instabilité thermo-visqueuse au sein du disque d’accrétion entourant la naine blanche. Les temps caractéristiques de ces éruptions posent de fortes contraintes sur les mécanismes de transport de moment cinétique pilotant l’accrétion dans le disque, mécanismes qui constituent l’objet de cette thèse. Il est souvent admis que l’instabilité magnéto-rotationnelle (MRI) est responsable du transport de moment cinétique via la turbulence qu’elle produit. Cependant, je montre ici, à l’aide de simulations locales de disque d’accrétion avec transfert radiatif, que le transport turbulent produit par la MRI ne permet pas de reproduire les courbes de lumière. En quiescence, où le disque est peu ionisé, il est même peu probable que de la turbulence MRI puisse survivre. Un des résultats majeurs de cette thèse est d’avoir mis en lumière que la MRI ne participe pas qu’au transport turbulent mais peut également lancer des vents magnéto-hydrodynamiques (MHD) qui transportent également du moment cinétique, voire dominent le transport dans l’état quiescent. Ces vents MHD induisent un couple magnétique de surface sur le disque et ne peuvent être réduits à une turbulence effective, en partie car ceux-ci ne déposent pas d’énergie thermique localement mais en emportent contrairement au transport turbulent. Nous avons inclus le transport de moment cinétique dû au couple du vent MHD dans un modèle d’instabilité de disque, modèle classiquement utilisé pour reproduire les éruptions de novæ naines. Avec ce nouveau modèle, nous avons montré qu’il est possible de reproduire les courbes de lumière des éruptions de novæ naines, en utilisant un champ magnétique à la surface de la naine blanche compatible avec ce qui est attendu. C’est la première fois que les éruptions de novæ naines sont modélisées avec succès en utilisant des prescriptions pour le transport de moment cinétique basées sur des simulations MHD et non sur les observations. / Dwarf novæ have been used for almost 50 years now as a test for the theory of accretion. These systems exhibit eruptions in optical light lasting approximately a week with a recurrence time scale of a month. Eruptions are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to put constraints on the mechanisms transporting angular momentum in the disk, which will be the subject of this thesis. Traditionally, transport is presumed to be turbulent where turbulence is due to the magneto-rotational instability (MRI). However, I show here, using local simulations of accretion disks with radiative transfer that there exists a discrepancy between observations and light curves obtained with MRI turbulence only. In quiescence, where the disk is poorly ionised, it is very unlikely that MRI can even survive. One of the key results of this thesis is that MRI do not participate to turbulent angular momentum transport only, but is also able to drive MHD outflows which extract angular momentum very efficiently, especially in quiescence. Wind-driven transport is, by nature, very different from turbulent transport, it induces a surface-torque on the disk and do not deposit thermal energy locally but extract energy from the disk instead. We included MHD wind-driven angular momentum transport in a disk instability model, model which is usually used to reproduce light curves of dwarf novæ. Using this new model, we were able to retrieve light curves looking alike observations, with a magnetic field consistent with what is expected from the dipolar magnetic field of a white dwarf. It is the first time that eruptions of dwarf novæ are modeled with success using prescriptions for angular momentum transport derived from first principles instead of ad hoc parameters.

HD 106906 is a young, binary stellar system, located at ~103.3 parsecs away in the Lower Centaurus Crux (LCC) group. This system is completely unique among known systems in that it contains an asymmetrical debris disk, as well as an 11 M(Jup) planet companion, at a separation of ~735 AU. Only 4 other systems are known to contain both a disk and detected planet, where HD 106906 is the only one in which the planet has apparently been ejected. Furthermore, the debris disk is nearly edge on, and extends roughly from 70 AU to >500 AU, where previous polarimetric studies with HST have shown the outer regions to have high asymmetry. The presence of a planet companion sparks questions about the origin of this asymmetry. To better understand the structure and composition of the disk, deeper data have been taken with the Gemini Planet Imager (GPI), which we have used to perform a deep polarimetric study of HD 106906’s asymmetrical debris disk. The data were taken in the H-band, and were supplemented with both J- and K1-band polarimetric data which have been obtained through one of GPI’s Large and Long Programs (LLP). Polarimetry is important in the study of debris disks in scattered light, as it helps us constrain their dust grain characteristics, as well as allowing us to obtain high-contrast images. Modelling the disk, along with an empirical analysis of our data, supports a disk that is asymmetrical in surface brightness and structure, as well as a disk that is highly eccentric. These results will be discussed in terms of possible sources of asymmetry, such as dynamical interaction with the planet companion HD 106906b. / Graduate / 2021-07-26

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第12813号 / 論理博第1539号 / 新制||理||1577(附属図書館) / 31300 / (主査)教授 嶺重 慎, 准教授 前田 啓一, 教授 長田 哲也 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20182号 / 理博第4267号 / 新制||理||1613(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 嶺重 慎, 准教授 前田 啓一, 教授 長田 哲也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20912号 / 理博第4364号 / 新制||理||1626(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 井岡 邦仁, 教授 川合 光, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20919号 / 理博第4371号 / 新制||理||1627(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 嶺重 慎, 准教授 前田 啓一, 教授 太田 耕司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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