This review paper is based on the invited talk presented by the same authors at the 8th International Workshop on Far-Infrared Technologies (IW-FIRT 2021). It overviews some well-known and novel and emerging applications of gyrotrons in diverse scientific and technological fields and presents both the current status and the prospects of the research in such a wide area worldwide.

We report enhanced broadband terahertz (THz) generation and detailed characterization from the interaction of femtosecond two-colour laser pulses with thin transparent dielectric tape target in ambient air. The proposed source is easy to implement and exhibits excellent scalability with laser energy. Spectral characterization using Fourier transform spectrometer reveals yield enhancement of more than

In this research article, a wideband antipodal Vivaldi antenna (AVA) is presented by incorporating a negative index metamaterial (NIM). The proposed antenna is designed using a linearly tapered shape structure. The overall performance of the designed antenna is improved by employing a negative index metamaterial (NIM). The ‘V’ shaped metamaterial unit cells are designed and placed at the top surface

Various regimes of hybrid, bulk-surface, modes being excited in the clinotron oscillator with a sheet electron beam and nonuniform grating have been studied theoretically. It is shown that higher output power of the hybrid mode in comparison with pure surface or bulk modes is due to an increase of a feedback and efficiency of the radiation output caused by radiating harmonic and due to high coupling

In the experimental study on the frequency tuning of a double-beam gyrotron, an unexpected abrupt increase of the output power was observed for some magnetic field values corresponding to the excitation of high-order axial modes (HOAM). This effect can be explained through an after-cavity interaction of the spent but bunched electron beam with the neighboring transverse mode which has the same azimuthal

As a promising terahertz (THz) source, two-color laser filamentation in gases attracts an exponential growth of interest because of the scalability of output THz pulse energy and bandwidth with input parameters. However, nonlinear effects within the laser filament on the THz pulse generation process, such as intensity-dependent refractive index change induced additional phase, have never been studied

Bioanalytical THz sensing techniques have proven to be an interesting and viable tool for the label-free detection and analysis of biomolecules. However, a major challenge for THz bioanalytics is to perform investigations in the native aqueous environments of the analytes. This review recapitulates the status and future requirements for establishing THz biosensing as a complementary toolbox in the

We introduce a generalized extrapolation framework as a constrained optimization problem to enhance the resolution in terahertz digital in-line holography. The alternating minimization method is employed to iteratively extrapolate the hologram beyond the actually detecting area and to reconstruct the complex amplitude distribution of the object wavefront. Within this framework, we propose a sparsity-based

Abnormally high level of methylation in the epigenome is a biomarker for various forms of cancer. It has been reported that a methylated nucleoside, 5-methylcytidine, when dissolved in water and then frozen, has a resonance absorption peak around 1.6 THz, which could allow for the determination of the degree of methylation using terahertz spectroscopy. This study attempts to replicate the experiment

A new numerical model of terahertz parametric process is demonstrated, which takes the spontaneous parametric down-conversion into consideration by adding tens of frequency components in the coupled-wave equations. To set the values of some parameters in this model, the residual pump energy and idler spectra during spontaneous parametric down-conversion are measured experimentally for different pump

We report on THz pulse generation through optical rectification of femtosecond laser pulses in sugar crystals. The sugar crystals were grown from an aqueous solution of caster cane sugar bought in a local grocery. Sugar exhibits rather good nonlinear coefficients (\(d_{23}\simeq 56\) pm/V), but the optical-THz conversion efficiency remains quite weak because of the lack of phase-matching, when pumped

We demonstrate experimentally the increase of optical-to-terahertz conversion efficiency for GaAs-based photoconductive terahertz emitters. This increase is achieved by preventing device breakdown through series resistors, which act as a current limiter. Pulsed photoexcitation and potential current fluctuations result in heat dissipation leading to local heating, which further increases the current

The present paper addresses numerical calculations on the eigenvalues of hybrid modes in corrugated circular waveguides with varying diameter and corrugation depth. Such calculations are essential for the numerical optimization of advanced mode converters and diameter tapers for future low-loss high-power microwave applications, like broadband high-power radar sensors for space debris observation in

Modifications of the geometry of a folded dipole antenna operating in the infrared were studied with the objective to increase the antenna input impedance. These modified folded dipoles were designed and optimized using numerical simulations. Candidate antennas were fabricated, and input impedance was measured experimentally using scattering-scanning near-field optical microscopy. Numerical and experimental

This paper presents a 120-GHz wideband phase-compensated variable gain amplifier (VGA) with a p-type metal–oxide–semiconductor (PMOS) switch using a 40-nm CMOS process. By applying a PMOS switch to the common source (CS) amplifier, the gain of the CS amplifier can be controlled by as much as 6.1 dB with 2° phase variations in 15.1-GHz bandwidth ranging from 100.9 to 115 GHz. The measured gain and 3-dB

A Correction to this paper has been published: https://doi.org/10.1007/s10762-021-00783-w

Terahertz radiation has many unique characteristics that make it useful for noninvasive mail inspection. While qualitative analysis of mail for suspicious objects is a relatively instantaneous process, quantitative analysis methods may be time-consuming. Multivariate analysis methods, including principal component analysis (PCA), partial least squares (PLS), and interval partial least squares (iPLS)

Epoxy resin, diglycidyl ether of bisphenol A (DGEBA) and triethylenetetramine (TETA), is known to be hygroscopic in nature, which eventually leads to mechanical performance degradation of the material. The long-standing debate on the nature of water-epoxy interaction has been investigated in this study with the help of terahertz spectroscopy, which is able to probe sub 100 cm− 1 wavenumbers in which

We explore background-free options to detect mid-infrared (MIR) electric transients. The MIR field and a near-infrared probe interact via sum- (SFG) and difference-frequency generation (DFG) in an electro-optic crystal. An intuitive picture based on a phasor representation and rigorous numerical calculations are used for analysis. It turns out that separating photons generated either by SFG or DFG

In this article, an improved fast Fourier transform (FFT)–based millimeter-wave imaging algorithm with range compensation is presented, which can be used to reconstruct 3D images for near-field multiple-input multiple-output synthetic aperture radar (MIMO-SAR). The frequency-domain interpolation is avoided and only one-step spherical-wave decomposition is employed in this algorithm. During the image

We recently proposed a multiphase oscillator utilizing a periodic pulse train generated and propagated in a closed one-dimensional lattice of resonant-tunneling diode (RTD) oscillators, each of which establishes the neighbor-to-neighbor coupling through lossy inductors. In this work, we develop the design criteria for the bias dependence of oscillation frequency through the bifurcation analysis based

Increasing data rates in wireless communications are accompanied with the need for new unoccupied and unregulated bandwidth in the electromagnetic spectrum. Higher carrier frequencies in the lower THz frequency range might offer the solution for future indoor wireless communication systems with data rates of 100 Gbit/s and beyond that cannot be located elsewhere. In this review, we discuss propagation

In this contribution, we describe the design, technology, and physical parameters of antenna-coupled microbolometer, used for broadband detection of terahertz electromagnetic spectrum. This microbolometer features an application of La0.67Sr0.33MnO3 layer grown on multilayered material stack ensuring lattice matching of the sensing LSMO layer to silicon. By virtue of bulk micromachining of silicon-on-insulator

The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical

In this paper, a terahertz metasurface supercell consists of two ring resonators with slightly different radii is proposed. Electromagnetic-induced transparency (EIT)–like resonance is excited with an ultrahigh figure of merit. Furthermore, the impact of the coupling between the two rings is investigated on the transmission amplitude response, the quality factor, and the EIT peak amplitude by varying

A novel type of near-field slit probe with high sensitivity and high spatial resolution is proposed. We tested the implementation of this in passive millimeter-wave microscopy at frequencies around 50 GHz. The slit probe comprises a standard rectangular waveguide incorporating a triple-screw tuner penetrating into the waveguide, followed by a four-section quarter-wave transformer, and a metal-coated

We demonstrate that the magneto-optical Kerr effect at normal incidence in the terahertz (THz) frequency range is useful for evaluating carrier transport properties of particular layers of a p-n junction. Since a single p-type thin film only exhibits a small magneto-optical Kerr effect, magneto-optical Kerr spectroscopy cannot be used to determine the carrier densities of such a film with high sensitivity

We applied microwave light with frequencies 10, 212, and 419 GHz to form octacalcium phosphate (OCP) intercalated with succinate ions (SCI). The composition of starting reagents (i.e., α-type tricalcium phosphate (TCP), calcium hydrogenphosphate dihydrate (DCPD), and SCI in pure water) did not enable us to form OCP intercalated with SCI in a thermostatic water bath. Although microwave irradiation typically

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues

A novel design is introduced for dual-band operation at 28/38 GHz for improving microstrip antennas. Also, the design support circularly polarized antenna that works in millimetre band applications. This proposed antenna is attached to a circular hole in the centre of the patch with four slits embedded in the corners. Also, small holes are etched at sides of the patch to achieve the resonant frequencies

The effect of reflection is studied experimentally and theoretically on a high-power 110-GHz gyrotron operating in the TE22,6 mode in 3μs pulses at 96kV, 40A. The experimental setup allows variation of the reflected power from 0 to 33% over a range of gyrotron operating conditions. The phase of the reflection is varied by translating the reflector along the axis. Operating at a higher efficiency point

The effect of delayed reflection on frequency stabilization and pulling in a gyrotron oscillator is studied. Simple theoretical formulas for start-oscillation current, oscillation frequency, and frequency stabilization rate are derived in the framework of the linear (or small-signal) theory and verified by numerical simulation for the gyrotron with fixed axial profile of the RF field. Capabilities

The demand of high-speed wireless communication has increased, which need the data rate to be in the order of Terabyte per second (Tbps) in the near future. Terahertz (THz) band communication is a key wireless communication technology to satisfy this future demand. This would also reduce the spectrum scarcity and capacity limitation of current wireless systems. Microfabricated Folded Waveguide TWTs

This work reports in first time a 100-Gb/s, ultra-low noise, variable gain multi-stagger tuned transimpedance amplifier (VGMST-TIA) over the D-band performance. The whole work is binding into two phases. The first phase involves the modeling and characterization of graphene field-effect transistor (GFET) with an optimized transition frequency of operation. While in the second phase, a TIA design employs

Mechanical tuning of a 3D-printed, polymer-based one-dimensional photonic crystal was demonstrated in the terahertz spectral range. The investigated photonic crystal consists of 13 alternating compact and low-density layers and was fabricated through single-step stereolithography. While the compact layers are entirely polymethacrylate without any intentional internal structures, the low-density layers

In the context of exploring the possibility of using Al-powder Selective Laser Melting to fabricate horn antennas for astronomical applications at millimeter wavelengths, we describe the design, the fabrication, the mechanical characterization, and the electromagnetic performance of additive manufactured horn antennas for the W-band. Our aim, in particular, is to evaluate the performance impact of

The low-voltage compact gyrotron is suitable for industrial applications. However, the beam-wave interaction efficiency is low in conventional low-voltage gyrotron. To improve the whole tube efficiency, a compact depressed collector is introduced in developing a 75 GHz low-voltage compact gyrotron. The compact depressed collector is directly connected to the output waveguide. It is grounded and isolated

In this paper, a novel design for a 5G base station (BS) antenna is proposed. The proposed antenna consists of two orthogonally polarized antennas. The two antennas are modified compact Vivaldi antennas operating in the two recommended 5G operating bands; 28 and 38 GHz with measured impedance bandwidth of 26.5–40 GHz. The orthogonality of the two antennas allows the use of two antennas on the same

To allow antenna movements in azimuth and elevation in high-power radar applications, rotary joints are essential. They allow the rotation of a transmission line and therefore are important transmission line components. In the present paper, a broadband rotary joint concept for high-power W-band radar applications is proposed. To avoid a twist of the polarization plane of a linearly polarized mode

The paper presents a new instrument—a portable spectroscopic quasi-optical (QO) THz ellipsometer-reflectometer (SQOTER). It combines two independent experimental techniques: THz ellipsometry and reflectometry. SQOTER is based on the hollow dielectric beamguide and beamguide components as a QO transmission line. This ultra-broadband line provides operation of the setup within 0.1…1 THz frequency range

This paper presents the design, construction, and testing of a 183-GHz FMCW transceiver module for wireless fuse applications. The transmitter consists of a square-triangle wave converter, a Ku-band VCO, a 91.5-GHz × 2 × 3 multiplying and amplifying chain, a 10-dB multi-hole directional coupler, and a 183-GHz frequency doubler. For the multiplying chain, the generated 91.5-GHz signal power is partly

Nonlinear dynamics of a gyrotron with high-order operating mode, driven by an external signal with harmonic or stepwise modulation of its parameters or by two monochromatic signals, was studied theoretically. Conditions for locking and control of the radiation frequency were found. Exit from the locking regime is accompanied by the beats of the operating mode or excitation of spurious modes. Simulations

The influence of a coaxial dielectric rod on eigenvalues, ohmic losses, transverse field structure, and beam-wave coupling coefficients is investigated for TE modes of a gyrotron cavity. It is shown that such dielectric insert, when made from a moderate-loss material, results in strong attenuation of all cavity modes, with the exception of those having caustic radii much larger than the insert radius

This article presents the design and prototyping of components for a modular multiple-input-multiple-output (MIMO) millimeter-wave radar for space applications. A single radar panel consists of 8 transmitters (TX) and 8 receivers (RX), which can be placed several times on the satellite to realize application-specific radar apertures and hence different cross-range resolutions. The radar chirp signals

We proposed and fabricated J-shaped planar structure metasurfaces to achieve excitation of dual-frequency toroidal dipole resonances simultaneously, which results in electromagnetically induced transparency effect in the terahertz band. The mutual coupling of two asymmetric J-shaped metal rings in the metasurface units excites low-frequency and high-frequency toroidal dipole resonances, and a transparent

Terahertz (THz) spectroscopy is regarded as a professional technology to probe collective molecular vibrational modes and intensity of bio–molecules, which is potential to analyze amino acids and short peptides. In this study, THz spectroscopy is utilized to investigate the correlations of glycine (Gly), Serine (Ser), amino acid mixtures of Gly/Ser, and dipeptides (GS and SG) in the frequency range

At present, the terahertz time-domain spectroscopic information of each component in multi-composition compounds detection is comprehensively combined. The lower resolution level of mixed spectra has posed many difficulties in signal analysis due to the overlapping characteristic information in the mixed ones. In this paper, to compare the performances of Nonnegative Matrix Factorization (NMF), Self-modeling

In this paper, a 4 × 4 60-GHz circularly polarized (CP) magnetoelectric (ME) dipole array based on TE340-mode substrate integrated cavity (SIC) and sequential rotation feed-network (SRFN) is proposed. The ME dipole array is constructed by 3 printed circuit board (PCB) layers with its assembled size of 21 × 23 × 2.4 mm3. The top layer is the radiation layer with 2 × 2 subarrays. The second layer includes

A new waveguide stepped septum-type circular polarizer (SST-CP) was developed to operate in the 500-GHz band for radio astronomical and planetary atmospheric observations. In a previous study, we developed a practical SST-CP for the 230-GHz band. However, several issues prevent this device being easily scaled down to the 500-GHz band, such as manufacturing dimensional errors and waveguide flange position

The phase stability of a 140-GHz, 1-kW pulsed gyro-amplifier system and the phase dependence on the cathode voltage were experimentally measured. To optimize the measurement precision, the amplifier was operated at 47 kV and 1 A, where the output power was ∼30 W. The phase was determined to be stable both pulse-to-pulse and during each pulse, so far as the cathode voltage and electron beam current

This paper presents the results of a study on improving the performance parameters such as the impedance bandwidth, radiation gain and efficiency, as well as suppressing substrate loss of an innovative antenna for on-chip implementation for millimetre-wave and terahertz integrated-circuits. This was achieved by using the metamaterial and the substrate-integrated waveguide (SIW) technologies. The on-chip

Solid-state sources consisting of cascaded Schottky diode-based frequency multipliers are commonly used to generate power at millimeter and submillimeter wave bands. In order to achieve increasing power at those frequencies, first-stage frequency multipliers have to handle a large amount of input power and generate enough output power to drive higher frequency multiplication stages. To that end, an

This article is intended for inclusion in the Special Issue on Interpretation of Solid-State Terahertz Spectra.

A magnetron-injection gun able to provide very high ratio tj≈ 0.5 of the beam current to the Langmuir current has been designed, while maintaining good enough quality and stability of a formed helical electron beam. The specific feature of the gun is the shape of the cathode with two singular points of the electric field distribution which ensure close to uniform electric field profile within the emitter

New method of measurement of dielectric constant and loss tangent for small samples of lossy dielectrics, including plain semiconductors, using non-oversized resonators being parts of the recently developed cavity switches for sub-terahertz bands is proposed. Strong influence of the tangent loss value to the Q-factor of the resonator is calculated and observed in experiments. A simple monosemantic

The original article contains mistakes specifically in the presentation of references.

Analysis of dielectric properties of several materials, BN, AlN with BN additive, crystalline quartz, MgAl2O4 spinel, 6H-SiC in the millimeter and submillimeter (terahertz) ranges, is presented. Basing on the experimentally measured dielectric losses of these materials and on their thermal and mechanical properties, we made a calculation to estimate the maximal achievable output power that can be transmitted

The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique

In order to better understand the variation mechanism of ozone abundance in the middle atmosphere, the simultaneous monitoring of ozone and other minor molecular species, which are related to ozone depletion, is the most fundamental and critical method. A waveguide-type multiplexer was developed for the expansion of the observation frequency range of a millimeter-wave spectroradiometer, for the simultaneous

The microstrip meander-line traveling wave tube (TWT) is a kind of small sized, low voltage, low cost, and easy to fabricate TWT. It is an attractive choice for many applications, such as communication systems, phased array radars, security detectors, and electronic counter measure (ECM) instruments. In this paper, a Ka band U-shape meander-line slow wave structure (SWS) supported by a diamond rod