Towards Faster Data Transfer by Spoof Plasmonics

With the emergence of complex architectures in modern electronics such as multi-chip modules, the increasing electromagnetic cross-talk in the circuitry causes a serious issue for high-speed, reliable data transfer among the chips. This thesis aims at developing a cross-talk resilient communication technology by utilizing a special form of electromagnetic mode, called spoof surface plasmon polariton for information transfer. The technique is based on the fact that a metal wire with periodic sub-wavelength patterns can support the propagation of confined electromagnetic mode, which can suppress cross-talk noise among the adjacent channels; and thus outperform conventional electrical interconnects in a parallel, high channel density data-bus. My developed model shows that, with 1 THz carrier frequency, the optimal design of cross-talk resilient spoof plasmon data-bus would allow each channel to support as high as 300 Gbps data, the bandwidth density can reach 1 Tbps per millimeter width of data-bus, and the digital pulse modulated carrier can travel more than 5 mm distance on the substrate. I have demonstrated that spoof plasmonic interconnects, comprised of patterned metallic conductors, can simultaneously accommodate electronic TEM mode, which is superior in cross-talk suppression at low-frequencies; and spoof plasmon mode, which is superior at high-frequencies. The research work is divided into two complementary parts: developing a theory for electromagnetic property analysis of spoof plasmon waveguide, and manipulating these properties for high-speed data transfer. Based on the theory developed, I investigated the complex interplay among various figure-of-merits of data transfer in spoof plasmonics, such as bandwidth density, propagation loss, thermal noise, speed of modulation, etc. My developed model predicts that with the availability of 1 THz carrier, the bit-error-rate of spoof plasmon data bus, subject to thermal noise would be $sim10^{-8}$ while the Shannon information capacity of the bus would be $10$ Tbps/mm. The model also predicts that, by proper designing of the modulator, it can be possible to alter the transmission property of the waveguide over one-fifth ($1/5$) of the spoof plasmon band which spans from DC frequency to the frequency of spoof plasmon resonance. To exemplify, if the spoof plasmon resonance is set at $1$ THz, then we can achieve more than $200$ Gbps speed of modulation with a very high extinction ratio, assuming the switching latency of the transistors at our disposal is negligible to the time-resolution of interest. We envision spoof plasmonic interconnects to constitute the next generation communication technology that will be transferring data at hundreds of Gigabit per second (Gbps) speed among different chips on a multi-chip module (MCM) carrier or system-on-chip (SoC) packaging.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163041/1/srjoy_1.pd

Novel Metamaterials and Their Applications in Subwavelength Waveguides, Imaging and Modulation

The development of metamaterials has opened the door for engineering electromagnetic properties by subwavelength artificial atoms , and hence accessing new properties and functionalities which cannot be found among naturally occurring materials. In particular, metamaterials enable the flexibility of independently controlling the permittivity and permeability to be almost any arbitrary value, which promises to achieve deep subwavelength confinement and focusing of electromagnetic waves in different spectrum regimes. The next stage of this technological revolution will be focused on the development of active and controllable metamaterials, where the properties of the metamaterials are expected to be tuned by external stimuli. In this sense, some natural materials are also promising to provide the tunable capability, particularly in the near infrared and terahertz domains either by applying a voltage or shining light on the materials. The objective of this dissertation is to investigate novel metamaterials and explore three important applications of them: subwavelength waveguiding, imaging and modulation. The first part of this dissertation covers the theory, design and fabrication of several different types of metamaterials, which includes artificially designed metamaterials and some naturally existing materials. The second part demonstrates metal gratings functioning as designer surface plasmonic waveguides support deep subwavelength surface propagation modes at microwave frequency. The third part proposes multilayered metal-insulator stack as indefinite metamaterial that converts evanescent waves to propagating waves, hence deep subwavelength image can be observed. The fourth part explores the tunability of several natural materials - gallium (Ga), indium tin oxide (ITO) and graphene, and demonstrates electro-optical (EO) modulators based on these materials can be achieved on nano-scale. The final part summarizes the work presented in this dissertation and also discusses some future work for photodetection, photovoltaics, and modulation

Surface plasmons for enhanced metal-semiconductor-metal photodetectors

Surface Plasmon Polaritons (SPPs) are quantized charge density oscillations that occur when a photon couples to the free electron gas of the metal at the interface between a metal and a dielectric. The extraordinary properties of SPP allow for sub-diffraction limit waveguiding and localized field enhancement. The emerging field of surface plasmonics has applied SPP coupling to a number of new and interesting applications, such as: Surface Enhanced Raman Spectroscopy (SERS), super lenses, nano-scale optical circuits, optical filters and SPP enhanced photodetectors. In the past decade, there have been several experimental and theoretical research and development activities which reported on the extraordinary optical transmission through subwavelength metallic apertures as well as through periodic metal grating structures. The use of SPP for light absorption enhancement using sub-wavelength metal gratings promises an increased enhancement in light collection efficiency of photovoltaic devices. A subwavelength plasmonic nanostructure grating interacts strongly with the incident light and potentially traps it inside the subsurface region of semiconductor substrates. Among all photodetectors, the Metal-Semiconductor-Metal photodetector (MSM-PD) is the simplest structure. Moreover, due to the lateral geometry of the MSM-PDs, the capacitance of an MSM-PD is much lower than capacitances of p-i-n PDs and Avalanche PDs, making its response time in the range of a few tens of picoseconds for nano-scale spacing between the electrode fingers. These features of simple fabrication and high speed make MSM-PDs attractive and essential devices for high-speed optical interconnects, highsensitivity optical samplers and ultra-wide bandwidth optoelectronic integrated circuits (OEIC) receivers for fibre optic communication systems. However, while MSM-PDs offer faster response than their p-i-n PD and avalanche PD counterparts, their most significant drawbacks are the high reflectivity of the metal fingers and the very-low light transmission through the spacing between the fingers, leading to very low photodetector sensitivity. This thesis proposes, designs and demonstrates the concept of a novel plasmonicbased MSM-PD employing metal nano-gratings and sub-wavelength slits. Various metal nano-gratings are designed on top of the gold fingers of an MSM-PD based on gallium arsenide (GaAs) for an operating wavelength of 830 nm to create SPP-enhanced MSM-PDs. Both the geometry and light absorption near the designed wavelength are theoretically and experimentally investigated. Finite Difference Time Domain (FDTD) simulation is used to simulate and design plasmonic MSM-PDs devices for maximal field enhancement. The simulation results show more than 10 times enhancement for the plasmonic nano-grating MSM-PD compared with the device without the metal nano-gratings, for 100 nm slit difference, due to the improved optical signal propagation through the nano-gratings. A dual beam FIB/ SEM is employed for the fabrication of metal nano-gratings and the sub-wavelength slit of the MSM-PD. Experimentally, we demonstrate the principle of plasmonics-based MSM-PDs and attain a measured photodetector responsivity that is 4 times better than that of conventional single-slit MSM-PDs. We observe reduction in the responsivity as the bias voltage increases and the input light polarization varies. Our experimental results demonstrate the feasibility of developing high-responsivity, low bias-voltage high-speed MSM-PDs. A novel multi-finger plasmonics-based GaAs MSM-PD structure is optimized geometrically using the 2-D FDTD method and developed, leading to more than 7 times enhancement in photocurrent in comparison with the conventional MSM-PD of similar dimensions at a bias voltage as low as 0.3V. This enhancement is attributed to the coupling of SPPs with the incident light through the nano-structured metal fingers. Moreover, the plasmonic-based MSM-PD shows high sensitivity to the incident light polarization states. Combining the polarization sensitivity and the wavelength selective guiding nature of the nano-gratings, the plasmonic MSM-PD can be used to design high-sensitivity polarization diversity receivers, integrating polarization splitters and polarization CMOS imaging sensors. We also propose and demonstrate a plasmonic-based GaAs balanced metalsemiconductor- metal photodetector (B-MSM-PD) structure and we measure a common mode rejection ratio (CMRR) value less than 25 dB at 830nm wavelength. This efficient CMRR value makes our B-MSM-PD structure suitable for ultra-high-speed optical telecommunication systems. In addition, this work paves the way for the monolithic integration of B-MSM-PDs into large scale semiconductor circuits. This thesis demonstrates several new opportunities for resonant plasmonic nanostructures able to enhance the responsivity of the MSM-PD. The presented concepts and insights hold great promise for new applications in integrated optics, photovoltaics, solidstate lighting and imaging below the diffraction limit. In Chapter 10 we conclude this thesis by summarizing and discussing some possible applications and future research directions based on SPP field concentration

Bio-inspired nanophotonics : manipulating light at the nanoscale with plasmonic metamaterials

textMetals interact very differently with light than with radio waves and finite conductivities and losses often limit the way that RF concepts can be directly transferred to higher frequencies. Plasmonic materials are investigated here for various optical applications, since they can interact, confine and focus light at the nanoscale; however, regular plasmonic devices are severely limited by frequency dispersion and absorption, and confined signals cannot travel along plasmonic lines over few wavelengths. For these reasons, novel concepts and materials should be introduced to successfully manipulate and radiate light in the same flexible way we operate at lower frequencies. In line with these efforts, optical metamaterials exploit the resonant wave interaction of collections of plasmonic nanoparticles to produce anomalous light effects, beyond what naturally available in optical materials and in their basic constituents. Still, these concepts are currently limited by a variety of factors, such as: (a) technological challenges in realizing 3-D bulk composites with specific nano-structured patterns; (b) inherent sensitivity to disorder and losses in their realization; (c) not straightforward modeling of their interaction with nearby optical sources. In this study, we develop a novel paradigm to use single-element nanoantennas, and composite nanoantenna arrays forming two-dimensional metasurfaces and three-dimensional metamaterials, to control and manipulate light and its polarization at the nanoscale, which can possibly bypass the abovementioned limitations in terms of design procedure and experimental realization. The final design of some of the metamaterial concepts proposed in this work was inspired by biological species, whose complex structure can exhibit superior functionalities to detect, control and manipulate the polarization state of light for their orientation, signaling and defense. Inspired by these concepts, we theoretically investigate and design metasurfaces and metamaterial models with the help of fully vectorial numerical simulation tools, and we are able to outline the limitations and ultimate conditions under which the average optical surface impedance concept may accurately describe the complex wave interaction with planar plasmonic metasurfaces. We also experimentally explore various technological approaches compatible with these goals, such as the realization of lithographic single-element nanoantenna and nanoantenna arrays with complex circuit loads, periodic arrays of plasmonic nanoparticles or nanoapertures, and stacks of rotated plasmonic metasurfaces. At the conclusion of this effort, we have theoretically analyzed, designed and experimentally realized and characterized the feasibility of using discrete metasurfaces to realize phenomena and performance that are not available in natural materials, oftentimes inspired by the biological world.Electrical and Computer Engineerin

Terahertz (THz) Waveguiding Architecture Featuring Doubly-Corrugated Spoofed Surface Plasmon Polariton (DC-SSPP): Theory and Applications in Micro-Electronics and Sensing

Terahertz (10^12 Hz) has long been considered a missing link between microwave and optical IR spectra. This frequency range has attracted enormous research attentions in recent years, with ever-growing anticipation for its applications in remote sensing, molecular spectroscopy, signal processing and next-generation high-speed electronics. However, its development has been seriously hindered by the lack of waveguiding and manipulating architectures that could support the propagation of THz radiations without excessive signal distortion and power loss. Facing this challenge, this work exploits the spoofed surface plasmon polariton (SSPP) mode of the THz oscillation and introduces the doubly corrugated SSPP (DC-SSPP) architecture to support sub-wavelength, low-dispersion THz transmission. DC-SSPP displays unique bandgap structure, which can be effectively modulated via structural and material variables. These unequaled properties make DC-SSPP the ideal solution to support not only signal transmission but also THz sensing and THz-electronics applications. In this thesis, theoretical analysis is carried out to thoroughly characterize the THz propagation, field distribution and transmission band structures in the novel architecture. Via numerical approximation and finite element simulations, design variations of the DC-SSPP are further studied and optimized to fulfill application-specific requirements. We demonstrate effective DNA sensing by adopting the Mach-Zehnder interferometer (MZI) or waveguide-cavity-waveguide insertions, which showed detectability with minuscule sample size even in the aqueous environment. We manifest high-speed analog-to-digital conversion via a combination of MZI DC-SSPP with nonlinear, partial-coupling detector arrays. Full characterization of the proposed ADC is carried out where high operation speed, small signal distortion, and great output linearity is shown. Also included in this work is a detailed review of the THz emitters and detectors, which are indispensable constituents of the THz system discussed herein. The future of the DC-SSPP in building THz bio-computing and THz digital circuits, considered as the next step of this research work, is also explored and demonstrated with the novel concept of directed logic network.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137130/1/xuzhao_1.pd

Implementation of the Finite Difference Time Domain algorithm for the analysis of terahertz waves

The aim of this report is to present results related to the application of the Finite Difference Time Domain (FDTD) method for the study of various devices at terahertz frequencies. The FDTD method has emerged as one of the most widely used numerical analysis methods used for electromagnetic simulations. The FDTD method can be used to solve numerous types of problems ranging from modelling the behaviour of microwave circuits, waveguides, and photonics to simulating terahertz devices and plasmas. In this study the FDTD approach is derived from first principles (finite differences) and explicit equations are shown based on Maxwell's equations. Furthermore, absorbing boundary conditions (ABCs) are discussed for the FDTD method. The algorithm is tested against a range of problems to ensure its validity and accuracy. The FDTD method is then applied for the numerical analysis of a range of devices at terahertz frequencies. The numerical results obtained from the application of the FDTD method to examine a microstrip circuit with filter loading and a Multi-mode Interference (MMI) coupler at terahertz frequencies are discussed in detail. Moreover, the results of the application of the FDTD method for the calculation of the dispersion characteristics of plasmonic waveguides are also presented. Finally, a summary of the work carried out is presented and an outline is given for future research which can be carried out by using the FDTD method for the study of terahertz frequency devices

Integrated Quantum Key Distribution sender unit for hand-held platforms

Mastering the generation, propagation and detection of electro-magnetic waves has enabled a technological breakthrough that has changed our entire society. World-wide communication through the telephone and the internet has become an integral part of our daily-life, which is expected to grow even further with the emergence of the internet of things. While secure communication was of concern mostly for governmental and financial institutions, digital security has now caught the attention of the general public. The weaknesses of cur- rent encryption protocols, such as the existence of back-doors or the predicted breakdown of popular algorithms such as RSA, reveal the need for alternative encryption schemes ensuring unconditional security on all types of devices. Quantum Key Distribution (QKD) has emerged as a powerful option to ensure a private communication between two users. Based on the laws of quantum mechanics, this class of protocols offers the possibility to detect the presence of a third party trying to intercept the key during its distribution, and even to quantify the amount of leaked information. While most research projects focus on long distance applications, little attention has been devoted to short distance schemes such as wireless payment, network access and authentication, which could highly benefit from QKD-enhanced security. This thesis focuses on the development of a miniature QKD sender add-on that could be embedded either in mobile devices or in existing optical communication platforms, thus allowing for a secure key exchange with a shared dedicated receiver over a free- space link. The proposed optics architecture (35 × 20 × 8 mm 3 ) is optimised for BB84-like protocols and uses an array of four Vertical-Cavity Surface-Emitting Lasers with highly similar properties to generate 40 ps long near-infrared faint coherent pulses at 100 MHz repetition rate. Under strong modulation, the polarisation of the pulses is not well defined and enables an external control of each diode’s emission by a wire-grid polariser. The four beams are spatially overlapped in a polarisation-insensitive femtosecond laser written waveguide array, and combined with a red beacon laser using an external beamsplitter to ensure a stable, synchronised optical link with the receiver. The complete module is compatible with current smartphone technology, allowing to run the classical post-processing over WLAN in the future. First tests with a free-space receiver indicate an average error ratio of 3.3 % and an asymptotic secure key rate of 54 kHz under static alignment. For the first time, a secure key exchange between a mobile platform held by a user and a receiver equipped with a dynamic alignment system could be demonstrated with an error ratio of 4.1 % and a secure key rate of 31 Hz. The further optimisation of the experimental parameters and the implementation of a decoy protocol will enhance the key generation rate as well as the general security of the system. The results of this thesis pave the way towards unprecedented security in wireless optical networks, as examplified for the communication between a mobile device and a dedicated receiver.Die Fähigkeit, elektromagnetische Strahlung kontrolliert zu erzeugen, gerichtet zu emittieren sowie zu detektieren stellte einen technologischen Durchbruch dar, der die ganze Gesellschaft verändert hat. Weltweite Kommunikation und Datenübertragung mittels Radiowellen, dem Telefon und dem Internet wurde zu einem wesentlichen Bestandteil des täglichen Lebens. Es ist zu erwarten, dass durch das Internet der Dinge die übertragene Datenmenge weiter zunehmen wird. Während zunächst vor allem Regierungen und Banken an Methoden für sichere Datenübertragung interessiert waren, wurde die Frage nach digitaler Sicherheit mit der Entwicklung mobiler Geräte, die immer mehr persönliche Daten sammeln und übertragen, in die breite Offentlichkeit getragen. Die Schwachpunkte aktueller Verschlüsselungstechnologien, wie z.B mögliche Hintertüren in existierenden Implementierungen oder das in absehbarer Zeit erwartete Brechen des weit verbreiteten RSA- Algorithmus, zeigen die Notwendigkeit alternativer Verfahren, deren Sicherheit nicht von zusätzlichen, mitunter nicht überprüfbaren Annahmen abhängt. Quantenschlüsselübertragung (engl. Quantum Key Distribution, QKD) stellt eine leistungsfähige Alternative dar, um verschlüsselte Kommunikation zwischen zwei Benutzern mithilfe eines gemeinsamen sicheren Schlüssels zu ermöglichen. Aufbauend auf den Gesetzen der Quantenmechanik ermöglicht es diese Klasse von Protokollen, eine dritte Partei beim Abhören des Schlüssels zu detektieren. Ebenso kann die Menge der möglicherweise abgefangenen Daten quantifiziert werden. Die meisten Forschungsprojekte konzentrierten sich bisher auf die Kommunikation über weite Strecken, wohingegen Anwendungen über kurze Entfernungen, wie z.B. handybasierte Bezahlmethoden oder Zugang und Authentifizierung in einem Netzwerk weitgehend vernachlässigt wurden, obwohl auch diese Anwendungen von der verbesserten Sicherheit durch QKD profitieren könnten. Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer miniaturisierten QKD Sendeeinheit für den sicheren Schlüsselaustausch über eine Freistrahlverbindung mit einem Empfänger, wie sie als Erweiterung für mobile Geräte oder bestehende optische Kommunikationsinfrastruktur verwendet werden könnte. Das vorgeschlagene Design des optischen Chips (35 × 20 × 8 mm³) ist für Protokolle, die sich an BB84 anlehnen optimiert und verwendet eine Anordnung von vier praktisch nicht zu unterscheidenden Oberflächenemittern (Vertical-Cavity Surface-Emitting Laser, VCSEL) die schwache, kohärente Pulse mit 40 ps Länge im nahen Infrarotbereich mit einer Wiederholrate von 100 MHz erzeugen. Bei starker Modulation ist die Polarisation der Pulse unbestimmt und kann daher mithilfe eines Gitterpolarisators für jede Diode separat eingestellt werden. Die vier Strahlen werden räumlich in einem optischen Wellenleiter, der mit einem Femtosekundenlaser geschrieben wurde und unabhängig von der Polarisation arbeitet, überlappt. Anschließend werden sie an einem externen Strahlteiler mit einem weiteren Laser im sichtbaren Bereich zusammengeführt. Dieser dient dazu, eine synchronisierte Verbindung zum Empfänger herzustellen. Das komplette Modul ist kompatibel mit aktueller Smartphonetechnik, wodurch ein klassischer Kanal, der für die Datennachbearbeitung benötigt wird, über LTE oder WLAN zur Verfügung gestellt werden kann. Erste Freistrahltests, bei denen Sender und Empfänger fixiert waren, ergaben eine durchschnittliche Fehlerrate von 3,3 % und eine asymptotische sichere Schlüsselrate von 54 kHz. Zum ersten Mal konnte auch ein sicherer Schlüsselaustausch zwischen einem vom Nutzer in der Hand gehaltenem mobilen Gerät und einem mit einem dynamischen Justagesystem ausgestatteten Empfänger gezeigt werden. Die Fehlerrate lag hierbei bei 4,1 % und die sichere Schlüsselrate bei 31 Hz. Durch eine weitere Optimierung der experimentellen Parameter sowie der Implementierung eines sogenannten Decoyprotokolls wird sich die Schlüsselrate sowie die Sicherheit des Systems noch deutlich erhöhen lassen. Zusammenfassend stellen diese Ergebnisse, exemplarisch gezeigt anhand der Kommunikation zwischen einem mobilen Gerät und einem stationren Empfänger, einen ersten Schritt hin zu bisher unerreichter Sicherheit in drahltlosen Netzwerken dar

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium