Broadband Characteristics Research Articles

Room-temperature infrared photoluminescence and broadband photodetection characteristics of Ge/GeSi islands on silicon-on-insulator.

In this study, molecular beam epitaxial growth of strain-driven three-dimensional self-assembled Ge/GeSi islands on silicon-on-insulator (SOI) substrates, along with their optical and photodetection characteristics, have been demonstrated. The as-grown islands exhibit a bimodal size distribution, consisting of both Ge and GeSi alloy islands, and show significant photoluminescence (PL) emission at room temperature, specifically near optical communication wavelengths. Additionally, these samples were used to fabricate a Ge/GeSi islands/Si nanowire (NW) based phototransistor using a typical e-beam lithography process. The fabricated device exhibited broadband photoresponse characteristics, spanning a wide wavelength range (300-1600 nm) coupled with superior photodetection characteristics and relatively low dark current (~ tens of pA). The remarkable photoresponsivity of the fabricated device, with a peak value of ~11.4 A/W (λ ~ 900 nm) in the near-infrared (NIR) region and ~ 1.36 A/W (λ ~ 1500 nm) in the short-wave infrared (SWIR) region, is a direct result of the photoconductive gain exceeding unity. The room-temperature optical emission and outstanding photodetection performance, covering a wide spectral range from the visible to the SWIR region, showcased by the single layer of Ge/GeSi islands on SOI substrate, highlight their potential towards advanced applications in broadband infrared Si-photonics and imaging. These capabilities make them highly promising for cutting-edge applications compatible with complementary metal-oxide-semiconductor (CMOS) technology.

Investigating pump harmonics generation in a SNAIL-based traveling wave parametric amplifier

Abstract Traveling wave parametric amplifiers (TWPAs) are extensively employed in experiments involving weak microwave signals for their highly desirable quantum-limited and broadband characteristics. However, TWPAs’ broadband nature comes with the disadvantage of admitting the activation of spurious nonlinear processes, such as harmonics generation, that can potentially degrade amplification performance. Here we experimentally investigate a Josephson TWPA device with superconducting nonlinear asymmetric inductive element-based unit cells focusing on the amplification behaviour along with the generation of second and third harmonics of the pump. By comparing experimental results with transient numerical simulations, we demonstrate the influence of Josephson junctions’ fabrication imperfections on the occurrence of harmonics and on the gain behaviour.

Wideband circularly polarized dielectric resonator antenna with multi-functional metasurface assistance for low profile

ABSTRACT A low-profile broadband circularly polarized (CP) dielectric resonator antenna (DRA) is proposed, utilizing DR reutilization and metasurface (MS) techniques. Two resonant frequencies and an AR minimum are achieved simultaneously, by exciting a rectangular DR with an inclined corner-truncated coupling slot, for it generates nearly degenerated TE111 modes. For significant bandwidth enhancement and low profile, a new resonator is formed by reusing the DR and loading an MS above, which works in orthogonal TM modes. To increase the axial ratio bandwidth (ARBW) further without enlarging the antenna size， two parasitic patches are printed on two adjacent side walls of the DR. To verify the rationality and feasibility, the antenna is fabricated and measured. The fabricated prototype possesses the profile of 0.06λ 0 (λ 0 is the free-space wavelength at the center frequency). The measured impedance bandwidth (IBW) and ARBW are 36.05% (4.23 ~ 6.09 GHz) and 22.46% (4.9 ~ 6.14 GHz), respectively. The proposed antenna has both broadband characteristics and a low profile, making it easy to integrate into space-limited wireless communication systems with high data rates.

Progress on Single-Feed Quality Wideband Linear Wire Array

This paper presents the latest developments regarding the single-feed Quality Wideband Linear (QWL) wire array antenna, known for its broadband and high-gain electromagnetic characteristics and robust design. A systematic review of recent advances in relation to the QWL antenna is provided, covering its driven element, director, reflector, low common-mode current interference connector, and array series-feed configuration. For the first time, an analytical expression and a quick design formula for the input impedance of the QWL antenna’s driven element, the linear Wideband High-gain Electromagnetic Structure (WHEMS) antenna, are presented. Theoretical analysis demonstrates the potential for broadband performance using the WHEMS antenna. The rugged design of the QWL array antenna offers engineering advantages such as simple feeding, low wind resistance, a lightweight construction, low cost, and structural robustness. The QWL antenna has already found applications in various industrial sectors, with potential for broader use in the future, contributing to further advancements in antenna technology.

Interictal stereo-electroencephalography features below 45 Hz are sufficient for correct localization of the epileptogenic zone and postsurgical outcome prediction.

Evidence suggests that the most promising results in interictal localization of the epileptogenic zone (EZ) are achieved by a combination of multiple stereo-electroencephalography (SEEG) biomarkers in machine learning models. These biomarkers usually include SEEG features calculated in standard frequency bands, but also high-frequency (HF) bands. Unfortunately, HF features require extra effort to record, store, and process. Here we investigate the added value of these HF features for EZ localization and postsurgical outcome prediction. In 50 patients we analyzed 30 min of SEEG recorded during non-rapid eye movement sleep and tested a logistic regression model with three different sets of features. The first model used broadband features (1-500 Hz); the second model used low-frequency features up to 45 Hz; and the third model used HF features above 65 Hz. The EZ localization by each model was evaluated by various metrics including the area under the precision-recall curve (AUPRC) and the positive predictive value (PPV). The differences between the models were tested by the Wilcoxon signed-rank tests and Cliff's Delta effect size. The differences in outcome predictions based on PPV values were further tested by the McNemar test. The AUPRC score of the random chance classifier was .098. The models (broad-band, low-frequency, high-frequency) achieved median AUPRCs of .608, .582, and .522, respectively, and correctly predicted outcomes in 38, 38, and 33 patients. There were no statistically significant differences in AUPRC or any other metric between the three models. Adding HF features to the model did not have any additional contribution. Low-frequency features are sufficient for correct localization of the EZ and outcome prediction with no additional value when considering HF features. This finding allows significant simplification of the feature calculation process and opens the possibility of using these models in SEEG recordings with lower sampling rates, as commonly performed in clinical routines.

Physical analysis of high refractive index metamaterial-based radiation aggregation engineering of planar dipole antenna for gain enhancement of mm-wave applications

A metamaterial-incorporated high-gain and broadband dipole antenna is proposed for 5G mm-wave applications. The designed high refractive index metamaterial (HRIM) properties are presented in detail with supporting results. The proposed dipole antenna achieved broadband features, and the antenna gain increased significantly by incorporating HRIM in the electromagnetic (EM) wave propagation path. Besides, the EM wave aggregation engineering of utilizing the HRIM on the dipole antenna is analyzed using the electric field, magnetic field, and power flow distributions. Both conventional and HRIM-based dipole antenna (HRIMDA) were fabricated on thin (0.254 mm) Rogers RT5880 substrate material with a low dielectric constant of 2.2, where the noticeable gain enhancement by HRIM is observed. The measured results show the operational bandwidth (BW) from 23 to 38 GHz frequency, and the highest gain of 9.5 dBi is accomplished at 35 GHz frequency. Finally, a four-element MIMO configuration is numerically and experimentally analyzed where the isolation of the two conjugated MIMO elements is < − 20 dB. The MIMO parameters like envelop correlation coefficient (ECC) < 1 × 10–3 and diversity gain (DG) value of > 9.9 were also achieved. Hence, the proposed HRIM base dipole antenna is a potential candidate for 5G mm-wave applications.

Multifunctional multifocal lens for simultaneous edge and polarisation imaging

ABSTRACT The method of inserting ultrathin devices in the Fourier plane of the 4F imaging system can achieve multifunctional imaging. However, such a method generally requires precise alignment in the system, which also makes the system more complex. Here, we present an imaging system that incorporates a multifocal Pancharatnam–Berry (PB) phase liquid crystal (LC) lens capable of achieving simultaneous edge and polarisation imaging. Without placing optical elements in the Fourier plane of the 4F system, the proposed imaging system replace a refracted lens in 4F system by multifocal LC lens, which integrates edge and polarisation imaging modes. The phase distribution of multifocal lens is obtained through manipulating the positions and polarisation states of four foci with holographic principles. Furthermore, the imaging system based on the proposed multifocal lens has been shown to be effective in simultaneously achieving polarisation and edge imaging with broadband characteristics. The proposed optical system effectively reduces the complexity of the imaging system, which is expected to be applied in the fields of biomedical imaging, optical interconnection and microscopic imaging.

Light funneling into localized acoustic graphene plasmons for extreme infrared and terahertz field enhancement and confinement

Enhancing light-matter interaction through deep subwavelength-scale confinement is crucial for numerous applications like molecular sensing, optoelectronic devices, and non-linear optics. Here, we report the excitation of localized acoustic graphene plasmons (LAGPs) confined in a sub-micro- wide, nanometer-thick layer using a metal slit antenna. This approach enables light funneling in the infrared and terahertz regimes, leading to strong field enhancement and confinement. LAGPs exhibit broad-band excitation characteristics, with the number of excited modes adjustable via the symmetry of the relative positioning between graphene and the metal slit. Detailed analysis indicates that the local field intensities of LAGPs are critically influenced by both the periodicity of the device structure and the electron relaxation time of graphene. These findings are effectively elucidated using temporal coupled mode theory. In comparison to conventional non-localized acoustic graphene plasmons, LAGPs demonstrate significantly improved field confinement and enhancement attributed to the funneling effect. Our study presents a promising avenue for achieving robust light-matter interaction and holds potential for various applications in the infrared and terahertz domains.

Numerical and experimental study on the aerodynamic noise characteristics of 600-km/h high-speed maglev trains

Abstract High-speed maglev trains represent a new mode of transportation; however, their 600-km/h traveling speed causes serious aerodynamic noise problems, with very little research on the aerodynamic noise of such trains. To clarify the aerodynamic noise source and radiation characteristics of high-speed maglev trains, both large eddy simulations and wind tunnel experiments with a 1:8 scale three-train model were conducted to obtain the aerodynamic noise source spectrum and spatial distribution characteristics of the maglev train at different speeds; the contribution of the dipole noise source was also analyzed. The results revealed that the train shape has a significant impact on the frequency distribution of the far-field aerodynamic noise below 1 kHz, with the highest sound power concentrated on the train head, especially in the areas around the track joint and streamlined sections. The surface dipole sound source energy increases linearly with speed and is mainly distributed at the bottom of the train head and tail train body. A short-streamlined train exhibited the highest ratio, especially in the trailing edge area, up to 1.42×1011 dB. The far-field sound pressure level of the short-streamlined maglev train is significantly higher, with a maximum difference of about 10 dB(A). The distribution pattern of far-field sound pressure level is influenced by the train type, reflected in the tail train and wake area. Different frequency intervals have different relationships between the acoustic pressure level and frequency; long-streamlined models may enhance broadband features in the sound field distribution of high-speed maglev trains. These findings could be used to guide the aero-acoustic design of maglev trains with a designed speed of 600 km/h.

Air-coupled ultrasound using broadband shock waves from piezoelectric spark igniters

We used an optomechanical sensor to study the ultrasound generated by manually operated piezoelectric spark igniters. These low-energy sparks produce short-duration acoustic shock-wave pulses, with sub-microsecond rise times and frequency content extending well beyond 2 MHz in air. The same source–receiver combination was then used to demonstrate broadband characterization of solid (polymer and glass) plates in a simple setup, where single spark events yielded high signal-to-noise ratio data without the need for critical alignment. This setup also enabled us to estimate pressure excursions approaching 105 Pa at millimeter-scale distances from the spark. The results are in large part made possible by the small size, wide bandwidth, and high sensitivity of the optomechanical sensor and might be of interest for air-coupled ultrasound applications in nondestructive testing.

Vanadium dioxide-assisted switching of a half-wave and quarter-wave plate to a dual-band absorber for the terahertz wave

In this study, we present a broadband, multi-functional, and switchable metamaterial for terahertz (THz) waves. The metamaterial design achieves high polarization conversion efficiency and can seamlessly transition between a polarization converter (PC) and a dual-band absorber through the phase transition behavior of vanadium dioxide (VO2). Our design facilitates linear to-cross (Half-wave plate) and linear to-circular (Quarter-wave plate) polarization conversion across a bandwidth extending from 0.58 THz to 0.88 THz, and 0.35 THz to 0.50 THz respectively. Near-perfect absorption at 0.35 THz and 0.99 THz was observed when VO2 changed its phase from an insulator to a conductor. These broadband features are realized through a straightforward metamaterial design incorporating only a reflector, a dielectric spacer, and an antenna (meta-atom resonator). Our designed structure demonstrates impressive angular stability for incident angles ranging from 0° to 45°. The proposed switchable metamaterial holds promise for advancing the development of tunable polarization rotating devices and on/off switching LPC devices, offering broad applications in THz detection, sensing, and communications.

Ultra-broadband and thin switchable multifunctional metamaterial for terahertz wave

An ultra-broadband thin multi-functional and switchable metamaterial is examined in the terahertz (THz) regime. The proposed design achieved high polarization conversion efficiency and can be switched from a polarization converter to an absorber using the phase change transitioning of vanadium dioxide (VO2). The linear polarization conversion is achieved from 6.0 THz to 15.0 THz, achieving a bandwidth of 9.0 THz, and the absorption is realized from 5.0 THz to 16.0 THz with a bandwidth of 11.0 THz. These broadband characteristics were achieved by a simple metamaterial design incorporating a few layers. The relative bandwidth was 85% for the polarization conversion and 105% for the absorption. Moreover, the angular stability of the designed structure is impressive for various incident angles from 0° to 45°. The proposed switchable design has the potential to contribute to the development of tunable polarization rotating devices, on/off switching LPC devices, which have wide application potentials in THz detection, sensing, adaptive optics, and communications.

Design and analysis of Minkowski fractal shaped metasurface absorber with broadband and polarization-insensitive characteristics using a tunable graphene layer for terahertz applications

The proposed research article’s main goal is to demonstrate a terahertz (THz) broadband absorber for high-speed wireless communication applications. The proposed structure is a compact one and possesses three layers. The ground layer acts as a metal reflector, lossy silicon acts as a dielectric material and finally a graphene layer in the shape of a minkowski fractal acts as a radiating patch for the proposed design. We have chosen a thickness of 5 μm for the lossy silicon which has a dielectric constant of 11.90. The bottom layer of the proposed design contains a good conductive material like gold with a conductivity of 4.561 e+007 s m−1 with a thickness of 0.2 μm. The thickness of a monolayer graphene is one nanometer, and the overall unit cell size of the proposed structure is 12 × 12 μm2. Because of its symmetrical nature, the proposed absorber offers a broad response to both TM and TE modes irrespective of any polarization angle. The proposed absorber can operate in the terahertz frequency range and has achieved two broad frequency bands from 3.34–3.98 THz and 4.6–5.30 THz, with an absorption percentage greater than 90. We can observe peak absorption frequencies at 3.60 THz and 5.04 THz, which exhibit an absorption percentage close to unity. Additionally, we validated the proposed broadband absorber using an equivalent circuit approach and verified it using the ADS tool.

Temperature- and pressure-responsive photoluminescence in a 1D hybrid lead halide

Low-dimensional hybrid lead halides with responsive emissions have attracted considerable attention due to their potential applications in sensing. Herein, a new one-dimensional hybrid lead bromide CyPbBr3 (Cy = cytosine cation) was synthesized to explore its emission evolution in response to temperature and pressure. The compound possesses an edge-sharing 1D double-chain structure and emits warm white light across nearly the entire visible spectrum upon ultraviolet excitation. This emission arises from the self-trapped excitons and its broadband feature is attributed to the strong electron-phonon coupling as revealed by the variable-temperature photoluminescence experiments. Moreover, a 4.5-fold pressure-induced emission enhancement was observed at 2.7 GPa which is caused by the pressure suppressed non-radiative energy loss. Furthermore, in-situ powder X-ray diffraction and Raman experiments reveal the maxima of the emission enhancement is associated with a phase transition at the same pressure. Our work demonstrates that low-dimensional metal halides are a promising class of stimuli-responsive materials which could have potential applications in temperature and pressure sensing.

Searching for QPOs in BATSE short gamma-ray bursts based on narrowband and broadband features

Gamma-ray bursts (GRBs), especially short GRBs, are often considered potential candidates for exhibiting kilohertz quasi-periodic oscillations (QPOs) due to their origin from binary mergers. It has already been discovered that two bursts exhibit QPOs. While systematic searches for QPOs in GRBs typically concentrate on the kilohertz range, there has been no comprehensive exploration in the hundred-hertz range. In this study, we systematically conducted QPO searches on all BATSE short burst data within the 0-1000 Hz range. Using nested significance tests, we observed that the reference distributions for different GRBs are quite similar. This observation prompted us to analyze the data by selectively focusing on those with larger statistical values, obviating the need to iterate through all the data and significantly reducing computational workload. Ultimately, our findings did not reveal any compelling evidence for QPOs, which may suggest that the GRB jet has lost the early merging memory.

3D-printed multi-material pyramids for broadband electromagnetic wave absorption

Designing and manufacturing broadband electromagnetic wave-absorbing materials represents one of the significant approaches to address electromagnetic pollution issues. This study utilizes three different PLA-based composite absorbing materials to prepare multilayer pyramid structures employing 3D printing technology. Results indicate that with material composition Z1-GT6-N8, material layer height (h) of 6 mm, and pyramid base length (L) of 36 mm, the fabricated structure achieves a minimum reflection loss of −24.19 dB with an effective absorption bandwidth of 14.21 GHz, demonstrating outstanding absorption performance. It is noteworthy that regardless of the incident angle increasing from 0° to 50°, the equivalent absorption bandwidth (EAB) for both transverse electric (TE) and transverse magnetic (TM) polarizations remains greater than 10 GHz, indicating excellent angle stability. The broadband and efficient electromagnetic wave (EMW) absorption characteristics exhibited by this absorber can be attributed to its outstanding impedance matching performance and the integration of multiscale loss mechanisms.

A Dual-Polarization Programmable Metasurface for Green and Secure Wireless Communication.

Dual-polarization programmable metasurfaces can flexibly manipulate electromagnetic (EM) waves while providing approximately twice the information capacity. Therefore, they hold significant applications in next-generation communication systems. However, there are three challenges associated with the existing dual-polarization programmable metasurfaces. This article aims to propose a novel design to address them. First, the design overcomes the challenge of element- and polarization-independent controls, enabling more powerful manipulations of EM waves. Second, by using more energy-efficient tunable components and reducing their number, the design can be nearly passive (maximum power consumption of 27.7mW), leading to a significant decrease in the cost and power consumption of the system (at least two orders of magnitude lower than the power consumption of conventional programmable metasurfaces). Third, the design can operate in a broad bandwidth, which is attractive for practical engineering applications. Both the element and array of the metasurface are meticulously designed, and their performance has been carefully studied. The experiments demonstrate that 2D wide-angle beam scanning can be realized. Moreover, secure communication based on directional information modulation can be implemented by exploiting the metasurface and an efficient discrete optimization algorithm, showing its programmable, multiplexing, broadband, green, and secure features.

High performance photodetectors by integrating CsPbBr3 perovskite directly on the germanium wafer

Lead halide perovskites are a promising candidate for the development of novel optoelectronic devices. Although substantial development has been achieved after years of research, it is still a challenge to prepare large-area high-quality CsPbBr3 thin films by simple processes. In this work, CsPbBr3 film can be used as a UV–Vis absorption active layer of heterojunction photodetector and a Ge-based infrared anti-reflective coating to optimize the photodetection performance of the device. CsPbBr3/p-Ge heterojunction photodetector exhibits self-powered (0 V) and broadband characteristics (365–1550 nm), the responsivity is 64 mA/W and the specific detectivity is 1.2 × 1012 Jones under 0 V bias and 1550 nm illumination. The rise and decay times of the device are 12.3 μs and 13.4 μs under 1310 nm illumination and 20 kHz operating frequency, respectively. This work further promotes the potential application and development of CsPbBr3 perovskites in integrated optoelectronic devices.

Nonlinear vibration energy harvesting via parametric excitation: Snap-through with time-varying potential wells

This study introduces a novel method to improve snap-through transitions in a parametrically excited cantilever-based energy harvester featuring repulsive magnetic coupling. The aim is to enhance the efficiency of a piezoelectric energy harvester while broadening its operational frequency range. The design incorporates an oscillating substructure that magnetically interacts with the beam, promoting easier transitions. Additionally, this setup capitalizes on the resonant energy of the substructure to optimize energy capture. This study addresses a previously unexplored limitation in the directly excited counterpart by optimizing the use of substructure’s resonant energy to significantly improve the energy harvesting device’s efficiency. We present a theoretical model based on the Hamilton principle, which is solved numerically. Comprehensive experimental studies are conducted to validate the theoretical results, showing a strong correlation between the experimental outcomes and theoretical predictions. The findings indicate that an oscillating substructure with a tailored natural frequency close to twice that of the parametrically excited piezoelectric beam can induce time-varying potential wells to facilitate snap-through transitions. Despite the damping coefficient of the piezoelectric beam being three times higher than that of its simple beam counterpart, periodic perturbations still effectively expand its instability region. This enlarged instability region, resulting from such perturbations, not only exhibits broadband characteristics but also facilitates efficient energy harvesting at lower excitation levels.

Broadband characteristics and bandpass acoustic focusing via movable second-order Helmholtz resonator

In this paper, a method is theoretically proposed to obtain a novel movable second-order Helmholtz resonator by adding a perforated plate inside the cavity of the Helmholtz resonator that can be freely tuned up and down. Through software simulation, it is concluded that the resonator meets the design objective of saving production costs so that when the manufactured physical object is required to change the structural parameters, it only needs to be dynamically adjusted on this basis, rather than to reproduce a new physical object with new structural parameters. Meanwhile, it has excellent acoustic characteristics, with a stop band in the range of 80–610 Hz at the low frequency and a bandpass acoustic focusing characteristic in the range of 4050–5250 Hz. It can achieve directional adjustable focus acoustic focusing for plane waves incident at any angle, and directional adjustable focus acoustic focusing for cylindrical wave incidence, with excellent focusing effect. The added perforated plate changes the resonator one-item structure model and can be reused to meet different structural parameters.