In recent years, the wireless data traffic grew exponentially, which was further accompanied by an increasing demand for higher data rates. Towards this aim, Terahertz band (0.1-10 THz) communication is envisioned as one of the key wireless technologies of the next decade. The THz band will help to overcome the spectrum scarcity problems and capacity limitations of current wireless networks, by providing an unprecedentedly large bandwidth. In addition, THz band communication will enable a plethora of long-awaited applications ranging from instantaneous massive data transfer among nearby devices in ultra-high-speed wireless personal and local area networks, to ultra-high-definition content streaming over mobile devices in 5G and beyond small cells. The objective of the thesis is to establish the physical layer foundations of the ultra- broadband communication in the THz band. First, a unified multi-path propagation channel is modeled in the THz band, based on ray-tracing techniques. The wideband characterization are analyzed, which include the distance-varying spectral windows, the delay spread, the wideband capacity and the temporal broadening effects. Second, a multi-wideband waveform design for the THz band is proposed to improve the distance and support ultra- high-speed transmissions. Third, two algorithms for timing acquisition in the pulse-based wireless systems are developed, namely the low-sampling-rate (LSR) algorithm, and the maximum likelihood (ML)-based approach. Fourth, the distance-aware bandwidth resource allocation schemes for the single-user and multi-user THz band networks are developed. Fifth, a three-dimensional (3-D) end-to-end model is developed and characterized, which includes the responses of the graphene-based reflectarray antenna and the 3-D multi-path propagation. The provided physical layer analysis in this thesis lays out the foundation for reliable and efficient ultra-high-speed wireless communications in the THz band.

Analysis has been made of the amplitudes of the second and third harmonics when pumping a discrete frequency to the Josephson-vortex photonic crystal within the THz range of the electromagnetic spectrum. The results of numerical simulations show that there are certain resonance frequencies for these harmonics where the amplitudes are strongly enhanced. The frequencies at which these resonances occur can be tuned by an applied magnetic field and tilting the material with respect to the incident radiation. For the second harmonic it has been possible to describe these resonances analytically with a resonance approximation which displays good agreement with numerical simulations at and near the resonances. A similar perturbative method has been used to simulate the nonlinear mixing of two discrete THz frequencies in the JV photonic crystal, producing resonances for harmonics at the sum and the difference of these two input frequencies. This method can allow a high degree of control over the harmonic frequencies produced.

Terahertz technology is rapidly growing for applications in various fields such as medical sciences, remote sensing, material characterization, and security. This accelerated growth has motivated engineers to develop compact, portable, and cost-effective terahertz sources and detectors. Terahertz generation and detection can be achieved using photoconductive antennas (PCAs), which have unique advantages. Notably, they do not require a vacuum or cryogenic cooling to function. PCAs operate on the principle of photoconductivity, which allows for compact integration with a fiber optic laser. It is also possible to launch THz radiation to a waveguide, which can be used for making a robust THz spectroscopy system. Ultra-short laser pulses are available in both 800 nm and 1550 nm wavelengths. However, the 1550 nm window has distinctive advantages such as availability of fiber amplifiers and fiber based electro-optical components at a relatively lower cost. The goal of this research is to introduce cost-effective and state-of-the-art solutions to develop THz transceivers for use in terahertz time-domain spectroscopy (THz-TDS) at 1550 nm wavelength. In this thesis we explore three approaches for enhancing THz emission and reception using PCAs. First, an array of hexagonal shape plasmonic nano-structures was used to increase the optical field coupling to the minimum depth of the substrate. Next, nano-structures also helped with enhancing the local electric field inside a low-cost semi-insulating GaAs substrate. This technique resulted in a 60% enhancement of the THz emission compared to a commercial LT-GaAs based PCA with antireflection coating. Moreover, the plasmonic nano-structures efficiently remove heat from the gap area allowing for operation at higher bias voltages. Plasmonic structures on LT-GaAs were investigated, which use a mid-gap Arsenic defect state to absorb 1550 nm light. The plasmonic devices were found to outperform existing InGaAs substrate based THz devices by factor of two. Finally, optimization of the LT-GaAs growth and annealing conditions was investigated to maximize the THz signal at 1550 nm. Outcomes of this research pave the way for designing cost-effective THz transceivers for time domain Terahertz spectroscopy systems at 1550 nm wavelength. / Graduate

archives@tulane.edu / In the past decades, the terahertz science and technology have been extensively studied due to their potential applications in fundamental physics, material characterization, communication, sensing and imaging. Although a lot terahertz optical devices have been proposed recently, but efficient, high-performance terahertz optical devices are still in great demand. With the development of plasmonic research and nano-/micro- fabrication techniques, plasmon and metasurface based terahertz optical devices demonstrate their capacity to fit these needs. It is crucial to learn the plasmon and metasurface mediated terahertz optical phenomena for designing such terahertz optical devices. This thesis will explore several plasmon and metasurface mediated terahertz optical phenomena and propose possible solutions for the design of terahertz optical devices. The plasmonic resonant responses of sub-wavelength metallic and dielectric gratings on Indium Antimonide (InSb) are first studied. The designed sub-wavelength metasurface structures are able to couple normal incident terahertz wave with the surface standing plasmon modes whose propagation constant is controlled by the period of the structure. The excited resonant mode on the metallic grating structure is sensitive to its ambient environment which could be potentially applied in molecular sensing. The high-refractive index dielectric grating on InSb wafer enables us to intentionally tune the plasmonic response of the structure which offers more flexibility for terahertz devices. The non-reciprocal reflection and reciprocal transmission of InSb wafer under weak external magnetic field is reported then. The surface plasmon theory of this non-reciprocal reflection and reciprocal transmission is reviewed and confirmed by the experiments. A high-performance THz optical isolator is then proposed based on this non-reciprocal reflection. A novel experiments setup to measure the quadratic terahertz nonlinearities using second-harmonic lock-in detection is proposed. The experimental method is demonstrated by measuring the THz Kerr effect on (110) Gallium Phosphide (GaP) crystal. The experimental design is extended to measure the second-harmonic generation of non-centrosymmetric media. We also design a split-ring-resonator (SRR) metasurface to enhance the second-harmonic generation from non-centrosymmetric media. / 1 / SHUAI LIN

Cette thèse présente et analyse, à température ambiante, plusieurs circuits dédiés à la détection et à la génération de signaux Terahertz en technologies CMOS 65nm bulk et 28 nm FDSOI. Ces travaux présentent la méthodologie de conception de matrices de détecteurs intégrés. La réalisation en technologies CMOS permet d’étudier la faisabilité de ces circuits comme solutions commerciales potentielles pour diverses applications Terahertz. Le domaine Terahertz (300 GHz – 3 THz) présente des caractéristiques très intéressantes permettant de nombreuses applications : l’imagerie médicale non-invasive (détection de cellules cancéreuses, imagerie dentaire, pharmaceutique, etc), la détection de produits chimiques, les portiques de sécurité, l’astronomie, la communication ultra haut débit et bien d’autres encore. Cependant, ce domaine est également connu pour le faible nombre de sources et de détecteurs commerciaux, ce qui a conduit à le surnommer le fossé THz. Les systèmes THz classiques souffrent notamment du faible niveau d’intégration et des coûts de réalisation très importants. Par conséquent, les produits THz actuels sont limités par un très faible nombre de pixels pour produire une image cadencée aux THz. Or, contrairement à l’état de l’art, le développement de circuits THz pour des produits grand public nécessite un haut niveau d’intégration, un débit important, une basse consommation et une utilisation à température ambiante. Les technologies en Silicium utilisées dans la majorité des produits électroniques grand public, sont une solution attirante pour combler ce fossé. Ces travaux de thèse vont de l’analyse théorique à l’optimisation de détecteurs réalisés dans différentes technologies et topologies d’éclairage, jusqu’à la conception d’un imageur vidéo de 1k-pixels, incluant le multiplexage, l’amplification et le traitement du signal. La conception de sources Terahertz basées sur des oscillateurs harmoniques visant à atteindre la plus haute fréquence possible en technologie CMOS est montrée. Les imageurs Terahertz sont aussi discutés dans le contexte de leurs applications correspondantes, bilan de liaison et faisabilité. / This PhD dissertation presents and analyses various room-temperature circuits for Terahertz detection and generation implemented in CMOS 65nm bulk and 28nm FDSOI throughout the course of the thesis. The work discusses the methodology of design and feasibility of fully-integrated focal-plane arrays of detectors in CMOS technologies as potential commercial solutions for various THz applications. The interesting characteristics of the Terahertz portion (300GHz-3THz) of the Electromagnetic spectrum incite plenty of applications ranging from safe and non-invasive medical imaging (cancer detection, dental imaging, pharmaceutical and other), security screening and chemical detection, safety inspection and quality control, astronomy, ultra-high data-rate communications and many others. However, this region of the Electromagnetic spectrum has been dubbed the THz-Gap due to the lack of commercial sources and detectors. Classical THz-systems, therefore, have been explicitly dominated by expensive technologies that suffer from low-integration levels and high operational costs. Consequently, current THz-products have been limited to single or few pixels only with raster-scanning techniques to produce single THz image-frames. Therefore, and contrary to the current state-of-the-art, developing such applications with commercial viability will require portability and high integration-levels, video-rate speeds, low power-consumptions as well as room-temperature operation. Reasonably, Silicon-based technologies that are the core of the vast majority of commercial and high-end electronic products seem to be a tempting solution to bring this THz-Gap. The investigations of this PhD thesis evolve from the theoretical analysis to the optimisation of naked detectors implemented in various technology nodes and illumination topologies, up to the implementation of a 1 k-pixel video imager that includes on-chip signal multiplexing, amplification and processing. Terahertz source design based on 5-push harmonic oscillators is discussed and aimed at attaining the highest frequencies possible in CMOS. Terahertz imaging systems are also discussed in the context of their corresponding applications, link budgets and feasibility.

Thesis (Ph. D.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 178-184). Also available on the World Wide Web.

This thesis focuses on the study of the electromagnetic properties of active spinplasmonic artificial materials. Artificial materials are composites having a macroscopic electromagnetic response that arises due to electromagnetic and electronic interactions between subwavelength sized elements. They are of practical engineering interest due to the wide range of free parameters such as the size, shape, density, and orientation of the individual elements, among others, thus providing the means to produce highly customizable photonic components. The thesis work can be categorized into four major sections: the design and construction of an advanced terahertz system capable of probing the electromagnetic response of such materials, the development of a class of artificial materials that permits the active, spin-dependent tuning of the position dependent phase accumulation of terahertz radiation, the study of spintronic-plasmonic artificial materials, and the discovery of a loss reduction mechanism for terahertz pulses transmitted through dense ensembles of bimetallic particles.

Nano-sensors, as an application of nanotechnology, are extremely important for environmental, medical and security applications. Terahertz science is an exciting new field that is set to impact the field of sensing to a large extent. I proposed to combine the fields of nanotechnology and terahertz science and develop a molecular nano-sensor that operates at terahertz frequencies. I focused our sensing on energetic materials, particularly nitromethane, and conducted an extensive analysis on its frequency spectrum. The study also focused on designing the nano-sensor and determining its terahertz operation characteristics. I subjected it to various conditions through the use of molecular dynamics simulations. Finally we analyzed the simulation results and provided a proof of the concept that we had a working molecular nano-sensor that operates at terahertz frequencies and senses energetic materials. The results from the frequency analysis of nitromethane showed that the frequency characteristics determined from our simulations were in close agreement with the ones determined experimentally. In addition to this we also successfully demonstrated the use of a Lennard Jones potential to model the CN bond scission of nitromethane. Finally, the results from the interactions between the nano-sensor and nitromethane showed that the presence of nitromethane causes sufficient change in the terahertz frequency characteristics of the nano-sensor providing a means to detect nitromethane.

Nano-sensors, as an application of nanotechnology, are extremely important for environmental, medical and security applications. Terahertz science is an exciting new field that is set to impact the field of sensing to a large extent. I proposed to combine the fields of nanotechnology and terahertz science and develop a molecular nano-sensor that operates at terahertz frequencies. I focused our sensing on energetic materials, particularly nitromethane, and conducted an extensive analysis on its frequency spectrum. The study also focused on designing the nano-sensor and determining its terahertz operation characteristics. I subjected it to various conditions through the use of molecular dynamics simulations. Finally we analyzed the simulation results and provided a proof of the concept that we had a working molecular nano-sensor that operates at terahertz frequencies and senses energetic materials. The results from the frequency analysis of nitromethane showed that the frequency characteristics determined from our simulations were in close agreement with the ones determined experimentally. In addition to this we also successfully demonstrated the use of a Lennard Jones potential to model the CN bond scission of nitromethane. Finally, the results from the interactions between the nano-sensor and nitromethane showed that the presence of nitromethane causes sufficient change in the terahertz frequency characteristics of the nano-sensor providing a means to detect nitromethane.

This thesis utilizes pulse shaping in characterization of high efficient wavelength tunable terahertz radiation. The influence of multi-pulses generation under different tunable chirp and time delay are characterized by Cross-FROG measurement. A strong enhancement of total power is observed from spectrum. Furthermore, the total power of terahertz radiation is reducing in same multi-pulses spacing experiment as chirp is increasing and negative chirp have higher THz total power than positive chirp. Meanwhile, we also theoretically and experimentally study the possibility of an alternative approach of enhancing the THz power. This enhancement is attributing to constructive interference between two independently generated THz pulses. This also reveals the nature of quantum coherence and potential of high power THz radiation.