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The objective of this work is to develop essential building blocks for the lab-on-a-chip optical sensing systems with high performance. In this study, the silicon-on-insulator (SOI) platform is chosen because of its compatibility with the mature microelectronics industry for the great potential in terms of powerful data processing and massive production. Despite the impressing progress in optical sensors based on the silicon photonic technologies, two constant challenges are larger sensitivity and better selectivity. To address the first issue, we incorporate porous materials to the silicon photonics platform. Two porous materials are investigated: porous silicon and porous titania. The demonstrated travelling-wave resonators with the magnesiothermically reacted porous silicon cladding have shown significant enhancement in the sensitivity. The process is then further optimized by replacing the thermal oxide with a flowable oxide for the magnesiothermic reduction. A different approach of making porous silicon using porous anodized alumina membrane leads to better flexibility in controlling the pore size and porosity. Porous titania is successfully integrated with silicon nitride resonators. To improve the selectivity, an array of integrated optical sensors are coated with different polymers, such that each incoming gas analyte has its own signature in the collective response matrix. A multiplexed gas sensor with four polymers has been demonstrated. It also includes on chip references compensating for the adverse environmental effects. On chip spectral analysis is also very critical for lab-on-a-chip sensing systems. For that matter, based on an array of microdonut resonators, we demonstrate an 81 channel microspectrometer. The demonstrated spectrometer leads to a high spectral resolution of 0.6 nm, and a large operating bandwidth of ~ 50 nm.

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Photonic integration and all-optical signal processing are promising solutions to the increasing demand for high-bandwidth and high-speed communication systems. III-V semiconductor materials, specially AlGaAs, have shown potentials for photonic integration and efficient nonlinear processes due to their low nonlinear absorption, flexibility at controlling the refractive index, and mature fabrication technology. In this thesis, we report the designs of AlGaAs microring resonators optimized for efficient four-wave mixing. Four-wave mixing (FWM) is a nonlinear optical phenomenon which can be used to realize many optical signal processing operations such as optical wavelength conversion and optical time division multiplexing and demultiplexing. Our designed AlGaAs microring resonators are expected to have good optical confinement, transmission characteristics, and efficient coupling between the ring and waveguide. Here we also present our fabrication efforts to fabricate the microring resonators device and the insights gained in the process. The microring resonators devices have a potential to be used in optical communication networks for all-optical signal processing operations.

Integrated Silicon Photonics has emerged as a powerful platform in the last two decades amongst high-bandwidth technologies, particularly since the adop- tion of CMOS compatible silicon-on-insulator(SOI) substrates. Microring res- onators are one of the fundamental blocks on a photonic integrated circuit chip o ering versatility in varied applications like sensing, optical bu ering, ltering, loss measurements, lasing, nonlinear e ects, understanding cavity optomechanics etc. This thesis covers the design and modeling of microring resonators for biosensing applications. The two applications considered are : homogeneous biosensing and wrist pulse pressure monitoring. Also, the designs have been used to fabricate ring resonator device using three different techniques. The results obtained through characterization of these devices are presented. Following are the observations made in lieu of this: 1) Design modeling and analysis - The analysis of ring resonator requires the study of both the straight and bent waveguide sections. Both rib and strip waveguide geometries have been considered for constructing the device as a building block by computing their respective eigen modes for both quasi-TE and quasi-TM polarizations. The non-uniform evanescent coupling between the straight and curved waveguide has been estimated using coupled mode theory. This method provided in estimating the quality-factor and free spec- tral range (FSR) of the ring-resonator. A case for optimizing the waveguide gap in the directional coupler section of a ring resonator has been presented for homogeneous biosensing application. On similar lines, a model of applying ring resonator for arterial pulse-pressure measurement has been analyzed. The results have been obtained by employing FD-BPM and FDTD including semi- vectorial eigen mode solutions to evaluate the spectral characteristics of ring resonator. The modeling and analytical results are supported by commercial software tools (RSoft). 2) Fabrication and Characterization - For the fabrication, we employ the design of ring resonator of radius 20 m on SOI substrate with two different waveguide gaps of 350 and 700 nm. Three different process sows have been used for fabricating the same device. The rst technique involved using negative e-beam resist HSQ which after exposure becomes SiO2, acts as a mask for Reactive-Ion Etching (RIE); helping in eliminating an additional step. The second technique involved the use of positive e-beam resist, PMMA for device patterning followed by metal deposition with lift-o . The third tech- nique employed was Focussed Ion-beam (FIB) which is resist-less patterning by bombarding Ga+ ions directly onto the top surface of the wafer with the help of a GDS le. The characterization process involved estimation of loss and observing the be- havior of optical elds in the device around the wavelength of 1550 nm using near-field scanning optical microscopy (NSOM) measurement. The estimation of roughness-induced losses has been made by performing Atomic Force Microscopy (AFM) measurements. In summary, the thesis presents novel design and analysis of SOI based microring resonators for homogeneous biosensing and wrist pulse pressure sensing applications. Also, the fabrication and characterization of 20 m radius ring- resonator with 500 500 nm rib cross-section is presented. Hence, this study brings forth several practical issues concerning application of ring resonators to biosensing applications.

Optical switchers can fulfill the same functions as all-electrical switching systems and are expected to play a key role in the near future. In this thesis an analysis if an optical system that can potentially behave as an optical switcher is discussed. This configuration consists of three microring resonators which are coupled and tangential to one another in a topology that is similar to the Leibniz packing or Apollonian gasket. The ray-transfer matrix approach is used in order to represent the whole system by a single matrix. The structure receives an initial input signal and gives an output signal, which is changed by only a scalar factor. This description is equal to an eigenvalue problem, where the matrix of the system operates over an initial vector and results a product of a scalar (the eigenvalue) times the initial vector. Due to its unique geometry each ring is divided into two unequal segments. We introduce the loss coefficients to express the attenuation along those segments. The relation between the loss coefficients is being examined and the results are verified by simulations.

Silicon photonics is a very promising technology for future low-cost high-bandwidth optical telecommunication applications down to the chip level. This is due to the high degree of integration, high optical bandwidth and large speed coupled with the development of a wide range of integrated optical functions. Silicon-based microring resonators are a key building block that can be used to realize many optical functions such as switching, multiplexing, demultiplaxing and detection of optical wave. The ability to tune the resonances of the microring resonators is highly desirable in many of their applications. In this work, the study and application of a thermally wavelength-tunable photonic switch based on silicon microring resonator is presented. Devices with 10µm diameter were systematically studied and used in the design. Its resonance wavelength was tuned by thermally induced refractive index change using a designed local micro-heater. While thermo-optic tuning has moderate speed compared with electro-optic and all-optic tuning, with silicon’s high thermo-optic coefficient, a much wider wavelength tunable range can be realized. The device design was verified and optimized by optical and thermal simulations. The fabrication and characterization of the device was also implemented. The microring resonator has a measured FSR of ~18 nm, FWHM in the range 0.1-0.2 nm and Q around 10,000. A wide tunable range (>6.4 nm) was achieved with the switch, which enables dense wavelength division multiplexing (DWDM) with a channel space of 0.2nm. The time response of the switch was tested on the order of 10 us with a low power consumption of ~11.9mW/nm. The measured results are in agreement with the simulations. Important applications using the tunable photonic switch were demonstrated in this work. 1×4 and 4×4 reconfigurable photonic switch were implemented by using multiple switches with a common bus waveguide. The results suggest the feasibility of on-chip DWDM for the development of large-scale integrated photonics. Using the tunable switch for output wavelength control, a fiber laser was demonstrated with Erbium-doped fiber amplifier as the gain media. For the first time, this approach integrated on-chip silicon photonic wavelength control.

<p>Kerr frequency comb, a sequence of equally spaced sharp lines in frequency domain generated via four-wave mixing process, has multiple applications such as spectroscopy, metrology, and atomic clocks. Conventional frequency combs generated from mode-locked laser have the limitations of low repetition rate and large volume. One novel platform, silicon nitride (SiN) microring resonator (MRR), can overcome such disadvantages. The SiN MRR is a low loss waveguide resonator and has good reliability and capacity for on-chip integration, which enables a portable solution for Kerr frequency comb.</p><p>This thesis focuses on the design and characterization of the SiN MRR to optimize the important performance characteristics for the applications.<br></p><p>In Kerr comb applications, phase coherence between the comb lines is required to eliminate unwanted signals in the systems. Therefore, the investigation of the coherent state in MRR based comb generation can benefit the development of comb generation techniques. In particular, dark pulses exhibit much higher comb conversion efficiency than the single soliton combs.<br></p><p>The tunability of Kerr comb is another important performance characteristic of the applications, which is useful for multiple applications, such as matching the comb line spacing to the wavelength multiplexing grid for coherent communication or aligning the on-chip laser wavelength and MRR resonance frequency during the integration. The theoretic analysis of thermal tuning and experimental characterization of resonance frequency tuning via an on-chip microheater are performed in this thesis to explore the thermal tuning efficiency and its limitation.<br></p><p>Another important performance characteristics of the frequency comb is the comb bandwidth. Large bandwidth comb will be beneficial for application like dual comb spectroscopy. In addition, octave-spanning Kerr comb is desired due to its capacity of f-2f self-referencing for comb line frequencies stabilization for the applications like atomic clocks. To demonstrate on-chip octave-spanning Kerr soliton, the dispersion engineering is utilized in the device design to optimize the pump dispersion and dispersive wave generation simultaneously. The octave-spanning solitons are achieved on SiN MRRs with around 900 GHz repetition rate.<br></p><p>Finally, two optical division approaches are demonstrated to read out the large repetition rate of the octave-spanning soliton on all-SiN platform with auxiliary combs to enable the locking of undetectable repetition rate with less complexity in the fabrication and integration. The first approach uses a 25 GHz soliton; whose repetition rate is directly detectable via a photodiode. The second approach employs a Vernier scheme with an 880 GHz soliton to provide an alternative optical division scheme with lower requirements in fabrication ultrahigh Q MRRs. The divided repetition rate can be locked to enable the fully stabilization of frequency comb to provide an on-chip high stability and low noise frequency comb source.<br></p><p></p>

This thesis presents a novel method of modulating silicon photonic circuits using 3D printed microfluidic devices. The fluids that pass through the microfluidic device interact directly with the silicon waveguides. This method changes the refractive index of the waveguide cladding, thus changing the effective index of the system. Through using this technique we demonstrate the shift in resonant wavelength by a full free spectral range (FSR) by increasing the concentration of the salt water in the microfluidic device from 0% to 10%. On a 60 μm microring resonator, this equals a resonant wavelength shift of 1.514 nm when the index of the cladding changes by 0.017 refractive index units (RIU), or at a rate of 89.05 nm/RIU. These results are confirmed by simulations that use both analytical and numerical methods. This thesis also outlines the development of a process that uses two-photon absorption(TPA) in silicon to produce a photoelectrochemical (PEC) etching effect. TPA induces free carriers in silicon that then interact with the Hydroflouric Acid (HF) solution that the wafer is submerged in. This interaction removes silicon away from the wafer, which is the etching observed in our experiments. Non-line-of-sight PEC etching is demonstrated. The optical assemblies used in these experiments are presented, as are several of the results of the etching experiments.

The scaling trend of microelectronics over the past 50 years, quantified by Moore’s Law, has faced insurmountable bottlenecks, necessitating the use of optical communication with its high bandwidth and energy efficiency to further improve computing performance. Silicon photonics, compatible with CMOS platform manufacturing, presents a promising means to achieve on-chip optical links, employing highly sensitive microring resonator devices that demand electronic feedback and control due to fabrication variations. Achieving the full potential of both technologies requires tight integration to realize the ultimate benefits of both realms of technology, leading to the convergence of microelectronics and photonics. A promising approach for achieving this convergence is the monolithic integration of electronics and photonics on CMOS platforms. A critical milestone was reached in 2015 with the demonstration of the first microprocessor featuring photonic I/O (Chen et al, Nature 2015), accomplished by integrating transistors and photonic devices on a single chip using a monolithic CMOS silicon-on-insulator (SOI) platform (GlobalFoundries 45RFSOI, 45 nm SOI process) without process modifications, thus known as the "zero-change" approach. This dissertation focuses on leveraging the fabrication capabilities of advanced monolithic electronic-photonic 45 nm CMOS platforms, specifically high-resolution lithography and small feature size doping implants, to realize photonic devices with subwavelength features that could potentially provide the next leap in integrated optical links performance, beyond microring resonator based links. Photonic crystal (PhC) nanobeam cavities can support high-quality resonance modes while confining light in a small volume, enhancing light-matter interactions and potentially enabling ultimate efficiencies in active devices such as modulators and photodetectors. However, PhC cavities have been overshadowed by microring resonators due to two challenges. First, their fabrication demands high lithography resolution, which excludes most standard SOI photonic platforms as viable options for creating these devices. Secondly, the standing-wave nature of PhC nanobeam cavities complicates their integration into wavelength-division multiplexing (WDM) optical links, causing unwanted reflections when coupled evanescently to a bus waveguide. In this work, we present PhC nanobeam cavities with the smallest footprint, largest intrinsic quality factor, and smallest mode volume to be demonstrated to date in a monolithic CMOS platform. The devices were fabricated in a 45 nm monolithic electronics–photonics CMOS platform optimized for silicon photonics, GlobalFoundries 45CLO, exhibiting a quality factor in excess of 100,000 the highest among fully cladded PhC nanobeam cavities in any SOI platform. Furthermore to eliminate reflections, we demonstrate an approach using pairs of PhC nanobeam cavities with opposite spatial mode symmetries to mimic traveling-wave-like ring behavior, enabling efficient and seamless WDM link integration. This concept was extended to realize a reflectionless microring resonator unit with two microrings operating as standing-wave cavities. Using this scheme with standing-wave microring resonators could lead to an optimum geometry for microring modulators with interdigitated p-n junctions in terms of modulation efficiency in a manner that allows for straightforward WDM cascading. This work also presents the first demonstration of resonant-structure-based modulators in the GlobalFoundries 45CLO platform. We report the first-ever demonstration of a PhC modulator in a CMOS platform, featuring a novel design with sub-wavelength contacts on one side allowing it to benefit from the "reflection-less"' architecture. Additionally, we also report the first demonstration of microring modulators. The most efficient devices exhibited electro-optical bandwidths up to 30 GHz, and 25 Gbps non-return-to-zero (NRZ) on-off-keyed (OOK) modulation with 1 dB insertion loss and 3.1 dB extinction ratio. Finally, as the complexity of silicon photonic systems-on-a-chip (SoC) increases to enable new applications such as low-energy data links, quantum optics, and neuromorphic computing, the need for in-situ characterization of individual components becomes increasingly important. By combining Near-field scanning optical microscopy (NSOM) with a flip-chip post-processing technique, this dissertation demonstrates a method to non-invasively perform NSOM scans of a photonic device within a large-scale CMOS-photonic circuit, without interfering with the performance and packaging of the photonics and electronics, making it a valuable tool for future development of high performance photonic circuits and systems.