Transparent and Flexible Radio Frequency (RF) Structures

With increasing demand for a wearable devices, medical devices, RFID, and small devices, there is a growing interest in the field of transparent and flexible electronics. In order to realize optically transparent and flexible microwave components, novel materials can be used. The combination of new materials and radio frequency (RF) structures can open interesting perspectives for the implementation of cost effective wireless communication system and wearable device design. The transparent and flexible RF structures can facilitate its application in the transparent and curved surfaces. In this dissertation, we present several demonstrations, all based on optically transparent and flexible materials and structures. We firstly demonstrate an optically transparent, flexible, polarization-independent, and broadband microwave absorber. The bow-tie shaped array which possesses double resonances is designed and measured. The combined resonances lead to more than 90% total absorption covering a wide frequency range from 5.8 to 12.2 GHz. Due to the use of thin metal and PDMS, the whole structure is optically transparent and flexible. Secondly, we demonstrate a new method for fabricating transparent and stretchable radiofrequency small antennas by using stretchable micromesh structures. Size reduction is achieved by using the zeroth-order resonant (ZOR) property. The antennas consist of a series of tortuous micromesh structures, which provides a high degree of freedom for stretching when encapsulated in elastomeric polymers and is optically transparent. Accordingly, these antennas can be stretched up to 40% in size without breaking. The resonant frequency of the antennas is linearly reconfigurable from 2.94 GHz to 2.46 GHz upon stretching. Next, we describe an ultra-low profile and flexible triple-polarization antenna. It is realized by using ZOR array antenna with high port-to-port isolation. This flexible antenna is fabricated with a flexible substrate and silver nanowire vias to be used in various wearable applications. Lastly, we demonstrate a dual-band tri-polarized antenna based on half-mode hexagonal (HMH) SIW structure. CRLH HMHSIW antenna and ZOR HMHSIW antenna are designed to have dual-band operating frequencies. This novel antenna can provide much improved wireless communication efficiency for the WBAN system under various incident field angles and polarizations.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147562/1/tjang_1.pd

Frontiers in microphotonics: tunability and all-optical control

The miniaturization of optical devices and their integration for creating adaptive and reconfigurable photonic integrated circuits requires effective platforms and methods to control light over very short distances. We present here several techniques an

Dynamic complex opto-magnetic holography

Computer-generated holograms with their animated, three-dimensional appearance have long appealed to our imagination as the path towards truly immersive displays with bi-directional natural parallax. Impressive progress in updateable 3-D imagery has been achieved with liquid crystal modulators and high-resolution, but quasi-static holograms are being recorded in photosensitive materials. However, the memory requirements and computational loads of real-time, large-area holography will be hard to tackle for several decades to come with the current paradigm based on a matrix calculations and bit-plane writing. Here, we experimentally demonstrate a conceptually novel, holistic approach to serial computation and repeatable writing of computer-generated dynamic holograms without Fourier transform, using minimal amounts of computer memory. We use the ultrafast opto-magnetic recording of holographic patterns in a ferrimagnetic film with femtosecond laser pulses, driven by on-the-fly hardware computation of a single holographic point. The intensity-threshold nature of the magnetic medium allows sub-diffraction-limited, point-by-point toggling of arbitrarily localized magnetic spots on the sample, according to the proposed circular detour-phase encoding, providing complex modulation and symmetrical suppression of upper diffractive orders and conjugated terms in holographically reconstructed 3-D images

Universal Three Dimensional Optical Logic

Modern integrated circuits are essentially two-dimensional (2D). Partial three-dimensional (3D) integration and 3D-transistor-level integrated circuits have long been anticipated as routes to improve the performance, cost and size of electronic computing systems. Even as electronics approach fundamental limits however, stubborn challenges in 3D circuits, and innovations in planar technology have delayed the dimensional transition. Optical computing offers potential for new computing approaches, substantially greater performance and would complement technologies in optical interconnects and data storage. Nevertheless, despite some progress, few proposed optical transistors possess essential features required for integration into real computing systems. Here we demonstrate a logic gate based on universal features of nonlinear wave propagation: spatiotemporal instability and collapse. It meets the scaling criteria and enables a 3D, reconfigurable, globally-hyperconnected architecture that may achieve an exponential speed up over conventional platforms. It provides an attractive building block for future optical computers, where its universality should facilitate flexible implementations.Comment: manuscript (5 pages, 3 figures) with supplementary information (6 pages, 5 figures

Parallel and Distributed Computing

The 14 chapters presented in this book cover a wide variety of representative works ranging from hardware design to application development. Particularly, the topics that are addressed are programmable and reconfigurable devices and systems, dependability of GPUs (General Purpose Units), network topologies, cache coherence protocols, resource allocation, scheduling algorithms, peertopeer networks, largescale network simulation, and parallel routines and algorithms. In this way, the articles included in this book constitute an excellent reference for engineers and researchers who have particular interests in each of these topics in parallel and distributed computing

Doctor of Philosophy

dissertationDriven by a myriad of potential applications such as communications, medical imaging, security, spectroscopy, and so on, terahertz (THz) technology has emerged as a rapidly growing technological field during the last three decades. However, since conventional materials typically used in microwave and optical frequencies are lossy or do not effectively respond at these frequencies, it is essential to find or develop novel materials that are suitable for device applications in the THz range. Therefore, there is wide interest in the community in employing novel naturally-occurring materials, such as 2D materials, as well as in designing artificial metamaterial structures for THz applications. Here, we combined both of these approaches so to develop reconfigurable THz devices capable of providing amplitude modulation, phase modulation, and resonance frequency tuning. First, graphene is employed as the reconfigurable element in metamaterial phase modulators. For this purpose, we propose the use of unit cells with deep-subwavelength dimensions, which can have multiple advantaged for beam shaping applications. The analyzed metamaterials have one of the smallest unit cell to wavelength ratios reported or proposed todate at THz frequencies. By systematic analysis of the geometrical tradeoffs in these devices it is found that there is an optimal unit cell dimension, corresponding roughly to ~λ/20, which can deliver the best performance. In addition to this, we explored other applications of graphene in metamaterial devices, including amplitude modulation and resonance-shifting. These studies motivated us to analyze what is the most suitable role of graphene from a THz device perspective: is graphene a good plasmonic material? Or it is better suited as a reconfigurable material providing tunability to otherwise passive metallic structures? Our studies show that the Drude scattering time in graphene is an important parameter in this regard. In order to attain strong plasmonic resonances graphene samples with τ >> 1ps are required, which is challenging in large area CVD samples. But graphene is just one example of a wider class of 2D materials. In this work we also studied for the first time the application of 2D materials beyond graphene as reconfigurable elements in THz devices. For this purpose, Molybdenum Disulfide (MoS2) was employed as the reconfigurable element in cross-slot metamaterial amplitude modulators. Our results evidence that smaller insertion loss is possible when employing 2D materials with a bandgap, such as MoS2, rather than a zero-gap material such as graphene. Furthermore, because of a stronger optical absorption active control of the metamaterial properties is possible by altering the intensity of an optical pump. We later investigate and discuss transparent conductive oxides (TCOs), which constitute an interesting choice for developing visible-transparent THz-functional metamaterial devices for THz applications. These materials show a metallic THz response and thus can substitute the metal patterns in metamaterial devices. In our particular studies we analyzed samples consisting of: (i) two-dimensional electron gases at the interface between polar/nonpolar complex oxides having record-high electron density, and (ii) thin-films of La-doped BaSnO3 having record-high conductivity in a TCO. These materials exhibit a flat THz conductivity across a broad terahertz frequency window. As a result of their metal-like broadband THz response, we demonstrate a visible-transparent THz-functional electromagnetic structure consisting of a wire-grid polarizer

Flatland plasmonics and nanophotonics based on graphene and beyond

In this paper, we review and discuss how the recently discovered two-dimensional (2D) Dirac materials, particularly graphene, may be utilized as new efficient platforms for excitations of propagating and localized surface plasmon polaritons (SPPs) in the terahertz (THz) and mid-infrared (MIR) regions. The surface plasmon modes supported by the metallic 2D materials exhibit tunable plasmon resonances that are essential, yet missing, ingredients needed for THz and MIR photonic and optoelectronic devices. We describe how the atomically thin graphene monolayer and metamaterial structures based on it may tailor and control the spectral, spatial, and temporal properties of electromagnetic radiation. In the same frequency range, the newly unveiled nonlocal, nonlinear, and nonequilibrium electrodynamics in graphene show a variety of nonlinear and amplifying electromagnetic responses, whose potential applications are yet unexplored. With these 2D material platforms, virtually all plasmonic, optoelectronic, and nonlinear functions found in near-infrared (NIR) and visible devices can be analogously transferred to the long-wavelength regime, even with enhanced tunability and new functionalities. The spectral range from THz to MIR is particularly compelling because of the many spectral fingerprints of key chemical, gas, and biological agents, as well as a myriad of remote sensing, imaging, communication, and security applications