Abstract-Broadband Terahertz Generation via the Interface Inverse Rashba-Edelstein Effect

C. Zhou, Y. P. Liu, Z. Wang, S. J. Ma, M. W. Jia, R. Q. Wu, L. Zhou, W. Zhang, M. K. Liu, Y. Z. Wu, and J. Qi

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.086801

Novel mechanisms for electromagnetic wave emission in the terahertz frequency regime emerging at the nanometer scale have recently attracted intense attention for the purpose of searching next-generation broadband THz emitters. Here, we report broadband THz emission, utilizing the interface inverse Rashba-Edelstein effect. By engineering the symmetry of the $\mathrm{Ag}/\mathrm{Bi}$ Rashba interface, we demonstrate a controllable THz radiation ($\sim 0.1–5\mathrm{THz}$) waveform emitted from metallic $\mathrm{Fe}/\mathrm{Ag}/\mathrm{Bi}$heterostructures following photoexcitation. We further reveal that this type of THz radiation can be selectively superimposed on the emission discovered recently due to the inverse spin Hall effect, yielding a unique film thickness dependent emission pattern. Our results thus offer new opportunities for versatile broadband THz radiation using the interface quantum effects.

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Abstract-Broadband in-line terahertz 2D imaging: comparative study with time-of-flight, cross-correlation, and Fourier transform data processing

Aleksandr Ushakov, Pavel Chizhov, Vladimir Bukin, Andrei Savel’ev, and Sergei Garnov

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-35-5-1159&origin=search

A pulsed broadband (from 0.1 to 1.5 THz), in-line, terahertz (THz) time domain electro-optical sampling is applied for the visualization of phase objects by using a large-aperture electro-optical crystal (ZnTe, 

1  cm×1  cm$\mathrm{}\mathrm{}$). The waveforms of the THz pulses are generated in a lithium niobate (LiNbO3${}_{\mathrm{}}$) crystal pumped by femtosecond pulses with a tilted amplitude front, and measured by a 2D THz imaging system. This system images the object under study in the THz range via a polytetrafluoroethylene (PTFE) lens in the 2𝑓−2𝑓$\mathrm{}\mathrm{}$ geometry onto a ZnTe crystal and then images the crystal surface on a CMOS camera using electro-optical sampling. Reconstruction of the image in such a scheme is implemented by processing the received 3D spatial-temporal distributions of the THz field in three different ways: (i) by detecting the displacement of the maximum peak position of the THz pulse due to a phase delay in the object under study; (ii) by using a cross-correlation function analysis; and (iii) by a Fourier transformation of a THz waveform and subsequent extraction of the phase difference at each THz frequency. Images of transparent PTFE objects were obtained. Main features of the resulting imaging system are discussed.

© 2018 Optical Society of America

Abstract-7 terahertz broadband GaP electro-optic sensor

Q. Wu, X. C. Zhang,

https://aip.scitation.org/doi/abs/10.1063/1.118691

We report a broadband coherent terahertz detection system using GaP as a free-space electro-optic field sensor with a demonstrated 3 dB bandwidth of 3.6 THz, useful bandwidth of 7 THz, and a pulse width of 185 fs. These figures represent new records in the coherent detection of free-space THz radiation. Sensor response as a function of crystal thickness and copropagation velocity mismatch dispersion in the THz regime is studied.

Abstract-Broadband THz radiation through tapered semiconductor gratings on high-index substrate

Truong Khang Nguyen, Phuc Toan Dang, Ikmo Park, and Khai Quang Le

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-3-583&origin=searchtapere

We numerically demonstrate an extraordinary optical transmission (EOT) through semiconductor gratings with plasmonic properties on a high-index substrate over a broad terahertz (THz) bandwidth at the plasmonic Brewster channel. The THz EOT is due to impedance matching, an inherently non-resonant mechanism, at the grating entrance and exit faces, which are periodically carved with tapered slits. The optimal grating geometry provides a transmission of over 85% over a broad 0.2–1.0 THz bandwidth at a Brewster angle of incidence of 75°. In addition, we introduce a perfect THz absorber with low-loss and metal-free properties over a broad operating bandwidth based on the plasmonic Brewster transmission concept.

© 2017 Optical Society of America

Abstract-Broadband amplification of low-terahertz signals using axis-encircling electrons in a helically corrugated interaction region

W. He, C. R. Donaldson, L. Zhang, K. Ronald, A. D. R. Phelps,  A. W. Cross,

https://journals.aps.org/prl/accepted/aa075Y48Q3c13a6851270ff5428f8a399615110c3


Experimental results are presented of a broadband, high power, gyrotron traveling wave amplifier (gyro-TWA) operating in the 75-110 GHz frequency band and based on a helically corrugated interaction region (HCIR). The second harmonic cyclotron mode of a 55 keV, 1.2 A, axis-encircling electron beam generated from a cusp electron gun was used to resonantly interact with a traveling TE21-like eigenwave, existing in the HCIR, achieving broadband amplification based on the cyclotron resonance maser instability. The gyro-TWA was measured to generate a maximum power of 3.4 kW with an unsaturated gain of 36-38 dB in the driving frequency band of 91-96.5 GHz.

Abstract-Metrology State-of-the-Art and Challenges in Broadband Phase-Sensitive Terahertz Measurements

 Mira Naftaly,  Roland G. Clarke,   David A. Humphreys, Nick M. Ridler,

http://ieeexplore.ieee.org/document/7829286/

The two main modalities for making broadband phase-sensitive measurements at terahertz (THz) frequencies are vector network analyzers (VNA) and time-domain spectrometers (TDS). These measuring instruments have separate and fundamentally different operating principles and methodologies, and they serve very different application spaces. The different architectures give rise to different measurement challenges and metrological solutions. This article reviews these two measurement techniques and discusses the different issues involved in making measurements using these systems. Calibration, verification, and measurement traceability issues are reviewed, along with other major challenges facing these instrument architectures in the years to come. The differences in, and similarities between, the two measurement methods are discussed and analyzed. Finally, the operating principles of electro–optic sampling (EOS) are briefly discussed. This technique has some similarities to TDS and shares application space with the VNA.

Abstract-Terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure on polyimide film

Xiao Wu  Huayue Li   Shixiong Liang   Jianjun Liu  Zhanghua Han  Zhi Hong 

http://www.opticsjournal.net/abstract.htm?aid=OJ1412290000238EaHdJ

A terahertz (THz) broadband polarizer using bilayer subwavelength metal wire-grid structure on both sides of polyimide film is simulated by the finite-difference time-domain method. We analyze the effect of film -thickness, material loss, and lateral shift between two metallic gratings on the performance of the THz -polarizer. Bilayer wire-grid polarizers are fabricated by a simple way of laser induced and non-electrolytic plating with copper. The THz time-domain spectroscopy measurements show that in 0.2–1.6 THz frequency range, the extinction ratio is better than 45 dB, the average extinction ratio reaches 53 dB, and the -transmittance exceeds 67%, which shows great advantage over conventional single wire-grid THz polarizer.

Abstract-Multi-Ray Channel Modeling and Wideband Characterization for Wireless Communications in the Terahertz Band

Bicen, A. ; Akyildiz, I.Han, C. 
Chong Han is with the Broadband Wireless Networking Laboratory (BWN-Lab), School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. (email: chong.han@ece.gatech.edu 
http://ieeexplore.ieee.org/xpl/abstractAuthors.jsp?reload=true&arnumber=6998944&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A4656680%29%26rowsPerPage%3D100

Terahertz Band (0.06 -10 THz) communication is envisioned as a key technology to satisfy the increasing demand for ultra-high-speed wireless links. In this paper, first, a unified multiray channel model in the THz Band is developed based on ray tracing techniques, which incorporates the propagation models for the line-of-sight, reflected, scattered and diffracted paths. The developed theoretical model is validated with the experimental measurements (0.06 - 1 THz) from the literature. Then, using the developed propagation models, an in-depth analysis on the THz channel characteristics is carried out. In particular, the distance-varying and frequency-selective nature of the Terahertz channel is analyzed. Moreover, the coherence bandwidth and the significance of the delay spread are studied. Furthermore, the wideband channel capacity using flat and water-filling power allocation strategies is characterized. Additionally, the temporal broadening effects of the Terahertz channel are studied. Finally, distance-adaptive and multi-carrier transmissions are suggested to best benefit from the unique relationship between distance and bandwidth. The provided analysis lays out the foundation for reliable and efficient ultra-high-speed wireless communications in the (0.06 - 10) THz Band.

Semi-OT DARPA closes in on high-speed wireless networks mounted on UAVs

My Note: I just saw this interesting article on the Virginia Diodes Facebook page
http://www.compoundsemiconductor.net/csc/news-details.php?cat=news&id=19737503 

Apr 09, 2014

The Mobile Hotspots program has progressed towards providing 1 Gb/s communications backbone to deployed units. During Phase 1, performers demonstrated output power exceeding 1 watt and 20 percent power added efficiency (PAE) from a single GaN chip operating at E-Band frequencies

Missions in remote, forward operating locations often suffer from a lack of connectivity to tactical operation centres and access to valuable intelligence, surveillance, and reconnaissance (ISR) data.

The assets needed for long-range, high-bandwidth communications capabilities are often unavailable to lower echelons due to theatre-wide mission priorities.

DARPA’s Mobile Hotspots program aims to help overcome this challenge by developing a reliable, on-demand capability for establishing long-range, high-capacity reachback that is organic to tactical units.

The program is building and demonstrating a scalable, mobile millimetre-wave communications backhaul network mounted on small unmanned aerial vehicles (UAVs) and providing a 1 Gb/s capacity. DARPA performers recently completed the first of three phases in which they developed and tested key technologies to be integrated into a complete system and flight tested in subsequent phases.

“We’re pleased with the technical achievements we’ve seen so far in steerable millimetre-wave antennas and millimetre-wave amplifier technology,” says Dick Ridgway, DARPA program manager. “These successes -and the novel networking approaches needed to maintain these high-capacity links - are key to providing forward deployed units with the same high-capacity connectivity we all enjoy over our 4G cell-phone networks.”

Phase 1 accomplishments include:

        Smaller, steerable millimetre-wave antennas: During field testing, the program successfully demonstrated steerable, compact millimetre-wave antennas that rapidly acquire, track, and establish a communications link between moving platforms. Steerable millimetre-wave antennas will enable the formation of a high-capacity backhaul network between aerial and ground platforms

        Low-noise amplifiers: Performers also demonstrated an advanced low-noise amplifier (LNA), which boosts the desired communications signal while minimising unwanted noise. The prototype achieved the record for one of the world’s lowest noise millimetre-wave LNAs at about half the noise figure of a typical LNA. 

        More efficient and capable power amplifiers:Efficient millimetre-wave amplification is required to achieve the long ranges (> 50 km) desired in the Mobile Hotspots programme.  During Phase 1, performers demonstrated output power exceeding 1 watt and 20 percent power added efficiency (PAE) from a single GaN chip operating at E-Band frequencies (71 GHz to 86 GHz). Output powers exceeding 20 watts and approaching 20 percent PAE were also achieved using power-combining techniques.

        New approaches for robust airborne networking: Mobile ad-hoc networking approaches were developed to maintain the high-capacity backhaul network among mobile air and ground platforms. Phase 1 performers developed unique solutions to overcome connectivity and network topology challenges associated with mobility and signal blockages due to terrain and platform shadowing.

        Low-Size, Weight, and Power (SWAP) pod design to carry it all: Performers created engineering designs for small, lightweight pods to be mounted on an RQ-7 Shadow UAV. The pods, with all of the Mobile Hotspots components inside, are designed to meet the challenging program goals of widths no more than 8 inches, weight less than 20 pounds, and power consumption less than 150 watts.

Phase 2 of the program began in March 2014. Two performers, L-3 Communications and FIRST RF, were chosen to lead teams comprising several Phase 1 performers. Phase 2 goals include the integration of the selected Phase 1 technologies into Shadow-compatible aerial pods and ground vehicles. 

Phase 2 will conclude with a ground demonstration of at least four Shadow-compatible pods, two ground vehicles and a fixed ground node. A planned third phase will encompass field testing of the Mobile Hotspot systems on networks of multiple SRQ-7 Shadow UAVs and mobile ground vehicles.

Abstract-Graphene as broadband terahertz antireflection coating

Zhou, Yixuan
State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, National Photoelectric Technology, Functional Materials and Application of Science and Technology International Cooperation Center, and Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China
Xu, Xinlong ; Hu, Fangrong ; Zheng, Xinliang ; Li, Weilong ; Zhao, Penghui ; Bai, Jintao ;Ren, Zhaoyu
http://ieeexplore.ieee.org/xpl/abstractAuthors.jsp?arnumber=6731433

We examined the potential of stacked multilayer graphene as broadband terahertz (THz) antireflection coating based on the impedance matching effect in experiment and theory. The reflected pulses from the quartz and silicon substrates were observed to change with the layer number and doping concentration of the graphene coating. Remarkable broadband impedance matching was achieved due to optimized THz conductivity. Theoretical analysis based on Drude model and thin film Fresnel coefficients have been used to explain the experimental phenomena, which indicated the shift of Fermi level caused by chemical doping. This work paves the way for graphene-based broadband THz antireflection coating.

Abstract-Broadband terahertz generation from metamaterials

• Liang Luo,
• Ioannis Chatzakis,
• Jigang Wang,
• Fabian B. P. Niesler,
• Martin Wegener,
• Thomas Koschny
• & Costas M. Soukoulis

http://www.nature.com/ncomms/2014/140108/ncomms4055/full/ncomms4055.html

The terahertz spectral regime, ranging from about 0.1–15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1–4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 μm (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility ~10⁻¹⁶ m² V⁻¹ that far exceeds thin films and bulk non-centrosymmetric materials.

Terahertz broadband metal-dielectric near-field antenna

http://www.cst.com/Content/Applications/Article/Terahertz+broadband+metal-dielectric+near-field

This antenna-based approach to near-field imaging and spectroscopy can be used for both continuous-wave and pulsed broadband electromagnetic radiation from microwave to terahertz frequencies. The model was constructed and simulated using CST MICROWAVE STUDIO® (CST MWS).

 

Figure 1: Picture of the near-field antenna 

The near-field antenna consists of a rectangular-shaped block of low-loss dielectric material sharpened to a pyramidal tip which is partially metallized and terminated by a micron-sized plane facet.

 

Figure 2: Field distribution in near-field antenna 

At this facet the entire energy of the incident wave is concentrated as a very high but strongly localized electric field, which can be used as a sensitive near-field microprobe for electromagnetic radiation. This is proven by the CST MWS simulation results shown in picture above .

 

Figure 3: Experimental Setup 

Currently, experiments in reflection geometry with pulsed terahertz radiation and continuous-wave radiation near 80  GHz reveal a frequency-independent spatial resolution of about 20  µm corresponding to  /200 at 80  GHz, which is only limited by the size of the facet terminating the tip.

 

Figure 4: 2D Scan of Water distribution in plant leafs at 80 GHz 

Potential applications of this device are:

  * Subcell resolution tissue imaging

 * THz spectroscopy on single cells

 * THz spectroscopy on single molecules

 * Contact - free spectroscopic imaging of ferroelectric domains

 * Fingerprint detection of very small amounts of hazardous   substances

 * Spatially resolved pump-probe experiments

 * Detection of water inclusion in minerals

All results are published with the permission of the FZ Jülich, ISG, and have been published in Journal of Applied Physics 98 (2005), 14910.

Abstract-Broadband Terahertz Wave Deflection Based on C-shape Complex Metamaterials with Phase Discontinuities

1. Xueqian Zhang¹, 
2. Zhen Tian^1,*, 
3. Weisheng Yue², 
4. Jianqiang Gu¹, 
5. Shuang Zhang³,
6. Jiaguang Han^1,*, 
7. Weili Zhang^1,4,*

1. ^{http://onlinelibrary.wiley.com/doi/10.1002/adma.201370210/abstract}

A terahertz metasurface is reported to exhibit broadband anomalous deflection with strong phase discontinuities. Weili Zhang, Jiaguang Han, Zhen Tian, and co-workers demonstrate on page 4567 that various frequency components ranging from 0.43 to 1.0 THz with polarization orthogonal to the incidence are deflected into a broad range of angles covering 25° to 84° under normal incidence. Based on this metasurface, a thin Fresnel zone plate was consequently developed that is capable of focusing terahertz radiation within a broad frequency range.