Abstract-Compensating Atmospheric Channel Dispersion for Terahertz Wireless Communication

Karl Strecker, Sabit Ekin,  John F. O’Hara,


https://www.nature.com/articles/s41598-020-62692-7

We report and demonstrate for the first time a method to compensate atmospheric group velocity dispersion of terahertz pulses. In ultra-wideband or impulse radio terahertz wireless communication, the atmosphere reshapes terahertz pulses via group velocity dispersion, a result of the frequency-dependent refractivity of air. Without correction, this can significantly degrade the achievable data transmission rate. We present a method for compensating the atmospheric dispersion of terahertz pulses using a cohort of stratified media reflectors. Using this method, we compensated group velocity dispersion in the 0.2-0.3 THz channel under common atmospheric conditions. Based on analytic and numerical simulations, the method can exhibit an in-band power efficiency of greater than 98% and dispersion compensation up to 99% of ideal. Simulations were validated by experimental measurements.

Abstract-Compensating Atmospheric Channel Dispersion for Terahertz Wireless Communication

Karl Strecker, Sabit Ekin, John F. O’Hara


https://www.nature.com/articles/s41598-020-62692-7

We report and demonstrate for the first time a method to compensate atmospheric group velocity dispersion of terahertz pulses. In ultra-wideband or impulse radio terahertz wireless communication, the atmosphere reshapes terahertz pulses via group velocity dispersion, a result of the frequency-dependent refractivity of air. Without correction, this can significantly degrade the achievable data transmission rate. We present a method for compensating the atmospheric dispersion of terahertz pulses using a cohort of stratified media reflectors. Using this method, we compensated group velocity dispersion in the 0.2-0.3 THz channel under common atmospheric conditions. Based on analytic and numerical simulations, the method can exhibit an in-band power efficiency of greater than 98% and dispersion compensation up to 99% of ideal. Simulations were validated by experimental measurements.

Abstract-Controllable broadband asymmetric transmission of terahertz wave based on Dirac semimetals

Linlin Dai, Yuping Zhang, John F. O’Hara, and Huiyun Zhang

 (a) PCR of linearly polarized wave, (b) the current densities in the top and bottom layers of the x-polarized wave at 1.389 THz and 1.668 THz, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-24-35784

We present a dynamic metamaterial based on Dirac semimetals and capable of realizing broadband and tunable asymmetric transmission in the terahertz region. The Dirac semimetal resonators have a chiral structure patterned with double-T resonators that results in partial polarization conversion of waves incident upon the material, leading to asymmetric transmission across a wide frequency range. We show how the gradual shift of the semimetal Fermi energy permits a method of control over the asymmetric total transmission.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Hello, 6G Testbed: Sub-THz Tinkerers, Welcome

  - ED TARGETT



https://www.cbronline.com/news/6g-sub-thz-ni

“Researchers need access to sub-THz testbeds to prototype multiple wireless use cases”

The world of 5G is barely beginning to be explored by businesses – its advocates tout a vision of smart factories; mobile game streaming; AR-augmented surgery; autonomous vehicles, and Pokemon Go on steroids – with network carriers rolling it out in phases this year and into 2020 in the UK, but experimental 6G work is already starting.

Texas-headquartered National Instruments (NI) gives a flavour of what that might look like this week, with the release of a sub-THz software defined radio (SDR) for 6G research, built on its mmWave Transceiver System and Virginia Diodes’ radio heads.

(The development cycle of a typical wireless standard is approximately 10 years, it notes; with 5G roll out gaining pace this year, 6G could emerge by 2029…)

Sub-THz, You Say?

With even 5G still rather more contested a term than it should be, what 6G might constitute is, at this stage, something of an open question.

Bets are being placed, however, on the use of the Terahertz (THz) frequency range (0.1 THz — 3 THz); the last span within the electromagnetic wave spectrum. (THz typically refers to 0.1–10 THz; sub-THz region is 0.1–0.3).

NI’s new gizmo, which is built on FPGAs (customisable and programmable chips) can be upgraded and customised to create a “real-time testbed for 6G research”, the company says, and is equipped with special radio heads that that can transmit/receive a wide variety of frequency bands in the sub-THz range.

Read this: Industry-First Xilinx Accelerator Card Runs in “Any Server, Any Cloud” – Fits Standard PCIe Slots

“Researchers need access to sub THz testbeds to prototype multiple wireless use cases. These testbeds must be highly flexible but also offer cutting edge performance in order to explore the boundaries of wireless performance at these very high frequencies,” said James Kimery, director of wireless research, NI.

He said the offering will “spur innovation on the way toward 6G.”

All signal processing, including coding, occurs in real-time on FPGAs added to the base transceiver system, the company said in a release today, with total system throughput is dependent on the frame structure and the number of channels used.

A typical expected throughput is 7.2 Gbps/channel, with a software front panel offering real-time visualisation of system level performance.

Can sub-THz/6G Networks… Work?

With massive MIMO-based 5G already a challenge for engineers (signals are easily block by physical structures, etc.) sub-THz/6G networks look set to require some ingenious materials developments.

As academics from Saudi Arabia’s King Saud University and France’s IETR University of Rennes put it in a joint paper, “Sub-THz Antenna for High-Speed Wireless Communication Systems” published March 2019: “Very high path loss is imposed as one of the main challenges at THz band frequencies, which poses a major constraint on communication distances.

“Additional challenges range from the implementation of compact high-power THz band transceivers, the development of efficient ultra-broadband antennas at THz Band frequencies, and characterization of the frequency-selective path loss of the THz band channel to the development of novel modulations, transmission schemes, and communication protocols tailored to the peculiarities of this paradigm.”

Other specialists at the coalface of network R&D agree.

“Admirable output powers, but only at cryogenic temperatures”

In a June 2019 paper “A Perspective on Terahertz Next-Generation Wireless Communications“, Oklahoma State University academics John F. O’Hara, Sabit Ekin, Wooyeol Choi and Ickhyun Song noted that the generation, reception, and conversion of terahertz waves in mobile devices requires “cutting-edge electronic, photonic, or hybrid approaches that push the limits of material properties and device capabilities.”

Modern photonic-based THz sources include quantum cascade lasers (QCLs); nonlinear optical-mixing sources, and ultrafast laser-driven pulsed sources and detectors.

They note: “The form factors and operational principles of these vary so widely that one-to-one comparisons in performance are very challenging. For example, terahertz QCLs can achieve admirable output powers, but only at cryogenic temperatures, whereas mid-infrared QCLs, used in conjunction with nonlinear crystals can make microwatts of tunable continuous wave (CW) terahertz (1-5 THz) at room temperature…

“All of these sources offer impressive performance in their own ways, but none so far
are easily integrated into larger digital electronic systems, which is arguably their biggest downfall for communication systems.”

Abstract-A Perspective on Terahertz Next-Generation Wireless Communications

John F. O’Hara, Sabit Ekin, Wooyeol Choi, Ickhyun Song

https://www.mdpi.com/2227-7080/7/2/43

In the past year, fifth-generation (5G) wireless technology has seen dramatic growth, spurred on by the continuing demand for faster data communications with lower latency. At the same time, many researchers argue that 5G will be inadequate in a short time, given the explosive growth of machine connectivity, such as the Internet-of-Things (IoT). This has prompted many to question what comes after 5G. The obvious answer is sixth-generation (6G), however, the substance of 6G is still very much undefined, leaving much to the imagination in terms of real-world implementation. What is clear, however, is that the next generation will likely involve the use of terahertz frequency (0.1–10 THz) electromagnetic waves. Here, we review recent research in terahertz wireless communications and technology, focusing on three broad topic classes: the terahertz channel, terahertz devices, and space-based terahertz system considerations. In all of these, we describe the nature of the research, the specific challenges involved, and current research findings. We conclude by providing a brief perspective on the path forward. 

Abstract-Comment on the Veracity of the ITU-R Recommendation for Atmospheric Attenuation at Terahertz Frequencies

 John F. O’Hara,  Daniel R. Grischkowsky,

https://ieeexplore.ieee.org/document/8331155/

The Radiocommunication Sector of the International Telecommunication Union (ITU-R) produces numerous global standards on the use and management of radiocommunication systems. Among these is Recommendation ITU-R P.676-11 (09/2016), which provides methods to estimate the attenuation by atmospheric gases for electromagnetic waves in the 1–1000 GHz frequency range. In this letter, we comment on the veracity of that recommendation in the 450–1000 GHz range. In particular, we compare ITU water vapor absorption estimates to the same data available from other sources: HITRAN and previously reported experimental measurements. We find that ITU estimates increasingly disagree with both sources as frequency and water vapor density increase, demanding a closer inspection as to the cause. This discrepancy is attributed to the method of inclusion of the summed contribution of resonance wings from absorption lines located in the 
THz range, and the method to account for continuum absorption.

Abstract-Active metasurface terahertz deflector with phase discontinuities

Xiaoqiang Su, Chunmei Ouyang, Ningning Xu, Wei Cao, Xin Wei, Guofeng Song, Jianqiang Gu, Zhen Tian, John F. O’Hara, Jiaguang Han, and Weili Zhang
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-21-27152

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.

© 2015 Optical Society of America

Full Article  |  PDF Article

Abstract-Limitation in thin-film sensing with transmission-mode terahertz time-domain spectroscopy

Withawat Withayachumnankul, John F. O’Hara, Wei Cao, Ibraheem Al-Naib, and Weili Zhang  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-1-972

Thin-film sensing with a film thickness much less than a wavelength is an important challenge in conventional transmission-mode terahertz time-domain spectroscopy (THz-TDS). Since the interaction length between terahertz waves and a sample film is short, a small change in the transmitted signal compared with the reference is considerably obscured by system uncertainties. In this article, several possible thin-film measurement procedures are carefully investigated. It is suggested that an alternating sample and reference measurement approach is most robust for thin-film sensing. In addition, a closed-form criterion is developed to determine the critical thickness, i.e., the minimal thickness of a film unambiguously detectable by transmission-mode THz-TDS. The analysis considers influences from the Fresnel transmission at interfaces and the Fabry-Pérot reflections, in addition to the propagation across the film. The experimental results show that typical THz-TDS systems can detect polymer films with a thickness down to a few microns, two orders of magnitude less than the wavelength. For reasonably accurate characterization, it is recommended that the film thickness be at least ten times above this limit. The analysis is readily extended to biomolecular and semiconductor films. The criterion can be used to estimate the system-dependent performance in thin-film sensing applications, and can help to ascertain whether an alternative terahertz sensing modality is necessary.

© 2014 Optical Society of America

Abstract: Dynamically reconfigurable terahertz metamaterial through photo-doped semiconductor

http://apl.aip.org/resource/1/applab/v99/i23/p231101_s1?isAuthorized=no

Dibakar Roy Chowdhury¹, Ranjan Singh¹, John F. O’Hara^1,2, Hou-Tong Chen¹, Antoinette J. Taylor¹, and Abul K. Azad¹

¹Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA

We demonstrate reconfigurable terahertz metamaterial (MM) in which constituent resonators can be switched from split-ring resonators (SRRs) to closed-ring resonators via optical excitation of silicon islands strategically placed in the split gap. Both the fundamental and the third-order resonance modes experience monotonic damping due to increasing conductive losses in the photo-doped silicon region. More importantly, increasing the optical fluence (>200 μJ/cm²) results in the excitation of the second-order resonance mode, which is otherwise forbidden in a split-ring resonator for the incidence polarization in our experiments. Such dynamical control of metamaterial resonances could be implemented in active terahertz devices to achieve additional functionalities.

© 2011 American Institute of Physics