Sub-terahertz Wave Research Articles

Sub-terahertz PAM4 modulator based on transmission characteristic reconstruction

In this paper, we propose a sub-terahertz PAM4 modulator based on transmission characteristic reconstruction by combining meta-unit, GaAs Schottky diode, and fan branch lines. This method combined the significant electromagnetic resonant characteristics of meta-unit, the high-speed controllability of GaAs Schottky diode, and the high integration of on-chip transmission line together to realize high-speed modulation. Then, we achieve transmission characteristic reconstruction by adjusting the resonance strength under different applied voltages through fan branch lines, enabling high-order amplitude modulation of sub-terahertz waves. The experimental results show that the PAM4 modulation of sub-terahertz waves is achieved with a nearly linear variation of the transmission coefficient in the whole voltage range and a maximum modulation rate of 21 Gbps, providing a promising prospect for the development and application of integrated sub-terahertz direct high-order modulation technology.

Sub-Terahertz Imaging-Based Real-Time Non-Destructive Inspection System for Estimating Water Activity and Foreign Matter Depth in Seaweed.

As the importance of hygiene and safety management in food manufacturing has been increasingly emphasized, research on non-destructive and non-contact inspection technologies has become more active. This study proposes a real-time and non-destructive food inspection system with sub-terahertz waves which penetrates non-conducting materials by using a frequency of 0.1 THz. The proposed system detects not only the presence of foreign matter, but also the degree of depth to which it is mixed in foods. In addition, the system estimates water activity levels, which serves as the basis for assessing the freshness of seaweed by analyzing the transmittance of signals within the sub-terahertz image. The system employs YOLOv8n, which is one of the newest lightweight object detection models. This lightweight model utilizes the feature pyramid network (FPN) to effectively detect objects of various sizes while maintaining a fast processing speed and high performance. In particular, to validate the performance in real manufacturing facilities, we implemented a hardware platform, which accurately inspects seaweed products while cooperating with a conveyor device moving at a speed of 45 cm/s. For the validation of the estimation performance against various water activities and the degree of depth of foreign matter, we gathered and annotated a total of 9659 sub-terahertz images and optimized the learning model. The final results show that the precision rate is 0.91, recall rate is 0.95, F1-score is 0.93, and mAP is 0.97, respectively. Overall, the proposed system demonstrates an excellent performance in the detection of foreign matter and in freshness estimation, and can be applied in several applications regarding food safety.

Structural characteristics of glycine aggregates grown under sub-Terahertz irradiation

Although crystal growth methods in which intramolecular bonds are induced by infrared irradiation have been investigated recently, there is very little reported on the use of electromagnetic waves to enhance growth rates. In this study, we have irradiated with 40 GHz sub-Terahertz waves during the solution growth of glycine and investigated the effect on crystal growth. Highly crystalline α-glycine grew with or without irradiation and aggregates were formed. The growth rate, however, was strongly affected by sub-Terahertz wave irradiation, with the growth rate from nucleation approximately doubling. Powder X-ray structure analysis showed that the unique diffraction peaks of glycine tended to partially shift to lower angles, with the (−110) plane and (011) plane spacing expanding by about 0.1 % and 0.15 %, respectively. The FTIR results in the terahertz region show a large absorption near 6 THz, suggesting that the structure contains water molecules.

Fabrication and testing of the transition section between modules of a wakefield accelerator

The fabrication process is presented for a typical transition section located between each cylindrical corrugated waveguide structure comprising the wakefield accelerator module. The transition section includes couplers for extracting the 180 GHz TM01 accelerating mode and separate couplers for extracting the 190 GHz HE11 dipole mode, both modes induced by the electron bunch traversing the cylindrical corrugated waveguide structure. Extraction of the high-power accelerating mode reduces the heat load due to the subterahertz wave power dissipation within the corrugated accelerating structure. Extraction of the low-power dipole mode serves the purpose of detecting the electron bunch transverse oscillations within the wakefield accelerator and identifying the onset of beam breakup instability. Comprehensive testing of the fully functional transition section with an electron beam was done at the Accelerator Test Facility in Brookhaven National Laboratory which verified the functionality of the transition section. Published by the American Physical Society 2024

Fundamental Properties of Sub-THz Reflected Waves for Water Content Estimation of Reinforced Concrete Structures

Water plays a significant role in the deterioration of reinforced concrete buildings; therefore, it is essential to evaluate the water content of the cover concrete. This study explores a novel non-destructive method for assessing the water content using sub-terahertz (sub-THz) waves. Among the four frequencies selected to evaluate the water content, an increase in reflectance was observed as the unit volume water content increased, and smaller data scatter was confirmed as the frequency increased. The derived empirical equation can classify the corrosion risk of the rebar environment based on the water content obtained using reflectance measurements. In other words, it can contribute to the diagnosis of the building integrity associated with rebar corrosion.

Frequency selection mechanism of sub-terahertz wave propagation within the sharp-coned plasma sheath

The propagation characteristics of sub-terahertz (sub-THz) waves through the sharp-coned plasma sheath are investigated, revealing a frequency selection phenomenon. Two significant electron density gradients within the sharp-coned plasma sheath, which result in high reflection coefficients, are identified. These strong reflective interfaces divide the plasma into distinct regions, and the frequency selection mechanism is analyzed using the improved scattering matrix method. This research finds that the combination of these reflective interfaces and the intervening plasma forms a “resonator structure,” leading to the observed frequency selection. A quantitative relationship between plasma parameters and the frequency selection phenomenon is analyzed. The results indicate that the reflection coefficients of the reflective interfaces increase, making the frequency selection more pronounced, when the thickness of the interfaces decreases or the peak electron density increases. In addition, a lower collision frequency leads to reduced absorption effects and a more pronounced frequency selection. The phenomenon suggests that enhancing transmissivity at lower frequencies may be feasible, providing a theoretical insight into the application of sub-THz waves in mitigating communication blackouts.

Non-destructive testing of reinforced concrete structures using sub-terahertz reflected waves

Terahertz (THz) and sub-terahertz (sub-THz) waves are unexplored waves between the infrared and microwave ranges. This range features unique characteristics of both light resolution and electromagnetic wave transmission. Sub-THz waves are part of attractive new diagnostic methods for inner objects because they are safer than normal non-destructive inspection methods involving high energy, such as X-rays. In this study, a novel non-contact and non-destructive inspection method for reinforced concrete (RC) structures using sub-THz reflection imaging is proposed to inspect the deterioration of inaccessible damaged RC structures. The results of this study confirmed that the proposed sub-THz imaging method can detect cracks and voids in concrete, as well as in metals inside or behind the concrete, based on differences in reflectance. In addition, the camera-based measurement system makes real-time on-site field measurements possible.

Current-Driven Terahertz Oscillations in Diamond TeraFET

We report on sub-terahertz plasmonic wave generation in the 2DEG channel of diamond TeraFET when biased by a DC current at the drain. Our numerical results demonstrated that p-diamond can support resonant oscillation of 300 GHz at room temperature, allowing it to function as a sub-THz emitter. We investigated the impact of different channel lengths, gate biases, drift velocities, and temperatures on the fundamental mode oscillation. The model incorporated the decay factors owing to electron scattering and electron fluid viscosity.

DaISy: diffuser-aided sub-THz imaging system.

Sub-terahertz (Sub-THz) waves possess exceptional attributes, capable of penetrating non-metallic and non-polarized materials while ensuring bio-safety. However, their practicality in imaging is marred by the emergence of troublesome speckle artifacts, primarily due to diffraction effects caused by wavelengths comparable to object dimensions. In addressing this limitation, we present the Diffuser-aided sub-THz Imaging System (DaISy), which utilizes a diffuser and a focusing lens to convert coherent waves into incoherent counterparts. The cornerstone of our progress lies in a coherence theory-based theoretical framework, pivotal for designing and validating the THz diffuser, and systematically evaluating speckle phenomena. Our experimental results utilizing DaISy reveal substantial improvements in imaging quality and nearly diffraction-limited spatial resolution. Moreover, we demonstrate a tangible application of DaISy in the scenario of security scanning, highlighting the versatile potential of sub-THz waves in miscellaneous fields.

Smart Radio Environments with Intelligent Reflecting Surfaces for 6G Sub-Terahertz-Band Communications

Technology for sixth-generation (6G) mobile communication system is now being widely studied. A sub-Terahertz band is expected to play a great role in 6G to enable extremely high data-rate transmission. This paper has two goals. (1) Introduction of 6G concept and propagation characteristics of sub-Terahertz-band radio waves. (2) Performance evaluation of intelligent reflecting surfaces (IRSs) based on beamforming in a sub-Terahertz band for smart radio environments (SREs). We briefly review research on SREs with reconfigurable intelligent surfaces (RISs), and describe requirements and key features of 6G with a sub-Terahertz band. After that, we explain propagation characteristics of sub-Terahertz band radio waves. Important feature is that the number of multipath components is small in a sub-Terahertz band in indoor office environments. This leads to an IRS control method based on beamforming because the number of radio waves out of the optimum beam is very small and power that is not used for transmission from the IRS to user equipment (UE) is little in the environments. We use beams generated by a Butler matrix or a DFT matrix. In simulations, we compare the received power at a UE with that of the upper bound value. Simulation results show that the proposed method reveals good performance in the sense that the received power is not so lower than the upper bound value.

Sub-terahertz photoacoustic effect enabling broadband ultrasound generation for underwater communication

Conventional wireless communication faces limitations in underwater environments due to the high electromagnetic wave attenuation. Ultrasound provides an alternative, enabling communication for underwater drones and intrabody communication for implantable devices. However, the limited bandwidth of resonance-based piezoelectric transducers remains a challenge. In addition, the significant acoustic impedance mismatch between water and air hinders cross-boundary communication and necessitates direct contact between the medium and the transducers. To overcome these challenges, we propose to use a sub-terahertz photoacoustic effect for the generation of broadband ultrasound in a non-contact manner. The photoacoustic effect generates acoustic signals in a medium through thermal expansion caused by optical irradiation. We demonstrate that the use of periodically modulated continuous-wave sub-terahertz waves around 0.1 THz enables broadband signal generation efficiently owing to their high attenuation in water. We characterize the frequency response of the generated acoustic signals and demonstrate a proof of concept for underwater communication.

Generating in vivo continuous ultrasound based on sub-terahertz photoacoustic effect

Non-contact ultrasound excitation based on the photoacoustic effect using short optical pulses has been widely used for biomedical and industrial inspections. However, generating and detecting photoacoustic signals in water or aqueous samples requires careful choice of the excitation wavelength. Here, we show that continuous-wave (CW) ultrasound can be directly generated in aqueous samples by irradiating them with the CW sub-terahertz waves modulated at acoustic frequencies, even when the stress confinement condition is not satisfied. The ultrasound generated at resonance can be detected even in the air using a microphone. The sub-terahertz waves exhibit a water absorption coefficient akin to peak near-infrared wavelengths while offering transmittance through diverse materials. Leveraging recent advances in high-frequency electronics, we develop a compact experimental system with the potential for further miniaturization. To demonstrate the potential of the proposed method, we present proof-of-concept applications of bulk modulus measurement of gelatin gels and in vivo anatomical imaging of human hands.

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.

Biothermal Heating on Human Skin by Millimeter and Sub-Terahertz Waves in Outdoor Environment—A Theoretical Study

The frequency band in the millimeter-wave (MMW) and sub-terahertz (sub-THz) range has shown great potential in mobile communication technology due to the advantages of ultra-large bandwidth and ultra-high data rates. Based on the increasing research activities on MMW/sub-THz waves, biological safety at relevant frequencies must be explored, especially when high-power illumination occurs. Here, its non-ionizing nature plays a vital role, which makes it safe for humans at low illumination powers. However, under high power, the biothermal heating on the skin surface is still a main concern, and lots of research has been conducted in a laboratory. In this article, we analyze the thermal heating effect of human skin in outdoor environments, where atmospheric conditions can significantly impact the propagation of MMW/sub-THz waves. Our analysis is based on rat skin, which has a similar structure to human skin. A theoretical model combining Pennes’ bioheat transfer equation (BHTE), the ITU model, and the Mie scattering theory is developed. Good agreement between calculation results and measured data confirms the efficiency of this model. The influence of rainfall rate, humidity, operating frequency, illumination time, power density, and propagation distance is presented and discussed.

Evaluation of sub-terahertz imaging for non-destructive inspection

In this article, we apply sub-terahertz waves to imaging a target about 1 meter away from the measurement equipment and evaluate image quality for non-destructive inspection. We show the results of imaging a range of items with sub-terahertz waves and confirm that some of them are penetrated.

Alignment-free twisted-split-ring metasurface on single substrate with 2π phase range for linearly polarized sub-terahertz wave.

To achieve high-speed, large-capacity communication, next-generation mobile communication systems will require manipulation of the propagation of sub-terahertz waves in the propagation channel. In this paper, we propose the use of a novel split-ring resonator (SRR) structure as a metasurface unit cell for manipulating the linearly polarized incident and transmission waves used in mobile communication systems. In this SRR structure, the gap is twisted by 90° to efficiently use cross-polarized scattered waves. By changing the twist direction and gap size of the unit cell, 2π phase designability can be achieved, which enables linear polarization conversion efficiencies of -2 dB with a backside polarizer and -0.2 dB with two polarizers. In addition, a complementary pattern of the unit cell was fabricated, and a measured conversion efficiency of more than -1 dB at the peak with only the backside polarizer on a single substrate was verified. In the proposed structure, the 2π phase designability and efficiency gain are obtained independently by the unit cell and polarizer, respectively, thus enabling alignment-free characteristics, which are highly advantageous from an industrial viewpoint. Metasurface lenses with binary phase profiles of 0 and π were fabricated using the proposed structure with a backside polarizer on a single substrate. The lenses' focusing, deflection, and collimation operations were experimentally verified with a lens gain of 20.8 dB, which agreed well with our calculated results. Our metasurface lens has the great advantages of easy fabrication and implementation, and it has the potential to enable dynamic control by combining it with active devices because of the simple design methodology, which entails only changing the twist direction and the gap's capacitance component.

Manipulation of sub-terahertz waves using digital coding metasurfaces based on liquid crystals.

This paper presents a novel sub-terahertz liquid crystal (LC) phase shifter based on digital coding metasurfaces. The proposed structure consists of metal gratings and resonant structures. They are both immersed in LC. The metal gratings function as reflective surfaces for electromagnetic waves and electrodes for controlling the LC layer. The proposed structure changes the state of the phase shifter by switching the voltage on every grating. It allows the deflection of LC molecules within a subregion of the metasurface structure. Four switchable coding states of the phase shifter are obtained experimentally. The phase of the reflected wave varies by 0°, 102°, 166°, and 233° at 120 GHz. Due to the presence of the transverse control electric field, modulation speed is approximately doubled compared to the free relaxation state. This work provides a novel idea for wavefront modulation of phase.

Detection of Terahertz Frequencies in S-Doped GaSe Crystals Using Laser Pulses at Telecom Wavelengths

Nonlinear optical crystals of gallium selenide are efficient up- and downconverters of infrared and terahertz frequencies. Their nonlinear properties have been investigated at wavelengths within the main transparency window. However, insufficient attention has been paid to studies at the telecommunication wavelength, especially for sulfur-doped crystals. Closing this gap, we report on the optical and electro-optical properties of GaSe(1–x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22). For this purpose, the refractive indexes of the ordinary waves at terahertz frequencies and at a wavelength of 1.55 μm have been measured. The detection efficiency of the subterahertz waves in the crystals was studied using Er-fiber laser pulses and compared with that of GaAs, the etalon electro-optical crystal, at this wavelength. This allows us to estimate the dependence of the electro-optic coefficient r22 of GaSe(1–x)Sx on the sulfur concentration. It was shown that the sample with x = 0.12 has the largest value of the electro-optical coefficient r22 = 1.26 pm/V and provides the highest detection efficiency among the samples. The potential of employing S-doped GaSe crystals as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

Fundamental research on non-destructive testing of reinforced concrete structures using sub-terahertz reflected waves

Terahertz and sub-terahertz waves are unexplored waves range between infrared waves and microwaves. This range features with unique characteristics of both light straightness and electromagnetic wave transmission. Terahertz and sub-terahertz waves are attractively new diagnose method of inner objects because they are safer compared to normal non-destructive inspection methods involving high energy, such as X-rays. Hereby, a novel non-destructive inspection method for concrete and reinforced concrete structures is proposed using sub-terahertz reflection imaging. In Japan, where earthquakes and other natural disasters frequently occur, this technique is expected to be used to remotely inspect the deterioration of damaged RC structures that are difficult to access to. The results of this study confirm that various types of condition changes in concrete can be found by using this sub-terahertz imaging method. The presence of cracks/voids in concrete and under the finishing layers was successfully detected based on differences in reflectance. Moreover, the presence of metals inside/behind the concrete specimens was able to be identified. It was also confirmed that the decline rate of the mechanical properties of concrete could be evaluated according to the decrease in reflectance due to the existence of fine cracks.

Flexible terahertz metamaterial filter with high transmission intensity and large tuning range for optical communication application

The resonant frequency of indoor wireless communication based on wireless fidelity (Wi-Fi) and Bluetooth is limited to several gigahertz (GHz). Following the international mobile telecommunications specification proposed by international telecommunication union in 2020 (ITUIMT-2020), communication links with sub-terahertz (sub-THz) waves are desired for indoor networks in the coming years. In this study, we present a flexible terahertz (THz) metamaterial (FTM) filter consisting of cross-shaped metamaterial configurations on a polydimethylsiloxanes (PDMS) substrate. As a band-stop filter, such designed FTM filters possess the selectivity of resonant frequency and thus provide a controllable transmission carrier for a broad frequency range in the sub-THz window. The FTM filters can generate a tuning range of over 85-GHz resonant shift from 360.6 GHz to 446.1 GHz by changing the distance between the top and bottom metamaterial layers. For specific frequency carriers in passband, the proposed FTM filters show the transmission intensity higher than 94%. This design will play a key role in the future indoor wireless communication with miniaturization, large tuning range, and high transmission intensity characteristics. • Tunable metamaterial filter (TMF) shows high transmission intensity and large tuning range. • Tuning range of TMF is 85 GHz from 360 GHz to 445 GHz. • For wireless THz communication from, the proposed TMF can modulate specific carriers. • TMF enables the transmission end of the communication system more flexible in data processing. • TMF provides a feasible scheme for the future ultra-high-speed wireless transmission system.