Modelling and Design of Efficient Photomixer Based Terahertz Antennas

The lack of unoccupied and unregulated bandwidth for wireless communication vanished at lower frequency spectrum and the increasing demand of high data transmission rate leads to an intensive interest in the research of THz technologies at 0.3THz to 30THz spectrum. However, the limitation of the low output power and low efficiency of current THz devices obstacles the utilization of THz technologies. Also, compared with microwave antenna, the signal generation and excitation of THz antenna require new simulation approach. Therefore, the motivation of this thesis is theoretically analyse the reason that cause the inefficiency of THz antenna, from which, the performance of such antennas is improved from the aspects of THz source with low efficiency, THz antenna with low match efficiency and THz antenna with low gain. These investigations are necessary for the development of the THz photomixer antenna in various applications . First of all, an new equation of the generated THz power from photomixer is developed from the equivalent circuit of photomixer fed antenna. Through this equation, various factors that affect the behaviour of photomixer is examined. Furthermore, a computational simulation process that solving both optoelectronic and electromagnetic problem in a full wave electromagnetic solver. This is a prerequisite for the analysis of improving the optical to THz conversion efficiency of photomixer. After that, the optical to THz conversion efficiency of the photomixer has been gradually improved through three different aspects, by optimizing photomixer electrodes, by utilizing reflectors underneath photomixer and by implementing superstrate. As a result, the highest enhancement factor of optical to THz conversion efficiency achieved is 494. Moreover, instead of exciting planar antenna with photomixer, the concept of truncating the photoconductive substrate of photomixer to form a dielectric resonator antenna is proposed. Such design eliminated the substrate effect to improve the radiation efficiency and to avoid using bulky lens. In addition, choke filter network and dielectric superstrate are used to improve the matching and radiation of these DRAs. The proposed DRA improved the matching efficiency and antenna gain by 10 times and 3dBi, respectively. Finally, a realization design that provide physically support to the dielectric superstrate and replace central feeding slot with coplanar waveguide is presented

Terahertz Technology and Its Applications

The Terahertz frequency range (0.1 – 10)THz has demonstrated to provide many opportunities in prominent research fields such as high-speed communications, biomedicine, sensing, and imaging. This spectral range, lying between electronics and photonics, has been historically known as “terahertz gap” because of the lack of experimental as well as fabrication technologies. However, many efforts are now being carried out worldwide in order improve technology working at this frequency range. This book represents a mechanism to highlight some of the work being done within this range of the electromagnetic spectrum. The topics covered include non-destructive testing, teraherz imaging and sensing, among others

Nanodevices for Microwave and Millimeter Wave Applications

The microwave and millimeter wave frequency range is nowadays widely exploited in a large variety of fields including (wireless) communications, security, radar, spectroscopy, but also astronomy and biomedical, to name a few. This Special Issue focuses on the interaction between the nanoscale dimensions and centimeter to millimeter wavelengths. This interaction has been proven to be efficient for the design and fabrication of devices showing enhanced performance. Novel contributions are welcome in the field of devices based on nanoscaled geometries and materials. Applications cover, but not are limited to, electronics, sensors, signal processing, imaging and metrology, all exploiting nanoscale/nanotechnology at microwave and millimeter waves. Contributions can take the form of short communications, regular or review papers

`THz Torch' technology: secure thermal infrared wireless communications using engineered blackbody radiation

The thermal (emitted) infrared frequency bands, from 20 to 40 THz and 60 to 100 THz, are best known for applications in thermography. This underused and unregulated part of the spectral range offers opportunities for the development of secure communications. The `THz Torch' concept, operating between the THz and mid-infrared ranges, was recently introduced. This technology fundamentally exploits engineered blackbody radiation, by partitioning thermally-generated spectral power into pre-defined frequency channels; the energy in each channel is then independently pulsed modulated to create a robust form of short-range secure communications in the far/mid-infrared. In the thesis, the development of `THz Torch' wireless communications systems will first be introduced. State-of-the-art THz technologies, infrared sources and detectors, as well as near-infrared and visible light communications technologies, will be reviewed in Chapter 2. Basic single-channel architecture of the `THz Torch' technology will be presented in Chapter 3. Fundamental limits for the first single-channel proof-of-concept demonstrator will be discussed, and possible engineering solutions will be proposed and verified experimentally. With such improvements, to date, octave bandwidth (25 to 50 THz) single-channel wireless links have been demonstrated with >2 kbit/s data rate and >10 cm transmission distance. To further increase the overall end-to-end data rate and/or the level of security, multiplexing schemes for `THz Torch' technologies are proposed in Chapter 4. Both frequency division multiplexing (FDM) and frequency-hopping spread-spectrum (FHSS) working demonstrators, operating between 10 and 100 THz spectral range, will be implemented. With such 4-channel multiplexing schemes, measured bit error rates (BERs) of <10−6 have been achieved over a transmission distance of 2.5 cm. Moreover, the integrity of such 4-channel multiplexing system is evaluated by introducing four jamming, interception and channel crosstalk experiments. Chapter 5 gives a detailed power link budget analysis for the 4-channel multiplexing system. The design, simulation and measurement of scalable THz metal mesh filters, which have potential applications for multi-channel `THz Torch' technology, will be presented in Chapter 6. The conclusions and further work are summarised in the last chapter. It is expected that this thermodynamics-based approach represents a new paradigm in the sense that 19th century physics can be exploited with 20th century multiplexing concepts for low cost 21st century ubiquitous security and defence applications in the thermal infrared range.Open Acces

High Bit Rate Wireless and Fiber-Based Terahertz Communication

RÉSUMÉ Dans le spectre électromagnétique, la bande des térahertz s’étend de 100 GHz à 10 THz (longueurs d’onde de 3 mm à 30 μm). Des décennies auparavant, le spectre des THz était connu sous le nom de « gap térahertz » en raison de l’indisponibilité de sources et détecteurs efficaces à ces fréquences. Depuis quelques années, la science a évolué pour faire migrer la technologie THz des laboratoires aux produits commerciaux. Il existe plusieurs applications des ondes THz en imagerie, spectroscopie et communications. Dans cette thèse, nous nous intéressons aux communications THz à travers deux objectifs. Le premier objectif est de développer une source THz de haute performance dédiée aux communications et basée sur les technologies optiques avec des produits commerciaux uniquement. Le second objectif est de démontrer l’utilisation de fibres optiques afin de renforcer la robustesse des communications THz sans fil. Nous débutons cette thèse avec une revue de la littérature scientifique sur le sujet de la communications THz sans fil et filaire. D’abord, nous discutons des deux méthodes communément utilisées (électronique et optique) pour démontrer des liens de communications THz avec leurs avantages et inconvénients. Nous présentons par la suite la possibilité d’utiliser un système de spectroscopie THz pour des applications en communications avec des modifications mineures au montage. Nous présentons ensuite plusieurs applications gourmandes en bande passante qui pourraient bénéficier du spectre THz, incluant la diffusion en continu (streaming) de flux vidéo aux résolutions HD et 4K non compressés. Ensuite, nous discutons de la motivation d’utiliser de longues fibres THz et notamment du fait qu’elles ne sont pas destinées à remplacer les fibres optiques conventionnelles de l’infrarouge, mais plutôt à augmenter la robustesse des liens THz sans fil. En particulier, les fibres THz peuvent être utilisées pour garantir le lien de communication dans des environnements géométriques complexes ou difficile à atteindre, ainsi que pour immuniser le lien THz aux attaques de sécurité. Plusieurs designs de fibres et guides d’onde précédemment démontrées dans la littérature sont discutés avec, entre autres, leurs méthodes de fabrication respectives. Nous discutons ensuite de la possibilité d’utiliser un simple guide d’onde diélectrique et sous-longueur d’onde pour transmettre l’information à un débit de l’ordre de plusieurs Gbps sur une distance de quelques mètres.----------ABSTRACT The Terahertz (THz) spectral range spans from 100 GHz to 10 THz (wavelength: 3 mm to 30 μm) in the electromagnetic spectrum. Decades ago, the THz spectral range is often named as ‘THz gap’ due to the non-availability of efficient THz sources and detectors. In the recent years, the science has evolved in bringing the THz technology from lab scale to commercial products. There are several potential applications of THz frequency band such as imaging, spectroscopy and communication. In this thesis, we focus on THz communications by addressing two objectives. The first objective is to develop a high-performance photonics-based THz communication system using all commercially available components. The second objective is to demonstrate the THz-fiber based communications, which can be used to increase the reliability of THz wireless links. We begin this thesis with a scientific literature review on the subject of THz wireless and fiber-based communications. First, the two different methodologies (all electronics based and photonics-based THz system) that is commonly used in the demonstration of THz communications is discussed along with their advantages and challenges. We then present the flexibility of photonics-based THz system where it is possible to switch it with minor modifications for THz spectroscopic studies and THz communication applications. Several bandwidth hungry applications that demands the use of THz spectrum for next generation communications is detailed. This includes the streaming of uncompressed HD/4K and beyond high-resolution videos, where the THz spectrum can be beneficial. Next, the motivation of using long THz fibers is discussed and we convince the readers that the THz fibers are not meant to replace the fibers in the optical-infrared region but to increase the reliability of THz wireless links. Particularly, the THz fibers can be used to provide connectivity in complex geometrical environments, secure communications and signal delivery to hard-to-reach areas. Several novel fiber/waveguide designs along with their fabrication technologies from the literature are presented. We then show that a simple solid core dielectric subwavelength fiber can be used to transmit the information in the order of several Gbps to a distance of a few meters

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies

Graphene-Assisted Integrated Nonlinear Optics

‎The unique linear and massless band structure of graphene in a purely two-dimensional Dirac fermionic structure has ignited intense research since the first monolayer graphene was isolated in the laboratory‎. ‎Not only does it offer new inroads into low-dimensional physics; graphene exhibits several peculiar properties that promise to widen the realm of opportunities for integrated optics and photonics‎. ‎This thesis is an attempt to shed light on the exceptional nonlinear optical properties of graphene and their potential applications in integrated photonics‎. ‎Following a theoretical exploration of light-graphene interaction‎, ‎disruptive new insight into the nonlinear optics of graphene was generated‎. ‎It now appears that graphene can efficiently enable photon-photon interaction in a fully integrated fashion‎. ‎This property‎, ‎taken together with ultrawideband tunability and ultrafast carrier dynamics could be fully exploited within integrated photonics for a variety of applications including harmonic generation and all-optical signal processing‎. ‎The multidisciplinary work described herein combines theoretical modeling and experimentation to proceed one step further toward this goal‎. ‎This thesis begins by presenting a semiclassical theory of light-graphene interaction‎. ‎The emphasis is placed on the nonlinear optical response of graphene from the standpoint of its underlying chiral symmetry‎. ‎The peculiar energy‎- ‎momentum dispersion of the quasiparticles in graphene entails a diverging field-induced interband coupling‎. ‎Following a many-body study of the carrier relaxations dynamics in graphene‎, ‎it will be shown that the charged carriers in the vicinity of the Dirac point undergo an unconventional saturation effect that can be induced by an arbitrarily weak electromagnetic field‎. ‎The perturbative treatment of the optical response of graphene is revisited and a theoretical model is developed to estimate the nonlinear optical coefficients including the Kerr coefficient of graphene‎. ‎The theoretical models are complimented by the experimental results‎. ‎The peculiar nonlinear optical properties of graphene together with its ablity to being integrated with optical platforms would render it possible to perform nonlinear optics in graphene integrated nanophotonic structures‎. ‎Here‎, ‎the suitability of graphene for nonlinear optical applications is investigated both theoretically and experimentally‎. ‎The emphasis is placed on an on-chip platform for ultrafast all-optical amplitude modulation‎. ‎The experimental results indicate strong all-optical modulation in a graphene-cladded planar photonic crystal nanocavity‎. ‎This development relies heavily on the unique properties of graphene‎, ‎including its fast carrier dynamics and the special phonon induced relaxation mechanism‎. ‎Finally‎, ‎the potential application of graphene based all-optical modulation in time resolved nonlinear spectroscopy is also discussed‎

Graphene-Assisted Integrated Nonlinear Optics

‎The unique linear and massless band structure of graphene in a purely two-dimensional Dirac fermionic structure has ignited intense research since the first monolayer graphene was isolated in the laboratory‎. ‎Not only does it offer new inroads into low-dimensional physics; graphene exhibits several peculiar properties that promise to widen the realm of opportunities for integrated optics and photonics‎. ‎This thesis is an attempt to shed light on the exceptional nonlinear optical properties of graphene and their potential applications in integrated photonics‎. ‎Following a theoretical exploration of light-graphene interaction‎, ‎disruptive new insight into the nonlinear optics of graphene was generated‎. ‎It now appears that graphene can efficiently enable photon-photon interaction in a fully integrated fashion‎. ‎This property‎, ‎taken together with ultrawideband tunability and ultrafast carrier dynamics could be fully exploited within integrated photonics for a variety of applications including harmonic generation and all-optical signal processing‎. ‎The multidisciplinary work described herein combines theoretical modeling and experimentation to proceed one step further toward this goal‎. ‎This thesis begins by presenting a semiclassical theory of light-graphene interaction‎. ‎The emphasis is placed on the nonlinear optical response of graphene from the standpoint of its underlying chiral symmetry‎. ‎The peculiar energy‎- ‎momentum dispersion of the quasiparticles in graphene entails a diverging field-induced interband coupling‎. ‎Following a many-body study of the carrier relaxations dynamics in graphene‎, ‎it will be shown that the charged carriers in the vicinity of the Dirac point undergo an unconventional saturation effect that can be induced by an arbitrarily weak electromagnetic field‎. ‎The perturbative treatment of the optical response of graphene is revisited and a theoretical model is developed to estimate the nonlinear optical coefficients including the Kerr coefficient of graphene‎. ‎The theoretical models are complimented by the experimental results‎. ‎The peculiar nonlinear optical properties of graphene together with its ablity to being integrated with optical platforms would render it possible to perform nonlinear optics in graphene integrated nanophotonic structures‎. ‎Here‎, ‎the suitability of graphene for nonlinear optical applications is investigated both theoretically and experimentally‎. ‎The emphasis is placed on an on-chip platform for ultrafast all-optical amplitude modulation‎. ‎The experimental results indicate strong all-optical modulation in a graphene-cladded planar photonic crystal nanocavity‎. ‎This development relies heavily on the unique properties of graphene‎, ‎including its fast carrier dynamics and the special phonon induced relaxation mechanism‎. ‎Finally‎, ‎the potential application of graphene based all-optical modulation in time resolved nonlinear spectroscopy is also discussed‎

Development of high performance microwaves, millimetres and terahertz antennas based on negative/gradient refractive index, and anisotropic metatronics

In this thesis, Metatronics have been applied to develop high performance antennas. This thesis added five new achievements to the scientific world. Firstly, a volumetric Negative Refractive Index (NRI) medium composed of Split Ring Resonators and Thin Wires (SRRs/TWs) is designed and incorporated with patch antenna operating at 10 GHz and 300 GHz. The gain is improved by 1.5dB. Secondly, a double sided NRI composed of Circular Split Ring Resonators and Thin Wires (CSRRs/TWs) employing a lens is proposed and characterized. The measured gain is improved from 6.5dB to 11.4dB. Thirdly, a new slotted waveguide antenna incorporated with Electrically Split Ring Resonator (ESRR) Metasurface (MTS) exhibiting NRI is proposed. The measured gain of the 10 GHz proposed antenna is improved from 6.7 dB to 10.5 dB. Furthermore, an anisotropic Low Epsilon Medium (LEM) ESRR-MTS is designed to focus the E-plane beam of the slotted antenna. The measured gain is improved by 4dB. Fourthly, high fabrication tolerance non-resonance and resonance GRIN MTS are proposed and characterized up to THz. The gain is improved from 6.7 to 11.3dB for 10 and 60 GHz antennas. Finally, a semi-analytical model based on transfer function is proposed to model THz Photoconductive antenna (PCA) excited by a femtosecond pulsed laser beam