Roadmapping the Next Generation of Silicon Photonics

Silicon photonics has developed into a mainstream technology driven by advances in optical communications. The current generation has led to a proliferation of integrated photonic devices from thousands to millions - mainly in the form of communication transceivers for data centers. Products in many exciting applications, such as sensing and computing, are around the corner. What will it take to increase the proliferation of silicon photonics from millions to billions of units shipped? What will the next generation of silicon photonics look like? What are the common threads in the integration and fabrication bottlenecks that silicon photonic applications face, and which emerging technologies can solve them? This perspective article is an attempt to answer such questions. We chart the generational trends in silicon photonics technology, drawing parallels from the generational definitions of CMOS technology. We identify the crucial challenges that must be solved to make giant strides in CMOS-foundry-compatible devices, circuits, integration, and packaging. We identify challenges critical to the next generation of systems and applications - in communication, signal processing, and sensing. By identifying and summarizing such challenges and opportunities, we aim to stimulate further research on devices, circuits, and systems for the silicon photonics ecosystem

Microwave Front-End Component Codesign: Filter-Amplifiers, Integrated Passives and Rectifying-Radiators

The ever-growing need for improved wireless communications motivates innovation in miniaturization and integration of microwave front-end components. This thesis addresses miniaturization by co-design methods that enable simultaneous size reduction, improved efficiency, diverse technology integration, and enhanced safety. First, a new theoretical treatment for simple design of narrowband filters with arbitrary complex impedance ports is introduced and validated through 2.4GHz designs with 2nd-order all-pole and 4th-order elliptical response. The theory is then extended to filters with port impedance tuning capabilities and validated with a 2nd-order Chebyshev filter with varactor-tunable input impedance over a pre-defined impedance range with maintained filter response. The theory is also applied to impedance-matching filters for power amplifiers (PAs) with the goal of improving efficiency and reducing footprint. A high-efficiency 4.7GHz single-stage 4-W hybrid GaN filter-PA (FPA) shows a measured gain of 15dB and PAE=55% with a pre-specified 9% fractional bandwidth. The approach is further validated on a GaAs MMIC FPA at 28GHz with a measured saturated gain of 8dB, 200mW of output power and PAE=30%. For further integration of components with miniaturized footprint, a heterogeneous integration process called metal-embedded chip assembly (MECA), developed by HRL, is exploited to combine ceramic passive circuits, surface-mount capacitors and GaN MMICs with a unique interconnect network. The interconnects outperform standard wirebonds and are also used to implement transmission lines, referred to as bridge-lines, with reduced loss and higher possible characteristic impedances compared to microstrip. Various couplers in the 8-10GHz range are designed and characterized to demonstrate the additional design capabilities provided by the MECA process. Thermal performance improvement of PAs is shown, and increased gain and efficiency for an X-band GaN MMIC is reported. Finally, the FPA design approach from the first part of the thesis is used to design a quasi-MMIC FPA with a predicted gain of 7.4dB, peak PAE of 23% and output power above 30dBm from 23.65 to 24.4GHz. Additionally, a 20-GHz dual-mode rectangular cavity resonator filter with insertion loss under 0.25dB is designed in the MECA process, and is in fabrication at the time of writing of this thesis. Wireless systems rely on batteries or wired charging, which limits the operational time. In the second part of the thesis, co-design and integration of wireless charging and harvesting is researched. Another application of wireless charging is for electric vehicles, where methods analogous to microwave antenna arrays, amplifiers and rectifiers can be used to provide a means for charging batteries of stationary or moving vehicles at lower frequencies and high power levels. A new method for reducing fringing fields in a capacitive wireless power transfer (CWPT) system using a near-field phased array is demonstrated using a multi-module approach on a 1.1kW system at 13.67MHz at a 25-cm energy-transfer distance with over 80% efficiency. To meet safety standards, a fringing field reduction of 24% with a two-module system and 43% with a four-module system, is demonstrated at 7, 14 and 29MHz. This system applied co-design of capacitor arrays with matching networks for the high-power inverters and rectifiers on the circuit side, and energy-transfer and fringing fields on the free-space side. In the low-power regime, co-design of both narrowband and broadband rectifiers and antennas for harvesting ambient power for wireless devices is demonstrated.Harvesting power from airplane altimeter radar antenna sidebands with a rectifier-antenna (rectenna) for aircraft health monitoring sensors demonstrates the possibility of charging a storage device at incident power levels below 2 W/cm2 at 4.3GHz. The narrowband harvesting device applies co-design to the antenna, rectifier and maximum power point tracking power-management circuit to provide a usable voltage level. For wideband energy harvesting from unknown and variable sources, wearable rectenna arrays screen-printed on clothing are demonstrated for harvesting 4-130W/cm2 power densities over more than an octave bandwidth in the sub-6GHz frequency range. Measurements on 36 and 64-element arrays show up to PDC=32W for incident power densities of 4W/cm2. For low incident power densities, the efficiency is in the 5-10% range, and reaches 32% for 100W/cm2. In these arrays, the rectifiers and tightly-coupled antennas are co-designed for broadband performance.</p

Heterogeneous integration of InP etched facet lasers to silicon photonics by micro transfer printing

Photonics Integrated Circuits allow optical functionalities and interconnects with small footprint, large band -width and -density, low heat generation. The silicon photonics platform (SOI) offers excellent waveguiding properties, large-area wafers and a highly developed CMOS infrastructure matured with electronics. Nevertheless, the key function of light amplification is missing due to the indirect band-gap of silicon. The light has to be provided to the SOI from a separate direct band-gap III-V material. InP based devices work in the infrared optical window of the electromagnetic spectrum and can be heterogeneously integrated to the SOI. This research deals with the development of the first stand-alone InP Fabry-Perot lasers heterogeneously integrated to SOI by Micro Transfer Printing (µTP). The lasers are pre-fabricated and tested before transfer and are optimized to reach excellent optical, electrical and thermal performance. Lasers printed on Si substrates emit over 20 mW optical power, have threshold current of 16 mA and series resistance of 6 Ω; the thermal impedance of 38 K/W is half of that for the same laser printed directly on the SOI. The transfer printable InP ridge lasers have been designed as rectangular coupons with both contacts at the top and etched facets at the sidewalls. Two main release technologies based on the FeCl3:H2O (1:2) solution and a InGaAs or a InAlAs sacrificial layer were developed for releasing the devices from the original InP substrate with selectivity to InP greater than 4000 at 1 ◦C. The working principle of a polymer anchor system which restrains the devices to the substrate during the undercut were determined. The devices were printed on different silicon photonic substrates with excellent adhesion, with and without adhesive layers. A process for creating recesses into the SOI was developed to allow edge coupling the laser waveguide to the SOI or a polymer waveguide. High alignment accuracy along the three spatial directions can be achieved with alignment markers, reference walls and the interposition of a metal layer beneath the devices. This work shows a possible path for the achievement of a laser source for silicon photonics and it has been the basis for the integration of others InP devices to PICs by micro transfer printing

Photonic platform and the impact of optical nonlinearity on communication devices

It is important to understand properties of different materials and the impact they have on devices used in communication networks. This paper is an overview of optical nonlinearities in Silicon and Gallium Nitride and how these nonlinearities can be used in the realization of optical ultra-fast devices targeting the next generation integrated optics. Research results related to optical lasing, optical switching, data modulation, optical signal amplification and photo-detection using Gallium Nitride devices based on waveguides are examined. Attention is also paid to hybrid and monolithic integration approaches towards the development of advanced photonic chips

Monolithic 1.3 μm InAs/GaAs Quantum Dot Lasers on Silicon: Simulation and Experiment

1.3 μm quantum dot (QD) lasers epitaxially grown on silicon have attracted great interest as light source for silicon photonics and other optical communication applications. This work focuses on improving the understanding of the physical mechanisms limiting the performance of these devices, and on studying the laser dynamics with respect to data transmission potential through simulation and experiment. Dislocation-induced carrier loss is a major concern for the performance of QD lasers on silicon. Part of this work aims, therefore, at identifying the processes degrading the laser characteristics as well as on understanding the performance disparity between silicon-based QD and quantum well (QW) devices. By using two specially extended types of rate equation travelling-wave models it is found that enhanced carrier loss at higher dislocation densities leads to a much larger laser threshold increase in QW than in QD lasers. The QD laser’s increased tolerance to dislocations can be explained based on efficient and ultrafast carrier capture into the QDs, where high energy barriers prevent them from migrating into defects. The carrier density reduction in the higher energy continuum layers is eventually reflected in a lower current injection efficiency and thus reduced light-current slope, confirming experimentally observed trends of 1.3 μm QD lasers on silicon. In particular, a large minority carrier diffusion length is identified as a key parameter inhibiting laser operation in QW-based devices. The other part of this work focuses on investigating the QD lasers’ dynamics by means of gain switching, small-signal and large-signal modulation. 150 ps short gain-switched pulses, modulation bandwidths of 1.6 GHz to 2.3 GHz, and optical eyes at 1.5 Gb/s are obtained from 2.5 mm long ridge-waveguide lasers grown and fabricated at University College London. Numerical simulations reveal that the observed high-speed limitations are a result of limited gain and a long photon lifetime, whereas suitability for 10 Gb/s operation is predicted in an optimised laser design. It was, furthermore, found that neither dislocation-induced carrier loss nor optical loss limit the modulation characteristics fundamentally. The reduced carrier lifetime is, however, reflected in stronger damping of the small-signal modulation curves. Apart from that, the overall device dynamics remain governed by the QD physics.Qualcomm Inc. School of Technology PhD scholarshi

Thin-film Lithium Niobate Photonics for Electro-optics, Nonlinear Optics, and Quantum Optics on Silicon

Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on lithium niobate. These thin-film LN waveguides offer over an order of magnitude improvement in optical confinement, and about two orders of magnitude reduction in waveguide bending radius, compared to conventional LN waveguides. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices are demonstrated in this work. First, two types of compact electrooptic modulators are presented – microring modulators, and Mach-Zehnder modulators. Next, two distinct approaches to nonlinear optical frequency converters are implemented – periodically poled lithium niobate, and mode shape modulation (grating assisted quasi-phase matching). Following this, stochastic variations are added to the mode shape modulation approach to demonstrate random quasi-phase matching. Afterward, broadband photon-pair generation is demonstrated in the miniaturized periodically poled lithium niobate, and spectral correlations of the biphoton spectrum are reported. Finally, extensions of the aforementioned results suitable for future work are discussed

Infrared Photodetectors based on InSb and InAs Nanostructures via Heterogeneous Integration-Rapid Melt Growth and Template Assisted Selective Epitaxy

Monolithic heterogeneous integration of III-V semiconductors with the contemporary Si Complementary Metal Oxide Semiconductor (CMOS) technology has instigated a wide range of possibilities and functionalities in the semiconductor industry, in the field of digitalcircuits, optical sensors, light emitters, and high-frequency communication devices. However, the integration of III-V semiconductorsis not trivial due to the differences in lattice parameters, polarity, and thermal expansion coefficient. This thesis explores two integrationtechniques to form III-V nanostructures with potential applications in the infrared detection field.The first technique implemented in this thesis work is the Rapid Melt Growth technique. InSb, which has a large lattice mismatch(19%) to Si, is used to demonstrate the RMG integration technique. A flash lamp with a millisecond annealing technique is utilized tomelt and recrystallize amorphous InSb material to form a single crystalline material. The development of the fabrication process andthe experimental results for obtaining a single crystalline InSb-on-insulator from a Si seed area through the RMG process are presented.Electron Back Scatter Diffraction (EBSD) technique was employed to understand the crystal quality, orientation, and defects in theRMG InSb nanostructures. The InSb nanostructures have a resistivity of 10 mΩ cm, similar to the VLS-grown InSb nanowires.Mobility ranging from 3490 - 877 cm2/ V sec was extracted through Hall and Van der Pauw measurements. Finally, we report the firstmonolithic integrated InSb nanostructure photodetector on Si through the RMG process. Detailed optical and electrical characterizationof the device, including the spectrally resolved photocurrent and the temperature-dependent dark current, is studied. The thesis presentsan InSb photodetector with a stable photodetector with a responsivity of 0.5 A/W at 16 nW illumination and millisecond time response.The second integration technique implemented in this thesis work is Template Assisted Selective Epitaxy. Here, the versatility ofTASE technique to integrate InAs nanowires on W metal seed is demonstrated. This technique enables the feasibility of integratingIII-V semiconductors to back -end of the line integration with Si CMOS technology. EBSD technique was utilized to study andobtain the statistics on the single crystalline InAs nanowires grown from different diameter templates. We also demonstrate thepossibility of achieving an nBn InAs detector using TASE on W approach. This technique is a promising step towards developinghigh operating temperature (HOT) monolithic integrated mid-infrared detectors. Thus, the results of this thesis provide theperspective into two viable CMOS-compatible III-V integration techniques that could be utilized for photodetector applications at areduced cost

2022 Roadmap on integrated quantum photonics

AbstractIntegrated photonics will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying optical quantum technologies can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration have enabled table-top experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. These advances have enabled integrated quantum photonic technologies combining up to 650 optical and electrical components onto a single chip that are capable of programmable quantum information processing, chip-to-chip networking, hybrid quantum system integration, and high-speed communications. In this roadmap article, we highlight the status, current and future challenges, and emerging technologies in several key research areas in integrated quantum photonics, including photonic platforms, quantum and classical light sources, quantum frequency conversion, integrated detectors, and applications in computing, communications, and sensing. With advances in materials, photonic design architectures, fabrication and integration processes, packaging, and testing and benchmarking, in the next decade we can expect a transition from single- and few-function prototypes to large-scale integration of multi-functional and reconfigurable devices that will have a transformative impact on quantum information science and engineering

Scalable designs and methods for heterogeneous electronic-photonic integrated circuitry

A set of semiconductor designs shown to be capable of facilitating scalable and reconfigurable layouts for electronic-photonic integrated circuitry is presented. Three emphases are established to outline and discuss the methods and advantages of merging stand-alone optical components into integrated heterogeneous systems, specifically for implementing optical sensing, efficient laser wavelength tuning, and III-V-on-Si semiconductor fabrication techniques together on a single platform. Considerations regarding the optical geometries and power efficiency of each design are reiterated to assure that each design is compatible with the goals of system-level integration in either biochemical point-of-use or telecommunications applications. These three approaches to scalable photonic designs are then investigated in their ability to offer dynamic controls of optical signals and their novel usage of heterogeneous material patterning. The optical sensing platform directly integrates multiple linear variable filters (LVFs) atop a CMOS image sensor for electronic controls of detecting a biochemical fluorescent or absorptive optical signal signature, enabling good wavelength resolution (3.77−6.08 nm) over a wide-band detection spectrum. Detection limits of 0.28 nM for Quantum Dot emitters and 32 ng/mL for near-infrared fluorescent dyes are found in this integrated design, providing comparable results in the compact optical platform to conventional laboratory spectrometers. The instrument is then extended in its usage by testing on point-of-use detection tests via discerning the concentration of free-chlorine in water colorimetrically. The tunable laser cavity design integrates together a GaN waveguide into a standard InGaAsP telecom (1550 nm) edge-emitting laser atop silicon, allowing for wide-band tuning via the strong anisotropic effects solved for in wurtzite GaN. A tuning parameter based off a refractive index variation, Δ, is found to be at |1.75∙10E−4|, based off the electro-optic effects in conjunction with an etched grating geometry designed directly into the coupled GaN waveguide, with the structure further extended into a Y-branch laser cavity to enable the Vernier effect for wideband tuning via mode-hopping. A separate GaN-based design, consisting of an RF signal modulator that launches a surface acoustic wave (SAW) into a cavity to produce a highly controllable refractive index variation, Δ, via the photo-elastic and photo-elastic effects, is found to produce a large tuning parameter of |1.84∙10E−3|. These effects are then described in their application to dynamically controllable effects for dense wavelength division multiplexing (DWDM) and how the underlying electronic platform enables this, providing advantages over larger footprint or less efficient designs. The fabrication techniques designed provide a method to enable bonding of III-V epitaxial wafers onto a silicon carrier wafer for large-scale processing before final bonding onto CMOS. A processing recipe takes bulk GaAs epitaxial structures and constructs a method for reversibly bonding and processing them on a silicon carrier wafer as III-V islands, ready for final large scale flip-chip bonding onto aligning CMOS features. Additional findings discuss the merits of various etch processes and techniques such that they are compatible to the heterogeneous III-V-on-Si patterning as laid out. The methods optimized allow for simultaneous, heterogeneous development of system-level device integration such that further processing can place various III-V devices side-by-side and process geometries in unison. Processing steps and their results are presented. The extension of this method to different III-V alloys beyond GaAs entirely is therefore considered for even larger-scale system design across photonic elements. Each set of findings presents both the relevant photonic device characteristics and also a method on how to intersect these devices with a paired CMOS electronic system on silicon, so that a single unified electronic-photonic schematic can be made. Accompanying these conclusions is a range of experimental work ranging from simulation studies, to full-scale integrable sensing designs and their testing, to detailed cleanroom-based fabrication processes for designing the system of III-V-on-Si patterns. A final set of conclusions relates the three tracks of research as being part of a common path forward in scalable photonics designs. Forecasts are then made on how the field of electronic-photonic integration and its applications utilized herein may yet evolve and potentially encompass findings or methodologies from this work