Browsing by Subject "Pockels"

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Optical and electro-optical phenomena in transition metal oxide thin film heterostructures
(2019-05-13) Ortmann, John Elliott, Jr.; Demkov, Alexander A.; Lai, Keji; Tsoi, Maxim; de Lozanne, Alejandro; Ekerdt, John G
Beginning in the mid-20th -Century and continuing to the present day, integrated circuit technology has advanced at a remarkable pace. Dedicated materials research has been at the heart of this advancement, with materials development preceding technological advancement at nearly every stage. As humanity barrels onward into the 21st -Century and our data and computational demands grow ever larger, new computing hardware designed to handle increasingly difficult computational challenges is quickly becoming necessary in order to continue the historical breakneck pace of advancement. Just as in the early days of the integrated circuit, materials advancement will likely be the key to developing the next generation of computing hardware. In this thesis, I investigate two materials systems well-suited for implementation in next-generation optical computing technologies: transition metal oxide quantum wells and Pockels-active BaTiO₃ thin film heterostructures. Both materials systems are promising for use in a wide variety of optical and electro-optical devices central to integrated photonic technologies, including quantum cascade lasers, photodetectors, electro-optic modulators and switches. For the case of transition metal oxide quantum wells, I focus on the famous SrTiO₃/LaAlO₃ materials system. I first investigate the structural and optical properties of arbitrarily thick, high-quality SrTiO₃/LaAlO₃ heterostructures grown on oxide substrates. Then, I demonstrate the monolithic integration of these heterostructures on silicon, bringing them one step closer to technological relevance. Finally, I present detailed simulations of the optical and electro-optical performance of integrated photonic devices based on SrTiO₃/LaAlO₃ heterostructures. In bulk form, the transition metal oxide BaTiO₃ has some of the largest known Pockels coefficients. However, early work suggests the coefficients are reduced by roughly a factor of ten when fabricated as a thin film. Here, I demonstrate the first BaTiO₃-based integrated devices showing bulk-like Pockels coefficients. Then, I iterate on the initial design of the devices in order to optimize them for ultra-low-power refractive index tuning. The resulting devices achieve refractive index tuning with power consumption many orders of magnitude less than previous reports. Taken together, the investigations in this thesis will hopefully open the door for the development of new kinds of optical and electrooptical devices for use in integrated photonics technologies
Theory of the electro-optic response in titanate perovskites
(2021-12-03) Paoletta, Therese Eileen; Demkov, Alexander A.; Macdonald, Allan H; Chelikowsky, James R; Ekerdt, John G; Bajaj, Chandrajit
Optical modulators are a key component in the booming industry of silicon photonics. As innovators strive to create devices that can overcome the shortcomings of silicon while still being compatible with preexisting technology, certain materials and mechanisms have come into focus. The linear electro-optic, or Pockels, effect measures how much an applied electric field changes the refractive index of light, and thus can modulate signals in a photonic integrated circuit. It is one of the fastest options for switches, has a broad bandwidth, and requires relatively little power to operate. This work focuses on two ABO₃ perovskites, barium titanate (BTO) and strontium titanate (STO), while others are already in practical use as Pockels devices, such as lithium niobate with electro-optic coefficient of approximately 30 pm/v. BTO has an exceptionally strong response (~1300 pm/V), and thus is an obvious subject of inquiry. Additionally, it has already been successfully integrated onto silicon photonic platforms to fabricate optical modulators. We delve into the origins of its strong response by analyzing two phases, the low-temperature rhombohedral and room-temperature tetragonal. In the rhombohedral study, we find what modes dominate the Pockels response and use molecular orbital theory to explain why those modes’ ionic displacements trigger such a response. The tetragonal phase boasts the most impressive response yet poses more of a challenge to calculate due to the lattice instability. We develop a workaround method that we suggest for materials that appear unstable in 0 Kelvin DFT simulations. Through this study, we find being near the morphotropic boundary, where a phonon is going soft, to be responsible for this phase’s singular Pockels response. In another study, we pair experiment with theory to fully characterize a strain-engineered phase of BTO. We explore how strain affects the crystal structure and thus Pockels response. Finally, we study the Raman spectrum of bulk and ultra-thin membranes of STO to investigate the effect membrane thickness has on its phonons. Throughout this work, phonons play a central role in the theoretical underpinnings of the response. We largely interpret our results in terms of the ionic behavior and find that understanding its importance is crucial to enhancing the Pockels response.