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To cover the so-called terahertz gap in powerful sources of coherent electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-free superconducting magnet has been designed, manufactured, and tested. With an operating voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuous-wave radiation at a frequency of 0.526 Terahertz with an output power of 250 Watts and an efficiency of 2.7%.

The terahertz gap attracted interest of researchers with its captivating properties such as its potential for ultrawideband communication and high-resolution imaging applications, which require robust and efficient terahertz components and circuits. Terahertz spoof surface plasmon polaritons are perfect alternatives to implement terahertz integrated circuits. We present the first-time demonstration of the terahertz spoof surface plasmon polariton delay lines for terahertz phase shifters, which present an unprecedented insertion phase versus loss performance. The electrical lengths of the proposed delay lines are tuned just by changing the corrugation depths, which allows the designer to keep the physical length of the delay lines constant. In order to verify the proposed idea, four different delay lines having the same length of 1052.5 mu m are designed, where the electrical lengths of the delay lines are selected to represent the four different phase states of a 2-bit phase shifter. The measurement results of the fabricated terahertz delay lines show almost perfect phase accuracy and very good agreement with the simulations, having an ultra-low average phase error of 0.6%, and average insertion and return losses of -2.3 and -18 dB, respectively. These measurements result in, to the best of our knowledge, a record-high figure-of-merit of 90 degrees/dB compared to all other kinds of planar waveguides at 0.3 THz. The proposed delay lines emerge as a very high-performance and promising candidate for all the waveguide-based components that are crucial for the terahertz integrated circuits.

With the right dimensions, a laser composed of a series of quantum wells emits hard-to-produce terahertz-frequency light without the usual need for cryogenic cooling.

The terahertz range has been barely exploited compared to the rest of the electromagnetic spectrum. But as Sidney Perkowitz argues, our ability to detect radiation at these wavelengths has proved vital in many areas of astronomy and cosmology

Since about one and half centuries ago, at the dawn of modern communications, the radio and the optics have been two separate electromagnetic spectrum regions to carry data. Differentiated by their generation/detection methods and propagation properties, the two paths have evolved almost independently until today. The optical technologies dominate the long-distance and high-speed terrestrial wireline communications through fiber-optic telecom systems, whereas the radio technologies have mainly dominated the short- to medium-range wireless scenarios. Now, these two separate counterparts are both facing a sign of saturation in their respective roadmap horizons, particularly in the segment of free-space communications. The optical technologies are extending into the mid-wave and long-wave infrared (MWIR and LWIR) regimes to achieve better propagation performance through the dynamic atmospheric channels. Radio technologies strive for higher frequencies like the millimeter-wave (MMW) and sub-terahertz (sub-THz) to gain broader bandwidth. The boundary between the two is becoming blurred and intercrossed. During the past few years, we witnessed technological breakthroughs in free-space transmission supporting very high data rates, many achieved with the assistance of photonics. This paper focuses on such photonics-assisted free-space communication technologies in both the lower and upper sides of the THz gap and provides a detailed review of recent research and development activities on some of the key enabling technologies. Our recent experimental demonstrations of high-speed free-space transmissions in both frequency regions are also presented as examples to show the system requirements for device characteristics and digital signal processing (DSP) performance. QC 20220614

In this preparatory chapter, a brief introduction to the generation, detection and processing of terahertz (THz) signals is given. Short descriptions of prospective applications of THz signals as well as some state-of-the-art electronic and photonic devices have also been included in this chapter. A chapter-wise overview of the entire book has been incorporated at the end of this introductory chapter.

Carbon allotropes such as fullerenes, carbon nanotubes (CNTs), and graphene have shown great promise for device applications in recent times. Nanomechanical oscillators based on CNTs are well-explored theoretically as well as experimentally. Investigations on the motion of C60 through CNTs have revealed oscillator frequencies of up to 74 GHz. However, the absence of mature technology in the terahertz regime has resulted in the so-called terahertz gap. On the basis of an analysis of the permeation of fullerenes through the nanopores of graphynes (GYs), herein, we report the theoretical design of nanomechanical oscillators in the 0.1–0.5 THz regime. The design strategy involves employing electronic structure methods as well as atomistic model potentials to probe the permeation process of a set of fullerenes, namely, C20, C42, C50, C60, C70, and C84 through the triangular and the rhombus-like nanopores of γ-GY-N (N = 4–6) and r-GY-N (N = 4–5), respectively. Considering the results from the electronic structu...

We investigated the continuum absorption of water vapour diluted in nitrogen, oxygen and air in the range of the pure rotational band of the water molecule (50-500 cm−1). Spectra recordings were performed at room temperature with a Fourier transform spectrometer associated to a 151-m multipass gas cell located at the AILES beamline of SOLEIL synchrotron facility. The study includes the first laboratory measurements in the wide 90-330 cm−1 interval. Tests of the baseline stability, crucial for the continuum determination, are reported together with the expected pressure dependences of the continuum absorption, measured over different pressure ramps. Retrieved foreign-continuum cross-sections are found in good agreement with literature values available in the lower and upper parts of the studied frequency range. The reported results validate the MT_CKD foreign-continuum empirical model, widely used in atmospheric applications, even if some overestimation of the MT_CKD values is noted in the centre of the band where experimental data were absent.

An accurate procedure for coupling (time-dependent) driven electron tunneling and electromagnetic propagation at terahertz frequencies cannot be developed neither with equivalent electric-circuit approximations nor using standard electromagnetic solvers. Alternatively, in this work, a full-physical time-dependent self-consistent algorithm for such coupling is presented. In order to demonstrate the numerical viability of the algorithm and to show the great interest of driven electron tunneling devices for terahertz applications, a transistorlike tunneling device (coupled to lossy transmission lines) is designed for developing a rectifier, a harmonic generator, and an amplitude modulator at terahertz frequencies.

arXiv:2006.10837v2, Generating and detecting radiation in the technologically relevant range of the so-called terahertz gap (0.1–10 THz) is challenging because of a lack of efficient sources and detectors. Quantum dots in carbon nanotubes have shown great potential to build sensitive terahertz detectors, usually based on photon-assisted tunneling. A recently reported mechanism combining resonant quantum dot transitions and tunneling barrier asymmetries results in a narrow linewidth photocurrent response with a large signal-to-noise ratio under weak THz radiation. That device was sensitive to one frequency, corresponding to transitions between equidistant quantized states. In this work we show, using numerical simulations together with scanning tunneling spectroscopy studies of a defect-induced metallic zigzag single-walled carbon nanotube quantum dot, that breaking simultaneously various symmetries in metallic nanotube quantum dots of arbitrary chirality strongly relaxes the selection rules in the electric dipole approximation and removes energy degeneracies. This leads to a richer set of allowed optical transitions spanning frequencies from 1 THz to several tens of THz, for a ∼10 nm quantum dot. Based on these findings, we propose a terahertz detector device based on a metallic single-walled carbon nanotube quantum dot defined by artificial defects. Depending on its length and contacts transparency, the operating regimes range from a high-resolution gate-tunable terahertz sensor to a broadband terahertz detector. Our calculations indicate that the device is largely unaffected by temperatures up to 100 K, making carbon nanotube quantum dots with broken symmetries a promising platform to design tunable terahertz detectors that could operate at liquid nitrogen temperatures., This work has been partially supported by the Spanish Ministry of Science and Innovation with PID2019-105488GB-I00 and PCI2019-103657 (A.A.) and FIS2017-82804-P (D.B.). The work of D.B. is partially supported by by the Transnational Common Laboratory QuantumChemPhys. The Basque Government supported this work through Project No. IT-1246-19 (A.A.). J.W.G. acknowledges financial support from FONDECYT: Iniciación en Investigación 2019 Grant N. 11190934 (Chile). A.A. acknowledge financial support by the European Commission from the NRG-STORAGE project (GA 870114). K.E., C.P. and D.P. acknowledge the Swiss National Science Foundation under Grant No. 200020_182015 and No. 200021_172527, and the NCCR MARVEL funded by the Swiss National Science Foundation (51NF40-182892). The Swiss National Supercomputing Centre (CSCS) under project ID s746 and s904 is acknowledged for computational resources.

A brief introduction to different advanced materials for terahertz (THz) devices, circuits and systems has been included in this introductory chapter. The chapter also contains short discussions regarding potential applications of THz waves and different recently developed electronic and optoelectronic devices operating at THz spectrum. Finally, an overview of all the chapters included in this book has been provided at the end of this chapter.

Terahertz radiation is the part of the electromagnetic spectrum ranging from hundreds of MHz up to several THz. Recent advances in detector technology enable us to develop compact and affordable systems able to generate and detect THz radiation, thus filling the so-called “Terahertz gap”. Terahertz cameras with thousands of pixels arranged in one or two dimensions allow for real-time imaging applications. THz imaging techniques can be used to detect materials that are opaque to that radiation, such as metal or ceramic objects embedded in THz-transparent materials (e.g. paper, textiles, etc.). Due to its non-ionising nature, THz radiation does not require special shielding precautions compared to X-ray systems, and can be safely applied to living tissue. Terahertz imaging can be employed to detect forbidden objects in security checks with parcels scanners and full body scanners. This paper will describe our recent developments towards an innovative application of a sub-THz imaging system based on a commercially available detector. The setup is used to detect objects inside parcels and packages using a conveyor belt and a THz imaging system. An automatic detection system based on convolutional neural networks has been developed allowing for a real-time selection of targets against a set of images. The performance of the setup will be shown and newest results on the recognition capability will be presented.

The Complementary Metal Oxide Semiconductor (CMOS) integrated circuits technology has emerged as a means for realization of capable and affordable systems that operate at 300 GHz and higher. This is bridging the Terahertz Gap and enabling everyday life applications utilizing this portion of the spectrum. Signal generation up to 1.3 THz, coherent detection up to 1.2 THz, and incoherent detection up to ~10 THz have been demonstrated using CMOS. Furthermore, a highly integrated rotational spectroscopy transceiver for electronic smelling operating up to near 300 GHz, a 30-Gbps 300-GHz QPSK transmitter for data communication with an output power of −6 dBm and an imaging array operating at 820 GHz have been demonstrated in CMOS. These along with the data in the literature suggest that the necessary terahertz electronics for everyday life applications can be affordably manufactured.

The high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 contains stacked ‘intrinsic’ Josephson junctions, with unrivaled packing density and a high superconducting gap energy. Cuboid ‘mesa’ devices constructed from this material are consequently a promising technology for coherent, continuouswave radiation in the ‘terahertz gap’ range, spanning from approximately 0.3-1.5 THz. A key issue for practical applications of such devices is their cryocooling requirements, and it is therefore highly desirable to optimize their performance at temperatures that can be achieved by nitrogen cryogenics. Here we report generation of 0.13 milliwatts of coherent emission power at 0.461 THz, at a bath temperature of 77.4 Kelvin. This was achieved by exciting the (3, 0) cavity mode of a stack containing 579 junctions, and with T c of 86.5 Kelvin. In order to minimize selfheating, the THz source was mounted on a copper substrate using PbSn solder. We will discuss the choice of mesa dimensions and cavity mode, and implications for the design of devices which are intended to operate close to the material’s superconducting critical temperature.

Terahertz frequency, also called terahertz gap, ranges from 0.1 to 10 THz is the area of the most up-to-date research spectrum useful for wireless communication. In this paper, performance of terahertz antenna using graphene and copper patch are analyzed. MATLAB codes are used to characterize the properties of graphene and are applied to high-frequency electromagnetic software which is used for modeling and simulation of the proposed antenna. Paper highlights the use of counterpart of copper, i.e. graphene as a patch material in terahertz antenna and also shows the performance comparison of graphene-based terahertz antenna and copper-based terahertz antenna. Return loss, directivity, and gain of the proposed antenna are compared. Results analysis shows that graphene is better option for the design of terahertz antenna.

This paper deals with a monolayer graphene based wideband reflective cross polarization converter metasurface structure effective over a wide range of incident angles in the lower terahertz gap of the electromagnetic spectrum. The unit cell of the metasurface comprises a modified split ring-shaped graphene pattern deposited over a gold backed silicon dioxide (SiO2) substrate. The proposed geometry converts incident linearly polarized (LP) wave to its cross polarized form with a near unity cross polarization conversion (CPC) ratio over a bandwidth of 2.53 THz in the lower terahertz gap ranging from 1.78 THz to 4.31 THz in which a near unity polarization conversion ratio (PCR) has been achieved. The PCR is greater than 0.9 over the band 2.5 THz to 4 THz while full width at half maxima (FWHM) fractional bandwidth of 92.89% with respect to center frequency of 3.05 THz has been realized. The wide bandwidth of CPC of the incident electromagnetic wave (EM) occurs till 40o incident angles under both TE and TM polarizations of the EM wave. The proposed CPC is ultrathin in nature by having a periodicity of λ/24 and thickness of λ/18.7 with respect to minimum frequency of the CPC bandwidth.

Asymmetric transmission of linearly or circularly polarized waves is a well-established property not only for three-layered chiral structures but for multi-layered ones. Here we show a method which can simultaneously implement asymmetric transmission for arbitrary base vector polarized wave in multi-layered chiral meta-surface. We systematically study the implemented method based on a multi-layered chiral structure consisting of a y-shape, a half gammadion and an S-shape in the terahertz gap. A numerical simulation was carried out, followed by an explanation of the asymmetric transmission mechanism in these structures proposed in this work. The simulated results indicate that the multi-layered chiral structure can realize a maximum asymmetric transmission of 0.89 and 0.28 for circularly and linearly polarized waves, respectively, which exhibit magnitude improvement over previous chiral metamaterials. Specifically, the maximum asymmetric transmitted coefficient of the multi-layered chiral structure is insensitivity to the incident angles from 0° to 45° for circularly polarized components. Additionally, we also study the influence of structural parameters on the asymmetric transmission effect for both linearly and circularly polarized waves in detail.

Terahertz radiation refers to a broad electromagnetic spectrum range between microwave and infrared waves, which is also known as the terahertz gap due to inadequate materials and technologies for its generation and manipulation. Atomically thin two dimensional (2D) materials such as graphene, black phosphorus (BP) and transition metal dichalcogenides (TMDs) provide a powerful platform for manipulation of the propagation and detection of terahertz waves. Furthermore, hybrid metamaterials that feature the combination of artificially engineered metamaterials and 2D materials greatly facilitate the dynamic modulation or manipulation of THz radiation towards novel terahertz applications. Herein, we review recent progress in 2D materials in the terahertz domain and hybrid metamaterials with engineered functionalities through the incorporation of graphene, TMDs and BP. The emerging THz devices based on the modulation, nonlinearity, filtering, and plasmonics of 2D materials and metamaterials will be highlighted, and a brief discussion with perspectives and the remaining challenges will be concluded.

Terahertz technology related to generation and detection of terahertz radiation, as well as its interaction with matter has been gaining much attention making it one of the most attractive research fields in turn of the 21st century and covering such major sectors as the semiconductor, medical, space and defense industries. With the advent of intense terahertz sources providing subpicosecond terahertz pulses with photon energies of 0.4–40 meV and maximum field strengths greater than 1 MV/cm, nonlinear effect studies of matter are now available. High-power terahertz sources open up a range of potential applications both in characterization and controlling of solid-state material properties that are elusive for other frequency domains. Recently, a new high-power terahertz source with a record-breaking electric field strength up to 100 MV/cm in the 0.1–5 THz range has been developed at JIHT RAS. In this paper, recent advances in terahertz technology related to intense terahertz field-matter interactions at field strengths above 1 MV/cm are discussed for the first time.

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1–50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities—a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids—a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium., Developing simple and efficient table-top sources of intense terahertz radiation is an ongoing pursuit. Here, Dey et al. demonstrate broadband terahertz generation from laser filamentation in liquids with an order of magnitude higher energy than from conventional two-color filamentation in air.

Terahertz radiation has a growing number of applications in material characterization, where spectral fingerprinting and diffractive effects are the carriers of information. On the other hand, electromagnetic waves in the range of millimeters exhibit strong unwanted specular reflections, resulting in uncontrolled interferences. This problem is especially disturbing in the goniometric time-domain spectroscopy (TDS) configuration, where angular distribution of the field modified by the sample is altered by unwanted reflections. For this reason, low-cost anti-reflection layers are desired. Here, we present a simple way of designing and manufacturing one-sided and two-sided anti-reflection polyamide layers for the THz range. The structures were fabricated using 3-D printers based on selective laser sintering. We demonstrate experimentally in the goniometric time-domain spectroscopy the significant reduction of wavelength-dependent oscillations in Fabry-Perot configuration in the range between 0.1 and 0.3 THz. We also examine the influence of the anti-reflection layers on the distribution of THz energy in reflected, transmitted, and diffracted fields.

Terahertz applications urgently require high performance and room temperature terahertz sources. The gyrotron based on the principle of electron cyclotron maser is able to generate watt-to-megawatt level terahertz radiation, and becomes an exceptional role in the frontiers of energy, security and biomedicine. However, in normal conditions, a terahertz gyrotron could generate terahertz radiation with high efficiency on a single frequency or with low efficiency in a relatively narrow tuning band. Here a frequency tuning scheme for the terahertz gyrotron utilizing sequentially switching among several whispering-gallery modes is proposed to reach high performance with broadband, coherence and high power simultaneously. Such mode-switching gyrotron has the potential of generating broadband radiation with 100-GHz-level bandwidth. Even wider bandwidth is limited by the frequency-dependent effective electrical length of the cavity. Preliminary investigation applies a pre-bunched circuit to the single-mode wide-band tuning. Then, more broadband sweeping is produced by mode switching in great-range magnetic tuning. The effect of mode competition, as well as critical engineering techniques on frequency tuning is discussed to confirm the feasibility for the case close to reality. This multi-mode-switching scheme could make gyrotron a promising device towards bridging the so-called terahertz gap.

We present the direct measurements of terahertz meta-atoms, an elementary unit of metamaterials, by using locally generated terahertz waves in the near-field region. In contrast to a conventional far-field terahertz spectroscopy or imaging, our technique features the localized emission of coherent terahertz pulses on a sub-wavelength scale, which has a potential for visualizing details of dynamics of each meta-atom. The obtained data show the near-field coupling among the meta-atoms and the impact of the electric field distribution from the excited meta-atom to neighbor meta-atoms. The observable LC resonance response is enhanced with an increase of numbers of meta-atoms. Furthermore, our approach also has a potential for visualizing the individual mode of meta-atom at different terahertz irradiation spots. These data can help us to understand the important role of the meta-atom in metamaterials and develop the novel terahertz components and devices such as active terahertz metamaterial and compact, high-sensitive bio-sensor devices.

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

Bridging the terahertz gap requires synergism between the microwave and photonic communities. Advances in each of these two communities are often complementary but sometimes overlooked. One example is the manipulation of waves, known as anomalous diffraction via the use of metamaterials. This is achieved by controlling the surface impedance of each pixel at the interface of two different materials or alternately the phase and magnitude of the wave diffracted off the pixel. In this paper, a review of developments in wave manipulation from microwave to optical frequencies is presented, together with our new results in the terahertz regime. Generation of phase curves for pixel design requires a priori information on material properties at terahertz frequencies. Fabrication of terahertz devices entails micromachining in the clean room while their experimental validation demands both amplitude and phase information. Through judicial selection of practices in microwave and photonic communities, we can further the exploration of wave phenomena at terahertz frequencies.

The absorption in graphene is rather low at terahertz frequencies. Here, we present a graphene-embedded photonic crystal structure to realize broadband terahertz absorption in graphene. The approach provides absorption enhancement in the whole terahertz regime (from 0.1 to 10 THz). It is shown that the average absorption in the graphene-embedded photonic crystal can be enhanced in the multiple propagating bands of the photonic crystals. The absorption efficiency can be further improved by optimizing the characteristic frequency, optical thickness ratio in a unit cell, and the angle of incidence on the photonic crystals. A maximum broadband absorption factor of 28.8% was achieved for fixed alternative dielectric materials. The graphene-embedded photonic crystal is promising for terahertz functional devices with broadband response.

In order to increase the terahertz output power of the photoconductive antenna, a new multiple terahertz power synthesis technology based on terahertz time domain spectroscopy waveform consistency is proposed. The dipole photoconductive antenna with 500-μm aperture and a plurality of delay lines are used to build multiple terahertz power synthesis system. A synthesis efficiency evaluation method is proposed on the basis of the temporal coherence degree. The terahertz time-domain spectroscopy signals can be generated synchronously by virtue of the precise control of multiple delay lines, which can be realized the maximizing frequency domain peak power. Experimental results showed that the synthesized time-domain spectroscopy signal peaks and frequency domain power peaks are maximized, when the terahertz optical path difference is consistent. Two channel, three channel, and four channel synthetic temporal coherence values are 99.47%, 90.45%, and 83.37%, respectively, and the terahertz radiation power peaks can reach 1.94, 2.27, and 2.45 times the peaks before synthesis, respectively.

A tunable beat source has been made using an optical switch module. A stably-tunable beat source for continuous wave terahertz spectroscopy system was implemented by simply connecting 16 coaxial distributed feedback laser diodes to an optical switch. The terahertz frequency was rapidly changed without frequency drifts by changing the optical path. The continuous wave terahertz frequency was tuned from 0.05 to 0.8 THz in steps of 50 GHz or 0.4 nm. We measured continuous wave terahertz waveforms emitted from the photomixers using the switched optical beat source. We also calculated the terahertz frequency peaks by taking fast Fourier transforms of the measured terahertz waveforms. By equipping the implemented tunable beat source with an optical switch, a continuous wave terahertz spectroscopy system was constructed and used to demonstrate the feasibility of continuous wave terahertz spectroscopy for nondestructive tests using the spectra of two type of Si wafers with different resistivity.

Enabling terahertz communications will alleviate the spectrum limitation of current wireless systems. This article presents an indoor multiuser terahertz system with array-of-subarrays architecture to accommodate hardware constraints as well as channel characteristics in the terahertz band. Specifically, the difference between terahertz and millimeter-wave communications is first clarified. Then the advantage of the array-of-subarrays structure is explained through comparison with the fully connected structure in both spectral and energy efficiency. Furthermore, a distance-aware multi-carrier scheme with the array-of-subarrays structure is introduced for wideband terahertz communications. Finally, the associated open issues and future research directions are discussed.

The extraordinary electronic, thermal and optical properties of graphene, together with its two dimensional nature, and with the possibility of facile integration, have enabled its application into a new generation of high-performance devices capable of extending the performance of existing terahertz communications technologies. Although promising for wireless communications applications, the terahertz region of the spectrum, i.e. the frequency range between 300 GHz and 10 THz, is in fact still characterized by a lack of efficient, compact, solid state components capable of operating at room temperature. In this regard, graphene-based terahertz components have shown very promising results in terms of modulation, detection, as well as generation of terahertz waves. This paper will review and discuss recent progress on graphene based devices for modulation, detection and generation of terahertz waves, which are among the key components for future terahertz band communications systems.

The ever increasing demand for higher wireless data rates has led to the development of new techniques to increase spectrum efficiency. However, the limited bandwidth of the frequency bands that are currently used for wireless communication bounds the maximum data rate possible. In the past few years, researchers have developed new devices that work as Terahertz (THz) transmitters and receivers. The development of these devices and the large available bandwidth of the THz band is a possible solution to this ever increasing demand. However, THz communication is still in its infancy and more research needs to be done to bring THz technology into every day life. This paper considers two specific problems — modulation of terahertz pulses and rate adaptation for terahertz communication systems. We describe a pulse modulation technique where multiple symbols are transmitted in each pulse. The technique is shown to achieve terabit/sec datarates. Subsequently, we examine the problem of rate adaptation which is significant at these frequencies due to the high distance based attenuation. Algorithms are presented to fairly share the channel among multiple users located at different distances from the access point. The research is based on simulations in Matlab and the channel model used is based on our own measurements using a time-domain terahertz system. The channel model is also presented in this paper.

We observe a broadband chaotic signal of Terahertz frequency emitted from a superconducting junction. The generated radiation has a wide spectrum reaching 0.7 THz and power sufficient to drive on-chip circuit elements. To our knowledge, this is the first experimental observation of a high-frequency chaotic signal emitted by a superconducting system which lies inside the Terahertz gap. Our experimental finding is fully confirmed by the numerical modeling based on the microscopic theory and reveals the unrealized potential of superconducting systems in chaos-based Terahertz communication, fast generation of true random numbers and non-invasive Terahertz spectroscopy applicable to physical, chemical and biological systems., 6 pages, 3 figures

A challenge of bridging the terahertz gap with semiconductor lasers faces an inevitable problem of enhanced non-radiative Auger recombination with reduction of photon energy. We show that this problem can be mitigated in mercury-cadmium-telluride quantum wells (HgCdTe QWs) wherein the Auger process is suppressed due to formation of quasi-relativistic electron-hole dispersion imposing strong energy-momentum restrictions on recombining carriers. Such dispersion is formed upon interaction of topological states at the two QW interfaces. We characterize the lasing properties of HgCdTe QWs quantitatively by constructing a microscopic theory for recombination, absorption, and gain, and show the feasibility of lasing down to ~ 50 $\mu$m at liquid nitrogen temperature with threshold currents two orders of magnitude lower than in existing lasers. Our findings comply with recent experimental data on stimulated far-infrared emission from HgCdTe QWs and show the directions toward achievement of maximum possible lasing wavelength.

Terahertz continuous-wave (CW) sources oscillating around the 1.0 THz band at room temperature have rapidly been developed to bridge the terahertz gap. However, reflectionless metasurfaces suitable for integration with terahertz CW sources as optical components have yet to be developed in the terahertz gap. Here, we propose a terahertz-focusing metalens consisting of reflectionless meta-atoms with a discrete distribution of negative refractive indices from − 1.1 to − 2.8 . The proposed 2D gradient-refractive-index metalens converts an incident terahertz Gaussian beam to a line focus. We also experimentally demonstrate a metasurface of reflectionless meta-atoms with a negative refractive index of − 2.8 adopted in the periphery of the metalens. The reflectionless metasurface in the terahertz gap would be a welcome contribution to the rapid growth of terahertz industrial applications with terahertz CW sources. Further, the design approach based on reflectionless meta-atoms with negative refractive indices could be applied to various 2D planar optical components with attractive functionalities such as collimating, arbitrary wavefront shaping, and light vortices.

This paper reports an elaborated study of a graphene-based transmittive-type triple-band linear to circular polarization converter (TTLCPC) within the lower mid infrared (MIR) region, covering the terahertz gap. The linearly polarized incident electromagnetic wave undergoes conversion into the circular polarization at three distinct frequencies viz., 5.72 THz, 13.49 THz and 18.90 THz. The axial ratios (AR) at these three THz bands are under 3 dB till 40°incident angles of the electromagnetic wave. The circular polarized nature of the transmitted electromagnetic wave was verified by generating the polarization ellipses at the three distinct frequency bands. The proposed prototype is designed in the sub-wavelength region by maintaining a thickness of λ /26.11 and a periodicity of λ /4.35 with respect to the lowest frequency band; making the proposed device ultra-thin and compact in nature. The proposed TTLCPC can be useful for electromagnetic measurement, THz detection, THz sensing, THz imaging, THz antenna and stealth technology, etc.

Gyrotrons are promising radiation sources for bridging the terahertz gap. They are based on the instability of electron cyclotron maser, where the harmonic operation is generally necessary to alleviate the need for a strong magnetic field. Unfortunately, the performance of a harmonic gyrotron is extremely sensitive to mode competition and magnetic tuning. In this study, to achieve highly efficient and mode-selective gyrotrons, inhomogeneous magnetic fields are applied to introduce a specified longitudinal distribution of the detuning frequency between the terahertz wave and the gyrating electron beam. This detuning frequency has different influences on the oppositely traveling forward wave (FW) and backward wave (BW) inside the cavity, from which optimized magnetic-field profiles for FW-favored and BW-favored interaction circuits are generalized accordingly. It is proposed that a negatively tapering magnetic field converts the energy-transfer rate of the FW interaction into a positive value, leading to highly efficient FW interaction. By contrast, a positively tapering magnetic field reduces the detuning frequency of BW interaction and extends its effective length. By controlling the detuning frequency, a scenario of suppressed mode competition is proposed in a 330-GHz second-harmonic gyrotron. A universal understanding of the influence of an inhomogeneous magnetic field—i.e., the detuning frequency—on the interaction dynamics would help to develop efficient and broadband tunable terahertz gyrotrons.

We studied experimentally and theoretically the vertical range of the confined electric field in the gap area of metamaterials, which was analyzed for various gap widths using terahertz time-domain spectroscopy. We measured the resonant frequency as a function of the thickness of poly(methyl methacrylate) in the range 0 to 3.2 µm to quantify the effective detection volumes. We found that the effective vertical range of the metamaterial is determined by the size of the gap width. The vertical range was found to decrease as the gap width of the metamaterial decreases, whereas the sensitivity is enhanced as the gap width decreases due to the highly concentrated electric field. Our experimental findings are in good agreement with the finite-difference time-domain simulation results. Finally, a numerical expression was obtained for the vertical range as a function of the gap width. This expression is expected to be very useful for optimizing the sensing efficiency.