Matching Items (12)

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Analysis, design, simulation, and measurements of flexible high impedance surfaces

Description

High Impedance Surfaces (HISs), which have been investigated extensively, have proven to be very efficient ground planes for low profile antenna applications due to their unique reflection phase characteristics. Another

High Impedance Surfaces (HISs), which have been investigated extensively, have proven to be very efficient ground planes for low profile antenna applications due to their unique reflection phase characteristics. Another emerging research field among the microwave and antenna technologies is the design of flexible antennas and microwave circuits to be utilized in conformal applications. The combination of those two research topics gives birth to a third one, namely the design of Conformal or Flexible HISs (FHISs), which is the main subject of this dissertation. The problems associated with the FHISs are twofold: characterization and physical realization. The characterization involves the analysis of scattering properties of FHISs in the presence of plane wave and localized sources. For this purpose, an approximate analytical method is developed to characterize the reflection properties of a cylindrically curved FHIS. The effects of curvature on the reflection phase of the curved FHISs are examined. Furthermore, the effects of different types of currents, specifically the ones inherent to finite sized periodic structures, on the reflection phase characteristics are observed. After the reflection phase characterization of curved HISs, the performance of dipole antennas located in close proximity to a curved HIS are investigated, and the results are compared with the flat case. Different types of resonances that may occur for such a low-profile antenna application are discussed. The effects of curvature on the radiation performance of antennas are examined. Commercially available flexible materials are relatively thin which degrades the bandwidth of HISs. Another practical aspect, which is related to the substrate thickness, is the compactness of the surface. Because of the design limitations of conventional HISs, it is not possible to miniaturize the HIS and increase the bandwidth, simultaneously. To overcome this drawback, a novel HIS is proposed with a periodically perforated ground plane. Copper plated through holes are extremely vulnerable to bending and should be avoided at the bending parts of flexible circuits. Fortunately, if designed properly, the perforations on the ground plane may result in suppression of surface waves. Hence, metallic posts can be eliminated without hindering the surface wave suppression properties of HISs.

Contributors

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Created

Date Created
  • 2013

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Terahertz Micro-Doppler Radar for Detection and Characterization of Multicopters

Description

The micromotions (e.g. vibration, rotation, etc.,) of a target induce time-varying frequency modulations on the reflected signal, called the micro-Doppler modulations. Micro-Doppler modulations are target specific and may contain information

The micromotions (e.g. vibration, rotation, etc.,) of a target induce time-varying frequency modulations on the reflected signal, called the micro-Doppler modulations. Micro-Doppler modulations are target specific and may contain information needed to detect and characterize the target. Thus, unlike conventional Doppler radars, Fourier transform cannot be used for the analysis of these time dependent frequency modulations. While Doppler radars can detect the presence of a target and deduce if it is approaching or receding from the radar location, they cannot identify the target. Meaning, for a Doppler radar, a small commercial aircraft and a fighter plane when gliding at the same velocity exhibit similar radar signature. However, using a micro-Doppler radar, the time dependent frequency variations caused by the vibrational and rotational micromotions of the two aircrafts can be captured and analyzed to discern between them. Similarly, micro-Doppler signature can be used to distinguish a multicopter from a bird, a quadcopter from a hexacopter or a octacopter, a bus from a car or a truck and even one person from another. In all these scenarios, joint time-frequency transforms must be employed for the analysis of micro-Doppler variations, in order to extract the targets’ features.

Due to ample bandwidth, THz radiation provides richer radar signals than the microwave systems. Thus, a Terahertz (THz) micro-Doppler radar is developed in this work for the detection and characterization of the micro-Doppler signatures of quadcopters. The radar is implemented as a continuous-wave (CW) radar in monostatic configuration and operates at a low-THz frequency of 270 GHz. A linear time-frequency transform, the short-time Fourier transform (STFT) is used for the analysis the micro-Doppler signature. The designed radar has been built and measurements are carried out using a quadcopter to detect the micro-Doppler modulations caused by the rotation of its propellers. The spectrograms are obtained for a quadcopter hovering in front of the radar and analysis methods are developed for characterizing the frequency variations caused by the rotational and vibrational micromotions of the quadcopter. The proposed method can be effective for distinguishing the quadcopters from other flying targets like birds which lack the rotational micromotions.

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Created

Date Created
  • 2018

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Fully passive wireless acquisition of neuropotentials

Description

The ability to monitor electrophysiological signals from the sentient brain is requisite to decipher its enormously complex workings and initiate remedial solutions for the vast amount of neurologically-based disorders. Despite

The ability to monitor electrophysiological signals from the sentient brain is requisite to decipher its enormously complex workings and initiate remedial solutions for the vast amount of neurologically-based disorders. Despite immense advancements in creating a variety of instruments to record signals from the brain, the translation of such neurorecording instrumentation to real clinical domains places heavy demands on their safety and reliability, both of which are not entirely portrayed by presently existing implantable recording solutions. In an attempt to lower these barriers, alternative wireless radar backscattering techniques are proposed to render the technical burdens of the implant chip to entirely passive neurorecording processes that transpire in the absence of formal integrated power sources or powering schemes along with any active circuitry. These radar-like wireless backscattering mechanisms are used to conceive of fully passive neurorecording operations of an implantable microsystem. The fully passive device potentially manifests inherent advantages over current wireless implantable and wired recording systems: negligible heat dissipation to reduce risks of brain tissue damage and minimal circuitry for long term reliability as a chronic implant. Fully passive neurorecording operations are realized via intrinsic nonlinear mixing properties of the varactor diode. These mixing and recording operations are directly activated by wirelessly interrogating the fully passive device with a microwave carrier signal. This fundamental carrier signal, acquired by the implant antenna, mixes through the varactor diode along with the internal targeted neuropotential brain signals to produce higher frequency harmonics containing the targeted neuropotential signals. These harmonics are backscattered wirelessly to the external interrogator that retrieves and recovers the original neuropotential brain signal. The passive approach removes the need for internal power sources and may alleviate heat trauma and reliability issues that limit practical implementation of existing implantable neurorecorders.

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Created

Date Created
  • 2014

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Design methodology for multiport antennas

Description

Multiport antennas offer greater design flexibility than traditional one-port designs. An antenna array is a special case of a multiport antenna. If the antenna's inter-element spacing is electrically small, the

Multiport antennas offer greater design flexibility than traditional one-port designs. An antenna array is a special case of a multiport antenna. If the antenna's inter-element spacing is electrically small, the antenna is capable of achieving superdirectivity. Superdirective antenna arrays are known to be narrow band and have low radiation resistance which leads to low radiation efficiency and high VSWR. However, by increasing the self-impedance of the antenna elements, the radiation resistance is increased but the bandwidth remains narrow. A design methodology is developed using the ability to superimpose electric fields and multi-objective optimization to design antenna feed networks. While the emphasis in this dissertation is on antenna arrays and superdirectivity, the design methodology is general and can be applied to other multiport antennas. The design methodology is used to design a multiport impedance-matching network and optimize both the input impedance and radiation pattern of a two-port superdirective antenna array. It is shown that the multiport impedance-matching network is capable of improving the input impedance of the antenna array while maintaining high directionality. The antenna design is critical for the methodology to improve the bandwidth and radiation characteristics of the array. To double the bandwidth of the two-port impedance matched superdirective antenna array, a three-port Yagi-Uda antenna design is demonstrated. The addition of the extra antenna element does not increase the footprint of the antenna array. The design methodology is then used to design a symmetrical antenna array capable of steering its main beam in two directions.

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Agent

Created

Date Created
  • 2012

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Holographic Metasurface Leaky Wave Antennas

Description

Articially engineered two-dimensional materials, which are widely known as

metasurfaces, are employed as ground planes in various antenna applications. Due to

their nature to exhibit desirable electromagnetic behavior, they are also used

Articially engineered two-dimensional materials, which are widely known as

metasurfaces, are employed as ground planes in various antenna applications. Due to

their nature to exhibit desirable electromagnetic behavior, they are also used to design

waveguiding structures, absorbers, frequency selective surfaces, angular-independent

surfaces, etc. Metasurfaces usually consist of electrically small conductive planar

patches arranged in a periodic array on a dielectric covered ground plane. Holographic

Articial Impedance Surfaces (HAISs) are one such metasurfaces that are capable of

forming a pencil beam in a desired direction, when excited with surface waves. HAISs

are inhomogeneous surfaces that are designed by modulating its surface impedance.

This surface impedance modulation creates a periodical discontinuity that enables a

part of the surface waves to leak out into the free space leading to far-eld radia-

tion. The surface impedance modulation is based on the holographic principle. This

dissertation is concentrated on designing HAISs with

Desired polarization for the pencil beam

Enhanced bandwidth

Frequency scanning

Conformity to curved surfaces

HAIS designs considered in this work include both one and two dimensional mod-

ulations. All the designs and analyses are supported by mathematical models and

HFSS simulations.

Contributors

Agent

Created

Date Created
  • 2017

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Gain and loss factor for conical horns, and impact of ground plane edge diffractions on radiation patterns of uncoated and coated circular aperture antennas

Description

Horn antennas have been used for over a hundred years. They have a wide variety of uses where they are a basic and popular microwave antenna for many practical applications,

Horn antennas have been used for over a hundred years. They have a wide variety of uses where they are a basic and popular microwave antenna for many practical applications, such as feed elements for communication reflector dishes on satellite or point-to-point relay antennas. They are also widely utilized as gain standards for calibration and gain measurement of other antennas.

The gain and loss factor of conical horns are revisited in this dissertation based on

spherical and quadratic aperture phase distributions. The gain is compared with published classical data in an attempt to confirm their validity and accuracy and to determine whether they were derived based on spherical or quadratic aperture phase distributions. In this work, it is demonstrated that the gain of a conical horn antenna obtained by using a spherical phase distribution is in close agreement with published classical data. Moreover, more accurate expressions for the loss factor, to account for amplitude and phase tapers over the horn aperture, are derived. New formulas for the design of optimum gain conical horns, based on the more accurate spherical aperture phase distribution, are derived.

To better understand the impact of edge diffractions on aperture antenna performance, an extensive investigation of the edge diffractions impact is undertaken in this dissertation for commercial aperture antennas. The impact of finite uncoated and coated PEC ground plane edge diffractions on the amplitude patterns in the principal planes of circular apertures is intensively examined. Similarly, aperture edge diffractions of aperture antennas without ground planes are examined. Computational results obtained by the analytical model are compared with experimental and HFSS-simulated results for all cases studied. In addition, the impact of the ground plane size, coating thickness, and relative permittivity of the dielectric layer on the radiation amplitude in the back region has been examined.

This investigation indicates that the edge diffractions do impact the main forward lobe pattern, especially in the E plane. Their most significant contribution appears in far side and back lobes. This work demonstrates that the finite edge contributors must be considered to obtain more accurate amplitude patterns of aperture antennas.

Contributors

Agent

Created

Date Created
  • 2014

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Fabrication and characterization of semiconductor nanolasers

Description

Semiconductor nanolasers, as a frontier subject has drawn a great deal of attention over the past decade. Semiconductor nanolasers are compatible with on-chip integrations towards the ultimate realization of photonic

Semiconductor nanolasers, as a frontier subject has drawn a great deal of attention over the past decade. Semiconductor nanolasers are compatible with on-chip integrations towards the ultimate realization of photonic integrated circuits. However, innovative approaches are strongly required to overcome the limitation of lattice-mismatch issues. In this dissertation, two alternative approaches are employed to overcome the lattice-mismatch issues. i) By taking advantage of nanowires or nanobelts techniques, flexibility in bandgap engineering has been greatly expanded, resulting in the nanolasers with wide wavelength coverage and tunability. Simultaneous two-color lasing in green and red is firstly achieved from monolithic cadmium sulfide selenide nanosheets. The wavelength separation is up to 97 nm at room temperature, larger than the gain bandwidth of a single semiconductor material in the visible wavelength range. The strategies adopted for two-color lasers eventually leads to the realization of simultaneous red, green and blue lasing and white lasing from a single zinc cadmium sulfide selenide nanosheet with color tunability in the full visible range, making a major milestone in the ultimate solution of laser illumination and laser display. In addition, with the help of nanowire techniques, material emission has been extended to mid-infrared range, enabling lasing at ~3µm from single lead sulfide subwavelength wires at 180 K. The cavity volume of the subwavelength laser is down to 0.44 λ3 and the wavelength tuning range is over 270 nm through the thermo-optic mechanism, exhibiting considerable potentials for on-chip applications in mid-infrared wavelength ranges. ii) By taking advantage of membrane transfer techniques, heterogeneous integration of compound semiconductor and waveguide material becomes possible, enabling the successful fabrication of membrane based nano-ring lasers on a dielectric substrate. Thin membranes with total thickness of ~200nm are first released from the original growth substrate and then transferred onto a receiving substrate through a generally applicable membrane transfer method. Nano-ring arrays are then defined by photolithography with an individual radius of 750 nm and a radial thickness of 400-500 nm. As a result, single mode lasing is achieved on individual nano-ring lasers at ~980 nm with cavity volumes down to 0.24 λ3, providing a general avenue for future heterogeneous integration of nanolasers on silicon substrates.

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Agent

Created

Date Created
  • 2016

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High-Directive Metasurface Printed Antennas for Low-Profile Applications

Description

Since the advent of High Impedance Surfaces (HISs) and metasurfaces, researchers

have proposed many low profile antenna configurations. HISs possess in-phase reflection, which reinforces the radiation, and enhances the directivity and

Since the advent of High Impedance Surfaces (HISs) and metasurfaces, researchers

have proposed many low profile antenna configurations. HISs possess in-phase reflection, which reinforces the radiation, and enhances the directivity and matching bandwidth of radiating elements. Most of the proposed dipole and loop element designs that have used HISs as a ground plane, have attained a maximum directivity of 8 dBi. While HISs are more attractive ground planes for low profile antennas, these HISs result in a low directivity as compared to PEC ground planes. Various studies have shown that Perfect Electric Conductor (PEC) ground planes are capable of achieving higher directivity, at the expense of matching efficiency, when the spacing

between the radiating element and the PEC ground plane is less than 0.25 lambda. To establish an efficient ground plane for low profile applications, PEC (Perfect Electric Conductor) and PMC (Perfect Magnetic Conductor) ground planes are examined in the vicinity of electric and magnetic radiating elements. The limitation of the two ground planes, in terms of radiation efficiency and the impedance matching, are discussed. Far-field analytical formulations are derived and the results are compared with full-wave EM simulations performed using the High-Frequency Structure Simulator (HFSS). Based on PEC and PMC designs, two engineered ground planes are proposed.

The designed ground planes depend on two metasurface properties; namely in-phase reflection and excitation of surface waves. Two ground plane geometries are considered. The first one is designed for a circular loop radiating element, which utilizes a

circular HIS ring deployed on a circular ground plane. The integration of the loop element with the circular HIS ground plane enhances the maximum directivity up to 10.5 dB with a 13% fractional bandwidth. The second ground plane is designed for a square loop radiating element. Unlike the first design, rectangular HIS patches are utilized to control the excitation of surface waves in the principal planes. The final design operates from 3.8 to 5 GHz (27% fractional bandwidth) with a stable broadside maximum realized gain up to 11.9 dBi. To verify the proposed designs, a prototype was fabricated and measurements were conducted. A good agreement between simulations and measurements was observed. Furthermore, multiple square ring elements are embedded within the periodic patches to form a surface wave planar antenna array. Linear and circular polarizations are proposed and compared to a conventional square ring array. The implementation of periodic patches results in a better matching bandwidth and higher broadside gain compared to a conventional array.

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Created

Date Created
  • 2020

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Design, modeling and simulation of nanoscale optoelectronic devices: semiconductor nano-lasers and plasmonic waveguides

Description

This thesis summarizes the research work carried out on design, modeling and simulation of semiconductor nanophotonic devices. The research includes design of nanowire (NW) lasers, modeling of active plasmonic waveguides,

This thesis summarizes the research work carried out on design, modeling and simulation of semiconductor nanophotonic devices. The research includes design of nanowire (NW) lasers, modeling of active plasmonic waveguides, design of plasmonic nano-lasers, and design of all-semiconductor plasmonic systems. For the NW part, a comparative study of electrical injection in the longitudinal p-i-n and coaxial p-n core-shell NWs was performed. It is found that high density carriers can be efficiently injected into and confined in the core-shell structure. The required bias voltage and doping concentrations in the core-shell structure are smaller than those in the longitudinal p-i-n structure. A new device structure with core-shell configuration at the p and n contact regions for electrically driven single NW laser was proposed. Through a comprehensive design trade-off between threshold gain and threshold voltage, room temperature lasing has been proved in the laser with low threshold current and large output efficiency. For the plasmonic part, the propagation of surface plasmon polariton (SPP) in a metal-semiconductor-metal structure where semiconductor is highly excited to have an optical gain was investigated. It is shown that near the resonance the SPP mode experiences an unexpected giant modal gain that is 1000 times of the material gain in the semiconductor and the corresponding confinement factor is as high as 105. The physical origin of the giant modal gain is the slowing down of the average energy propagation in the structure. Secondly, SPP modes lasing in a metal-insulator-semiconductor multi-layer structure was investigated. It is shown that the lasing threshold can be reduced by structural optimization. A specific design example was optimized using AlGaAs/GaAs/AlGaAs single quantum well sandwiched between silver layers. This cavity has a physical volume of 1.5×10-4 λ03 which is the smallest nanolaser reported so far. Finally, the all-semiconductor based plasmonics was studied. It is found that InAs is superior to other common semiconductors for plasmonic application in mid-infrared range. A plasmonic system made of InAs, GaSb and AlSb layers, consisting of a plasmonic source, waveguide and detector was proposed. This on-chip integrated system is realizable in a single epitaxial growth process.

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Created

Date Created
  • 2012

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Analysis, design and measurements of flat and curved circularly symmetric high impedance surfaces for curvilinear antenna applications

Description

In this dissertation a new wideband circular HIS is proposed. The circular periodicity made it possible to illuminate the surface with a cylindrical TEMz wave and; a novel technique is

In this dissertation a new wideband circular HIS is proposed. The circular periodicity made it possible to illuminate the surface with a cylindrical TEMz wave and; a novel technique is utilized to make it wideband. Two models are developed to analyze the

reflection characteristics of the proposed HIS.

The circularly symmetric high impedance surface is used as a ground plane for the design of a low-profile loop and spiral radiating elements. It is shown that a HIS with circular periodicity provides a wider operational bandwidth for curvilinear radiating elements such, such as loops and spirals, compared to canonical rectangular HISs.

It is also observed that, with the aid of a circular HIS ground plane the gain of a loop and a spiral increases compared to when a perfect magnetic conductor (PMC) or rectangular HIS is used as a ground plane. The circular HIS was fabricated and the loop and spiral elements were placed individually in close proximity to it.

Also, due to the growing demand for low-radar signature (RCS) antennas for advanced airborne vehicles, curved and flexible HIS ground planes, which meet both the aerodynamic and low RCS requirements, have recently become popular candidates within the antenna and microwave technology. This encouraged us, to propose a spherical HIS where a 2-D curvature is introduced to the previously designed flat HIS.

The major problem associated with spherical HIS is the impact of the curvature on its reflection properties. After characterization of the flat circular HIS, which is addressed in the first part of this dissertation, a spherical curvature is introduced to the flat circular HIS and its impact on the reflection properties was examined when it was illuminated with the same cylindrical TEMz wave. The same technique, as for the flat HIS ground plane, is utilized to make the spherical HIS wideband. A loop and spiral element were placed in the vicinity of the curved HIS and their performanceswere investigated. The HISs were also fabricated and measurements were conducted to verify the simulations. An excellent agreement was observed.

Contributors

Agent

Created

Date Created
  • 2018