Sub-terahertz Frequency Range Research Articles

Faraday Rotation Due to Quantum Anomalous Hall Effect in Cr-Doped (Bi,Sb)2Te3

Quantum anomalous Hall effect (QAHE) represents a quantized version of the classical anomalous Hall effect. In the latter case the magnetization takes over the role of magnetic field and induces nonzero off-diagonal elements in the conductivity matrix. In magnetic topological insulators with the band inversion the QAHE can be reached due to quantized conduction channel at the sample edge if the Fermi energy is tuned into the surface magnetic gap. In the static regime the QAHE is seen as a zero-field step in the Hall resistivity. At optical frequencies this step is transformed into a quantized value of the polarization rotation approaching the fine structure constant α=e2/2ε0hc≈1/137. However, due to material issues the steps reach the predicted values at millikelvin temperatures only. In this work we investigate the Faraday polarization rotation in thin films of Cr-doped topological insulator and in the sub-terahertz frequency range. Well defined polarization rotation steps can be observed in transmittance in Faraday geometry. At temperatures down to T=1.85 K the value of the rotation reached about 20% of the fine structure constant and disappeared completely for T>20 K.

A new approach to the development of perspective compact frequency multipliers of the subterahertz and terahertz bands for on-board electronic equipment

Currently, the increasing interest of researchers is attracted by the theoretical and practical problems of mastering the sub-terahertz and terahertz frequency range. Electronic devices operating in these ranges find effective applications in various fields of science and technology: aerospace equipment, security systems, spectroscopy, medicine, biology and many others. The purpose of this work is to focus on a frequency multiplication device that allows using basic sources of relatively low frequency generation to enter the terahertz frequency range. The results of recent years obtained both on the basis of solid-state effects and with the help of vacuum electronics, in particular, magnetron-type devices, which are characterized by compactness, high resistance to radiation loads, mechanical influences, which is important for on-board equipment, are considered. It is known that at high frequencies, vacuum devices require super-precision manufacturing of decelerating systems. This is essentially the main difficulty. The article proposes a new approach based on the hypothesis of P.L. Kapitsa, which allows to significantly simplify the anode structure of a magnetron multiplier with an acceptable level of output parameters. The achievements of recent years in the field of creating sub-terahertz and terahertz frequency multipliers, mainly for on-board equipment of mobile platforms, taking into account the requirements of aerospace systems, first of all, are noted.

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

Acoustic properties of strained SiGe/Si layers in the sub-terahertz frequency range

This work studies the scattering of coherent acoustical phonons within the frequency range of 30−200 GHz in strained SiGe/Si heterostructures with uniform SiGe layers and layers where the initial stage of self-organized islands formation was observed. Coherent phonon pulses reflected by single SiGe layers were detected, and phonon interference in the systems composed of two thin (approximately 10 nm) SiGe layers was observed. Acoustical properties were determined for single SiGe layers, and lateral acoustical inhomogeneity of the layers was estimated in the subterahertz frequency range. The results show that within the range of germanium content of 10%−32% acoustical properties of an approximately 10-nm SiGe layer are insensitive to internal strains governed by lattice mismatch and non-uniformities caused by initial stage of Stranski–Krastanov growth. The sound velocity and wave impedance of SiGe layers can be determined within 5% error, using the corresponding parameters of relaxed SiGe solid solutions with the same germanium content.

ABS and PLA sub-terahertz absorbers for 3D-printing technology

The results of research of the dielectric properties of absorbers based on commercial composite plastics for 3D printing in the sub-terahertz frequency range are presented. Spectra of the complex dielectric constant of plastics based on acrylonitrile butadiene styrene and polylactide with the addition of carbon nanoparticles in the frequency range 115-258 GHz are shown. The calculated transmission, reflection, and absorption spectra for the layers of the adsorbent with a thickness of 0.5 to 2 mm are presented.

Gated Bow-Tie Diode for Microwave to Sub-Terahertz Detection.

We propose a new design microwave radiation sensor based on a selectively doped semiconductor structure of asymmetrical shape (so-called bow-tie diode). The novelty of the design comes down to the gating of the active layer of the diode above different regions of the two-dimensional electron channel. The gate influences the sensing properties of the bow-tie diode depending on the nature of voltage detected across the ungated one as well as on the location of the gate in regard to the diode contacts. When the gate is located by the wide contact, the voltage sensitivity increases ten times as compared to the case of the ungated diode, and the detected voltage holds the same polarity of the thermoelectric electromotive force of hot electrons in an asymmetrically shaped n-n+ junction. Another remarkable effect of the gate placed by the wide contact is weak dependence of the detected voltage on frequency which makes such a microwave diode to be a proper candidate for the detection of electromagnetic radiation in the microwave and sub-terahertz frequency range. When the gate is situated beside the narrow contact, the two orders of sensitivity magnitude increase are valid in the microwaves but the voltage sensitivity is strongly frequency-dependent for higher frequencies.

Spontaneous super-radiative cascade undulator emission from short dense electron bunches

We propose to use super-radiative self-compression of a short dense electron bunch to provide the cascade two-undulator regime of spontaneous emission from the bunch. At the first stage of this cascade, the spontaneous super-radiative emission of a relatively long-wavelength wave results in compression of the bunch by the radiated field. This results in high-efficiency spontaneous radiation of a short-wavelength wave at the second stage. According to the simulations performed for electron bunches with the parameters typical for modern photoinjectors, the cascade regime ensures radiation in the subterahertz frequency range with efficiencies from 10% (in regular systems) up to 30%–50% (in profiled systems).

Development of Wireless Communication Systems in the Subterahertz Frequency Range

The subterahertz and terahertz frequency ranges are promising for the creation of high-speed wireless communication systems because of the possibility to achieve a bandwidth of about several dozen gigahertz, which ensures a high channel capacity. However, rapid attenuation of a signal during its propagation in the atmosphere complicates the operation of communication systems in these ranges. The use of fixed narrow-beam antennas with a high power gain provides a direct surface communication distance of up to a few kilometers. The communication distance limitation can be partially removed by decreasing the frequency down to 200 GHz and narrowing the channel bandwidth down to a few gigahertz. In this paper, we present a radically new approach that was developed to create a wideband (up to one gigahertz) communication channel based on rapid modulation of a centimeter wavelength signal followed by the carrier-frequency multiplication up to 230 GHz without the modulating-signal distortion. This approach was not used in the previous communication systems. The model of a transmit–receive system (200–220 GHz) based on modern semiconductor devices is described in detail. The possibility of digital signal transmission at a speed of up to 1 Gbit/s is experimentally shown. According to calculations, an output power of the transmitter about several hundreds of microwatt is enough for data transmission to a distance of up to 1.5 km with an antenna gain of no less than 50 dB.

Spontaneous parametric down-conversion of photons at 660 nm to the terahertz and sub-terahertz frequency range.

We report on spontaneous parametric down-conversion (SPDC) in periodically poled lithium niobate (PPLN) using 660 nm pump wavelength and the type 0 phase-matching condition to the terahertz and even sub-terahertz frequency range. Detection of the frequency-shifted signal photons is achieved by using highly efficient and narrowband volume Bragg gratings and an uncooled sCMOS camera. The acquired frequency-angular spectrum shows backward and forward generation of terahertz and sub-terahertz photons by SPDC, as well as up-conversion and higher order quasi phase-matching (QPM). The frequency-angular spectrum is theoretically calculated using a Monte-Carlo integration scheme showing a high agreement with the measurement. This work is one important step toward quantum sensing and imaging in the terahertz and sub-terahertz frequency range.

A New Method for Dielectric Characterization in Sub-THz Frequency Range

A new method for broadband characterization of dielectrics in the sub-terahertz (sub-THz) frequency range is presented. Quasi-optical free space measurement setup is used to determine the complex reflection coefficient in a wide frequency band. This method makes it possible to characterize every dielectric by using only the measured complex reflection coefficient without accurate information about the thickness of dielectric sample. This method is useful in case of lossy and/or thick dielectric samples and is based on modern measurement equipment feasibility, with the possibility to make measurements in high number of frequency points. The mathematical model of the method is formulated. Finally, results of measurements for different dielectrics are presented. The obtained permittivity values of these dielectrics are comparable to those reported in the literature.

Interaction of surface plasmon polaritons and acoustic waves inside an acoustic cavity.

In this Letter, we introduce an approach for manipulation of active plasmon polaritons via acoustic waves at sub-terahertz frequency range. The acoustic structures considered are designed as phononic Fabry-Perot microresonators where mirrors are presented with an acoustic superlattice and the structure's surface, and a plasmonic grating is placed on top of the acoustic cavity so formed. It provides phonon localization in the vicinity of the plasmonic grating at frequencies within the phononic stop band enhancing phonon-light interaction. We consider phonon excitation by shining a femtosecond laser pulse on the plasmonic grating. Appropriate theoretical model was used to describe the acoustic process caused by the pump laser pulse in the GaAs/AlAs-based acoustic cavity with a gold grating on top. Strongest modulation is achieved upon excitation of propagating surface plasmon polaritons and hybridization of propagating and localized plasmons. The relative changes in the optical reflectivity of the structure are more than an order of magnitude higher than for the structure without the plasmonic film.

Modulation of TE propagation modes in rectangular metal waveguide with integrated graphene structure in the sub-terahertz frequency range

The propagation of TE modes in a rectangular metal waveguide with an integrated structure of finite length containing a graphene monolayer was studied by a modal decomposition method. The modal decomposition method generates a number of linear algebraic equations related to waveguide modes taken into account. The system of linear algebraic equations for the amplitudes of all modes was derived taking into account the propagation as well as a number of local evanescent modes. Truncation errors in the function of the number of modes taken were quantified by using the discontinuities of the boundary conditions for the transverse electric and magnetic field components at the interface of the discontinuities of the structure. The transmitted and reflected TE01 mode amplitudes were calculated versus the graphene layer length, its position inside the waveguide cavity and the graphene electrons chemical potential. It was shown that the symmetrical positioning of the graphene layer inside the waveguide cavity is the most suitable to achieve the largest amplitude modulation, whereas an asymmetrical positioning is suitable to achieve a large phase modulation of the transmitted mode. It was shown that the phase modulation increases with a transmission coefficient modulation increase and decreases with a transmission coefficient increase. It was found that the phase modulation firstly increases almost proportionally with the graphene layer length for relatively short structures; however, after passing through a maximum, the phase modulation becomes almost constant and hence only weakly dependent on the graphene layer length. Typical phase modulation values turn out to be about 15–20 degrees for minimal transmission coefficients of about 0.6–0.4; however, the phase modulation reaches a value of about 45 degrees if the transmission coefficient modulation equals 50%.

Interaction of electromagnetic radiation with magnetically functionalized CNT nanocomposite in the subterahertz frequency range

The interaction of electromagnetic radiation with a magnetically functionalized nanocomposite based on carbon nanotubes (CNTs) is considered using the model of random distribution of ferromagnetic nanoparticles in the carbon matrix characterized by the presence of resistive–inductive–capacitive coupling (contours). The model is based on the representation of the nanocomposite as a system consisting of the CNT matrix, ferromagnetic nanoparticles, and the interfaces between CNTs and nanoparticles. The wide range of possible resonant phenomena caused both by the presence of contours and the properties of the CNT nanocomposite is shown.

Characterization of CFRP Thermal Degradation by the Polarization-Frequency Reflectometry Method in Subterahertz Frequency Range

We investigate the carbon fiber reinforced plastics (CFRP) thermal degradation using the polarization-frequency reflectometry (PFR) method in subterahertz frequency range. The setup that realizes the PFR method is developed and includes the sample scanning by a quasi-optical wave beam with various polarizations and frequencies. The CFRP thermal degradation can be detected in the 260 ° C-300 ° C temperature range using the PFR method confidently enough. Applying full-wave simulation of a plane wave scattering from a CFRP sample with the aid of the integral functional technique, we show that the temperature dependence of the reflection coefficient is due to the change of the CFRP filler permittivity.

Theoretical investigation of nonlinear damping and nonlinear phase shift of spin-electromagnetic waves propagating in infinite multiferroics at sub-terahertz frequencies

The nonlinear phase shift and nonlinear damping of spin-electromagnetic waves were theoretically studied for the first time in sub-terahertz frequency range in infinite homogeneous longitudinal magnetized multiferroics. The research was based on the solution of the Ginzburg-Landau equation. It is shown that the saturation of the phase shift occurs due to the nonlinear damping if the nonlinear damping coefficients exceed v1=108 s-1 and v2=109 s-1.

An Improved Broadband Boundary Condition for the RF Field in Gyrotron Interaction Modeling

Gyrotrons are the only RF sources providing significantly high output powers at continuous-wave operation in the sub-terahertz frequency range. A prerequisite for a proper study of gyrotron behavior is the accurate time-dependent self-consistent simulation of the interaction between the electron beam and the RF wave inside the interaction region (cavity). That requires an accurate description of the RF fields at the boundaries of the interaction region. In this work, an improved broadband boundary condition is proposed. Based on polynomial series expansion of the load impedance and of the wavenumber, this boundary condition not only features a significantly improved broadband matching, but also allows a customizable frequency-dependent reflection coefficient at the boundaries. The boundary condition has a general formulation, i.e., it does not rely on a special numerical method. For the case of perfect matching, the new formulation is validated through comparing the numerically calculated reflection coefficients with those obtained by the state-of-the-art matched gyrotron broadband boundary condition, whereas the validation in the case of a given frequency-dependent reflection is done by comparing the numerically calculated reflection with the prescribed one. Although this concept is initially intended for the simulation of gyrotron cavities, it can be easily extended to any other open-cavity resonators, for which proper definition of the boundary conditions is required.

Dual electronic tunability of the dispersion characteristics of electromagnetic-spin waves in high-anisotropic layered multiferroic structures

Electric and magnetic tuning of phase, frequency, and delay time of electromagnetic-spin waves in thin-film layered multiferroic structure (metal-dielectric gap-ferroelectric material-hexaferrite-dielectric substrate) is numerically simulated. The results are used to demonstrate prospects for application of layered multiferroic hexaferrite-ferroelectric structures in the construction of basically new devices for generation and processing of microwave signals under dual (electric and magnetic) tuning of the working characteristics in the subterahertz frequency range.

On the possibility of a short subterahertz pulse amplification in a plasma channel created in air by intense laser radiation

The evolution of the electron energy distribution function in the plasma channel created in air by a UV-laser pulse of femtosecond duration is studied. It is shown that such a channel can be used to amplify few-cycle electromagnetic pulses in the subterahertz frequency range at the time of relaxation of the energy spectrum in air determined by the vibrational excitation of the nitrogen molecules. The gain factor as a function of time, electron concentration and frequency of the amplifying radiation is obtained. The propagation of few-cycle radio-frequency pulses through the amplifying medium is analysed.

High Frequency Dynamic Nuclear Polarization

During the three decades 1980-2010, magic angle spinning (MAS) NMR developed into the method of choice to examine many chemical, physical, and biological problems. In particular, a variety of dipolar recoupling methods to measure distances and torsion angles can now constrain molecular structures to high resolution. However, applications are often limited by the low sensitivity of the experiments, due in large part to the necessity of observing spectra of low-γ nuclei such as the I = 1/2 species (13)C or (15)N. The difficulty is still greater when quadrupolar nuclei, such as (17)O or (27)Al, are involved. This problem has stimulated efforts to increase the sensitivity of MAS experiments. A particularly powerful approach is dynamic nuclear polarization (DNP) which takes advantage of the higher equilibrium polarization of electrons (which conventionally manifests in the great sensitivity advantage of EPR over NMR). In DNP, the sample is doped with a stable paramagnetic polarizing agent and irradiated with microwaves to transfer the high polarization in the electron spin reservoir to the nuclei of interest. The idea was first explored by Overhauser and Slichter in 1953. However, these experiments were carried out on static samples, at magnetic fields that are low by current standards. To be implemented in contemporary MAS NMR experiments, DNP requires microwave sources operating in the subterahertz regime, roughly 150-660 GHz, and cryogenic MAS probes. In addition, improvements were required in the polarizing agents, because the high concentrations of conventional radicals that are required to produce significant enhancements compromise spectral resolution. In the last two decades, scientific and technical advances have addressed these problems and brought DNP to the point where it is achieving wide applicability. These advances include the development of high frequency gyrotron microwave sources operating in the subterahertz frequency range. In addition, low temperature MAS probes were developed that permit in situ microwave irradiation of the samples. And, finally, biradical polarizing agents were developed that increased the efficiency of DNP experiments by factors of ∼4 at considerably lower paramagnet concentrations. Collectively, these developments have made it possible to apply DNP on a routine basis to a number of different scientific endeavors, most prominently in the biological and material sciences. This Account reviews these developments, including the primary mechanisms used to transfer polarization in high frequency DNP, and the current choice of microwave sources and biradical polarizing agents. In addition, we illustrate the utility of the technique with a description of applications to membrane and amyloid proteins that emphasizes the unique structural information that is available in these two cases.

Plasma channel produced by femtosecond laser pulses as a medium for amplifying electromagnetic radiation of the subterahertz frequency range

The electron energy distribution function in the plasma channel produced by a femtosecond laser pulse with a wavelength of in atmospheric-pressure gases was considered. Conditions were determined whereby this channel may be employed for amplifying electromagnetic waves up to the terahertz frequency range over the energy spectrum relaxation time . Gains were calculated as functions of time and radiation frequency. The effect of electron – electron collisions on the rate of relaxation processes in the plasma and on its ability to amplify the electromagnetic radiation was investigated.