Graduate Thesis Or Dissertation
 

Ultra-energy-efficient Silicon Photonic Modulators Driven by Transparent Conductive Oxides

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0r967b10s

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  • Silicon photonics has become the most promising platform for future large-scale optical interconnect and optical computing systems due to its inherent CMOS compatibility, which brings exclusive advantages in bandwidth density, energy efficiency, and cost effectiveness. Parallel optical interconnects based on photonic integrated circuits (PICs) have the capacity to meet the high bandwidth density requirement of parallel computing systems, however, are facing the same challenge in energy efficiency and bandwidth limit as their electrical counterparts because the margin shrinks unfavorably for shorter distance optical interconnects. Unprecedented requirement in energy efficiency has been outlined, which poses tremendous challenges to existing PIC devices, even to the state-of-the-arts silicon photonics. In recent years, transparent conductive oxides (TCOs) have emerged as increasingly favorable tunable materials for active photonic devices. TCOs exhibit a large refractive index tunability on the order of unity, which enables unique epsilon-near-zero (ENZ) light confinement and significant enhancement in light-matter interaction. These intriguing optical properties offer us the potential to expand the functionality and improve the device performance of the silicon photonics platform. This dissertation presents design and demonstration of novel active photonic devices driven by TCOs on silicon photonics platform, with a focus on achieving ultra-energy-efficient silicon photonic modulator. Three types of photonic devices are investigated. Firstly, an electrically tunable plasmonic subwavelength grating based on a metallic subwavelength slit array coupled with a Si/SiO2/ITO MOS capacitor is designed and demonstrated. We show that large modulation depth can be achieved for both transmission and reflection modes through modifying the electron concentration within 0.5 nm thick TCO accumulation layer. In the second part, we develop a novel device platform of TCO-gated silicon micro-resonators. A Si-TCO photonic crystal (PC) nanocavity modulator is designed and demonstrated. We achieve extreme large wavelength tuning of 250 pm/V, single digit femto-joule per bit energy efficiency, and 2.2 GHz operation bandwidth with a deep sub-λ ultra-small modulation volume. We also propose a strategy to improve bandwidth to over 23GHz and reduce energy consumption to atto-joule per bit level. Besides, TCO-gated microring resonators are investigated for two applications. We design and demonstrate a tunable microring filter with an unprecedented wavelength tuning of 271 pm/V, a large electrical tuning range of 2 nm, and a negligible static energy consumption, which can be used for wavelength division multiplex (WDM) application. A TCO-gated microring modulator is also designed, which can potentially achieve a large operation bandwidth over 50GHz. Lastly, a sub-micron, sub-pico-second, femto-joule level all-optical switch (AOS) using hybrid plasmonic-silicon waveguides driven by high mobility TCOs is proposed. By defining a comprehensive metric using the product of device size, switching energy and switching time, the proposed device shows superior performance than any existing on-chip AOS device. In addition to the device research, we systematically analyze the energy efficiency and bandwidth limit of resonator-based silicon photonic modulators from three fundamental perspectives: free carrier dispersion strength of the active materials, Purcell factors of the resonators, and electrical configuration of the capacitors. The analysis lays the theoretical foundation and identifies possible routes for achieving atto-joule per bit energy efficiency and approaching the bandwidth limit of silicon photonic modulators. In summary, TCOs could play an important role in the development of future photonics technology, which provide a CMOS compatible solution to overcome the intrinsic weak E-O effect of the silicon photonics platform, lead to unprecedented reduction in energy consumption, increasing bandwidth, as well as enable novel functionalities. Future researches should include, but not limited to, optimizing the design and fabrication of TCO-driven modulators to reduce series resistance and increasing overlapping factor, integrating TCO-driven devices with photonics foundry fabricated PICs, and developing of high mobility TCOs.
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  • Pending Publication
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  • 2020-07-23 to 2021-08-24

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