High-Q CMOS-integrated photonic crystal microcavity devices

Integrated optical resonators are necessary or beneficial in realizations of various functions in scaled photonic platforms, including filtering, modulation, and detection in classical communication systems, optical sensing, as well as addressing and control of solid state emitters for quantum technologies. Although photonic crystal (PhC) microresonators can be advantageous to the more commonly used microring devices due to the former's low mode volumes, fabrication of PhC cavities has typically relied on electron-beam lithography, which precludes integration with large-scale and reproducible CMOS fabrication. Here, we demonstrate wavelength-scale polycrystalline silicon (pSi) PhC microresonators with Qs up to 60,000 fabricated within a bulk CMOS process. Quasi-1D resonators in lateral p-i-n structures allow for resonant defect-state photodetection in all-silicon devices, exhibiting voltage-dependent quantum efficiencies in the range of a few 10 s of %, few-GHz bandwidths, and low dark currents, in devices with loaded Qs in the range of 4,300–9,300; one device, for example, exhibited a loaded Q of 4,300, 25% quantum efficiency (corresponding to a responsivity of 0.31 A/W), 3 GHz bandwidth, and 30 nA dark current at a reverse bias of 30 V. This work demonstrates the possibility for practical integration of PhC microresonators with active electro-optic capability into large-scale silicon photonic systems.United States. Defense Advanced Research Projects Agency. Photonically Optimized Embedded MicroprocessorsUnited States. Dept. of Energy (Science Graduate Fellowship

Photonic crystal cavity based optical induced transparency

Nowadays, information technology has been deeply integrated in our daily life. However, within its rapid development, it faces a serious bottleneck due to the prohibitive power consumption and limited transmission bandwidth of electrical interconnects. Silicon photonics introduces a potential solution for information technology based on optical communication. In this field, delay-bandwidth devices offer a high bandwidth optical interconnection and low power consumption for the next generation information communication technology. Through introducing the slow light effect, I can realise time domain control and store the light to achieve a new functional component, which is the optical buffer for optical information processing. The optical buffer allows us to control and store the light, using as the optical information process and transit. However, the current optical buffer devices are limited by high optical loss and the ability to produced tunable group delay of the light. In this thesis, I examine different configurations of the coupled photonic crystal resonator system and then introduce a novel tuneable delay line, based on photonic crystal cavity structures. Through the optical analog to electromagnetically induced transparency (EIT), an EIT-like transmission spectrum has been achieved in coupled photonic crystal cavities. By tuning the phase difference between two coupled resonators and resonance wavelength, I can achieve the desired analog conditions and reach to a maximum group delay of 360 ps. By adding thermal tuning pattern, I have demonstrated a tuning of the group delay of over 120 ps range at a low input power and a maximum delay of 300 ps group delay in coupled photonic crystal cavities system. All devices are with a footprint at only 200 μm², and with integrated compatibles as well. By employing a new vertical coupling technique, a record low loss 15 dB/ns is presented making this system very promising for practical optical information applications

Compact and low power consumption tunable photonic crystal nanobeam cavity

A proof-of-concept for a new and entirely CMOS compatible tunable nanobeam cavity is demonstrated in this paper. Preliminary results show that a compact nanobeam cavity (~20 μm^2) with high Q-factor (~50,000) and integrated with a micro-heater atop, is able of tuning the resonant wavelength up to 15 nm with low power consumption (0.35nm/mW), and of attaining high modulation depth with only ~100 μW. Additionally, a tunable bi-stable behavior is reported

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

Tunable and Broadband Nanostructured Photonic Devices:Fabrication and Characterization

The main topic of this thesis is the fabrication and characterization of structures smaller than the wavelength of light, for operation in the visible or near infrared spectral range. In the first part of this thesis, the fabrication of periodic structures in one and two dimensions with an interference photolithography technique is described in detail. The structure periodicities that have been fabricated with this process ranges between 270 nm and a few microns on areas that can be as large as 100 cm2. Typical structure thicknesses are approximately equal to the period. In particular, a square lattice of pillars with a period of 270 nm has been created on the (non-flat) surface of a quartz microlens array and exhibits anti-reflective properties. Experimental results show a 15 % attenuation of the reflectivity and a 3 % enhancement of the transmissivity over the visible spectral range. In the second part of the thesis the structures are fabricated by e-beam lithography to ensure very precise devices shapes. The experimental results are obtained in the infrared range with two different structures, called photonic crystals. The first structure, a superprism, is a triangular lattice of pillars infiltrated by liquid crystals. A displacement of the output light spot is measured to be 20.5 µm for a wavelength variation of 27 nm. The structure length is 70 µm. The device, based on standard silicon technology, should allow integration of the device as a multiplexer/demultiplexer system into optical micro-circuits. The second structure, a tunable resonant cavity, is a wavelength filter working in the near infrared spectrum. The active area is composed of a photonic waveguide with a triangular lattice made of sub-micrometer holes. Additional nanostructuring in the light waveguide acts as a resonant cavity. The tunability of the device is obtained due to the liquid crystals which are infiltrated into the nanostructure. A 32 nm shift of the transmitted light peak inside the photonic band gap is measured by changing the temperature from room temperature to 45 °C