Nanoantennas Design for THz Communication: Material Selection and Performance Enhancement

In the development of terahertz (THz) communication systems, the nanoantenna is the most significant component. Especially, the focus is to design highly directive antennas, because it enhances the performance of the overall system by compensating the large path loss at THz and thus improves the signal-to-noise ratio. This paper presents suitable material for nanoantenna design and the advancement in their performance for THz communications. Copper, Graphene, and carbon nanotube materials are used as promising candidates for nanoantenna design. The performance of nanoantennas is carried out by analyzing the properties and behavior of the material at THz. Results show that the Graphene nanoantenna provides better performance in terms of miniaturization, directivity, and radiation efficiency. Further, the performance enhancement of the nanoantenna at THz is studied by dynamically adjusting the surface conductivity via the chemical potential of Graphene using the electric field effect. The performance of the nanoantenna is enhanced in terms of high miniaturization, high directivity, low reflection, frequency reconfiguration, and stable impedance. The THz nanoantennas using Graphene have the potential to be used for THz communication systems. In view of the smart THz wireless environment; this paper finally presents a THz Hypersurface using Graphene meta-atoms. The user-side Graphene nanoantennas and environment-side Graphene Hypersurface can build a promising smart THz wireless environment

From Terahertz Imaging to Terahertz Wireless Communications

Terahertz (THz) technology is probably best known to the public as a powerful tool for imaging, since it has been applied in security and medical scanning, resulting in numerous impressive images that would be unobtainable using other technologies. With the roll-out of 5G mobile networks, research into 6G wireless communications is heating up. It is envisioned that THz technology will be used for 6G and future wireless communications. In this paper, we review how THz technology has been employed for imaging and wireless communications, identify state-of-the-art developments in the field, and then examine and compare common devices and issues in both applications. The possibility of integrating THz imaging/sensing and wireless communications is considered, and challenges and future perspectives are presented and discussed. It is shown that THz technology is indeed a key enabling technology for both imaging and wireless communications in the future

Tunnel field-effect transistors for sensitive terahertz detection

The rectification of electromagnetic waves to direct currents is a crucial process for energy harvesting, beyond-5G wireless communications, ultra-fast science, and observational astronomy. As the radiation frequency is raised to the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics becomes challenging and requires alternative rectification protocols. Here we address this challenge by tunnel field-effect transistors made of bilayer graphene (BLG). Taking advantage of BLG's electrically tunable band structure, we create a lateral tunnel junction and couple it to an antenna exposed to THz radiation. The incoming radiation is then down-converted by the tunnel junction nonlinearity, resulting in high-responsivity (> 4 kV/W) and low-noise (0.2 pW/$\sqrt{\mathrm{Hz}}$}) detection. We demonstrate how switching from intraband Ohmic to interband tunneling regime can raise detectors' responsivity by few orders of magnitude, in agreement with the developed theory. Our work demonstrates a potential application of tunnel transistors for THz detection and reveals BLG as a promising platform therefor

Enhancement of terahertz photoconductive antenna operation by optical nanoantennas

Photoconductive antennas are promising sources of terahertz radiation that is widely used for spectroscopy, characterization, and imaging of biological objects, deep space studies, scanning of surfaces, and detection of potentially hazardous substances. These antennas are compact and allow for generation of both ultrabroadband pulses and tunable continuous wave terahertz signals at room temperatures, with no need for high-power optical sources. However, such antennas have relatively low energy conversion efficiency of femtosecond laser pulses or two close pump wavelengths (photomixers) into the pulsed and continuous terahertz radiation, correspondingly. Recently, an approach to solving this problem that involves known methods of nanophotonics applied to terahertz photoconductive antennas and photomixers has been proposed. This approach comprises the use of optical nanoantennas for enhancing the absorption of pump laser radiation in the antenna gap, reducing the lifetime of photoexcited carriers, and improving the antenna thermal efficiency. This Review is intended to systematize the main results obtained by researchers in this promising field of hybrid optical-to-terahertz photoconductive antennas and photomixers. We summarize the main results on hybrid THz antennas, compare the approaches to their implementation, and offer further perspectives of their development including an application of all-dielectric nanoantennas instead of plasmonic ones

Plasmonic Devices in the Terahertz and Optical Frequency Domains

We are living in an age where the evolution of semiconductor devices and components is contingent upon their miniaturization and seamless integration with the rest of the circuitry. Unfortunately, it is anticipated that electronic systems will soon approach the theoretical design limits of size and bandwidth, and it poses to be a serious concern for the development of high-speed information technologies. Replacement of electronic pulses that act as communication signals with electromagnetic surface waves offers a very promising solution, particularly in terms of device miniaturization and the heart of this optimism are the plasmonic waves arising due to collective electron oscillations at the surface of a conductor. Surface plasmon polaritons propagating along a metal-dielectric interface at optical frequencies have lately been a subject of immense research interest, mainly due to their reduced wavelength at least by an order of magnitude. Hence, miniaturized wave devices can be created at optical frequencies. Terahertz plasma waves, on the other hand, exist in infinitesimally thin plasma regions formed inside a transistor substrate, and are observed at much lower frequencies in the far-infrared regime. Due to essentially a two-dimensional nature of the plasma region, a much higher wavelength reduction factor that can exceed well beyond 100 is achievable. Furthermore, the boundary conditions due to the transistor terminals along with electric biasing create unstable resonance conditions that eventually lead to radiation in the terahertz frequency range. Such phenomena provide bright prospects for creating highly miniaturized terahertz devices. A reliable and efficient electromagnetic (EM) analysis for multilayer geome tries has gained further significance due to the emergence of plasmonic structures in the optical as well as terahertz frequency domains. In this regard, integral equation (IE) techniques are ideally suited due to their efficient handling of mutilayer structures. Although the presence of thin layers poses a challenge to any EM analysis technique, here the procedure is simplified due to the infinitesimally thin nature of the plasma region, which can be analyzed as a conducting sheet, with the same current flowing on either side of the sheet. Essential to any IE technique is an efficient and systematic formulation of Green functions (GFs) and their subsequent computation. In this dissertation, a transmission-line network based approach is adopted to derive spectral domain GFs for an infinitesimally thin sheet in a layered medium. The associated spatial domain counterparts are then computed through the Sommerfeld integrals (SIs). The extraordinary electromagnetic properties of plasmonic devices are demonstrated by a presentation of the properties of plasmonic antennas and a super-resolution imaging scheme which is able to resolve objects separated only by a few nanometers

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future

Efficient Antennas for Terahertz and Optical Frequencies.

The inherent low conductivity of metals and low skin depth at terahertz (THz) frequencies and above decrease antennas radiation efficiency at such high frequencies. In addition to the high surface impedance, due to the small dimensions of metallic antennas at THz frequencies and above can further reduce the radiation efficiency. Nevertheless, attention has been drawn to metallic antennas that are designed to operate at optical frequencies due to their ability to create substantial field confinement and enhancement at their terminals. In this thesis, a highly conductive nanomaterial, Bundled Carbon Nanotubes (BCNTs) is examined to overcome the low efficiency of metallic antenna at THz frequencies. Due to their axial current, BCNTs are modeled by a 2D anisotropic resistive sheet having a tensor surface resistivity. Using a numerical method, the radiation efficiency of antennas consisting of BCNTs and gold are compared and it is concluded that BCNTs should be packed up about 1,000 times more than the current density of BCNTs to outperform gold at 2 THz. Efficient nanoantennas near infrared (IR) frequencies are also studied to enhance the performance of uncooled IR detectors and thermophotovoltaics (TPVs) power transducers. A gold bowtie dipole antenna topology loaded with a low bandgap indium gallium arsenide antimonide (InGaAsSb) p-n junction is investigated for this purpose. Through optimized arrangements, it is shown that a large array of flexible load bowtie nanoantennas can produce an efficient TPV system that can absorb 95% of the incident power. Similarly, a focal-plane array of nano-bowtie antennas used as an uncooled IR detector is demonstrated to enhance the sensitivity of the detector by a factor equal to the field enhancement factor, approximately 23 when compared to a detector made from a thick layer of the same material, InGaAsSb. Finally, a more advanced antenna topology using a cross tapered-bowtie antenna for detecting circularly polarized (CP) IR signals is designed and its perfect CP property is verified experimentally for the microwave range. A conceptual full‐Stoke's vector polarimetric imager using focal planar arrays of the nanoantennas with vertical, horizontal, 45°-tilted, and right-hand circular polarization is proposed.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107316/1/sangjo_1.pd

Nano-Dielectric Resonator Antenna Reflectarray/Transmittarray for Terahertz Applications

Nanoantennas have introduced wide bandwidth for fast data communications. The material properties of good conducting metals introduce plasmonic behavior at Terahertz frequencies. The material property of good conducting metals using Drude Lorentz model has been investigated. The radiation characteristics of nano-dielectric resonator antenna (NDRA) reflectarray at 633 nm have been investigated. A parametric study for the nano DRA unit cell dimensions and material has been introduced. A NDR with silver ground plane have been designed and analyzed. A nano-transmitarray unit-cell has been introduced for the analysis. A comparison between the radiation characteristics of 17×17 and 21×21 NDRA transmitarray has been given. A compromise between the nano-transmitarray size, maximum gain, and operating bandwidth is applied to Terahertz applications. The finite integral technique is used to carry a full wave analysis to design a NDRA reflectarray and a NDRA transmitarray

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