Dual-band Response Research Articles

3D-printed polarization-independent low-cost flexible frequency selective surface based dual-band microwave absorber

Abstract A 3D-printed polarization-independent low-cost lightweight and flexible frequency selective surface based dual-band microwave absorber is presented in this paper. Two concentric square loops fabricated at different heights using 3D printing technology are responsible for exhibiting dual-band responses at 3.32 GHz (S-band) and 5.46 GHz (C-band) with more than 97% absorptivities. The corresponding full widths at half maximum bandwidths are observed as 230 MHz (3.21–3.44 GHz) and 450 MHz (5.27–5.72 GHz). The proposed topology is polarization-insensitive owing to the four-fold symmetry. The absorption phenomenon is explained with the analysis of current distributions at the surface and impedance curves at the frequencies of resonance. Further, the performance has been evaluated for both planar and curved surfaces with different angles of curvature, and the good agreement between the measured and simulated responses confirms the flexible behavior of the proposed structure.

Compact dual-band enhanced bandwidth 5G mm – wave MIMO dielectric resonator antenna utilizing metallic strips

A compact dual-band Multiple-Input Multiple-Output Dielectric Resonator Antenna with improved bandwidth for 5G mm-wave applications is presented. Two rectangular DRAs mounted on a substrate backed ground plane are excited by microstrip feed through aperture coupling. Notches cut out from center of the DRAs helps to achieve a dual-band response with improved bandwidth by exciting several modes. The isolation between the DRAs is achieved by appropriately positioning metallic strips on walls on of the DRAs. A dual-band response with enhanced bandwidth and mutual coupling reduction. The impedance bandwidth for the proposed design is between 26.5–30 GHz (12.4 %) and 34.2–40 GHz (15.6 %). The maximum isolation achieved in the lower band is 32 dB and above 45 dB in the upper band. The overall size of the design is 1.88λo x 0.94λo x 0.20λo. The design procedure is described, and the role of decoupling strips are explained. A prototype is fabricated and measured to validate the proposed design.

A closely spaced dual band polarization insensitive FSS for 5G applications

This study presents a novel single layer Frequency Selective Surface (FSS) for fifth generation (5G) applications with a polarization independent closely spaced dual band response. The proposed design consists of four split ring apertures etched on a square patch printed on a RT5880 dielectric substrate. The FSS has two stop-bands at 24.78 GHz and 28 GHz center frequencies with attenuations around 55 dB. The S21<−10 dB bandwidths (BW) of these bands are 14.48 % and 9.25 %, respectively and these closely spaced bands succeed 1.13 frequency ratio. It shows a stable frequency response for both TE and TM polarizations. Furthermore, the FSS represents a single layered and quite thin (thickness 0.042λl) structure with its unit cell size (0.70λl × 0.70λl, where λl is the free-space wavelength at lower frequency). The novelty of the presented FSS is not only achieving a small closely spaced band ratio in the mmWave band and exhibiting polarization insensitivity but also providing these features with a simple geometry and, uncomplicated single layer structure. The simulation results were confirmed by well accordant measurement results. All these results make the presented FSS a good candidate for 5G electromagnetic interference (EMI) shielding applications.

Nanophotonic Bragg grating assisted Mach–Zehnder interferometers for O-band add-drop filters

Spectral filters are fundamental building blocks in integrated photonics. Bragg grating filters have been demonstrated in silicon waveguides with a wide range of spectral responses and are suitable for wavelength division multiplexing applications. However, retrieving Bragg grating reflections typically requires external components such as fiber optic circulators. In this work, we develop fully integrated add-drop filters based on cladding-modulated Bragg gratings incorporated in a Mach–Zehnder interferometer configuration. We design complex spectral filtering devices with single and dual-band flat-top responses for the specified bandwidth. Additionally, we propose a novel design methodology which aims to minimize phase errors within the filters. We experimentally demonstrate add-drop filters with single-band and two-band rejection spectra at the datacom O-band, fabricated on a 220-nm thick silicon-on-insulator platform. Our results show an insertion loss below 1 dB and a crosstalk of around −20 dB at the channel center for a 4.5-nm wavelength grid and 3-nm wide channels.

High quality factor and unity circular dichroism enabled by dielectric-Weyl semimetal-dielectric stack

A mid-infrared (mid-IR) chiral absorber based on multilayer films of silicon (Si)/Weyl semimetal (WSM)/Si is proposed and numerically studied. The proposal exhibits high quality factor circular dichroism (CD) with different absorptance properties for right-hand circular polarization (RCP) and left-hand circular polarization (LCP) light. The absorption characteristics have been explored with the generalized 4 × 4 transfer matrix algorithm, and the effects of structural parameters on the circularly polarized (CP) absorptance are systematically investigated. The circular polarization efficiency (η) and CD are defined as crucial parameters for quantifying the chirality response. The results prove that the chiral absorber exhibits extraordinary efficiencies of 1 and 0 for LCP and RCP light at a wavelength of 11.5 μm, respectively. Thus a maximum CD in absorption of unity with a resonant quality factor (Q-factor) of 295 can be obtained. The performance of dual-band chiral responses can be engineered via the thickness of each layer. Moreover, it is identified that the design results in angle-independent performance, which is essential trait for an efficient chiral absorber.

Low-cost and efficient all group-IV visible/shortwave infrared dual-band photodetector.

Low-cost broadband photodetectors (PDs) based on group-IV materials are highly demanded. Herein, a vertical all group-IV graphene-i-n (Gr-i-n) structure based on sputtering-grown undoped Ge0.92Sn0.08/Ge multiple quantum wells (MQWs) on n-Ge substrate was proposed to realize efficient visible/shortwave infrared (VIS/SWIR) dual-band photoresponse. Harnessing Gr-germanium tin (GeSn)/Ge MQWs van der Waals heterojunctions, an extended surface depletion region was established, facilitating separation and transportation of photogenerated carriers at VIS wavelengths. Consequently, remarkable VIS/SWIR dual-band response ranging from 400 to 2000 nm with a rapid response time of 23 μs was achieved. Compared to the PD without Gr, the external quantum efficiency at 420, 660, and 1520 nm was effectively enhanced by 10.2-, 5.2-, and 1.2-fold, reaching 40, 42, and 50%, respectively. This research paves the way for the advancement of all group-IV VIS/SWIR broadband PDs and presents what we believe to be a novel approach to the design of low-cost broadband PDs.

Terahertz dual-band bandpass filter based on spoof surface plasmon polaritons with wide upper stopband suppression

In this paper, a terahertz (THz) dual-band bandpass filter based on spoof surface plasmon polaritons (SSPPs) with wide upper stopband suppression is proposed. The filter utilizes two types of SSPP unit cells loaded on a double-sided quasi-SSPPs transmission line to achieve dual-band filtering responses and wide upper stopband suppression simultaneously. The performance and bandwidth of the filter's passbands can be adjusted by modifying the SSPP unit cells within the dual-band filtering part. The detailed design method is provided to enhance understanding of the operating principle of the presented filter. The simulation results of the designed THz filter demonstrate its excellent dual-band filtering performance. It features a wide stopband spanning from 0.91 to 1.45 THz with |S21| &lt; -35 dB. To validate the proposed approach, a microwave filter with a similar design is implemented. Finally, the simulated and measured S-parameter responses of the filter are provided for comparison and evaluation.

Pattern Diversity Based Four-Element Dual-band MIMO Patch Antenna for 5G mmWave Communication Networks

This study presents a planar dual-band multiple-input multiple-output (MIMO) antenna design for the prospective fifth-generation (5G) frequency bands of 28 and 38 GHz. The antenna element is designed by utilizing a rectangular patch with an offset microstrip feeding technique. A dual-band response is achieved by placing semi-circular slots on each side of the patch element. To tune the frequency response and improve impedance matching, vertical rectangular slits are etched in the rectangular patch and the ground plane, respectively. The results show that the single antenna element offers an impedance bandwidth of 2.52 GHz (26.32–28.84 GHz) and 7.5 GHz (34–41.5 GHz). In addition, a MIMO configuration based on pattern diversity using four antenna elements is designed and fabricated. The designed MIMO configuration achieves an impedance bandwidth of 3 GHz (27–30 GHz) and 5.46 GHz (35.54–41 GHz) at operating bands of 28 and 38 GHz. The peak realized gain for the single element at 28 and 38 GHz is noted to be 7.4 dBi and 7.5 dBi, respectively. Furthermore, the polarization diversity configuration illustrates an isolation of > 15 dB and > 25 dB for the 28 and 38 GHz frequency bands, respectively. Moreover, the MIMO configuration attains appropriate values for the envelope correlation coefficient (ECC) and diversity gain (DG), Total Active Reflection Co-efficient (TARC), Channel Capacity Loss (CCL) and Mean Effective Gain (MEG) for the operating frequency bands. The proposed MIMO system based on results seems to be potential choice for mmwave Ka Band Applications.

Dual-band and spectrally selective infrared absorbers based on hybrid gold-graphene metasurfaces

In this paper, we propose a dual-band and spectrally selective infrared (IR) absorber based on a hybrid structure comprising a patterned graphene monolayer and cross-shaped gold resonators within a metasurface. Rooted in full-wave numerical simulations, our study shows that the fundamental absorption mode of the gold metasurface hybridizes with the graphene pattern, leading to a second absorptive mode whose properties depend on graphene’s electrical properties and physical geometry. Specifically, the central operation band of the absorber is defined by the gold resonators whereas the relative absorption level and spectral separation between the two modes can be controlled by graphene’s chemical potential and its pattern, respectively. We analyze this platform using coupled-mode theory to understand the coupling mechanism between these modes and to elucidate the emergence and tuning of the dual band response. The proposed dual-band device can operate at different bands across the IR spectrum and may open new possibilities for tailored sensing applications in spectroscopy, thermal imaging, and environmental monitoring.

Tailoring intrinsic chiroptical responses via twisted bilayer α-MoO3 separated by a VO2 film

Flexible control of intrinsic chiroptical responses within compact nanostructures is crucial for flat optics, topological photonics, and chiroptics. However, previous approaches require complicated patterns with both in-plane and out-of-plane mirror symmetry breaking to achieve intrinsic chirality, and their chiroptical responses cannot be dynamically controlled as well. Herein, we demonstrated that near-perfect intrinsic circular dichroism (CD) can be achieved within a lithography-free structure consisting of the twisted bilayer α-MoO3 separated by a vanadium dioxide (VO2) film. By twisting the bilayer α-MoO3, dual-band intrinsic chiroptical responses can be realized due to the excitations of the hyperbolic phonon polaritons modes in the mid-infrared. It is the spin-selected average electric-field enhancement instead of the chiral absorption that is responsible for the intrinsic CD of the device. In addition, the chiroptical responses are insensitive to the variation of the thickness of the structure as well as the incident angle, and high contrast CD can be dynamically tuned by varying the volume fraction of VO2.

Self-powered SnSx/TiO2 photodetectors (PDs) with dual-band binary response and the applications in imaging and light-encrypted logic gates

In this work, we report the design and fabrication of self-powered binary response PDs based on II-type heterostructures consisting of SnSx nanoflakes (NFs) and rutile TiO2 nanorod arrays (NRs). The TiO2 NRs effectively block light with wavelengths below 400 nm from reaching SnSx. Under 385 nm light, the photoelectrons in TiO2 recombine with holes in SnSx at the interface due to the energy band bending, resulting in a positive photocurrent. Under 410 nm light, the photoelectrons in SnSx and the photogenerated holes in TiO2 accumulate at the interface, overcoming the interfacial potential barriers induced by the higher Fermi levels of SnSx and inducing a negative photocurrent. Based on the bipolar response, the dual-band imaging capability without external filters and the light-encrypted OR, AND, and NOT logic gates using a single device are demonstrated. This work provides a blueprint for the development of multifunctional self-powered PDs that can simplify system architecture, reduce the energy consumption, and improve accuracy for applications, such as visual systems, light-controlled logic circuits, and encrypted optical communications.

Design nano cluster high Q-factor perfect plasmonic absorber for bacteria detection in the optical range

Plasmonic nanostructures enable strong light absorption at the nanometer scale, making them highly attractive for a variety of applications including sensing, imaging, and spectroscopy. In this work, we propose and simulate a dual-band plasmonic absorber based on a Pentamer and split ring resonator (SRR) hybrid structure. The absorber operates at terahertz frequencies of 209 and 229 THz, corresponding to free-space wavelengths of 1310 nm and 1430 nm. At the lower frequency band, the absorber provides perfect absorption up to 100 %, while at the higher band, the absorption is 95 %. The high absorption selectivity at the two target bands is enabled by the hybrid heptamer-SRR design. The central Pentamer consists of a central nanodisk surrounded by four peripheral nanodisks with precise dimensions and spacing. This Pentamer provides a dual-band response tuned to the desired frequencies. To further enhance the quality factor and absorption, the Pentamer is encircled by an SRR designed to boost the Q-factor. At 229 THz, the SRR pushes the quality factor to around 635, significantly higher than typical plasmonic absorbers. The final optimized design enables strong absorption together with high Q-factor and frequency selectivity. It exhibits sensitivity around 450 nm/RIU and a Figure of Merit of 227 RIU−1. The full-wave simulation is used for designing the absorber in CST Microwave Studio to optimize the dimensions for maximum dual-band absorption for biosensing applications. This absorber can be realistically fabricated for terahertz sensing applications such as detecting bacteria as studied.

Eight element dual-band MIMO array antenna for modern fifth generation mobile phones

In this paper, an eight-element multiple-input multiple-output (MIMO) antenna with dual-band response is designed for fifth-generation (5G) mobile devices. The proposed MIMO systems’ radiators are placed in the inner side frame of the mobile phone board, which has a size of 150 × 7 mm2, each spanning an area of 21.5 × 5.5 mm2. All eight antenna elements are symmetrical to each other and are fed with a 50Ω feeding line. The proposed MIMO antenna covers two distinct 5G frequency bands, i.e., 3.5 and 5.3 GHz, and offers −6 dB impedance bandwidth in a frequency range of 3.4–3.65 and 4.8–5.8 GHz. It is worth mentioning that an isolation of more than 18 dB is achieved over the bands of interest without the use of a decoupling network. Similarly, the proposed MIMO antenna achieved an efficiency of more than 60% across the bands, an envelope correlation coefficient (ECC) below 0.04, antenna gain better than 5 dBi, a mean effective gain (MEG) between any two radiating elements found to be better than 3 dB, and a channel capacity (CC) above 38.5 bps/Hz, which is three times better compared to a 2 × 2 MIMO antenna system. Upon fabrication and testing of the proposed antenna, it was noted that the measured results are in good agreement with the simulated data. Furthermore, the MIMO antenna response was simulated in the presence of a human user and shows how the user phantom affects the radiation characteristics of the MIMO antenna. These performance parameters showed promising results, making the proposed design a better option to be deployed in modern-day 5G communication application scenarios, specifically cell phones.

DUAL-BAND BANDSTOP FILTERS BASED ON ULTRA THIN FREQUENCY SELECTIVE SURFACES

In this paper, we present designs for terahertz filters with two independent stopbands based on periodic metallic resonant structures patterned on the dielectric substrate. Two simple and thin structures are proposed to realize required characteristics for single-polarized and dual polarized filter designs. To realize single polarized one, a unit cell of the proposed periodic structure consists of two sets rectangular shape conducting metal patches patterned on the top of the dielectric substrate, and the dual-band response is due the lattice resonances observed in the unit cell. To realize dual polarized filter design, a modified unit cell is considered with additional patches on the bottom of the dielectric substrate. The patches on the bottom layer are the same ones as on the top layer but orthogonally oriented. For this case, polarization-independent frequency response can be obtained after geometric optimization of the unit cell because the top and the bottom layers resonantly interact with only the TE or TM incident plane wave to minimize cross coupling effects between the layers. In order to intensively understand the transmission performance of the proposed filters, a large number of simulations using integral equation method are performed based on the different values for permittivity, period of the unit cell, dielectric thickness, and geometric dimensions. The electric field distributions are analyzed to understand the mechanism of the resonance behavior.

Visible-ultraviolet dual-band photodetectors based on an all-inorganic CsPbCl3/p-GaN heterostructure.

All-inorganic metal halide perovskites (MHPs) have attracted increasing attention because of their high thermal stability and band gap tunability. Among them, CsPbCl3 is considered a promising semiconductor material for visible-ultraviolet dual-band photodetectors because of its excellent photoelectric properties and suitable band gap value. In this work, we fabricated a visible-ultraviolet dual-band photodetector based on a CsPbCl3/p-GaN heterojunction using the spin coating method. The formation of the heterojunction enables the device to exhibit obvious dual-band response behavior at positive and negative bias voltages. At the same time, the dark current of the device can be as low as 2.42 × 10-9 A, and the corresponding detection rate can reach 5.82 × 1010 Jones. In addition, through simulation calculations, it was found that the heterojunction has a type II energy band arrangement, and the heterojunction response band light absorption is significantly enhanced. The type II energy band arrangement will separate electron-hole pairs more effectively, which will help improve device performance. The successful implementation of visible-ultraviolet dual-band photodetectors based on a CsPbCl3/p-GaN heterojunction provides guidance for the application of all-inorganic MHPs in the field of multi-band photodetectors.

Multi-Band Band-Pass Filter with Independently Controlled Asymmetric Dual-band Response Based on Metacell

Multi-Band Band-Pass Filter with Independently Controlled Asymmetric Dual-band Response Based on Metacell

UV–visible dual-band photodetector based on an all-inorganic Mn-doped CsPbCl3/GaN type-II heterojunction

The all-inorganic perovskite CsPbCl3 has attracted significant attention for its excellent carrier mobility and high optical absorption coefficient, demonstrating great potential for use as a high-performance photodetector. However, the severe toxicity of lead and its narrow-band response in the UV region pose a huge challenge to the application of the perovskite. In this study, we reduce the toxicity of CsPbCl3 by substituting Mn atoms for Pb atoms and introduce a wide-bandgap semiconductor p-GaN to improve the photoresponse of CsPbCl3 in the UV region. Mn-doped CsPbCl3/GaN heterojunction is prepared by transferring the synthesized Mn-doped CsPbCl3 precursor solution onto p-GaN substrates via the spin-coating method. The morphology and optical properties of the heterojunction are characterized in various ways, and the current–voltage characteristics and optical response properties of the Mn-doped CsPbCl3/GaN heterojunction photodetector are also measured. The rectification behavior and UV–visible dual-band response of the heterojunction are demonstrated, with the response in the UV region extending to 320 nm. Furthermore, the calculations, which are based on density functional theory, confirm that the optical absorption of the Mn-doped CsPbCl3/GaN heterojunction is significantly enhanced compared with that of the isolated material. Meanwhile, a type-II energy band arrangement is found at the heterojunction interface. The results of this study suggest that Mn-doped CsPbCl3/GaN heterojunctions are potential candidates for use as UV–visible dual-band photodetectors.

Side-edge dual-band MIMO antenna system for 5G cellular devices

An eight-element multiple-input multiple-output (MIMO) antenna system is presented for two distinct 5G allocated bands, i.e., 3.5 and 5.2 GHz. The MIMO radiating elements are printed on each side of the mobile phone chassis, having dimensions of 150 × 7 mm2, while the microstrip feeding lines fed through a coaxial connector are printed on the main board, having dimensions of 150 × 75 mm2. The single element of the MIMO antenna system consists of a modified slotted rectangular patch radiator. One slot is composed of a hybrid shape (which consists of both T- and L-shaped patterns), and the second one is a typical inverted L-shaped slot. The placement of slots generated two different current paths, which led to dual-band response at 3.5 and 5.2 GHz. In the MIMO configuration, the arrangement of radiating elements ensures pattern diversity characteristics, which are useful for MIMO systems. Furthermore, the MIMO antenna system offers a minimum isolation of 13.5 and 21.5 dB at 3.5 and 5.2 GHz, respectively. From the results, it is also observed that the radiating elements offer a radiation efficiency of ≥70% at 3.5 GHz, while at 5.2 GHz, the radiation efficiency is noted to be >55% with acceptable gain. A comprehensive analysis of key performance parameters, e.g., envelope correlation coefficient (ECC), mean effective gain (MEG), and channel capacity (CC), was conducted, and it is noted that the proposed MIMO antenna system exhibits an ECC of less than 0.015, MEG of < −3 dB, and CC of 39.5 bps/Hz. In addition, the specific absorption rate (SAR) analysis of the proposed MIMO antenna was conducted and observed a value of 1.35 and 1.05 W/Kg at 3.5 and 5.2 GHz, respectively. To verify the computed results, a prototype of the proposed MIMO antenna was fabricated and subjected to thorough measurements. It was observed that the measured results satisfied the simulated data and aligned excellently with the measured data. Based on the performance of the proposed MIMO antenna, it became evident that the MIMO antenna structure can be used as a potential candidate for future fifth-generation (5G)-enabled smart phones.

Inverse design of metasurfaces with customized transmission characteristics of frequency band based on generative adversarial networks.

In recent years, deep learning (DL) has demonstrated significant potential in the inverse design of metasurfaces, and the generation of metasurfaces with customized transmission characteristics of frequency band remains a challenging and underexplored area. In this study, we propose a DL-assisted method for the inverse design of transmissive metasurfaces. The method consists of a generative adversarial network (GAN)-based graph generator, an electromagnetic response predictor, and a genetic algorithm optimizer. By integrating these components, we can obtain customized metasurfaces with desired transmission characteristics of frequency band. We demonstrate the effectiveness of the proposed method through examples of inverse-designed three-layer cascaded transmissive metasurfaces with wideband, dual-band, and stopband responses in the 8∼12 GHz frequency range. Specifically, we realize three different types of dual-band metasurfaces, namely double-wide, front-wide and rear-narrow, and front-narrow and rear-wide configurations. Additionally, we analyze the accuracy and reliability of the inverse design method by employing data from the training dataset, self-defined objectives, and bandwidth-reduced target responses scaled from the wideband type as design inputs. Quantitative evaluation is performed using metrics such as mean absolute error and average precision. The proposed method successfully achieves the desired effect as intended.

Independently switchable dual-passband frequency-selective surface design based on characteristic mode analysis.

High-order bandpass filtering responses are highly desirable for frequency-selective surfaces (FSSs) in narrowband antenna/radar systems. In this paper, the design process of a reconfigurable dual-band FSS with second-order response is presented. Initially, the basic dumbbell-shaped resonator used in this design is theoretically investigated using the characteristic mode analysis (CMA) method to study the relation between geometric design parameters and the excited orthogonal resonance modes in different frequency bands. Then, an additional CMA process was performed on a unit cell with four such patch resonators arranged with 90-degree rotation between adjacent ones. This detailed analysis leads to a polarization-independent FSS design with a high-order dual-band response. Two of these composite resonators are combined back-to-back through coupling apertures on the middle layer. Finally, PIN diodes are loaded on separate layers to realize independent pass-band switching. The loading place of the PIN diodes is carefully chosen based on electromagnetic field analysis. A prototype was also fabricated and experimentally tested. Experimental results show that this FSS has two independently switchable passbands centered at 3 GHz and 4.8 GHz.