Toward accurate and large-scale silicon photonics

Thesis (Ph. D. in Electrical Engineering)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-163).Silicon photonics, emerging from the interface of silicon technology and photonic technology, is expected to inherit the incredible integration ability of silicon technology that has boomed the microelectronic industry for half a century, as well as the unparalleled communication capability of photonic technology that has revolutionized the information industry for decades. Being a prevailing research topic in the past decade, silicon photonics has seen tremendous progresses with the successful demonstrations and commercializations of almost all of the key components, including on-chip light source, low-loss silicon waveguide, and ultrafast silicon modulators and detectors. It seems silicon photonics is ready to take off by following the successful path the microelectronic industry has been traveling through to achieve a large-scale integration of millions of photonic devices on the silicon chip with the aide of the well-established complementary metal-oxide-semiconductor (CMOS) technology. However, there remain some substantial challenges in silicon photonics, including the reliable design and fabrication of silicon photonic devices with unprecedented accuracy, and the large-scale integration of otherwise discrete silicon photonic devices. To this end, this thesis explored several examples as possible means of addressing these two challenges in silicon photonics. Two different ways of improving silicon photonic device accuracy were presented from perspectives of fabrication and device design respectively, followed by a successful integration demonstration where more than 4,000 components worked together on a silicon chip to form a functional large-scale silicon photonic system, representing the largest silicon photonic integration demonstrated to date.by Jie Sun.Ph.D.in Electrical Engineerin

Development and characterization of metallo-dielectric hybrid nanomaterials

The rational combination of dielectric and metallic nano particles brings novel optical properties to conventional subwavelength structures. This thesis introduces the optoplasmonic geometries demonstrating versatile ability in both far and near field modification within nano scale. Template-assisted self-assembly approaches are applied creating nano entities with titanium dioxide and gold nano spheres. A top-bottom mono hybrid unit and interdigitated array are developed. With the examination of the elastic and inelastic response of these hybrid materials, physical models are simulated to depict the scenario of varied geometry and combination of nano particles. In contrast to solely metal or dielectric particle arrays, this type of artificial material not only enhances the near electric field intensity within the metal nano cluster hot spots, but also expands the overall volume of enhanced electric field. Further study reveals that the additional enhancement and redistribution of near field are derived from the coupling between the nano gold cluster plasmon resonance and the in-plane diffractive mode of the dielectric array. The redirected emission profile of the fluorescent dyes within the hybrid array is explored

Microwave Photonic Applications - From Chip Level to System Level

Die Vermischung von Mikrowellen- und optischen Technologien – Mikrowellenphotonik – ist ein neu aufkommendes Feld mit hohem Potential. Durch die Nutzung der Vorzüge beider Welten hat die Mikrowellenphotonik viele Anwendungsfälle und ist gerade erst am Beginn ihrer Erfolgsgeschichte. Der Weg für neue Konzepte, neue Komponenten und neue Anwendungen wird dadurch geebnet, dass ein höherer Grad an Integration sowie neue Technologien wie Silicon Photonics verfügbar sind. In diesem Werk werden zuerst die notwendigen grundlegenden Basiskomponenten – optische Quelle, elektro-optische Wandlung, Übertragungsmedium und opto-elektrische Wandlung – eingeführt. Mithilfe spezifischer Anwendungsbeispiele, die von Chipebene bis hin zur Systemebene reichen, wird der elektrooptische Codesign-Prozess veranschaulicht. Schließlich werden zukünftige Ausrichtungen wie die Unterstützung von elektrischen Trägern im Millimeterwellen- und THz-Bereich sowie Realisierungsoptionen in integrierter Optik und Nanophotonik diskutiert.The hybridization between microwave and optical technologies – microwave photonics – is an emerging field with high potential. Benefitting from the best of both worlds, microwave photonics has many use cases and is just at the beginning of its success story. The availability of a higher degree of integration and new technologies such as silicon photonics paves the way for new concepts, new components and new applications. In this work, first, the necessary basic building blocks – optical source, electro-optical conversion, transmission medium and opto-electrical conversion – are introduced. With the help of specific application examples ranging from chip level to system level, the electro-optical co-design process for microwave photonic systems is illustrated. Finally, future directions such as the support of electrical carriers in the millimeter wave and THz range and realization options in integrated optics and nanophotonics are discussed

Fabrication of Sub-Wavelength Photonic Structures by Nanoimprint Lithography

Nanoimprint lithography (NIL) is a novel but already a mature lithography technique. In this thesis it is applied to the fabrication of nanophotonic devices using its main advantage: the fast production of sub-micron features in high volume in a cost-effective way. In this thesis, fabrication methods for conical metal structures for plasmonic applications and sub-wavelength grating based broad-band mirrors are presented. Conical metal structures, nanocones, with plasmonic properties are interesting because they enable concentrating the energy of light in very tight spots resulting in very high local intensities of electromagnetic energy. The nanocone formation process is studied with several metals. Enhanced second harmonic generation using gold nanocones is presented. Bridged-nanocones are used to enhance Raman scattering from a dye solution. The sub-wavelength grating mirror is an interesting structure for photonics because it is very simple to fabricate and its reflectivity can be extended to the far infrared wavelength range. It also has polarization dependent properties which are used in this thesis to stabilize the output beam of infrared semiconductor disk laser. NIL is shown to be useful a technique in the fabrication of nanophotonic devices in the novel and rapidly growing field of plasmonics and also in more traditional, but still developing, semiconductor laser applications

Nanoscale Field Emission Devices for High-Temperature and High-Frequency Operation

Field emission—the quantum-mechanical tunneling of electrons from the surface of a material into vacuum by means of a strong electric field—has been studied for over a century. However, the usage of devices based on this mechanism has been limited to a handful of niche applications such as high-power RF systems and field emission displays. The preference for solid-state devices relies on their low cost, long lifetimes, reduced power consumption, ease of integrability, and simple and scalable fabrication. Nonetheless, with the advent of modern fabrication techniques, it has been possible to build field emission devices with nanoscale dimensions that offer several advantages over traditional semiconductor devices. The use of vacuum allows ballistic transport with no lattice scattering. As device capacitance can be engineered by tuning the geometry, these devices are appealing for high-frequency operation. Vacuum is also inherently immune to harsh operating conditions such as high temperature and radiation, which is desirable for aerospace, nuclear, and military applications. In addition, even though field emission requires substantial electric fields, by exploiting the nanoscale gaps that can be easily fabricated with state-of-the-art lithographic capabilities, we can expect operating voltages comparable to CMOS. Thus, vacuum emission devices have the potential to greatly improve upon the limitations of current technologies. In this work, we experimentally demonstrate various design paradigms to develop nanoscale field emission devices for high-temperature environments and high-frequency operation. First, we propose suspended lateral two- and four-terminal devices. By removing the underlying solid substrate, we aim to increase the resistance of the leakage current pathways that emerge at elevated temperatures. Tungsten is the chosen electrode material due to its low work function and ability to withstand high temperatures. Our next architecture consists of a multi-tip two-terminal array, which exclusively relies on the inherent fast response of field emission. Due to the strong non-linearity in the emission characteristic, frequency mixing is measured. Lastly, we combine field emission with plasmonics to conceive devices that can be modulated both electrically and optically at telecommunication wavelength. By taking advantage of the strong confinement and significant optical field enhancement of surface plasmon polaritons, we seek to minimize the applied voltages required for field emission as well as the necessary laser powers for photoemission towards the development of high-speed, low-power, nanoscale optoelectronic systems.</p

DESIGN AND OPTIMIZATION OF MICRO/NANO PHOTONICS AIMING AT SENSOR APPLICATIONS

Ph.DDOCTOR OF PHILOSOPH

Optically Induced Nanostructures

Nanostructuring of materials is a task at the heart of many modern disciplines in mechanical engineering, as well as optics, electronics, and the life sciences. This book includes an introduction to the relevant nonlinear optical processes associated with very short laser pulses for the generation of structures far below the classical optical diffraction limit of about 200 nanometers as well as coverage of state-of-the-art technical and biomedical applications. These applications include silicon and glass wafer processing, production of nanowires, laser transfection and cell reprogramming, optical cleaning, surface treatments of implants, nanowires, 3D nanoprinting, STED lithography, friction modification, and integrated optics. The book highlights also the use of modern femtosecond laser microscopes and nanoscopes as novel nanoprocessing tools

Scalable Nanophotonic Light Management Design for Solar Cells

The current trend in wide adoption of solar energy is encouraging in the context of current projections of increasing energy consumption and the dire need to decrease carbon emissions. The solar industry has expanded due to scientific advances in the power conversion efficiency of solar modules. In order maintain a rapid pace of adoption and further decrease electricity costs, converting each photon becomes increasingly important. This work focuses on nanophotonic approaches to increasing the power conversion efficiency of different solar photovoltaic designs. The projects voluntarily impose certain design constraints in order to be compatible with the large scale manufacturing needed by the solar industry. A focus was given to designs that can leverage the promising technology of nanoimprint lithography. Amorphous silicon tandem cells with embedded nanophotonic patterning attempted to increase absorption while minimizing materials and time costs. Simulated designs of Copper Indium Gallium Diselenide absorbers showed that the management of excited carriers is equally as important as light management in decreasingly thin absorber layers. Near perfect anti-reflection structures were given a detailed physical analysis to better describe the fundamental physics of near zero reflection due to nanocones printed on solar cell encapsulation glass. Experimental results agreed with the theoretical analysis, and showed that these nanostructures further increased absorbed photocurrent by trapping light in the encapsulation glass. Finally, a unique device in the form of a tandem luminescent solar concentrator/silicon solar module was proposed and analyzed as a low cost and adaptable technology for increased solar power conversion efficiency. Key to this design was discovery of new, near-perfect components for light management. Exciting and innovative designs are proposed to control the light-matter interaction within these devices. Study of a photonic luminescent solar concentrator predicted that luminescence can be trapped in photonic crystal slab waveguides with near zero loss. Rigorous experimental efforts to characterize a multitude of near-perfect samples help guide these designs toward their final goals

Nonlinear terahertz spectroscopy of carbon nanomaterials and semiconductor nanostructures

This thesis will cover my work relating to the developing field of terahertz (THz) science and technology. It will present experimental and theoretical studies investigating the optical and electrical properties of various material systems using novel THz imaging and spectroscopy techniques. Due to its low photon energy, THz imaging and spectroscopy are useful tools for non-contact, non-destructive probing of materials. Broadband, single-cycle THz pulses are prepared using modern THz generation technology. Using the THz detection techniques of THz raster imaging and THz time-domain spectroscopy (THz-TDS), the local carrier dynamics of nanomaterials such as graphene and carbon nanotubes were determined. THz measurements on single-layer graphene grown with different recipes and on various substrates exhibit sub-millimeter spatial inhomogeneity of sheet conductivity. THz transmission data reveals that a thin plastic, polymethyl methacrylate (PMMA), layer in contact with single-layer graphene induces a small yet noticeable reduction in conductivity. Ulterior THz measurements performed on vertically-aligned multi-walled carbon nanotubes (V-MWCNT) employ time-resolved THz transmission ellipsometry. The angle- and polarization-resolved transmission measurements reveal anisotropic characteristics of the THz electrodynamics in V-MWCNT. The anisotropy is, however, unexpectedly weak: the ratio of the tube-axis conductivity to the transverse conductivity, σ_z / σ_ xy ≅ 2.3, is nearly constant over the broad spectral range of 0.4-1.6 THz. The relatively weak anisotropy and the strong transverse electrical conduction indicate that THz fields readily induce electron transport between adjacent shells within the multi-walled carbon nanotubes. In-depth coverage of the development of a high-field THz generation system based on a lithium niobate prism will be presented. The evolution of techniques in the realm of high power THz generation is ongoing. The resolved issues throughout implementation include: magnesium doping, phase matching, and wave front distortion. The high power, broadband THz emitter (maximum THz field, E_max > 1 MV/cm) allows for nonlinear THz spectroscopy of various material systems including single-layer graphene and high-resistivity, bulk GaAs. THz-induced transparency is observed in two types of single-layer graphene samples: (i) suspended graphene-PMMA layer and (ii) graphene embedded in dielectrics. THz-induced transparency is shown to be significantly higher in suspended graphene than in graphene on a Si substrate. The experimental observation leads to a universal nonlinear THz property of graphene that the sheet conductivity undergoes two-fold reduction when THz fields reach 0.8 MV/cm. We confirm the generality of this result by measuring different grapheme samples on different substrates. Time-resolved THz transmission measurements show that the THz-induced transparency in graphene is dynamic; the transient conductivity gradually decreases throughout the pulse duration. The large THz fields induce sub-picosecond electron thermalization and subsequent carrier-carrier scattering, transiently modulating the electrical and optical properties, in effect reducing the electrical conductivity of graphene by an order of magnitude. Nonlinear THz spectroscopy methods are also applied to the investigation of a nano-antenna patterned, high-resistivity, intrinsic GaAs wafer. The antenna near-field reaches 20 MV/cm due to a huge field enhancement in the plasmonic nanostructure. Thus, the nonlinear THz interactions take place in the confined nanometer-scale region adjacent to the antenna. As a result of the huge THz fields, nano-antenna patterned GaAs demonstrates remarkably strong nonlinear THz effects. The fields are strong enough to generate high density free carriers (N_e > 10¹⁷ cm⁻³) via high-energy interband excitations associated with a series of impact ionizations (n_I ≈ 33-37); thus inducing large absorption of THz radiation (> 35%).Keywords: Carbon Nanotubes, CNT, High Field, Lithium Niobate, Nano Antenna, GaAs, Terahertz, Graphene, Nonlinear, High Power, Gallium Arsenid