Title: Radiation Damage and Radiation-Based Device-Fabrication Techniques in LiNbO3
Description: The study of radiation effects in materials and devices has been of much interest for a broad variety of scientific reasons and applied purposes. Examples are the investigation of the robustness of devices under radiation environments, interstellar space, or the use of irradiation for device-fabrication methods such as ion implantation. Thus, a fundamental understanding of the irradiation-induced change in materials properties is crucial of applied science. In this thesis, investigation of radiation damage in complex oxides, especially LiNbO3 and its related device, was carried out. Different radiation sources were examined, including light and heavy ion, electron and gamma rays. A set of instruments were employed to probe induced damage from different perspective. These instruments include optical methods (optical microscopy and micro-Raman spectroscopy), ion beam analysis (Rutherford backscattering spectroscopy), and direct visualization (SEM, TEM and AFM). It was found that the degree of ion-matter interaction (stopping power) plays a major role in initiating damage.

Different forms of damage, including surface deformation, material amorphization, defect clusters, and long-range strain field, were observed. Probing the response of oxide crystals with these tools provides a comprehensive basis for better comprehension of radiation damage. The detailed studies and their findings are described in more details in Chapters 4, 5, and 7. In addition to the materials characterization, the impact of radiation effects on one specific active device (thin-film electro-optic modulator) was also studied. It was found that radiation dose and distribution both affect device performance. More details are in Chapter 6. Our knowledge of these radiation effects allow us to utilize and engineer this damage for the development of advanced radiation-enabled device fabrication techniques. Both light- and heavy-ion irradiation were studied and it was found that the nature of their different damage mechanisms lead to essentially a different response to wet chemical etching.

Using radiation-enabled selective etching allowed use to explore new or improved techniques for fabricating sub-um-thick LiNbO3 thin film and high-resolution patterning. Co-irradiation of both ion species was also investigated and this method shows easy fabrication of patterning on freestanding thin film. The characteristics of these methods make them useful for the fabrication of the current and future photonic integrated platforms. More details are in Chapters 3 and 8. In addition to radiation damage study, other materials-based fabrication methods were investigated - particularly those that were a consequence of our materials fabrication studies. For example, domain engineering of ferroelectric materials (poling) for nonlinear-optic applications was investigated and it was found that domain broadening can be sufficiently suppressed using a thinned sample. More details are in Chapters 3.