Narrow Bandwidth Research Articles

Reverse design of load-bearing broadband metamaterial absorber assisted by deep learning

Abstract In response to the current challenges of narrow absorption bandwidth, weak load-bearing capacity, and low design efficiency in absorbing structures, this study focuses on the reverse design of load-bearing broadband metamaterial absorber. A parameterized model of load-bearing metamaterial absorber was developed by integrating the composite sandwich structure with the electromagnetic resonant layers. The resonant layer was constructed using the combination of Vicsek-fractal and circular rings, with resistive films employed to broaden the absorption bandwidth. A deep learning-based forward prediction model was established to accurately predict the absorbance of the metamaterial absorber. The SHapley Additive exPlanations (SHAP) framework was utilized to analyze the forward prediction network, revealing the influence of various design parameters on the absorbance at center frequencies across the L to K band spectrum. Additionally, the Group Teaching Optimization Algorithm (GTOA) was introduced into the design process, leading to the development of an automated reverse design method for metamaterial absorber that can achieve specific design objectives. Using the GTOA-based reverse design method, a metamaterial absorber capable of effectively absorbing vertically incident electromagnetic waves within the 3-20 GHz frequency range was designed. The designed absorbing structure was fabricated, and its absorption performance was measured using the arch method. The measurement results were found to be in good agreement with the simulation data. The absorbing mechanism of the designed metamaterial absorber was analyzed based on the calculation of equivalent electromagnetic parameters and the electromagnetic resonance observed at the resonant frequency. It was determined that the primary absorbing effect is induced by electric resonance triggered by electromagnetic waves. The proposed metamaterial absorber can be applied to radar stealth design for military targets such as naval vessels. The research methodology and approach demonstrate significant generalizability and engineering applicability.

Angle insensitive filters based on Fabry–Pérot resonance structures

Filters based on plasmon resonance suffer from low efficiency due to inevitable metal loss. Moreover, their operational limitations, particularly their inability to function effectively at large incident angles, significantly restrict their practical applications. To address these challenges, we propose an all-dielectric filter characterized by relatively small Ohmic loss and remarkably high efficiency, even at wide incident angles. The filter is based on Fabry–Pérot cavity resonance, with narrow bandwidth and high transmittance. The transmittance in the near-infrared band is as high as 99%, and the transmittance in the shortwave band is highly suppressed. This filter offers ease of manufacture coupled with exceptional efficiency. It is expected to gain application prospects in different fields such as liquid crystal displays, optical communications, sensor detection, and imaging.

An enhanced broadband piezoelectric energy harvester via elastic amplification structure for multidirectional vibration

Abstract Vibration energy harvesting using a piezoelectric mechanism has significant potential for powering wireless sensors. However, most current vibration energy harvesters face limitations such as bidirectionality, narrow bandwidth, and high operating frequencies. To address these issues, we propose an enhanced broadband piezoelectric energy harvester utilizing an elastic amplification structure for multidirectional vibration (EB-PVEH). By utilizing the multidirectional rotation capacity of the excitation block and the amplified foundation excitation provided by springs, the EB-PVEH effectively captures broadband vibrations in 2D space under low-frequency excitation. Additionally, its design features long-term durability, as the piezoelectric beams are smoothly excited by the pendulum-induced motion of the block without a tip mass. The practical feasibility and the impact of structural parameters on the output behavior of EB-PVEH were investigated through theoretical analysis and experimental testing. The results revealed that the introduction of springs dynamically amplified the harnessed electrical power output. Moreover, EB-PVEH could harvest the multidirectional vibration, and it exhibited different power-generating characteristics in various directions. Furthermore, the resonance frequency could be efficiently tuned by adjusting the flexible arm length and proof mass, with different optimal arm lengths identified for each vibration direction to maximize working bandwidth. The harvester achieved an optimal output power of 3.98 mW. Practical applications, such as charging a capacitor by driving an e-bike or a bike, demonstrate the potential of the proposed harvester to provide power for micro-electrical devices.

Experimental Demonstration of Mode-Coupled and High-Brightness Self-Amplified Spontaneous Emission in an X-Ray Free-Electron Laser

X-ray free-electron lasers (FELs) are modern research tools with applications in multiple scientific fields. Standard x-ray FEL pulses are produced by the self-amplified spontaneous emission (SASE) mechanism. SASE-FEL pulses have high power, short duration, and excellent transverse coherence but exhibit poor temporal coherence with power and spectral profiles consisting of multiple randomly distributed spikes. Here, we present the demonstration of two modes that enhance the temporal coherence of SASE-FEL radiation: mode-coupled and high-brightness SASE. Both schemes are based on delaying the electron beam with magnetic chicanes placed between the undulator modules of the FEL facility. First, we show the generation of frequency combs with tunable peak separation via the mode-coupled SASE scheme. Second, we present a proof-of-principle demonstration of the high-brightness SASE mechanism, producing FEL pulses with a bandwidth reduction up to a factor of 3 with respect to standard SASE pulses. The demonstration was done at Athos, the soft x-ray beamline of SwissFEL, for photon energies between 500 and 600 eV. Our work will benefit current applications and may open up new research areas requiring frequency combs or narrow bandwidths. Published by the American Physical Society 2024

Spectrally selective and interleaved water imaging and fat imaging (siWIFI).

To develop a novel imaging sequence that independently acquires water and fat images while being inherently insensitive to motion. The new sequence, termed spectrally selective and interleaved water imaging and fat imaging (siWIFI), uses a narrow bandwidth RF pulse for selective excitation of water and fat separately. The interleaved acquisition method ensures that the obtained water and fat images are inherently coregistered. A radial sampling strategy further reduces motion-induced artifacts. Phantoms with lipid concentrations ranging from 0% to 50% were scanned to measure fat fraction. Moreover, healthy volunteers were scanned to assess the in vivo feasibility of fat fraction measurement at the hip, knee, and liver. In vivo fat fraction measurements were compared with those from vendor-provided iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) scans. Furthermore, a magnetization transfer (MT) preparation module was incorporated to demonstrate the feasibility of simultaneous measurement of fat fraction and MT ratio utilizing the siWIFI framework. The phantom fat fractions measured by siWIFI showed excellent correlation with lipid concentrations (R2 = 0.9995, p < 0.0001). In vivo studies demonstrated that the fat fractions obtained from siWIFI were comparable to those from IDEAL. Additionally, siWIFI demonstrates reduced motion artifacts from pulsatile flow in knee imaging compared to IDEAL scans and exhibits less sensitivity to respiratory motion in liver imaging compared to IDEAL scans without breath-hold. The knee imaging study demonstrated that MT-prepared siWIFI is capable of generating fat fraction and MT ratio maps simultaneously. The proposed siWIFI sequence allows selective water-fat imaging and quantification with reduced motion artifacts.

A fully enclosed wave energy converter and effects of cable-based PTO on power absorption

Abstract Existing wave energy converters (WECs) are limited by high unit energy costs, narrow bandwidths and vulnerability to seawater corrosion, making it difficult for wave energy to compete with solar and wind energy. This paper presents a fully enclosed WEC based on cable-driven parallel mechanisms, which absorbs energy from multiple directions to enhance the total energy absorption efficiency and bandwidth. The configuration of the inner cable-parallel mechanism (CDPM) is the key to this improved absorption efficiency. A configuration synthesis method of cable-driven parallel mechanisms is proposed to generate more configurations for the fully enclosed WEC. The motion equation of the fully enclosed WEC is derived to evaluate the power absorption performance. Moreover, a configuration synthesis method for realizing n-DOF configurations in the fully constrained CDPMs is proposed, and the configurations of these n-DOF fully-constrained CDPMs with (n + 1) cables are acquired. Then, the power absorption performance under different PTO parameters and wave frequencies is calculated, and the effects of the CDPM configuration are investigated. The results indicate that the proposed WECs exhibit excellent performance in mean absorbed power and bandwidth. Several principles for configuration selection are provided to further improve the performance of the CDPM-based fully enclosed WEC.

Ultra‐Broadband Visible‐DUV Supercontinuum Generation by Non‐Resonant Coherent Raman Scattering in Air

AbstractTo date, supercontinuum light in the visible and near‐infrared ranges is readily realizable by the optical Kerr effect through self‐phase modulation of ultrashort laser pulses in transparent media. However, it is still a challenge to extend the supercontinuum spectrum down to the deep‐ultraviolet (DUV) range, which is particularly needed for exploring ultrafast dynamics in chemistry, materials, and biology. Here, an approach of non‐resonant coherent Raman scattering is developed to generate ultra‐broadband visible‐DUV supercontinuum in ambient air with a spectral range spanning over 250 nm and a wavelength down to 220 nm. A rovibrational coherence is established in air molecules by filamentation of a near‐infrared femtosecond 800 nm pulse and two femtosecond Raman laser pulses at 267 and 400 nm are introduced into the coherent media to induce non‐resonant coherent Stokes and anti‐Stokes Raman scatterings, which serve as the spectral bridges to link the neighboring Raman pump laser spectra, resulting in ultra‐broadband supercontinuum light. The mechanism is further verified by examining the broadening of picosecond N2+ laser lines with narrow bandwidths (10–30 cm−1), which forms a supercontinuum spectrum spanning over 150 nm. The work provides a viable route for the establishment of coherent DUV supercontinuum in the gas media at designed wavelength ranges.

THz biosensing for early detection of influenza and coronaviruses using dielectric metamaterials

Abstract The world has faced a significant challenge since December 2019, when coronavirus (COVID-19) was found. Viruses of this type are causing pandemics all over the world right now. For this purpose, a biosensor is designed to operate based on metamaterial (MM) incorporated and can detect different coronaviruses. The proposed metamaterial absorber (MMA) includes two bands with perfect absorption characteristics and a narrow absorption bandwidth: 4.093 THz and 3.647 THz. The circuit model’s transmission line technique is another approach to verifying the absorber’s functionality. The proposed design consists of a square-shaped graphene ring (SGR) and a silicon-based square ring resonator (SSRR) to provide unique and narrow band absorption characteristics. Changing the graphene’s chemical potential suggests MMA tuneability and control capability. The suggested MMA can detect several coronaviruses because of its extremely narrow absorption spectrum behavior. It has unique features such as ultra-narrow absorption bandwidth, polarization insensitivity, and simplicity. The MMA is designed with a compact structure, good sensitivity (s), exceptional figure of Merit (FOM), and superior Quality factor (Q) values.

A centrifugal spring mechanism empowers self-adjusting in piezoelectric wind energy harvesting

Piezoelectric wind energy harvesters (PWEHs) offer promises in sustainable green energy supply for micropower electronics. However, traditional PWEHs encounter challenges in terms of high start-up wind speeds, subpar output performance, and narrow bandwidth, hampering their widespread adoption. To enhance the energy capture efficiency, a novel self-adjusting PWEH integrating a centrifugal spring mechanism (CSM) is presented. The CSM consists of a sliding rod, a 304 stainless steel spring, an exciting magnet, and a mass block. Additionally, the PWEH employs a vertical-axis blade design, allowing it to capture wind from different directions. Via theoretical analyses, finite element simulations, and experiments, key parameters of the PWEH are optimized including the CSM additional mass, spring wire diameter, and the configuration of multi-frequency piezoelectric transducers. Results demonstrate that the optimized PWEH works at 1.5m/s and generates 0.45mW, 2.742mW, and 5.973mW at wind speeds of 2m/s, 3.3m/s, and 4.75m/s, respectively. Compared to the unoptimized PWEH, the introduction of the CSM results in an 80.7% enhancement in open-circuit voltage and a 90.9% increase in short-circuit current. Additionally, by utilizing four pairs of piezo-transducers with different resonance frequencies (6.83Hz, 9.38Hz, 12.1Hz, and 14.8Hz), the resonance bandwidth of the PWEH is expanded to 1.5~5.5m/s. The feasibility of PWEH for powering signage, electronic screens, and Bluetooth thermohygrometer is demonstrated. The potential of PWEH in constructing high-precision intelligent wind speed monitoring systems is tested via convolutional neural networks. The study offers a strategy for designing the PWEH for both powering of the microelectronic devices, and intelligent sensing and monitoring of wind.

Tunable underwater sound absorption via piezoelectric materials with local resonators

In recent years, piezoelectric composite materials have been widely used in the design of underwater anechoic coatings due to their adaptability to tuning parameters. However, there are also some shortcomings, such as a single dissipation mechanism, narrow bandwidth, and poor low-frequency sound absorption. This work proposes an acoustic composite structure combining piezoelectric composite materials with local resonance units, which effectively enhances the sound absorption performance of the structure through the coupling effect of the piezoelectric energy consumption mechanism and local resonance mechanism. Compared to conventional acoustic structures, the proposed acoustic composite structure not only has a strong low-frequency sound absorption effect but also enriches the mid-high frequency sound absorption modes by connecting shunt damping circuits. On this basis, the effect of piezoelectric parameters and resonator morphological properties on structural sound absorption performance is further investigated, and the results show that the designed structure has the characteristic of sound absorption performance that is tunable. In addition, key factors affecting the sound absorption performance of the structure have been optimized to achieve better broadband sound absorption performance. This work may provide valuable ideas for the development of low-frequency broadband adjustable underwater sound-absorbing coatings.

Cavity-enhanced induced coherence without induced emission

This paper presents a theoretical study of the enhancement of Zou-Wang-Mandel (ZWM) interferometry through cavity-enhanced spontaneous parametric down-conversion (SPDC) processes producing frequency-entangled biphotons. The ZWM interferometry shows the capability to generate interference effects between single signal photons via indistinguishability between the entangled idler photons. This paper extends the foundational principles of ZWM interferometry by integrating cavity-enhanced SPDCs, aiming to narrow photon bandwidths for improved coherence and photon pair generation efficiency, which is critical for applications in quantum information technologies, quantum encryption, and quantum imaging. This work explores the theoretical implication of employing singly resonant optical parametric oscillators within the ZWM interferometer to produce narrow-band single photons. By combining cavity-enhanced SPDCs with ZWM interferometry, this study fills a gap in current theoretical proposals, offering significant advancements in quantum cryptography and network applications that require reliable, narrow-band single photons.

Cellular Internet of Things: Principles, Potentials and Use Cases

Internet of things (IoT) can either be deployed over existing cellular networks or their own custom-built standalone networks. Based on the infrastructure, IoT can be classified into two types: cellular and non-cellular categories. In the cellular form, IoT network needs the support of cellular infrastructure of the mobile service providers. Currently, three forms of cellular IoT are being deployed across the world. They are: narrowband Internet of things (NBIoT), extended coverage GSM (EC-GSM) and long term evolution for machines (LTE-M). Out of these three, NBIoT and EC-GSM are low energy and low resource consuming versions of cellular IoT. They need narrow bandwidths for their operations. Their energy consumption is also very low and thus suitable for low energy applications. Both NBIoT and EC-GSM are compatible with all types of cellular communication infrastructure such as 2G, 3G, 4G and 5G. They can cover a large area with a very small amount of power. Both these forms are popular low power wide area (LPWA) technologies. Due to their LPWA features, they are popular for the connected living applications at home and workplace surroundings. Their LPWA features make them popular green technology for digital transformation. LTE-M uses comparatively larger bandwidth and higher power. It is suitable for higher bandwidth and higher data rate applications. We survey the recent literature on cellular IoT and present their key principles, potentials and applications. We provide their main characteristics, deployment options, standards, and some specific applications in different sectors.

All-Dielectric Dual-Band Anisotropic Zero-Index Materials

Zero-index materials, characterized by near-zero permittivity and/or permeability, represent a distinctive class of materials that exhibit a range of novel physical phenomena and have potential for various advanced applications. However, conventional zero-index materials are often hindered by constraints such as narrow bandwidth and significant material loss at high frequencies. Here, we numerically demonstrate a scheme for realizing low-loss all-dielectric dual-band anisotropic zero-index materials utilizing three-dimensional terahertz silicon photonic crystals. The designed silicon photonic crystal supports dual semi-Dirac cones with linear-parabolic dispersions at two distinct frequencies, functioning as an effective double-zero material along two specific propagation directions and as an impedance-mismatched single-zero material along the orthogonal direction at the two frequencies. Highly anisotropic wave transport properties arising from the unique dispersion and extreme anisotropy are further demonstrated. Our findings not only show a novel methodology for achieving low-loss zero-index materials with expanded operational frequencies but also open up promising avenues for advanced electromagnetic wave manipulation.

One-Dimensional Photonic Crystals Comprising Two Different Types of Metamaterials for the Simple Detection of Fat Concentrations in Milk Samples.

In this study, we demonstrate the reflectance spectrum of one-dimensional photonic crystals comprising two different types of metamaterials. In this regard, the designed structure can act as a simple and efficient detector for fat concentrations in milk samples. Here, the hyperbolic and gyroidal metamaterials represent the two types of metamaterials that are stacked together to construct the candidate structure; meanwhile, the designed 1D PCs can be simply configured as [G(ED)m]S. Here, G refers to the gyroidal metamaterial layers in which Ag is designed in a gyroidal configuration form inside a hosting medium of TiO2. In contrast, (ED) defines a single unit cell of the hyperbolic metamaterials in which two layers of porous SiC (E) and Ag (D) are combined together. It is worth noting that our theoretical and simulation methodology is essentially based on the effective medium theory, characteristic matrix method, Drude model, Bruggeman's approximation, and Sellmeier formula. Accordingly, the numerical findings demonstrate the emergence of three resonant peaks at a specified wavelength between 0.8 μm and 3.5 μm. In this context, the first peak located at 1.025 μm represents the optimal one regarding the detection of fat concentrations in milk samples due to its low reflectivity and narrow full bandwidth. Accordingly, the candidate detector could provide a relatively high sensitivity of 3864 nm/RIU based on the optimal values of the different parameters. Finally, we believe that the proposed sensor may be more efficient compared to other counterparts in monitoring different concentrations of liquid, similar to fats in milk.

Dual-emissive carbon dots with high-color purity from sweet basil leaves: synthesis, characterization and optical properties

Abstract In this study, we synthesized dual-emission carbon dots (CDs) from sweet basil leaves dissolved in hexane using the hydrothermal method. Extensive analyses were carried out on their morphology, structure, and optical properties. The CDs show a spherical shape and highly disordered structure with an average diameter of 2 nm. They predominantly comprise carbon surrounded by a dense shell layer of oxygen and nitrogen-related functional groups. Under excitation at a single wavelength of 380 nm, the CDs emit two distinct peaks at 450 and 675 nm demonstrating a narrow bandwidth emission with full width at half maximum (FWHM) of 72 and 27 nm, respectively. The emission characteristics of the CDs are ascribed to the combined effects of radiative recombination of the carbon-core and fully passivated surface states, resulting in two distinct emission peaks and excitation-independent emission property. We present a highly effective and eco-friendly approach approach to fabricate luminescent CDs exhibiting dual emission properties derived from sustainable resources, holding promise for utilization in bioimaging.&amp;#xD;

Solvent mediated synthesis of multicolor narrow bandwidth emissive carbon quantum dots and their potential in white light emitting diodes

The development of efficient and environmentally sustainable materials for white light-emitting diodes (WLEDs) is of paramount importance in the field of lighting technology. In this study, we present a solvent-modulated synthesis approach for the fabrication of multicolor narrow-bandwidth emissive carbon quantum dots (CQDs) as a promising solution for constructing WLEDs. The synthesis method involves the controlled reaction of organic precursors in different solvent environments, leading to the formation of CQDs with distinct emission wavelengths with a relatively small full width at half maximum, ranging from 28 to 42 nm. Moreover, these synthesized multicolor CQDs demonstrate a remarkably high fluorescence quantum yield of up to 65%, indicating their potential for constructing efficient WLED when incorporated in polymer matrix and coated on the surface of blue light-emitting diode (LED).

A low-profile wideband meta-surface based microstrip antenna for 5G wireless communications

This article presents a low-profile meta surface microstrip radiator operating at 34 GHz with high performance for the operation of 5G wireless communications. The meta-surface radiator entails an improved patch that is present between the 4 × 4 proportioned square ring meta-surface and a ground plane. Initially, the rectangular slot of the patch would be aligned to improve the inherent narrow bandwidth. But the multifunctional properties of the proposed meta surface antenna with a convinced operating frequency increases the functionality and size of the network. Moreover, this paper outlines how the unit cell meta surface is implemented as a superstrate with a single U-slot for a rectangular patch antenna. The proposed antenna is simulated and measured on FR4 epoxy substrate material with a modified patch with a specific air-gap yields an overall antenna size of 1.36λ0 × 1.36λ0 × 0.1818λ0 especially to operate at 34 GHz resonating frequency. The results demonstrate that the meta surface radiator can achieve a wide-bandwidth range from 28 to 41 GHz for S11 with a return loss of 60 dB along with a maximum valued gain of 10dBi with a 3 dB AR bandwidth.

Performance Analysis and Prediction of 5G Round-Trip Time Based on the VMD-LSTM Method

With the increasing level of industrial informatization, massive industrial data require real-time and high-fidelity wireless transmission. Although some industrial wireless network protocols have been designed over the last few decades, most of them have limited coverage and narrow bandwidth. They cannot always ensure the certainty of information transmission, making it especially difficult to meet the requirements of low latency in industrial manufacturing fields. The 5G technology is characterized by a high transmission rate and low latency; therefore, it has good prospects in industrial applications. To apply 5G technology to factory environments with low latency requirements for data transmission, in this study, we analyze the statistical performance of the round-trip time (RTT) in a 5G-R15 communication system. The results indicate that the average value of 5G RTT is about 11 ms, which is less than the 25 ms of WIA-FA. We then consider 5G RTT data as a group of time series, utilizing the augmented Dickey–Fuller (ADF) test method to analyze the stability of the RTT data. We conclude that the RTT data are non-stationary. Therefore, firstly, the original 5G RTT series are subjected to first-order differencing to obtain differential sequences with stronger stationarity. Then, a time series analysis-based variational mode decomposition–long short-term memory (VMD-LSTM) method is proposed to separately predict each differential sequence. Finally, the predicted results are subjected to inverse difference to obtain the predicted value of 5G RTT, and a predictive error of 4.481% indicates that the method performs better than LSTM and other methods. The prediction results could be used to evaluate network performance based on business requirements, reduce the impact of instruction packet loss, and improve the robustness of control algorithms. The proposed early warning accuracy metrics for control issues can also be used to indicate when to retrain the model and to indicate the setting of the control cycle. The field of industrial control, especially in the manufacturing industry, which requires low latency, will benefit from this analysis. It should be noted that the above analysis and prediction methods are also applicable to the R16 and R17 versions.

Resonant Raman Auger spectroscopy on transient core-excited Ne ions

Abstract Short-lived core-ionized neon atoms were investigated by measuring under resonant Raman conditions the Auger decay following the excitation of a second core electron into a Rydberg state. Making use of intense and narrow bandwidth x-ray free-electron laser pulses, the photoexcitation spectrum of the femtosecond-lived Ne+1s02s22p6np series was characterized. Energy position and lifetimes of the lower-lying Rydberg states were determined and the final state configurations following the decay of the Ne+1s02s22p63p double-core hole resonance were partially resolved.

Performance Comparison of Seed Generation Techniques of Stimulated Brillouin Scattering-Based Microwave Photonics Amplifier and Filter

This work presents a Brillouin amplification performance comparison of seed generation techniques using double-sideband suppressed carrier (DSB-SC) and single-sideband suppressed carrier (SSB-SC) modulations. The SSB-SC is obtained using an optical bandpass filter (OBPF) and in-phase and quadrature Mach-Zehnder modulator (IQ-MZM). All three techniques provide high amplification performance with optical signal-to-noise ratio (OSNR) enhancement of 37.47 dB, 33.14 dB, and 32.67 dB using DSB-SC, SSB-SC/OBPF, and SSB-SC/IQ-MZM, respectively. The best seed generation technique is using the DSB with a signal amplification of 62.47 dB. The technique presents ~4 dB higher OSNR enhancement due to the dual-energy transfer obtained from the beating process of the DSB than SSB. A ~3 dB OSNR reduction is found when pump linewidth (LW) was changed from 1kHz to 50 MHz, which suggests using a low-cost pump source whenever the OSNR reduction is not critical. The work also shows that the three techniques required 10 dBm stimulated Brillouin scattering threshold (SBST) to stimulate the process. An additional analysis of DSB-SC shows that a high-carrier suppression during the seed generation technique using MZMs is insignificant to the amplification performance. The high-carrier suppression produces a high seed signal power that distorts the Brillouin gain spectrum (BGS) and the pump depletion region, hence reducing the Brillouin gain (BG). Since carrier suppression is not a primary consideration, a cost-effective MZM with a modest extinction ratio requirement is allowed. The relaxed requirement of the pump’s linewidth and MZM’s extinction ratio suggest a cost-effective development of the SBS-based optical amplifier with narrow filter bandwidth.