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Abstract Digital Micromirror Devices, extensively employed in projection displays offer rapid, polarization-independent beam steering. However, they are constrained by microelectromechanical system limitations, resulting in reduced resolution, limited beam steering angle and poor stability, which hinder further performance optimization. Liquid Crystal on Silicon technology, employing liquid crystal (LC) and silicon chip technology, with properties of high resolution, high contrast and good stability. Nevertheless, its polarization-dependent issues lead to complex system and low efficiency in device applications. This paper introduces a hybrid integration of metallic metasurface with nematic LC, facilitating a polarization-independent beam steering device capable of large-angle deflections. Employing principles of geometrical phase and plasmonic resonances, the metallic metasurface, coupled with an electronically controlled LC, allows for dynamic adjustment, achieving a maximum deflection of ± 27.1°. Additionally, the integration of an LC-infused dielectric grating for dynamic phase modulation and the metasurface for polarization conversion ensures uniform modulation effects across all polarizations within the device. We verify the device’s large-angle beam deflection capability and polarization insensitivity effect in simulations and propose an optimization scheme to cope with the low efficiency of individual diffraction stages.

Bound states in the continuum (BICs) are widely exploited in all‐dielectric metasurfaces to significantly enhance the Q‐factor and second harmonic generation (SHG). However, a higher SHG conversion efficiency is overshadowed by the narrow transparency window of all‐dielectric materials and strict requirement of polarization‐matching. Herein, an augmented SHG assisted is demonstrated by symmetry‐protected quasi‐BICs from z‐cut wurtzite GaN metasurfaces, whose wide transparent range suppresses self‐absorption of second harmonic. Strong resonances emerge under both X‐ and Y‐polarized excitations, where the multipole characters of each eigenmode are quantitatively identified via decomposition simulations. The local field enhancement enabled by the extreme energy confinement of quasi‐BICs contributes to an efficient SHG, recording a conversion efficiency up to 2 × 10−7 at moderately low input peak intensity (≈1 GW cm−2), orders of magnitude larger than bulk GaN. With proper design, the overlapping X‐ and Y‐polarized resonances can support SHG from excitation with arbitrary polarizations. The alleviation of polarization‐matching remarkably enhances the SHG power by 66.2% under unpolarized illumination condition. The results of intense SHG reveal GaN as a promising candidate for high‐performance nanoscale nonlinear photonic devices. The weak polarization sensitivity of the designed structure will chart a novel course for the advancement of next‐generation efficient all‐dielectric metasurfaces. [ABSTRACT FROM AUTHOR]

Summary: This article presents a novel wearable frequency selective surface (FSS) for shielding the communication link of implantable devices with the external environment. The proposed shield blocks a dual‐band frequency operating at 404 MHz and 2.45 GHz in the medical implant communication service (MICS) and industrial, scientific, and medical (ISM) bands. The two distinct stop bands provide a high‐frequency band ratio of 6.02 in an ultra‐miniaturized physical form factor of 0.024λo × 0.024λo, with at least 76.96% miniaturization when compared to the existing art. Each of the unit cells is imprinted on a conformal 50 μm polyester substrate in a symmetrical configuration. The polyester adheres with jean to realize its compatibility with wearable applications. The designed shield system offers a low profile of only 0.00146λo. By suppressing the grating lobe, the proposed shield achieves a high angular stability of 80° and polarization insensitivity of 90° in both transverse electric (TE) and transverse magnetic (TM) modes. The FSS is fabricated and measured results validate its performance. To evaluate the physical insight, an equivalent circuit model (ECM) analysis is done. To ensure durability, the proposed shield system is tested with Amitech SDR04. [ABSTRACT FROM AUTHOR]

In this paper, we propose an absorber with adjustable single-layer graphene multi-band perfect absorption, which has the advantages of simple structure, polarization insensitivity, tunability, multi-band absorption, and high sensitivity. The device can achieve four perfect absorption peaks at the same time, and the absorption rate of all absorption peaks is above 99%. The absorption effect of the absorber can be efficiently adjusted and controlled by adjusting the geometric parameters of the single-layer graphene array and the thickness of the dielectric layer. In addition, by changing the strength of the applied magnetic field, the Fermi level, and the relaxation rate of graphene, the absorption of the device can be dynamically adjusted, and high absorption can be maintained in the range of 0°–70° wide incidence angle. Finally, considering the potential sensing applications of the device, we measured maximum sensitivity (S) of 3.06 THz/RIU and a maximum figure of merit (FOM) of 54.6 when exposed to different ambient refractive indices, suggesting that the device can be used as a refractive index sensor. These results show that this study provides a new idea for the design of the tunable multi-band perfect metamaterial absorber based on graphene, which has great application value in many fields and provides a new reference for future research. [ABSTRACT FROM AUTHOR]

In this paper, a dual-passband frequency selective surface (FSS) with high angular stability and polarization insensitivity is proposed. The unit structure consists of a circular aperture, two annular apertures and four cross apertures. The designed FSS can achieve a double-passband at the interested frequencies of 8.45 GHz and 12.76 GHz with an insertion loss of less than 1 dB, and it can retain a stable transmission characteristic with the incident angle ranging from 0° to 86° for TE mode and from 0° to 83° for TM mode. Good agreement between the experimental results and the simulated response verifies the feasibility of the proposed FSS. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel type of hollow-core anti-resonance fiber (HC-ARF). The cladding region of this fiber is formed by a combination of nested tubes and U-shaped tubes, and the centrally symmetric arrangement significantly reduces sensitivity to polarization. The influence of parameters on the performance of the designed HC-ARF LMA is analyzed by a finite element algorithm. The simulation results demonstrate that the designed structure achieves a large mode area of 3180 µm2, bending loss of 2 × 10−2 dB/km, and confinement loss of 5 × 10−3 dB/km at a wavelength of 1064 nm. Similarly, at a wavelength of 1550 nm, the large mode area, bending loss, and confinement loss are 3180 µm2, 1.4 × 10−2 dB/km, and 4 × 10−2 dB/km, respectively. These results indicate unprecedentedly large mode areas and ultra-low losses compared to previous studies. Within the bending radius under consideration, the bending loss remains below 1.35 × 10−2 dB/km. Furthermore, by increasing the fiber radius, the large mode area can reach an extraordinary 6250 µm2. The proposed device exhibits excellent mode area and outstanding polarization insensitivity, along with favorable bending performance. We believe that the designed fiber holds promising applications in high-power miniaturized fiber lasers, fiber amplifiers, and various high-power fiber communication systems, and it can be applied in sensors that require polarization insensitivity and better bending performance. [ABSTRACT FROM AUTHOR]

As graphene-related technology advances, the benefits of graphene metamaterials become more apparent. In this study, a surface-isolated exciton-based absorber is built by running relevant simulations on graphene, which can achieve more than 98% perfect absorption at multiple frequencies in the MWIR (MediumWavelength Infra-Red (MWIR) band as compared to the typical absorber. The absorber consists of three layers: the bottom layer is gold, the middle layer is dielectric, and the top layer is patterned with graphene. Tunability was achieved by electrically altering graphene's Fermi energy, hence the position of the absorption peak. The influence of graphene's relaxation time on the sensor is discussed. Due to the symmetry of its structure, different angles of light source incidence have little effect on the absorption rate, leading to polarization insensitivity, especially for TE waves, and this absorber has polarization insensitivity at ultra-wide-angle degrees. The sensor is characterized by its tunability, polarisation insensitivity, and high sensitivity, with a sensitivity of up to 21.60 THz/refractive index unit (RIU). This paper demonstrates the feasibility of the multi-frequency sensor and provides a theoretical basis for the realization of the multi-frequency sensor. This makes it possible to apply it to high-sensitivity sensors. [ABSTRACT FROM AUTHOR]

In this paper, a broadband reconfigurable active frequency selective rasorber is proposed, which can be used as a radome for antenna radar cross section (RCS) reduction. By tuning the active components, the radome can achieve in‐band RCS reduction and reconfigurable absorptive/transmissive states. In the absorptive state, the max bandwidth with an absorption rate greater than 90% spanning from 1.62 to 5.37 GHz; in the absorptive/transmissive state, 2.2 GHz is the transmissive frequency with the insertion loss of 93% while absorbing in 0.94–2.06 GHz and 2.63–6.5 GHz, which demonstrate impressive characteristics of broadband and a high absorption rate. Additionally, the radome can maintain performance at ±35° angular incidence and is insensitive to incident wave polarization. Furthermore, it features a lightweight and low‐profile design, with a cell electrical size of just 0.078λ × 0.078λ × 0.066λ. The prototype test shows a noteworthy correlation with simulation. This study has wider applications in the electromagnetic stealth of communication antennas and aircraft, as well as in the control of electromagnetic pollution signals in space. [ABSTRACT FROM AUTHOR]

This paper presents an ultra-wideband microwave absorber with a composite of carbonyl iron powder (CIP) and thermoplastic polyurethane (TPU) material and a wide-angle absorption stacked cross structure. The simulation results show that although the total thickness is less than 3.21 mm, the proposed device can achieve high absorption (over 90%) in the range of 7.55–100 GHz, and the total bandwidth is 92.45 GHz, with a relative bandwidth of 172%. In addition, the absorber is insensitive to the polarization characteristics of incident electromagnetic waves and can achieve a wide incident angle absorption of up to 60∘. [ABSTRACT FROM AUTHOR]

In this paper, a frequency-selective absorber with large-angular stability under oblique incidence is present. The absorptivity of the structure can be reached to maximum by impedance matching and parameter optimization method under a specific angle (40°), which ensures an excellent absorption response within 60° oblique incidence. This method solves the problem that the absorptivity of traditional absorbers deteriorates with the increase of incident angle. The absorber consists of a three-layer structure: two lossy layers and a metal ground layer, the equivalent circuit model is discussed and the operating frequency band is widened by generating electric resonances and magnetic resonances. Due to the high symmetry of the structure, the absorber has good polarization insensitivity. Numerical simulation shows that from 4.4 to 20 GHz, the absorptivity of the structure is above 90% within 40° oblique angle of incidence. Finally, the design plays an important role in military stealth and preventing electromagnetic interference. [ABSTRACT FROM AUTHOR]

Metamaterial absorbers have garnered significant interest due to their unique electromagnetic properties, which facilitate the efficient absorption of electromagnetic radiation in a specific frequency range or multiple frequency ranges. The metamaterial absorbers are designed based on artificially engineered structures that enable tailored absorption properties. These structures might include multiple resonances or incorporate electrical resistive components to achieve broadband absorption. This review paper provides a comprehensive analysis of the progress made in the field of the bandwidth of metamaterial absorbers, encompassing the underlying design principles, key performance characteristics, diverse applications, and perspectives for further research. The paper draws to a conclusion by outlining the potential future directions for further advancements in this exciting area of research, and highlighting the challenges that need to be addressed for the widespread adoption of metamaterial absorbers. [ABSTRACT FROM AUTHOR]

This paper presents a novel metasurface design strategy to realize broadband radar cross section (RCS) reduction. The phase distribution across the metasurface aperture can be regarded as applying an additional parabolic phase upon periodically arranged parabolic subarrays. Such a design fully utilizes the diffusive scattering nature of the parabolic phase distribution. Since the proposed metasurface is governed by only two focal lengths, the optimization procedure is quite easy compared to metasurface with random coding sequence. Experimental results show that the proposed metasurface can achieve more than 10 dB RCS reduction from 7.52 GHz to 19.66 GHz with a fractional bandwidth of 89.3% under both linearly and circularly polarized normal incidences, and keeping a performance of more than 7 dB RCS reduction until the incident angle increases to 40° for both x -polarized and y -polarized incidences in the frequency range of 8–19.35 GHz. When the incident angle increases to 60°, 7 dB RCS reduction can still be obtained for x -polarized incidence from 7.9 GHz to 19.35 GHz with a fractional bandwidth of 84%. [ABSTRACT FROM AUTHOR]

In this article, a broadband, polarization-independent and ultra-thin radar-absorbing structure (RAS) is proposed for the radar cross-section reduction (RCSR) of both planar and conformal scattering hotspots. The absorption characteristics and radar cross-section (RCS) performance of the novel metasurface-based RAS are analyzed in detail for both TE and TM polarization over a wide range of incident angles. The equivalent circuit model corresponding to the proposed unit cell is included as well. Further, the novel ultra-thin RAS (thickness of 0.043λ at the lowest operating frequency) was fabricated using a flexible substrate in both planar and conformal configurations, and the periodic pattern remained intact (without delamination) when applied over conformal geometries. The measurement results show that the percentage of power absorbed by the proposed structures (both planar and conformal) is greater than 80% in the frequency range of 8.2–12.2 GHz for both polarizations even at oblique angles of incidence. In addition, they provide more than 10 dB RCSR in comparison with their metallic counterparts of identical dimensions. [ABSTRACT FROM AUTHOR]

In this paper, a dual-passband frequency selective surface (FSS) with high angular stability and polarization insensitivity is proposed. The unit structure consists of a circular aperture, two annular apertures and four cross apertures. The designed FSS can achieve a double-passband at the interested frequencies of 8.45 GHz and 12.76 GHz with an insertion loss of less than 1 dB, and it can retain a stable transmission characteristic with the incident angle ranging from 0° to 86° for TE mode and from 0° to 83° for TM mode. Good agreement between the experimental results and the simulated response verifies the feasibility of the proposed FSS.

As graphene-related technology advances, the benefits of graphene metamaterials become more apparent. In this study, a surface-isolated exciton-based absorber is built by running relevant simulations on graphene, which can achieve more than 98% perfect absorption at multiple frequencies in the MWIR (MediumWavelength Infra-Red (MWIR) band as compared to the typical absorber. The absorber consists of three layers: the bottom layer is gold, the middle layer is dielectric, and the top layer is patterned with graphene. Tunability was achieved by electrically altering graphene’s Fermi energy, hence the position of the absorption peak. The influence of graphene’s relaxation time on the sensor is discussed. Due to the symmetry of its structure, different angles of light source incidence have little effect on the absorption rate, leading to polarization insensitivity, especially for TE waves, and this absorber has polarization insensitivity at ultra-wide-angle degrees. The sensor is characterized by its tunability, polarisation insensitivity, and high sensitivity, with a sensitivity of up to 21.60 THz/refractive index unit (RIU). This paper demonstrates the feasibility of the multi-frequency sensor and provides a theoretical basis for the realization of the multi-frequency sensor. This makes it possible to apply it to high-sensitivity sensors.

Since the use of chemical fuels is permanently damaging the environment, the need for new energy sources is urgent for mankind. Given that solar energy is a clean and sustainable energy source, this study investigates and proposes a six-layer composite ultra-wideband high-efficiency solar absorber with an annular microstructure. It achieves this by using a combination of the properties of metamaterials and the quantum confinement effects of semiconductor materials. The substrate is W–Ti–Al2O3, and the microstructure is an annular InAs-square InAs film–Ti film combination. We used Lumerical Solutions' FDTD solution program to simulate the absorber and calculate the model's absorption, field distribution, and thermal radiation efficiency (when it is used as a thermal emitter), and further explored the physical mechanism of the model's ultra-broadband absorption. Our model has an average absorption of 95.80% in the 283–3615 nm band, 95.66% in the 280–4000 nm band, and a weighted average absorption efficiency of 95.78% under AM1.5 illumination. Meanwhile, the reflectance of the model in the 5586–20,000 nm band is all higher than 80%, with an average reflectance of 94.52%, which has a good thermal infrared suppression performance. It is 95.42% under thermal radiation at 1000 K. It has outstanding performance when employed as a thermal emitter as well. Additionally, simulation results show that the absorber has good polarization and incidence angle insensitivity. The model may be applied to photodetection, thermophotovoltaics, bio-detection, imaging, thermal ion emission, and solar water evaporation for water purification. [ABSTRACT FROM AUTHOR]

A single layer frequency selective rasorber exhibiting dual-band transmissive and dual-band absorber characteristics is proposed. The dual-band absorber characteristics are achieved by two concentric circular rings with circular stubs on outer ring and circular slots in inner ring. The dual-band transmissive characteristics are achieved by two concentric circular ring apertures with additional quad diagonally circular rings. The absorber rings and transmissive aperture rings are separated by 0.8 mm thick FR-4 substrate. The Equivalent Circuit Model of the proposed Frequency Selective Rasorber (FSR) structure has been rigorously analyzed to understand the behavior of the quad-band transmissive and absorber characteristics. The measurement results demonstrated that a pass band with a minimal insertion loss of 0.21 dB can be achieved at 7.38 GHz and 10.80 GHz. The absorption rates of higher than 95% were obtained at 3.28 GHz and 16.618 GHz. The proposed quad-band FSR also verifies the polarization-insensitive behavior from 0° to 45°. Measurement results of proposed quad-band FSR prototype demonstrated superior agreement with simulated results and analytically ECM results. As compared to earlier reported geometries, novelty lies in its compact topology, higher angular stability, polarization insensitivity, and in band transmission response at desired range for filtering out unwanted signals in metrological satellites, surface ship radars, and various Radome applications. [ABSTRACT FROM AUTHOR]

In this paper, a tunable ultra-broadband terahertz metamaterial absorber is proposed based on the phase transition material of vanadium dioxide (VO _2 ). The absorber cell consists of a petal-like monolayer vanadium dioxide, a dielectric layer, and a metal layer. The terahertz absorption bandwidth of more than 90% absorptance reaches 4.2 THz, which covers from 1.99 to 6.19 THz, and a relative bandwidth attains to 102.7%. By changing the conductivity of VO _2 , the absorbance of this structure can be dynamically adjusted from 2.4% to 98.96%. The physical mechanism of the perfect absorption in this paper is investigated by the impedance matching theory and electric field distributions. The results show that the strong coupling effect in the petal-like structure contributes to the broadening of the absorption spectrum, and the absorber is polarization-insensitive and wide-angle incidence-insensitive due to the symmetry of the cell structure. The metamaterial absorber designed in this paper is expected to have a wide range of applications in the fields of terahertz imaging, stealth, sensing and detection.

Metamaterial absorbers have garnered significant interest due to their unique electromagnetic properties, which facilitate the efficient absorption of electromagnetic radiation in a specific frequency range or multiple frequency ranges. The metamaterial absorbers are designed based on artificially engineered structures that enable tailored absorption properties. These structures might include multiple resonances or incorporate electrical resistive components to achieve broadband absorption. This review paper provides a comprehensive analysis of the progress made in the field of the bandwidth of metamaterial absorbers, encompassing the underlying design principles, key performance characteristics, diverse applications, and perspectives for further research. The paper draws to a conclusion by outlining the potential future directions for further advancements in this exciting area of research, and highlighting the challenges that need to be addressed for the widespread adoption of metamaterial absorbers.

In this paper, by engineering the filling factor of the resonator of a hybrid frequency selective surface (FSS), a low-profile and application-targeted microwave metamaterial absorber is proposed with polarization and angle insensitivity. The proposed hybrid FSS of the absorber on FR4 substrate comprises four identical concentric copper sub SRRs and a defected copper ground. The square-cut area at the center of the ground and the filling factor of the resonator were tuned to get desired absorption peaks at some IEEE defined frequencies at C and X bands, which any other available perfect metamaterial absorbers cannot obtain. The measurement of the array absorber in anechoic chamber and the equivalent circuit analysis has demonstrated that the filling factor engineering technique illustrated an efficient way to achieve maximum absorptivity with the negative refractive index for perfect absorptions at the desired frequencies. The Wide-angle and polarization insensitivity to both co- and cross-polar waves are described by numerical analysis and EM responses. The thickness and dimension of the unit cell at the lowest operating frequency are 0.022 λ and 0.22 λ, respectively, and make it subwavelength compact. Besides normal and oblique incidence up to 80° and 180°, respectively, the absorber showed similar performance for cross-polar waves with an average of 99.5% absorptivity at 4.196 GHz, 5.24 GHz, 8.632 GHz, 9.264 GHz, and 10.152 GHz. The absorber has the potential for real-life RCS reduction, stealth, and sensing applications at C and X band antenna, satellite, and fighter aircraft stealth systems.

Water-based absorbers have shown great development potential in the past few years. In this paper, an all-dielectric transparent bi-directional water-based broadband metamaterial absorber is designed. The simulation results indicate that absorptance of the absorber is over 90% in 5.7–41.6 GHz, and its fraction bandwidth is 151.8%. The experimental results are greatly consistent with the simulations. The designed absorber has excellent performances of polarization insensitivity, oblique incidence stability and thermal stability. When the absorptance is more than 0.8, the maximum incident angle reaches 40° in TE mode and is over 60° in TM mode. In 0–80 °C, absorptance of the absorber is hardly changed. Because of the optical transparency of the designed absorber, it can be extensively used in stealth window weapons and electromagnetic compatibility equipment. [ABSTRACT FROM AUTHOR]

The power generation efficiency of conventional photovoltaic (PV) devices is restricted to a low level due to the Shockley-Queisser (SQ) limit. Fortunately, the solar thermophotovoltaic (STPV) technology can serve as an alternative to produce electricity efficiently. To further improve the efficiency of the STPV system, a novel absorber-emitter pair based on metamaterials is designed to match the InGaAsSb cells. According to the detailed balance calculation model, the performance analysis of the proposed STPV system is carried out and it achieves the maximum efficiency of 33.26% with the high-temperature effect considered. Besides, the intrinsic absorption and emission mechanisms are revealed through electromagnetic field distributions. The absorber achieves ultrahigh broadband absorption through slowlight mode and the emitter shows narrowband emission excited by the localized surface plasmon resonance. The angle and polarization insensitivity is also investigated, illustrating the excellent performance of the proposed system. The methods and mechanisms explored in this work are of great significance for researchers to understand and design similar structures. [ABSTRACT FROM AUTHOR]

The proposed FSS structure employs two square annular rings for absorption functionalities and two square aperture slots for transmission properties. Through comprehensive simulations and analysis, the FSS structure demonstrates remarkable performance in selectively absorbing and transmitting electromagnetic waves across four distinct frequency bands. Moreover, the design exhibits excellent TE and TM polarizations angular stability, consistently maintaining constant absorption and transmission characteristics across varying incidence angles. A comprehensive analysis of the equivalent circuit model of the FSS structure has also been performed to understand the quad-band transmission and absorption properties. The measurement results underscore the effectiveness of the design of a 250*250 mm array, with absorption peaks notably occurring at 4.2 GHz and 7.23 GHz, boasting peak absorption rates of 92% and 85%, respectively. Moreover, the insertion loss within the pass band at 8.8 GHz and 12.2 GHz stands impressively low at 0.2 dB and 0.32 dB, respectively. Notably, the proposed quad-band FSS showcases polarization-insensitive behavior across a range from 0° to 60° and successfully undergoes conformal testing with a curvature radius up to 50 mm. The results of experimental testing on a quad-band FSS prototype demonstrate superior agreement with those obtained through simulations and analytically derived equivalent circuit model (ECM) results, validating its practical applicability in real-world scenarios. [ABSTRACT FROM AUTHOR]

• High-Q Dual-Resonance Metasurface : We propose a novel square nanoblock octamer all-dielectric metasurface capable of generating dual-resonance modes with exceptional Q-factors in the mid-infrared spectrum. • Physical Mechanism Analysis: Through multipole decomposition and electromagnetic field distribution analysis, we elucidate the physical mechanisms underlying the dual-resonance modes, which are dominated by toroidal dipole (TD) and magnetic quadrupole (MQ), respectively. • Exceptional Sensing Performance: Our result demonstrates outstanding refractive index sensing performance, achieving a maximum sensitivity of 3666 nm/RIU and a figure of merit (FoM) of 55,302 RIU-1. • Gas Sensing Capabilities: By incorporating gas-sensitive films, our metasurface serves as highly sensitive gas sensors capable of detecting hydrogen and methane with sensitivities of −1.304 nm/% and −0.484 nm/% for hydrogen, and −2.366 nm/% and −8.917 nm/% for methane, respectively. • Significance: Our research contributes to the development of high-performance mid-infrared refractive index, hydrogen, and methane sensor devices, with potential applications in various fields including environmental monitoring, medical diagnostics, and industrial safety. The attainment of dual-resonance boasting high Q-factors in the mid-infrared spectrum stands as a captivating domain of inquiry. In this paper, a square nanoblock octamer all-dielectric metasurface is proposed. It generates dual-band responses in the mid-infrared band, exhibiting exceptional Q-factors of up to 15,149 and 120,856 respectively. The data evince a remarkable alignment between the Q-factor derived from fitting the Fano resonances and that computed through FWHM. Multipole decomposition and electromagnetic field distributions indicate dual-resonance modes are controlled by toroidal dipole (TD) and magnetic dipole (MD). The maximum sensitivity to the refractive index is 3666 nm/RIU, the maximum FoM is 55,302 RIU−1, and the polarization is insensitive. Gas sensors can be developed by filling the metasurface structure with hydrogen and methane-sensitive films. This sensor has a maximum sensitivity of −1.304 nm/% for hydrogen and −8.917 nm/% for methane. This research provides an important and effective approach to achieving high-performance mid-infrared refractive index, hydrogen, and methane sensor devices. [ABSTRACT FROM AUTHOR]

In this letter, a Jaumann-like rasorber is examined, which offers wideband absorption, angular stability, and polarization insensitivity. The structure is defined by three layers with two resistive layers placed on top of a lossless layer. Also, all three layers use capacitors to ensure compactness for the metasurface rasorber. These features provide broad angular stability up to 55 $^{\circ }$ while also maintaining low insertion losses of about 1 dB and only 0.6 dB for measured broadside incidence. The rasorber is also shown to work as a radome in the near field of a planar antenna array. [ABSTRACT FROM AUTHOR]

Since the use of chemical fuels is permanently damaging the environment, the need for new energy sources is urgent for mankind. Given that solar energy is a clean and sustainable energy source, this study investigates and proposes a six-layer composite ultra-wideband high-efficiency solar absorber with an annular microstructure. It achieves this by using a combination of the properties of metamaterials and the quantum confinement effects of semiconductor materials. The substrate is W–Ti–Al2O3, and the microstructure is an annular InAs-square InAs film–Ti film combination. We used Lumerical Solutions’ FDTD solution program to simulate the absorber and calculate the model’s absorption, field distribution, and thermal radiation efficiency (when it is used as a thermal emitter), and further explored the physical mechanism of the model’s ultra-broadband absorption. Our model has an average absorption of 95.80% in the 283–3615 nm band, 95.66% in the 280–4000 nm band, and a weighted average absorption efficiency of 95.78% under AM1.5 illumination. Meanwhile, the reflectance of the model in the 5586–20,000 nm band is all higher than 80%, with an average reflectance of 94.52%, which has a good thermal infrared suppression performance. It is 95.42% under thermal radiation at 1000 K. It has outstanding performance when employed as a thermal emitter as well. Additionally, simulation results show that the absorber has good polarization and incidence angle insensitivity. The model may be applied to photodetection, thermophotovoltaics, bio-detection, imaging, thermal ion emission, and solar water evaporation for water purification.

Based on the Si-Si3N4 hybrid photonic platform, an integrated edge coupler embedded with both sub-wavelength grating (SWG) waveguide and suspended structure was demonstrated. The proposed device structure and geometrical parameters were fully optimized to realize efficient optical interconnections with standard single-mode fiber (SMF-28). The coupling loss of 0.86 dB/port for TE mode and 0.94 dB/port for TM mode was achieved with a remarkable polarization dependent loss (PDL) less than 0.1 dB at the input wavelength of 1600 nm. It revealed that the integrated edge coupler assisted by sub-wavelength grating can further reduce the PDL. Moreover, the device also exhibited the advantages of large alignment tolerance. The alignment tolerances for 1-dB excess loss are ±1.8 and $\pm 2.5~\mu \text{m}$ in horizontal and vertical direction, respectively. [ABSTRACT FROM AUTHOR]

A three-layer wide-band reflective Frequency Selective Surface (FSS) reflector with a high roll off band edge is proposed in this paper. Each layer of the structure is patch type FSS. The proposed FSS has merits of wideband response over S band and lower C band with bandwidth of 3.6 GHz for transmission less than -10 dB with 94% bandwidth and significant stability for different incident angles up to 40 degree. The polarization insensitivity of the FSS also added an extra dimension of the structure. The simulation process as well as the experimental measurement process and ECM (equivalent circuit model) analysis of the FSS have been done. The good agreement in simulated, measured and ECM results verifies the wide stop band for the proposed FSS. The tunable and switchable transmission behavior of the FSS for variation in width of the internal air gap and lateral sliding of the middle layer of the structure respectively are also presented here.

A circular ring-shaped metamaterial (CRM) absorber was designed to harvest radio frequency (RF) energy in the ultra-wideband (UWB) frequency band applications. The proposed metamaterial unit cell features a circular shaped structure, with rectangular strip lines connected in the form of a cross leaving a square shaped slot at center. The unit cell dimensions are 15×15×1.6 mm. The absorber was etched on a low cost FR4 substrate having a dielectric constant of 4.4. Ansys high frequency structure simulator (HFSS) software was used for simulation and the analysis were carried out for unit cell, 2×2, 3×3, and 4×4 array structures. The absorber parameters plotted are absorption characteristics and reflection characteristics. Also, the metamaterial parameters (μeff) and (εeff) are also retrieved from the absorber parameters and analyzed. From the analysis, the values (μeff) and (εeff) were found to be negative, leaving refractive index also negative (n [ABSTRACT FROM AUTHOR]

We theoretically proposed an ultra-broadband infrared light absorber and demonstrated that a high absorption could be obtained in the range of ultra-broadband infrared light using the iron thin layers. The simulation results showed that the proposed absorber could achieve a close-to-unity absorption rate across a wide infrared spectral range, and that the average absorption rate would reach 90.6% over the range of 1.4-11.4μm. The high absorption rate resulted from the excitation of strong localized surface plasmon resonance in the iron layers and the superposition of different resonance frequencies. Numerical analysis results demonstrated that our proposed absorber had a broader efficient absorption range than most previously reported broadband infrared light absorbers.

A flexible multi-band metamaterial absorber for S and C band applications is proposed in this study. For flexibility and durability, the metamaterial structure is made of nylon reinforced neoprene rubber. The suggested absorber is compact and insensitive to polarization. The absorber exhibits more than 90% absorption at 2.87, 3.93, 5.58, 6.12 and 6.89 GHz. The RCS analysis of the absorber is presented in this study to validate the absorber's unique multi-band behavior. This paper also includes a comprehensive study on the importance of the effect of nylon reinforcing in neoprene rubber. The structure is fabricated using photolithography and tested using the free space measurement method. The proposed absorber can be used for stealth applications in aircrafts to reduce their visibility from airport surveillance radar used in ATC which falls in the range of 1.55 to 5.2GHz. [ABSTRACT FROM AUTHOR]

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Water-based absorbers have shown great development potential in the past few years. In this paper, an all-dielectric transparent bi-directional water-based broadband metamaterial absorber is designed. The simulation results indicate that absorptance of the absorber is over 90% in 5.7–41.6 GHz, and its fraction bandwidth is 151.8%. The experimental results are greatly consistent with the simulations. The designed absorber has excellent performances of polarization insensitivity, oblique incidence stability and thermal stability. When the absorptance is more than 0.8, the maximum incident angle reaches 40° in TE mode and is over 60° in TM mode. In 0–80 °C, absorptance of the absorber is hardly changed. Because of the optical transparency of the designed absorber, it can be extensively used in stealth window weapons and electromagnetic compatibility equipment.

A transmissive water-based metamaterial absorber(TWMA) is designed, simulated, and measured, which uses 3D-printing to manufacture. The TWMA uses a circular column and rectangular water layer structure and has no metal backplane. We have proved theoretically and experimentally that the TWMA has absorption rate exceeds 90% in the frequency ranging from 6.5 to 21.4 GHz. Furthermore, the TWMA is thermo-tunable when temperatures change from 10 to 60 °C and has the characteristics of wide angle incidence and polarization insensitivity. It can be applied to a variety of electromagnetic applications, such as reducing broadband scattering and collecting electromagnetic energy in a variable temperature environment. [ABSTRACT FROM AUTHOR]

Development of metamaterial absorbers that can operate in the terahertz spectrum has become the latest research interest of the electromagnetic fraternity. Temperature tunable absorbers are a special type of absorbers whose absorption spectra shift with the temperature. Initially, single-band and dual-band temperature tunable absorbers were designed and with progressive research in this field more sophisticated designs with multi-band absorption spectra are being developed. In this paper a seven-band temperature tunable terahertz metamaterial absorber is proposed, whose unit cell consists of a slotted flower–shaped resonator (S-FSR) on InSb dielectric substrate. The peak absorption's offered by the structure at 210 K are 99.12% at 0.895 THz, 97.63% at 1.042 THz, 96.97% at 1.268 THz, 99.7% at 1.401 THz, 89.37% at 1.551 THz, 97.57% at 1.646 THz, and 94.89% at 1.831 THz. Its temperature sensitivity is found to be 12.6 GHz/K, 10.9 GHz/K, 9.2 GHz/K, 8.2 GHz/K, 7.7 GHz/K, 7.2 GHz/K, and 6.5 GHz/K respectively. The novel design has several applications in the design of terahertz temperature sensors. [ABSTRACT FROM AUTHOR]

This article presents a dual notched Frequency Selective Surface (FSS) using a compact structure. Four sub-cell square split ring resonators (SSRRs) are imprinted over 8×8 mm2 FR-4 substrate in a 90° clockwise rotational symmetric manner. Dual resonant bands are obtained at 3.08 GHz ranging from 2.5 to 3.6 GHz and 10.2 GHz ranging from 7.95 GHz to 12.3 GHz. The proposed FSS covers the WiMAX band ranging from 2.5 to 2.7 GHz and 3.3 to 3.6 GHz and entire X-band ranging from 8 to 12 GHz. The structure presents a good angular response for both TE and TM incident polarized wave for normal and oblique angles. The pass band from 4.3 GHz to 5 GHz frequency with resonant dip at 4.6 GHz is obtained. The maximum relative deviation in between transmission zero frequency and angularly incident frequency is 1.2% and 4.2% at 70° TM and TE polarized wave, respectively. [ABSTRACT FROM AUTHOR]

To reduce the polarization sensitivity, a T-type grating with the material SiO2 is proposed on a reflective layer made of aluminum material. Based on structural parameters derived from rigorous coupled-wave analysis and simulated annealing, the described reflection beam splitter enables a nine-port separation under normal incidence. In addition, it demonstrates exceptional and uniform diffraction efficiency. For transverse electric polarization, the diffraction efficiencies of 0th, ±1st, ±2nd, ±3rd, and ±4th are 10.02%, 11.70%, 10.85%, 11.00%, and 10.03%, respectively. For transverse magnetic polarization, the diffraction efficiencies of 0th, ±1st, ±2nd, ±3rd, and ±4th are 12.54%, 10.13%, 9.51%, 10.46%, and 11.89%, respectively. The finite element technique is used to verify the diffraction efficiency values. In addition, a thorough analysis of the fabrication tolerance, wavelength bandwidth, and electromagnetic field dispersion is conducted. The outcome shows that the suggested grating structure has outstanding uniformity and efficiency, and these studies greatly enhance our knowledge of and progress toward the creation of the nine-port beam splitter. Consequently, there is a lot of room for the implementation of the suggested grating in multi-channel optical communication systems. [ABSTRACT FROM AUTHOR]

In this paper, a broadband transparent microwave absorber with a fractional bandwidth of 118.1% was designed. In the proposed absorber, a gear-shaped unit cell consisting of optically transparent indium tin oxide-polyethylene terephthalate films fabricated on a polymethyl methacrylate is developed to guarantee electromagnetic wave to well enter into and is dissipated in the absorbing structure. Two resonant frequencies at 4.8 GHz and 13.6 GHz, which correspond to electromagnetic and electric resonances, respectively, are generated to achieve a wide absorbing band from 3.86 GHz to 15.04 GHz covering S, C, X and Ku bands. The proposed absorber has the advantages of low profile, polarization insensitivity, and angular stability. The prototype of the absorber has been fabricated and measured, and the experimental results are in good agreement with the simulation results.

A three-layer wide-band reflective Frequency Selective Surface (FSS) reflector with a high roll-off band edge is proposed in this paper. Each layer of the structure is patch type FSS. The proposed FSS has merits of wideband response over S band and lower C band with bandwidth of 3.6 GHz for transmission < -10 dB with 94% bandwidth and significant stability for different incident angles up to 40 degree. The polarization insensitivity of the FSS also added an extra dimension of the structure. The simulation process as well as the experimental measurement process and ECM (equivalent circuit model) analysis of the FSS have been done. The good agreement in simulated, measured and ECM results verifies the wide stop band for the proposed FSS. The tunable and switchable transmission behavior of the FSS for variation in width of the internal air gap and lateral sliding of the middle layer of the structure respectively are also presented here. [ABSTRACT FROM AUTHOR]

In this letter, a novel design of a narrow bandpass frequency selective surface (FSS) based on the utilization of the coupled method, which resonates at Ka-band frequencies between 29 and 30.3 GHz, is proposed and experimentally evaluated. A high selectivity was achieved, by coupling two metallic layers, which were based on the structure of a cross aperture and a rectangle grid patch, respectively. The simple symmetric patterns used in the proposed work have also made the FSS independent of polarization properties. Moreover, a high angular stability was also achieved since for electromagnetic (EM) wave incident at degrees from 0° to 75°, the frequency deviations were only 0.1% and 0.4% for TE and TM polarizations, respectively. For performance evaluation, an experimental FSS prototype was fabricated by a double-sided printed circuit board (PCB), which was composed of 50 × 50 unit cells in a dimension of 18 cm × 18 cm. A Keysight N5227A PNA and two-horn antennas in a chamber were setup to measure the performance. The experimental measurement results have shown a good agreement with the numerical simulations. [ABSTRACT FROM AUTHOR]

In this article, by deliberately controlling multiple resistive electric and magnetic resonances in terms of dispersion and dissipation, a low-profile, wideband, microwave metamaterial absorber with wide-angle and polarization-independent responses is proposed. The proposed absorber comprises a planar array with resistance-loaded metallic cross patterns, and a vertical periodic crossed mesh array with resistance-loaded metallic ring patterns. The vertical periodic crossed mesh array was inserted between the planar array and the metal ground to improve the wide-angle polarization-independent absorption. The involved dispersion-engineered design strategy for angular- and polarization-insensitive responses is described with numerical evidences and electromagnetic response behaviors. A proof-of-concept absorber was fabricated and measured for verification. At quasi-normal incidence, the measured bandwidth characterized by more than 90% absorption was 2.11–3.89 GHz, i.e., a fractional bandwidth (FBW) of 59.3%. At the incident angle of 50°, the FBW of the absorption larger than 90% was 48.6%. The absorber was thin with a thickness of 13 mm, corresponding to $0.09\lambda _{0}$ at the lowest operating frequency. The numerical and experimental results demonstrated that our proposed strategy provides an effective way to achieve wide-angle and polarization-independent responses in a broadband; these responses are very promising for most strategic applications. [ABSTRACT FROM AUTHOR]

A highly transparent polarization-insensitive metamaterial absorber with wideband microwave absorption is presented. The broadband absorption (6.0~16.7 GHz, absorptance > 85%) is achieved using three patterned resistive metasurfaces. The visible light transmittance of the absorber is as high as 85.7%. The thickness of the absorber is 4.42 mm, which is only 0.088 times of the upper-cutoff wavelength. A prototype sample is fabricated and measured to demonstrate its excellent performance. The experimental results agree well with the simulation results. In view of its wide band absorption, high transmittance, low profile, polarization insensitivity and wide incidence angle stability, the presented absorber has a wide range of potential applications. [ABSTRACT FROM AUTHOR]

In this paper, a new octagonal close ring resonator (OCRR)-based dumbbell-shaped tuning fork perfect metamaterial absorber for C- and Ku-band applications is presented. This design is a new combination of an octagonal ring close ring resonator with two dumbbell-shaped tuning forks metal strips integrated on epoxy resin dielectric substrate. The proposed perfect metamaterial absorber (PMA) is assessed by finite-integration technique (FIT)-based electromagnetic simulator-Computer simulation technology (CST) software. The anticipated assembly reveals dual resonance frequencies of 6.45 GHz and 14.89 GHz at 99.15% and 99.76% absorption, respectively, for TE incidence. The projected design is augmented through various types of parametric studies, such as design optimization, the effect of the octagonal ring resonator width, and varying the split gap of the double tuning fork. The numerical results are also investigated and verified using the equivalent circuit model, another electromagnetic simulator high frequency structural simulator (HFSS), and different array combinations that showed very negligible disparity. The TE polarization wave is applied to analyze the absorption separately and oblique incidence angle showing polarization insensitivity up to 30° and wide incident angle up to 60°. The presented metamaterial absorber is suitable for satellite communication bands, stealth-coating technology, and defense and security applications.

In this paper, an integrated polarization-insensitive optical comb filter with flat passband and tunable central wavelength is designed based on cascaded Mach-Zehnder structure. It is realized by employing three specific directional couplers with square waveguides. The central wavelength of the proposed filter for both TE and TM modes can be equally shifted. Simulation results show that the designed filter has a 3-dB bandwidth of 2.8 nm, a free spectral range of 6.09 nm, and a minimum insertion loss of 0.315 and 0.325 dB for TE and TM mode, respectively. Moreover, the polarization-dependent loss is ~0.05 dB over the passband.

We have designed a polarization conversion coding metasurface (CM) for broadband radar cross-section reduction. The CM is made up of a kind of deformed bowtie-shaped particle and its mirror, named "0" and "1", respectively, which can realize cross-polarization conversion under the vertical incidence of a linearly polarized wave. According to the coding sequence optimized by the genetic algorithm, the two kinds of unit are arranged to form a design, called a 1-bit CM, which can scatter electromagnetic waves in various directions. The simulation results demonstrate that the two kinds of unit can achieve more than a 90% polarization conversion ratio, and the whole CM can realize more than a 10-dB reduction of monostatic radar cross-section (RCS) from 8.8 GHz to 20.9 GHz, while the maximum RCS reduction of 32.5 dB can be obtained at 14 GHz under the normal incidence of x- and y-polarized waves. The experimental results are almost identical to the simulation. Thus, we believe that the designed 1-bit CM can be well applied for the stealth business. [ABSTRACT FROM AUTHOR]

This article reports a compact ultrathin metamaterial absorber with three resonances in S‐band, X‐band, and Ku‐band. The geometry of the proposed absorber consists of a square ring, a splitted square ring, and a plus shaped resonating structure. The plus shaped structure is enclosed within a split ring resonator and it is further enclosed by an outer square ring. The proposed absorber is fabricated on a low cost FR4 substrate of thickness 1 mm that is, 0.011 λlowest (where, λ is the lowest resonating frequency). The size of unit cell is 10 × 10 mm2. It exhibits 99.6%, 99.1%, and 99.1% absorption at 3.4, 9.6, and 13 GHz, respectively. Moreover, the physical insight of the design is explained by surface current distribution and equivalent circuit analysis. Stability of the proposed design is validated with different incident (for transverse electric (TE) and transverse magnetic (TM) modes) angles and different polarization angles. Finally, a prototype of the absorber is fabricated and validated experimentally. [ABSTRACT FROM AUTHOR]