Silicon Photonic Modulators for Low-power Applications

In this book, silicon photonic integrated circuits are combined with electro-optic organic materials for realizing energy-efficient modulators with unprecedented performance. These silicon-organic hybrid Mach-Zehnder modulators feature a compact size, sub-Volt drive voltages, and they support data rates up to 84 Gbit/s. In addition, a wet chemical waveguide fabrication scheme and an efficient fiber-chip coupling scheme are presented

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

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

Enabling integrated nanophotonic devices in hybrid cmos-compatible material platforms for optical interconnection

Recent impactful advances in integrated photonics undoubtedly owe much to silicon and its associated enabling platform (SOI). Although silicon has proved to be an indispensable element in many photonic systems yet it seems that it is not the ultimate solution to address all the challenges facing the photonics community. Therefore, integration of silicon with other optical materials featuring diverse properties is highly desirable. Such integration will be conducive to platforms which are naturally more capable and are suited for implementation of a wider range of optical devices and diverse functionalities. This dissertation is dedicated to design and implementation of integrated optical elements for hybrid material platforms. The basic theoretical foundation of integrated photonics is laid out in Chapter 2. In Chapter 3, an interlayer grating coupler for a specific hybrid material platform is designed, and demonstrated. Considering the fact that in almost all integrated photonic platforms, fabrication imperfections lead to an unpredictable shift in the wavelength of operation of individual devices, post fabrication tuning/trimming is inevitable. A number of widely used post fabrication trimming/tuning methods are briefly reviewed in Chapter 4 with special emphasis on a method based on electron beam exposure. In Chapter 5, an ultra-fast, low-power, and self-trimmable electro-optic modulator in demonstrated on a Si-based multilayer platform. Due to its remarkable optical and electronic properties, graphene has become a valuable material for opto-electronic applications. Integration of this novel 2D material with SOI platform is investigated in Chapter 6. Graphene-based electro-optic modulation through absorption and refractive-index change is successfully demonstrated using electrostatic gating mechanism. Chapter 7 is devoted to demonstration of a field-programmable 2 by 2 optical switch on a vertically stacked Si/SiO2/SOI platform. In Chapter 8, the peak-dragging phenomenon in a nanobeam photonic crystal cavity is studied. The optical bistability associated with this nonlinear phenomenon is of great interest for all-optical processing and sensing application. Future directions of this thesis are also discussed in the last Chapter.Ph.D

Photodetectors for graphene-based integrated photonics

The development of integrated optical circuits has enabled a diverse portfolio of chipscale photonic applications—ranging from data communication over sensing to imaging—that is set to grow further as new device concepts in, for example, quantum information processing and optical neural networks mature. While silicon photonics has emerged as a viable candidate to translate proof-of-principle demonstrations to mass-manufacturing, the fabrication of photonic integrated circuits (PICs) and their subcomponents remains highly heterogeneous, which drives up cost and slows down progress towards faster and more power-efficient performance. In this dissertation, I demonstrate how single-layer graphene (SLG) can bring active functionality to arbitrary passive waveguide platforms, offering a more universal approach to developing integrated photonic components. SLG can be transferred to PICs without the limitations or complexity of traditional deposition or bonding processes. It also stands to outperform conventional semiconductors in terms of speed and spectral operating range due to its ultra-fast carrier dynamics, high carrier mobility, and gapless bandstructure. Using the key component for optical-to-electrical signal conversion—the photodetector—as an example, here I demonstrate graphene-based components on three different PIC platforms. First, I show plasmonic enhanced graphene photodetectors (GPDs) on silicon nitride waveguides, which generate a voltage from optically generated hot-carrier distributions in SLG via the photo-thermoelectric (PTE) effect. Then, I demonstrate PTE-GPDs—fabricated from layered material heterostructures—on a silicon-on-insulator microring resonator. Both detectors operate at Telecom wavelengths (λ = 1.55 μm), are compatible with high-speed (> 10 GHz) operations, and hold the current records in voltage responsivity—R ∼ 12 V/W and R ∼ 90 V/W—for waveguide-integrated GPDs fabricated from chemical vapour deposited and mechanically exfoliated SLG, respectively. I then go on to show how established GPD concepts can be translated to an integrated mid-infrared (λ = 3.8 μm) platform based on sub-wavelength grating waveguides in silicon and study how light-graphene interaction under in-plane incidence can be further optimised to improve GPD performance. Finally, I develop new approaches towards high-quality, scalable SLG on PICs, critically required to advance graphene-based integrated photonics towards industrial production

MORPHIC : programmable photonic circuits enabled by silicon photonic MEMS

In the European project MORPHIC we develop a platform for programmable silicon photonic circuits enabled by waveguide-integrated micro-electro-mechanical systems (MEMS). MEMS can add compact, and low-power phase shifters and couplers to an established silicon photonics platform with high-speed modulators and detectors. This MEMS technology is used for a new class of programmable photonic circuits, that can be reconfigured using electronics and software, consisting of large interconnected meshes of phase shifters and couplers. MORPHIC is also developing the packaging and driver electronics interfacing schemes for such large circuits, creating a supply chain for rapid prototyping new photonic chip concepts. These will be demonstrated in different applications, such as switching, beamforming and microwave photonics

High-Speed, Low-Power and Mid-IR Silicon Photonics Applications

In this book, the first high-speed silicon-organic hybrid (SOH) modulator is demonstrated by exploiting a highly-nonlinear polymer cladding and a silicon waveguide. By using a liquid crystal cladding instead, an ultra-low power phase shifter is obtained. A third type of device is proposed for achieving three-wave mixing on the silicon-organic hybrid (SOH) platform. Finally, new physical constants which describe the optical absorption in charge accumulation/inversion layers in silicon are determined

Silicon-Organic Hybrid Platform for Photonic Integrated Circuits

Die vorliegende Arbeit erkundet das technische Potential einer neuen Plattform für hochintegrierte Optik. Diese Plattform kombiniert ausgewählte organische Materialien mit optischen, integrierten Schaltungen der Silizium-Photonik zu silizium-organischen Hybrid-Bauteilen (SOH) mit außergewöhnlichen Eigenschaften, d.h. Eigenschaften, welche Silizium allein nicht besitzt. Es werden Schlüsselkomponenten wie Laser und Modulatoren konzipiert und mit CMOS-basierten Technologien hergestellt