Abstract-Terahertz Waveguides: Broadband Single‐Mode Hybrid Photonic Crystal Waveguides for Terahertz Integration on a Chip

Haisu Li, Mei Xian Low, Rajour Tanyi Ako, Madhu Bhaskaran, Sharath Sriram, Withawat Withayachumnankul, Boris T. Kuhlmey, Shaghik Atakaramians,

https://onlinelibrary.wiley.com/doi/abs/10.1002/admt.202000117

Broadband, low‐loss, low‐dispersion propagation of terahertz pulses in compact waveguide chips is indispensable for terahertz integration. Conventional 2D photonic crystals (PCs) based terahertz waveguides are either all‐metallic or all‐dielectric, having either high propagation losses due to the Ohmic loss of metal, or a narrow transmission bandwidth restricted by the range of single‐mode operation in a frequency range defined by the PC bandgap, respectively. To address this problem, a hybrid (metal/dielectric) terahertz waveguide chip is developed, where the guided mode is completely confined by parallel gold plates and silicon PCs in vertical and lateral directions, respectively. A unique multiwafer silicon‐based fabrication process, including gold–silicon eutectic bonding, micropatterning, and Bosch silicon etching, is employed to achieve the self‐supporting hybrid structure. Theoretical and experimental investigations demonstrate that the hybrid waveguide supports a single‐mode transmission covering 0.367–0.411 THz (bandwidth of 44 GHz, over twice wider than that of all‐silicon PC waveguides) with low loss (below 0.05 dB mm−1) and low group velocity dispersion (from −8.4 to −0.8 ps THz−1 mm−1). This work enables more compact, wideband terahertz waveguides and auxiliary functional components that are integratable in chips toward ultra‐high‐density integrated terahertz devices in particular in the field of wireless communications.

Abstract-Dielectrics for Terahertz Metasurfaces: Material Selection and Fabrication Techniques

Rajour Tanyi Ako, Aditi Upadhyay, Withawat Withayachumnankul,   Madhu Bhaskaran, Sharath Sriram,

Correct choice of dielectric materials can overcome low efficiency and low operational bandwidth in terahertz metasurface devices. A guide to their selection based on properties and fabrication compatibility is presented. State‐of‐the‐art examples of dielectrics demonstrated as spacers and substrates and as resonant structures are covered, highlighting that selection and handling of dielectric materials can determine the performance of terahertz devices.

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900750

Manipulation of terahertz radiation opens new opportunities that underpin application areas in communication, security, material sensing, and characterization. Metasurfaces employed for terahertz manipulation of phase, amplitude, or polarization of terahertz waves have limitations in radiation efficiency which is attributed to losses in the materials constituting the devices. Metallic resonators‐based terahertz devices suffer from high ohmic losses, while dielectric substrates and spacers with high relative permittivity and loss tangent also reduce bandwidth and efficiency. To overcome these issues, a proper choice of low loss and low relative permittivity dielectric layers and substrates can improve field confinement and reduce dissipation. Alternatively, replacing metallic resonators with a moderate relative permittivity dielectric material that supports cavity mode resonances also reduces dissipation due to the absence of conduction current. Herein, an overview of dielectric materials employed as spacers and dielectric resonators is provided, and the fabrication methods employed to realize these devices at the terahertz frequency range are also presented. Material selection guidelines, material‐specific and application‐specific fabrication quality metrics are outlined, and new techniques are proposed.

Abstract-Terahertz multi-beam antenna using photonic crystal waveguide and Luneburg lens

APL Photonics

Daniel Headland,  Withawat Withayachumnankul, Ryoumei Yamada, Masayuki Fujita,  Tadao Nagatsuma,

Luneburg lens concept, showing the reciprocal principle of operation that relates a point on the circumference to a plane wave on the opposite side. It is noted that this behavior is represented with a ray-tracing diagram, but in practice, the lens must be significantly larger than a wavelength for this to be truly valid.

https://aip.scitation.org/doi/abs/10.1063/1.5060631

Recent years have seen the emergence of efficient, general-purpose terahertz photonic-crystal waveguides etched from high-resistivity silicon. Systems founded upon this platform will require antennas in order to interface with free-space fields. Multi-beam antennas are desirable to this end, as they are capable of interacting with a number of distinct directions simultaneously. Such functionality can be provided by Luneburg lenses, which we aim to incorporate with the terahertz photonic crystal waveguide. A Luneburg lens requires a precisely defined gradient-index, which we realize using effective medium techniques that are implemented with micro-scale etching of silicon. Thus, the photonic crystal waveguides can be integrated directly with the Luneburg lens and fabricated together from the same silicon wafer. In this way, we develop a planar Luneburg-lens antenna with a diameter of 17 mm and seven evenly spaced ports that cover a 120° field of view. Numerical and experimental characterization confirm that the antenna functions as intended over its operation bandwidth, which spans from 320 to 390 GHz. The Luneburg-lens antenna is subsequently deployed in a demonstration of terahertz communications over a short distance. The device may therefore find applications in terahertz communications, where multiple point-to-point links can be sustained by a given transceiver node. This form of terahertz beam control may also be useful for short-range radar that monitors several directions simultaneously.

Abstract-Tutorial: Terahertz beamforming, from concepts to realizations

Daniel Headland, Yasuaki Monnai, Derek Abbott, Christophe Fumeaux, Withawat Withayachumnankul,

https://aip.scitation.org/doi/am-pdf/10.1063/1.5011063?class=chorus+notVisible

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertzrange beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared light. Thereafter, noteworthy experimental demonstrations of beam control in the terahertz range are discussed, and these include geometric optics, phased array devices, leaky-wave antennas, reflectarrays, and transmitarrays. These techniques are compared and contrasted for their suitability in applications of terahertz waves. 

Abstract-Integrated Silicon Photonic Crystals Toward Terahertz Communications

Withawat Withayachumnankul,   Masayuki Fujita, Tadao Nagatsuma,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800401

The terahertz frequency range locates between 0.1 and 10 THz. This range accommodates atmospheric windows with staggering absolute bandwidth. It holds a potential for point‐to‐point wireless communications with an aggregate capacity reaching terabit per second in a range up to a kilometer. This unique capability is envisaged for backhauls between base stations and for local area networks. To this end, efficiency and compactness of the transceivers are crucial for successful large‐scale adoption. However, state‐of‐the‐art terahertz front ends are based on radio‐frequency or photomixing technologies that are inefficient, bulky, or complicated. In principle, as a neighbor of the microwave and optics domains, the terahertz band can leverage technologies from both sides to overcome those challenges. Recently, low‐loss integrated circuits based on photonic crystal waveguides are developed for routing terahertz waves. Here, a progress report on core components, including waveguides and diplexers, is presented. Additionally, the interfacing of the platform with electronic sources and detectors on one end, and with antennas for free‐space coupling on the other end, is discussed. Currently, the platform can support terahertz communications at a data rate over 10 Gbit s−1. Challenges and opportunities are discussed in the light of future development in this area.

Abstract-Dielectric-resonator metasurfaces for broadband terahertz quarter- and half-wave mirrors

Wendy S. L. Lee, Rajour T. Ako, Mei Xian Low, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnankul

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-11-14392

Polarization conversion of terahertz waves is important for applications in imaging and communications. Conventional wave plates used for polarization conversion are inherently bulky and operate at discrete wavelengths. As a substitute, we employ reflective metasurfaces composed of subwavelength resonators to obtain similar functionality but with enhanced performance. More specifically, we demonstrate low-order dielectric resonators in place of commonly used planar metallic resonators to achieve high radiation efficiencies. As a demonstration of the concept, we present firstly, a quarter-wave mirror that converts 45° incident linearly polarized waves into circularly polarized waves. Next, we present a half-wave mirror that preserves the handedness of circularly polarized waves upon reflection, and in addition, rotates linearly polarized waves by 90° upon reflection. Both metasurfaces operate with high efficiency over a measurable relative bandwidth of 49% for the quarter-wave mirror and 53% for the half-wave mirror. This broadband and high efficiency capabilities of our metasurfaces will allow to leverage maximum benefits from a vast terahertz bandwidth.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-All-dielectric rod antenna array for terahertz communications

Withawat Withayachumnankul, Ryoumei Yamada, Masayuki Fujita, Tadao Nagatsuma

https://aip.scitation.org/doi/abs/10.1063/1.5023787

The terahertz band holds a potential for point-to-point short-range wireless communications at sub-terabit speed. To realize this potential, supporting antennas must have a wide bandwidth to sustain high data rate and must have high gain and low dissipation to compensate for the free space path loss that scales quadratically with frequency. Here we propose an all-dielectric rod antenna array with high radiation efficiency, high gain, and wide bandwidth. The proposed array is integral to a low-loss photonic crystal waveguide platform, and intrinsic silicon is the only constituent material for both the antenna and the feed to maintain the simplicity, compactness, and efficiency. Effective medium theory plays a key role in the antenna performance and integrability. An experimental validation with continuous-wave terahertz electronic systems confirms the minimum gain of 20 dBi across 315–390 GHz. A demonstration shows that a pair of such identical rod array antennas can handle bit-error-free transmission at the speed up to 10 Gbit/s. Further development of this antenna will build critical components for future terahertz communication systems.

Abstract- Efficient terahertz metasurface-based flat lens

Daniel Headland.   Chun-Chieh Chang, Derek Abbott,  Withawat Withayachumnankul,  Hou-Tong Chen,

http://ieeexplore.ieee.org/document/8067113/

We present a nonuniform metasurface that operates as a flat lens at 400 GHz. Efficiency is enhanced by the use of a novel, tri-layer polarization-converting nonuniform meta-surface structure. The device is fabricated and experimentally characterized. The results confirm that the flat lens is indeed capable of focusing radiation to a focal spot with ∼68% efficiency. Constrained by spatial dispersion, the −3 dB bandwidth is 150 GHz.

Abstract-Demonstration of a highly efficient terahertz flat lens employing tri-layer metasurfaces

Chun-Chieh Chang, Daniel Headland, Derek Abbott, Withawat Withayachumnankul, and Hou-Tong Chen

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-42-9-1867&origin=search

We demonstrate a terahertz flat lens based on tri-layer metasurfaces allowing for broadband linear polarization conversion, where the phase can be tuned through a full 2𝜋$2\pi$ range by tailoring the geometry of the subwavelength resonators. The lens functionality is realized by arranging these resonators to create a parabolic spatial phase profile. The fabricated 124-μm-thick device is characterized by scanning the beam profile and cross section, showing diffraction-limited focusing and ∼68%$\sim 68\%$ overall efficiency at the operating frequency of 400 GHz. This device has potential for applications in terahertz imaging and communications, as well as beam control in general.

© 2017 Optical Society of America

Abstract-Editorial Introduction to the Special Issue: Terahertz Metamaterials and Photonic Crystals

Ibraheem Al-Naib, Withawat Withayachumnankul,

https://link.springer.com/article/10.1007/s10762-017-0425-7

The dawn of this century saw an emerging field of engineered materials called metamaterials. The concept promises a great degree of freedom for wave manipulation with these artificial materials across the electromagnetic spectrum. In the last decade, there has been a rapid progress in this field towards many applications. Underlying those applications are metamaterial-based components such as filters, absorbers, antenna structures, sensors, and high-frequency modulators. For terahertz (THz) science and technology, these metamaterials have been at the core, as various functionalities cannot be derived from natural materials with relatively weak electromagnetic responses. Motivated by this rapid development, this special issue features some latest progresses in terahertz metamaterials and photonic crystals. It consists of ten invited articles, the content of which is briefly outlined below. These contributions represent some of the ongoing research in this area.

1. 1.
   Tailoring terahertz propagation by phase and amplitude control in metasurfacesby J. Zheng, X. Zhang, L. Liu, Q. Li, L. Singh, J. Han, F. Yan, and W. Zhang (a collaboration between USA and Chinese universities) presents a review of their recent progress in metasurface-mediated propagation control of terahertz free-space and surface waves.
    
2. 2.
   Review on polarization selective terahertz metamaterials: from chiral metamaterials to stereometamaterials by E. Philip, M. Z. Güngördü, S. Pal, P. Kung, and Margaret Kim (the University of Alabama, USA) thoroughly reviewed recent development of sophisticated metamaterial structures called chiral metamaterials and stereometamaterials in the terahertz region and their abilities to perform as polarization waveplates, filters, and modulators.
    
3. 3.
   Recent progress in terahertz metasurfaces by I. Al-Naib and W. Withayachumnankul (University of Dammam, Saudi Arabia, and The University of Adelaide, Australia) reviews recent progress in terahertz metamaterials relating to wavefront control, high-Q sensing, and graphene-based structures.
    
4. 4.
   Terahertz sensor using photonic crystal cavity and resonant tunneling diodes by K. Okamoto, K. Tsuruda, S. Diebold, S. Hisatake, M. Fujita, and T. Nagatsuma (Osaka University and ROHM Co. Ltd., Japan) demonstrates a photonic crystal cavity integrated with resonant tunneling diodes for a source and detector. The integrated system achieves a quality factor of more than 10,000 for sensing applications.
    
5. 5.
   Influence of distance between metal squares in checkerboard patterns on transmittance characteristics in the infrared region by T. Higashira, T. Kageyama, K. Kashiwagi, H. Miyashita, K. Takano, M. Nakajima, and S.-S. Lee (Tottori University and Osaka University, Japan) investigates the influence of the metal square configuration in self-complementary metallic checkerboard patterns in the infrared region.
    
6. 6.
   Direct measurements of terahertz meta-atoms with near-field emission of terahertz waves by K. Serita, J. Darmo, I. Kawayama, H. Murakami, and M. Tonouchi (Osaka University, Japan, and Vienna University of Technology, Austria) presents direct measurements of terahertz meta-atoms by using locally generated terahertz waves in the near-field region.
    
7. 7.
   Narrowband metamaterial absorber for terahertz secure labeling by M. Nasr, J. T. Richard, S. A. Skirlo, M. S. Heimbeck, J. D. Joannopoulos, M. Soljacic, H. O. Everitt, and L. Domash (Triton Systems Inc.; Massachusetts Institute of Technology, U.S. Army Aviation and Missile RD&E Center, and IERUS Technologies, USA) describes flexible metamaterial films for securely labeling objects.
    
8. 8.
   Metamaterial demonstrates both a high refractive index and extremely low reflection in the 0.3-THz band by K. Ishihara and T. Suzuki (Ibaraki University and Tokyo University of Agriculture and Technology, Japan) reports the implementation of an unprecedented metamaterial with a high effective refractive index of 6.66 + j0.123, an extremely low reflection power of 1.16%, and a high figure of merit (FOM = |nreal/nimag|) of above 300 at a frequency of 0.3 THz.
    
9. 9.
   A brief review on metamaterial-based vacuum electronics for terahertz and microwave science and technology by T. Matsui (Mie University, Japan) summarizes recent research activities on metamaterial-inspired vacuum electronics for microwave and THz science and technology.
    
10. 10.
    Fiber drawn metamaterial for THz waveguiding and imaging written by S. Atakaramians, A. Stefani, H. Li, Md. S. Habib, J. G. Hayashi, A. Tuniz, X. Tang, J. Anthony, R. Lwin, A. Argyros, S. C. Fleming, and B. T. Kuhlmey (The University of Sydney, Australia; Technical University of Denmark, Denmark; and Beijing Jiaotong University, China) reviews their research activities in metamaterials fabricated by fiber drawing for terahertz waveguides and imaging applications.
     

The guest editors would like to thank all contributors and reviewers for their valuable contributions. We hope that the readers will find this special issue useful for their research.

Abstract-Fabry-Pérot interferometer for sensing polar liquids at terahertz frequencies

David Jahn1, Amin Soltani1, Jan C. Balzer1, Withawat Withayachumnankul2, and Martin Koch

http://aip.scitation.org/doi/abs/10.1063/1.4983780

We propose and validate a sensor for polar liquids that operates in conjunction with terahertz time-domain spectroscopy. The sensor is constructed from an optically thick silicon wafer and a ground plane, separated by a gap into which the liquid is injected. This arrangement represents a Fabry-Pérot interferometer that causes a sharp minimum in the reflection spectrum. Compared to resonance-based sensors, this sensor design can maintain its sharp spectral response when loaded with highly absorbing polar liquids. This overcomes an issue of damped resonance caused by material losses in resonance-based sensors. We report a reflection minimum shift of 8 GHz per percent ethanol in water. The sensor can be readily integrated with a microfluidic channel for real-time fluid monitoring.

Abstract-Recent Progress in Terahertz Metasurfaces

Ibraheem Al-Naib, Withawat Withayachumnankul,

https://link.springer.com/article/10.1007%2Fs10762-017-0381-2

In the past decade, the concept of metasurfaces has gradually dominated the field of metamaterials owing to their fascinating optical properties and simple planar geometries. At terahertz frequencies, the concept has been driven further by the availability of advanced micro-fabrication technologies that deliver sub-micron accuracy, well below the terahertz wavelengths. Furthermore, terahertz spectrometers with high dynamic range and amplitude and phase sensitivity provide valuable information for the study of metasurfaces in general. In this paper, we review recent progress in terahertz metasurfaces mainly in the last 5 years. The first part covers nonuniform metasurfaces that perform beamforming in reflection and transmission. In addition, we briefly overview four different methodologies that can be utilized in realizing high-quality-factor metasurfaces. We also describe two recent approaches to tuning the frequency response of terahertz metasurfaces using graphene as an active medium. Finally, we provide a brief summary and outlook for future developments in this rapidly progressing field.

Abstract-Terahertz near-field imaging of dielectric resonators

Wendy S. L. Lee, Korbinian Kaltenecker, Shruti Nirantar, Withawat Withayachumnankul, Markus Walther, Madhu Bhaskaran, Bernd M. Fischer, Sharath Sriram, and Christophe Fumeaux

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3756

As an alternative to metallic resonators, dielectric resonators can increase radiation efficiencies of metasurfaces at terahertz frequencies. Such subwavelength resonators made from low-loss dielectric materials operate on the basis of oscillating displacement currents. For full control of electromagnetic waves, it is essential that dielectric resonators operate around their resonant modes. Thus, understanding the nature of these resonances is crucial towards design implementation. To this end, an array of silicon resonators on a quartz substrate is designed to operate in transmission at terahertz frequencies. The resonator dimensions are tailored to observe their low-order modes of resonance at 0.58 THz and 0.61 THz respectively. We employ a terahertz near-field imaging technique to measure the complex near-fields of this dielectric resonator array. This unique method allows direct experimental observation of the first two fundamental resonances.

© 2017 Optical Society of America

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Abstract-Low-Profile Terahertz Radar Based on Broadband Leaky-Wave Beam Steering

Kosuke Murano,  Issei Watanabe,  Akifumi Kasamatsu,  Safumi Suzuki,  Masahiro Asada,  Withawat Withayachumnankul

http://ieeexplore.ieee.org/document/7762933/

We demonstrate short-range terahertz radar based on a leaky-wave antenna with a beam steering capability. As a proof of concept, we develop a microstrip-based periodic leaky-wave antenna driven by a vector network analyzer. By sweeping the frequency from 235 to 325 GHz, beam steering from −23∘ to +15∘ across the broadside can be achieved with a nearly constant beam width of 4 ∘. Small target detection is demonstrated by locating a metal cylinder with a diameter of 12 mm placed 46–86 mm in front of the antenna with a mean error of 2.4 mm. The use of a leaky-wave antenna can pave the way for developing a low-loss, low-profile, and wide-aperture terahertz radar. Importantly, it can be integrated with a solid-state source and a detector. The proposed approach is particularly promising for use with emerging small devices such as drones or wearable devices, where millimeter-wave radar is not suitable in terms of the resolution and system footprint.

Abstract-Analysis of 3D-printed metal for rapid-prototyped reflective terahertz optics

Daniel Headland, Withawat Withayachumnankul, Michael Webb, Heike Ebendorff-Heidepriem, Andre Luiten, and Derek Abbott
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-15-17384

We explore the potential of 3D metal printing to realize complex conductive terahertz devices. Factors impacting performance such as printing resolution, surface roughness, oxidation, and material loss are investigated via analytical, numerical, and experimental approaches. The high degree of control offered by a 3D-printed topology is exploited to realize a zone plate operating at 530 GHz. Reflection efficiency at this frequency is found to be over 90%. The high-performance of this preliminary device suggest that 3D metal printing can play a strong role in guided-wave and general beam control devices in the terahertz range.

© 2016 Optical Society of America

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Abstract-Dielectric Resonator Reflectarray as High-Efficiency Nonuniform Terahertz Metasurface

Daniel Headland*†, Eduardo Carrasco‡¶, Shruti Nirantar§∥, Withawat Withayachumnankul†⊥, Philipp Gutruf§∥,James Schwarz§∥, Derek Abbott†, Madhu Bhaskaran§∥, Sharath Sriram§∥, Julien Perruisseau-Carrier‡, and Christophe Fumeaux†

† School of Electrical and Electronic Engineering, The University of Adelaide, Adelaide, SA 5005, Australia

‡ École Polytechnique Fédérale de Lausanne, EPFL, 1015 Lausanne, Switzerland

¶ Foundation for Research on Information Technologies in Society, IT’IS, 8004 Zürich, Switzerland

§Functional Materials and Microsystems Research Group and ∥MicroNano Research Facility, RMIT University, Melbourne, Victoria 3000, Australia

⊥ Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan

ACS Photonics, Article ASAP

DOI: 10.1021/acsphotonics.6b00102

Publication Date (Web): May 13, 2016

Copyright © 2016 American Chemical Society

*E-mail: daniel.headland@adelaide.edu.au.

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00102

Advances in terahertz technology rely on the combination of novel materials and designs. As new devices are demonstrated to address the terahertz gap, the ability to perform high-efficiency beam control will be integral to making terahertz radiation a practical technology. Here, we use a metasurface composed of nonuniform dielectric resonator antennas on a ground plane to achieve efficient beam focusing at 1 THz. The dielectric resonators are made of high-resistivity silicon, which is a low-loss, nondispersive material for terahertz waves. The resonators operate around the resonance of the displacement current in the silicon, which is crucial to attaining high efficiency. The reflectarray’s capacity to focus terahertz radiation is experimentally verified, and hence by the principle of antenna reciprocity, it can also be employed as a terahertz collimator. The demonstrated device can therefore be deployed for high-gain terahertz antennas. Further measurements show that the loss of the reflectarray is negligible, which confirms the high efficiency of the dielectric resonators. This finding will enable the design of efficient flat-profile terahertz reflectarrays and metasurfaces to serve arbitrary beam control requirements in the near and far fields.