## Monday, July 15, 2019

### Abstract-A broad dual-band switchable graphene-based terahertz metamaterial absorber

Limei Qi, Chang Liu, Syed Mohsin, Ali Shah

https://www.sciencedirect.com/science/article/pii/S0008622319306980

A switchable graphene-based terahertz metamaterial absorber is proposed by using nonstructured graphene loaded with simple dielectric resonators, which can achieve both the broad and dual-band absorption with polarization-independent and wide-angle characteristics. The relative bandwidth of the two bands above 80% absorption reaches 97.8% and 31% in the frequency range of 0.473-1.407 THz and 2.273-3.112 THz, respectively. By changing the chemical potential of graphene, the state of the absorber can be switched from absorption (>80%) to reflection (>91%) over the two broad bands. Physical mechanisms of the broad dual-band switchable absorber are investigated by the impedance matching theory and the coupled-mode theory. The dual bands are caused by the Fabry-Perot resonance of dielectric substrate, the broad and high absorption originates from the appropriate impedance match between the absorber and free space. As the absorber is based on a monolayer nonstructured graphene loaded with simple dielectric resonators, the processing difficulty will be reduced greatly and it is easy to tune it through the bias voltage. This structure provides a new perspective to design broad multi-band absorbers and would have promising applications in multiple amplitude modulators, imaging and sensing.

### Abstract-High-efficiency Huygens’ metasurface for terahertz wave manipulation

Ruiqiang Zhao, Zheng Zhu, Guohua Dong, Tingting Lv, Yuxiang Li, Chunying Guan, Jinhui Shi, and Han Zhang

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-14-3482

A fair amount of theoretical work has shown that Huygens’ metasurfaces well modulate electromagnetic waves by properly designing electrical impedance ${Z}_{es}$ and magnetic admittance ${Y}_{ms}$; however, the transmissive Huygens’ metasurface is still challenging in the terahertz band. In this work, a transmission-type Huygens’ metasurface with bilayer metallic patches has been proposed and theoretically demonstrated to show a reflectionless phase modulation for a linearly polarized terahertz wave. The simulation results show that the metasurface can achieve $2\pi$ phase coverage, and importantly the phase change can be simply achieved by changing a single geometric parameter of the metamolecule, along with a similar transmission effect. We design a high-efficiency beam deflector to realize an anomalous refraction with an angle of 19.8°. The proposed metasurface will provide a simple and direct way to realize efficient terahertz devices for wavefront manipulation.
© 2019 Optical Society of America

### Abstract-Design of Terahertz CMOS Integrated Circuits for High-Speed Wireless Communication

Minoru Fujishima, Shuhei Amakawa,

https://digital-library.theiet.org/content/books/cs/pbcs035e

Communications technology at a frequency range into Terahertz (THz) levels has attracted attention because it promises near-fibre-optic-speed wireless links for the 5G and post-5G world. Transmitter and receiver integrated circuits based on CMOS, which has the ability to realize such circuits with low power consumption at a low cost, are expected to become increasingly widespread, with much research into the underlying electronics currently underway. This book describes recent research on terahertz CMOS design for high-speed wireless communication. The topics covered include fundamental technologies for terahertz CMOS design, amplifier design, physical design approaches, transceiver design, and future prospects. This concise source of key information, written by leading experts in the field, is intended for researchers and professional circuit designers working in RFIC and CMOS design for telecommunications.

## Sunday, July 14, 2019

### Abstract-Characterization and Water Content Estimation Method of Living Plant Leaves Using Terahertz Waves

Adnan Zahid,  Hasan T. Abbas,  Muhammad A. Imran,  Khalid A. Qaraqe,  Akram Alomainy.  David R. S. Cumming, Qammer H. Abbasi,

https://www.mdpi.com/2076-3417/9/14/2781/htm

An increasing global aridification due to climate change has made the health monitoring of vegetation indispensable to maintaining the food supply chain. Cost-effective and smart irrigation systems are required not only to ensure the efficient distribution of water, but also to track the moisture of plant leaves, which is an important marker of the overall health of the plant. This paper presents a novel electromagnetic method to monitor the water content (WC) and characterisation in plant leaves using the absorption spectra of water molecules in the terahertz (THz) frequency for four consecutive days. We extracted the material properties of leaves of eight types of pot herbs from the scattering parameters, measured using a material characterisation kit in the frequency range of 0.75 to 1.1 THz. From the computed permittivity, it is deduced that the leaf specimens increasingly become transparent to the THz waves as they dry out with the passage of days. Moreover, the loss in weight and thickness of leaves were observed due to the natural evaporation of leaf moisture cells and change occurred in the morphology of fresh and water-stressed leaves. It is also illustrated that loss observed in WC on day 1 was in the range of 5% to 22%, and increased from 83.12% to 99.33% on day 4. Furthermore, we observed an exponential decaying trend in the peaks of the real part of the permittivity from day 1 to 4, which was reminiscent of the trend observed in the weight of all leaves. Thus, results in paper demonstrated that timely detection of water stress in leaves can help to take proactive action in relation to plants health monitoring, and for precision agriculture applications, which is of high importance to improve the overall productivity.

### Abstract-Characteristics of terahertz waves from laser-created plasma with an external electric field

T. Fukuda, T. P. Otsuka, T. Kobayashi, T. Asai, Y. Yoshida, K. Yamamoto, T. Nagami, H. Yamanaka, S. Endo,  N. Yugami

https://iopscience.iop.org/article/10.7567/1347-4065/ab2c31/pdf

We investigated the terahertz radiation from laser-created plasma in the presence of an external electric field parallel to the laser direction in the frequency range 0.14 to 0.33 THz. A narrow cone radiation pattern was observed by measuring the angular distribution, and polarizations were quasi-radial polarization. The terahertz intensities depended on the square of the electric field in the range 2 to 5 kV cm−1.

### Abstract-Thermally switchable terahertz wavefront metasurface modulators based on the insulator-to-metal transition of vanadium dioxide

 Fig. 1 Schematic image of the setup used for studying operational characteristics of the THz wavefront modulators as a function of temperature. (a) The device has no effect on the THz wave in its off-state (e.g., at T = 20 °C < TC). The THz device acts on the THz wave in its on-state (e.g., at T = 70 °C > TC) as (b) a multiple foci lens or (c) a Ariy beam generator. TTML: Temperature controlled THz multi-focus lens. TTAG: Temperature controlled THz Ariy beam gererator.

Active use of phase transition phenomena for reversibly tuning the properties of functional materials in devices currently is an attractive research area of materials science. We designed and fabricated two kinds of metasurface modulators for dynamically controlling the wavefront of terahertz (THz) radiation based on the temperature-induced insulator-to-metal phase transition of vanadium dioxide (VO2). The modulators designed are based on the C-shaped slot antenna array. The slot antennas are made of the VO2 films on c-sapphire substrates. The C-shaped slot antennas are active only when the VO2 is in its metallic phase, i.e. at temperatures T > TC ∼68 °C. At T > TC, the first kind acts as a THz multi-focus lens which converges an incident THz plane wave into four focal spots and the second kind as an Airy beam generator. We characterized the function of two THz wavefront modulators over a broad frequency range, i.e. from 0.3 to 1.2 THz. Such thermally switchable THz wavefront metasurface modulators with a capability of dynamically steering THz fields will be of great significance for the future development of THz active devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement