sub-THz Wave Research Articles

The absorption properties of ZrO2 nanoparticles in the THz and sub-THz frequency ranges.

As terahertz (THz) and sub-THz region electromagnetic waves are becoming vital for industrial applications such as 5G wireless communication, so too are THz and sub-THz wave absorbing materials. Herein, we report the optical properties of monoclinic zirconia (m-ZrO2) nanoparticles in these frequency regions, with different crystalline sizes. The crystalline sizes of the three samples, measured by transmission electron microscopy, are 93 ± 23 nm (denoted 1), 28 ± 14 nm (denoted 2) and 2.6 ± 0.7 nm (denoted 3). X-ray diffraction and Raman spectra show that 1 and 2 have high crystallinity whereas 3 shows peak broadening due to its small crystalline size. Terahertz time-domain spectroscopy (THz-TDS) measurements of pelletised samples show that the small crystalline size sample exhibits larger absorption, e.g., the absorbance value at 300 GHz is 0.18 mm-1 (1), 0.04 mm-1 (2) and 1.11 mm-1 (3), and the related dielectric loss value (ε'') is 0.04 (1), 0.01 (2) and 0.82 (3), respectively. This is considered to be due to the proportional increase in surface water molecules for the small particle size sample due to the relative increase in surface area and under-coordinated atoms, shown by IR spectra. These results show that small crystalline size m-ZrO2 nanoparticles have potential as THz and sub-THz wave absorbing materials, which are crucial for noise reduction in THz and sub-THz wave technologies.

Fast Beam Search Based on Recorded Combinations for Distributed Massive MIMO Systems Using Sub-THz Wave Bands

Massive multiple-input multiple-output (mMIMO) systems using sub-Terahertz (sub-THz) wave bands are viewed as a promising way to increase greatly the capacity of future mobile communications because they can provide a wider bandwidth than conventional wireless communications. Distributed mMIMO systems can improve the spatial multiplexing transmission performance by reducing spatial correlations among the MIMO channels in line-of-sight environments. Unfortunately, beam search is more complex than in conventional wireless communications, because each access point (AP) requires a very narrow beamwidth to compensate for the significant propagation attenuation in sub-THz waves and beam search must involve all distributed APs. To address this problem, this paper proposes a fast beam search method to limit the beam-sweep directions of beam sweep based on the beam combinations selected for the distributed APs in the past. Simulations show that the proposed method maintains the transmission capacity while greatly reducing the number of search beams compared to conventional all-AP exhaustive beam search. The results show based on 16 APs that while matching the transmission capacity attained by all-AP exhaustive or two-stage beam search methods, the proposed method reduces the number of search beams by APs (excluding the initial AP executed exhaustive beam search) by 90.8% at the 50th percentile and 62.5% at the 95th percentile compared to two-stage beam search for 50,000 sets of user equipment used for recording beam combinations before evaluating the performance.

Imaging and Mode Content Analysis of the Wave Beam From a Short-Pulse High-Power Gyrotron Using the Millimeter-Wave-Induced Gas Breakdown Initiated by a Metal–Dielectric Screen

The wave beam of a sub-terahertz (THz) gyrotron with a pulsed magnetic field was imaged in a single pulse using a microwave-induced gas breakdown initiated by a metal–dielectric (MD) screen. Images of sub-THz beams, which were recorded at the output of a quasi-optical mode converter, were compared with data obtained by numerical simulation of propagation and transformation of the wave beam from the gyrotron. The analysis of the waveguide mode content in the gyrotron was done. Based on experimental and simulation data, it was shown that the distortion of the sub-THz wave beam is caused by the deformation of the Teflon window at the gyrotron output. Based on the results of the study, the gyrotron was modified, and the quality of its wave beam was improved.

High overall performance uni-traveling carrier photodiodes for sub-THz wave generation.

Modified near-ballistic uni-traveling-carrier photodiodes with improved overall performances were studied theoretically and experimentally. A bandwidth up to 0.2THz with a 3dB bandwidth of 136GHz and large output power of 8.22dBm (99GHz) under the -2V bias voltage were obtained. The device exhibits good linearity in the photocurrent-optical power curve even at large input optical power, with a responsivity of 0.206A/W. Physical explanations for the improved performances have been made in detail. The absorption layer and the collector layer were optimized to retain a high built-in electric field around the interface, which not only ensures the smoothness of the band structure but also facilitates the near-ballistic transmission of uni-traveling carriers. The obtained results may find potential applications in future high-speed optical communication chips and high-performance terahertz sources.

High-gain and low-profile dielectric-image-line leaky-wave-antenna for wide-angle beam scanning at sub-THz frequencies

A low-profile THz leaky-wave antenna (LWA) enabled by a Dielectric Image-Line (DIL) with wide beam steering capability is reported for the first time in this paper. The DIL as a low-loss dielectric waveguide synthesized directly on a substrate with high dielectric permittivity is an appropriate candidate for emerging directive compact antennas for future sub-THz wave communications. The dominant mode of DIL is disturbed by printing two periodic set of radiating elements on the top wall of DIL, including three barrier strips and two anti-reflection strips with an extensive tuning range for open stop-band (OSB) suppression. The operation bandwidth of the proposed antenna spans from 142 GHz to 225 GHz providing an impedance bandwidth of 48.8% in the fast wave region. It is observed from numerical simulations that the proposed 35-cell leaky wave antenna permits a beam steering angle of 128° (−68° to 60° (including the broadside)) in the operation frequency band with the gain varying from 14 to 19.3 dBi in the fast wave region. Detailed design guidelines along with simulation results are presented. The proposed antenna has the capability of being directly scaled to other frequency bands by considering its corresponding dispersion characteristic.

Performance Enhancement of Silicon-Based Sub-Terahertz Detector by Highly Localized Plasmonic Wave in Nano-Ring FET

A compact monolithic trantenna (transistor-antenna) device is presented for a high-performance sub-THz wave detector using 28-nm CMOS foundry process. Based on a highly localized plasmonic wave in a silicon nano-ring field-effect transistor (FET), we obtained a total 535-fold photoresponse (Δu) enhancement in an on-chip measurement as compared with our previous works using the same asymmetry ratio (ηa= 30). The inner contact diameter (din) was scaled down from 8 to 0.13 μm for the parasitic resistance limit case. By changing the ground source from inside to outside the nano-ring FET, we could generate different Δu polarities, which in turn reduced the junction leakage with improved |Δu|. From a fabricated nano-ring FET with the outer ring grounded source, we observed 5× of additional |Δu| enhancement followed by 107× with din scaling. In addition, based on the highly localized plasmonic wave nano-ring FET without any external gain, a record-high free-space responsivity of 12.4 kV/W and a reduced noise equivalent power of 1 pW/Hz0.5 were experimentally demonstrated under 0.12-THz radiation.

A filterless frequency 32-tupling photonic scheme to generate Sub-Terahertz wave signal enabled by optical polarization modulators

A scheme based on polarization modulators to generate Sub-Terahertz (Sub-THz) wave signal with frequency 32-tupling is proposed. The system consists of two subsystems in cascade, and each subsystem consists of 4 paralleled polarization modulators. In this scheme, by properly controlling two subsystems, an optical signal with optical carrier and ±16th-order optical sidebands is achieved. Next, by adjusting the optical attenuator and optical phase shifter, the optical carrier can be canceled. Finally, the Sub-THz wave with frequency 32-tupling can be obtained after beaten in the photodetector. The results show the ±16th-order optical sideband suppression ratio is 53.38 dB and the radio frequency sideband suppression ratio of the 32-tupling Sub-THz wave is 42 dB, which are consistent with the theoretical analysis very well. Meanwhile, the effects of phase offset, modulation index, and attenuator deviation on suppression ratio are analyzed.

High-refractive index, low-loss oxyfluorosilicate glasses for sub-THz and millimeter wave applications

Terahertz (THz) time-domain spectroscopy was used to study the optical properties of two series of oxyfluorosilicate (OFS) glasses. The experimentally measured refractive indices are analyzed by using the Clausius–Mossotti equation to retrieve information about polarizability of the glass. Compared with previously studied oxide-based glasses and chalcogenides, OFS glasses exhibit a balance of relatively low absorption coefficients (6–9 cm−1) and high refractive indices (2.9–3.7) at 0.5 THz. The value of 3.7 is the highest among silicate glasses reported to date and are comparable to those of La3+:chalcogenide glasses. The relatively high values of refractive indices have been attributed to the great increase of the glass polarizability in OFS glasses, which offsets the effect of an increase in molecular volume caused by multicomponent modification of the glass structure. Comparatively low THz absorption of OFS glasses is explained by the structural relaxation effect of fluorine, which effectively suppress the charge fluctuation in the glass structure. The high refractive index and low absorption loss properties of the present OFS glasses should be useful for quasi-optic components such as lens and waveguide devices application in the sub-THz and millimeter wave region.

Studies on the transmission of sub-THz waves in magnetized inhomogeneous plasma sheath

There have been many studies on the sub-terahertz (sub-THz) wave transmission in reentry plasma sheaths. However, only some of them have paid attention to the transmission of sub-THz waves in magnetized plasma sheaths. In this paper, the transmission of sub-THz waves in both unmagnetized and magnetized reentry plasma sheaths was investigated. The impacts of temporal evolution of the plasma sheath on the wave transmission were studied. The transmission of “atmospheric window” frequencies in a magnetized plasma sheath was discussed in detail. According to the study, the power transmission rates (Tp) for the left hand circular (LHC) and the right hand circular modes in the magnetized plasma sheath are obviously higher and lower than those in the unmagnetized plasma sheath, respectively. The Tp of LHC mode increases with both wave frequency and external magnetic field strength. Also, the Tp of LHC mode in both magnetized and unmagnetized plasma sheaths varies with time due to the temporal evolution of the plasma sheath. Moreover, the performance of sub-THz waves in magnetized plasma sheath hints at a new approach to the “blackout” problem. The new approach, which is in the capability of modern technology, is to utilize the communication system operating at 140 GHz with an onboard magnet installed near the antenna.

Design of a Broadband E-Plane Power Combiner Based on Quarter-Arc Bent Rectangular Waveguides for Sub-THz and THz Wave**Supported by the National Natural Science Foundation of China under Grant No 11075032, and the Fundamental Research Funds for the Central Universities under Grant No ZYGX2014J033.

A method of designing an E-plane power combiner composed of two quarter-arc bent rectangular waveguides is proposed for sub-THz and THz waves. The quarter-arc bent-waveguide power combiner has a simple geometry which is easy to design and fabricate. By HFSS codes, the physical mechanism and performance of the power combiner are analyzed, and the relationship between the output characteristics and the structure/operating parameters is given. Simulation results show that our power combiner is suitable for the combining of two equal-power and reversed-phase signals, the bandwidth of the combiner is wide and can be adjusted by the radius of the quarter-arc, and the isolation performance of the combiner can be improved by adding thin film resistive septa at the junction of two quarter-arc bent waveguides. Meanwhile, an approximate method based on the analytic geometrical analysis is given to design this power combiner for different frequency bands.

All-Optical Generation and Switching of Few-Cycle Millimeter-Wave Pulses

We conducted a comparative study of two schemes of photonic generation and switching of few-cycle sub-THz or millimeter wave (MMW) pulses by use of a photonic-transmitter-mixer (PTM) module with a broadband and high-power near-ballistic uni-traveling carrier photodiode (NBUTC-PD). In the first scheme, we performed all-optical ultra-fast switching (bias modulation) of the PTM injected with a 93 GHz optical local-oscillator signal. Sub-2-cycle short MMW pulses with central frequency at 93 GHz were generated. To compare, in scheme 2, we employed femtosecond optical short pulses to directly excite the PTM under a DC bias (optical modulation). The former approach is shown to be capable of providing much less signal distortion and much shorter pulse duration than the latter.

Design and Experiment Results of High-Speed Wireless Link Using Sub-terahertz Wave Generated by Photonics-Based Technology

Using a sub-terahertz (sub-THz) wave generated using a photonics-based technology, a high-speed wireless link operating at up to 10 Gbps is designed and demonstrated for realization of seamless connectivity between wireless and wired networks. The sub-THz region is focused upon because of the possibility to obtain sufficient bandwidth without interference with the allocated RF bands. To verify the high-speed wireless link, such dynamic characteristics as the eye diagrams and bit error rate (BER) are measured at up to 10 Gbps for non-return-to-zero pseudorandom binary sequence data. From the measurement results, a receiver sensitivity of –23.5 dBm at is observed without any error corrections when the link distance between the transmitter and receiver is 3 m. Consequently, we hope that our design and experiment results will be helpful in implementing a high-speed wireless link using a sub-THz wave.

Development of DNP-Enhanced High-Resolution Solid-State NMR System for the Characterization of the Surface Structure of Polymer Materials

A dynamic nuclear polarization (DNP)-enhanced cross-polarization/magic-angle spinning (DNP/CP/MAS) NMR system has been developed by combining a 200 MHz Chemagnetics CMX-200 spectrometer operating at 4.7 T with a high-power 131.5 GHz Gyrotron FU CW IV. The 30 W sub-THz wave generated in a long pulse TE \( _{{41}}^{{(1)}} \) mode with a frequency of 5 Hz was successfully transmitted to the modified Doty Scientific low-temperature CP/MAS probe through copper smooth-wall circular waveguides. Since serious RF noises on NMR signals by arcing in the electric circuit of the probe and undesired sample heating were induced by the continuous sub-THz wave pulse irradiation with higher powers, the on-off sub-THz wave pulse irradiation synchronized with the NMR detection was developed and the appropriate setting of the irradiation time and the cooling time corresponding to the non-irradiation time was found to be very effective for the suppression of the arcing and the sample heating. The attainable maximum DNP enhancement was more than 30 folds for C1 13 C-enriched D-glucose dissolved in the frozen medium containing mono-radical 4-amino-TEMPO. The first DNP/CP/MAS 13 C NMR spectra of poly(methyl methacrylate) (PMMA) sub-micron particles were obtained at the dispersed state in the same frozen medium, indicating that DNP-enhanced 1H spins effectively diffuse from the medium to the PMMA particles through their surface and are detected as high-resolution 13 C spectra in the surficial region to which the 1H spins reach. On the basis of these results, the possibility of the DNP/CP/MAS NMR characterization of the surface structure of nanomaterials including polymer materials was discussed.

Sub-terahertz imaging of defects in building blocks

A compact sub-terahertz (THz) imaging system with a 0.2 THz-band GaAs TUNNETT diode oscillator and its application for non-destructive and harmless inspections of timbers, concrete and ceramic tiles are shown. Sub-THz transmission characteristics of various kinds of woods were investigated. It is shown that wood and concrete show a high transmittance in this frequency range, and that the measured absorption coefficients correlate well with the densities of woods. Then, the invisible grains, knots and diffused water inside the timbers were investigated by sub-THz transmission imaging. It is also shown that the sub-THz wave is a very efficient tool for defect recognition in concrete. Invisible cracks, diffused water and the quality of adhesion of tiles were investigated. It is concluded that the sub-THz wave has shown a high sensitivity in detecting these defects in building blocks.

A simple terahertz spectrometer based on a low-reflectivity Fabry-Perot interferometer using Fourier transform spectroscopy

A simple terahertz (THz) Fourier transform spectrometer is theoretically and experimentally demonstrated with a low-reflectivity Fabry-Perot interferometer. Composed with only two parallel low reflectivity surfaces, this simple spectrometer has the ability to measure the spectrum over more than one octave with a controllable resolution. The emission spectra of a wavelength-tunable photonic transmitter excited by an optical coherent control system are determined by the spectrometer. With a simple algorithm for the spectral reconstruction and a compact structure with easy alignment, the demonstrated spectrometer will meet the needs of many important applications where a compact and convenient spectrometer is required in the frequency range from sub-THz wave up to even visible light.

Tunable-frequency high-field electron paramagnetic resonance

A tunable-frequency methodology based on backward wave oscillator sources in high-frequency and -field EPR (HFEPR) is described. This methodology is illustrated by an application to three non-Kramers transition metal ion complexes and one Kramers ion complex. The complexes are of: Ni(II) ( S = 1) as found in dichlorobistriphenylphosphanenickel(II), Mn(III) ( S = 2) as found in mesotetrasulfonatoporphyrinatomanganese(III) chloride, Fe(II) ( S = 2) as found in ferrous sulfate tetrahydrate, and Co(II) ( S = 3/2) as found in azido(tris(3- tert-butylpyrazol-1-yl)hydroborate)cobalt(II). The above Ni(II) and Mn(III) complexes have been studied before by HFEPR using the multifrequency methodology based on Gunn oscillator sources, but not by the present method, while the Fe(II) and Co(II) complexes presented here have not been studied by any form of HFEPR. Highly accurate spin Hamiltonian parameters can be obtained by the experimental methodology described here, in combination with automated fitting procedures. This method is particularly successful in determining g-matrix parameters, which are very difficult to extract for high-spin systems from single frequency (or a very limited set of multi-frequency) HFEPR spectra, but is also able to deliver equally accurate values of the zero-field splitting tensor. The experimental methods involve either conventional magnetic field modulation or an optical modulation of the sub-THz wave beam. The relative merits of these and other experimental methods are discussed.

Sub-THz Wave Generation Using Photonics Technology and Its Application

We describe a technique for generating millimeter- and submillimeter-waves by using photonics technology focusing on the generation of optical beat signals with such a frequency range. We also describe an optical beat signal generation technique that utilizes an optical frequency comb with a spectral bandwidth of over 1 THz. To obtain a low-noise optical frequency comb, a phase modulator and a single-mode fiber are used for optical pulse generation. The frequency interval of the optical frequency comb can be tuned from 30 to 40 GHz. We can obtain an optical beat signal with an arbitrary frequency from 100 GHz to 1 THz by selecting and mixing two appropriate line spectra from the optical frequency comb.

Pseudooctahedral complexes of vanadium(III): electronic structure investigation by magnetic and electronic spectroscopy.

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.

Finite size effect of transmission property for metal hole arrays in subterahertz region

The mechanism of the high band-pass transmission property of metal hole arrays (MHA), which is metal slabs perforated periodically with circular holes, in the subterahertz (sub-THz) wave region has been investigated. We measured the transmission spectra varying the number of holes using a THz time domain spectroscopy and observed that the peak transmittance normalized by the porosity of holes increases with increasing the number of holes. This finite size effect of the transmission characteristics of the MHA is attributed to the increase of the coupling efficiency between the incident THz wave and the surface mode excited on the MHA surface with increasing the number of holes.

Generation of sub-terahertz waves using a semiconductor photomixer

A low-temperature-grown GaAs (LT-GaAs) photoconductive switch has been utilized as a photomixer in the sub-terahertz (sub-THz) region. The usefulness of the switch has been demonstrated by detecting the radiated sub-THz wave with a high-TC Josephson junction detector consisting a series of nine junctions and a gold-coated log-periodic antenna. The sub-THz waves are radiated from the photoconductive switch by introducing the optical heterodyne signal of two semiconductor lasers. Although the radiation power of the photoconductive switch is three orders of magnitude lower than a conventional photodiode, the sub-THz waves from the switch were successfully observed in the difference voltage response of the junctions. This result suggests that the LT-GaAs switch can be used as the photomixer in the optical-microwave converter for superconductive electronics. We have also studied the properties of a radiated sub-THz frequency spectrum using a Martin–Puplett interferometer.