Geometric Phase Based Diffuse Scattering Metasurface with High-efficient Transmission Window

Professor Yongmeng Liu from the Harbin Institute of Technology in China talks about the research behind his Letter ‘Optical-transparent Wi-Fi bandpass mesh-coated frequency selective surface’ on page 381. Professor Yongmeng Liu Our group's work is focused on designing novel frequency selective surfaces (FSSs) as security wallpaper for optical windows of electromagnetic filtering and interference shielding. A FSS is an array of periodic apertures perforated on a conducting sheet or metallic patches which exhibits bandpass or bandstop responses. A traditional FSS generally has very low optical transmittance and cannot be easily applied as a dual-mode filter to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The solution is to create novel periodic structures that have unique and fascinating features attracting us to this field of research. We have reported a FSS periodic structure fabricated using a UV lithography technique to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The FSS is a resonance cavity structure with a periodic array of apertures removed from an optically transparent conductive mesh coating. Simulation and experimental results show that the resonance centre frequency of the mesh-coated FSS is about 5.3 GHz with transmittance loss of −2dB, while its optical transmittance is greater than 87.5% in a waveband of 1.35 to 3.3 µm. Such a FSS has the merits of ameliorable electromagnetic shielding effectiveness and improvable optical transmittance by optimising the thickness of the mesh layer. Our FSS can be used as security wallpaper for optical windows to solve the problem of transmission security of Wi-Fi communications. Generally, transparent conductive mesh coating is widely used to provide both strong RF/microwave shielding and optical transmittance, however this seriously attenuates Wi-Fi transmittance. A bandpass FSS is used to provide spatial filtering, but suffers from low optical transmittance due to the large fraction of the area obscured by metallic films. Neither of these existing technologies can meet the dual requirement of optics/Wi-Fi filtering for security wallpaper. It is a significant advancement to develop a periodic structure to achieve simultaneous optical transparency and stable Wi-Fi bandpass. One potential application is as security wallpaper for securing confidential business data. Another application is to enhance the transmission of the specific signal of interest through optical windows. State-of-the-art FSS technology includes indium tin oxide (ITO). This kind of ITO-FSS is used for energy saving windows since the ITO layer reflects a significant portion of long and short infrared wave energy. However, the restrictive relationship between conductivity and optical transparency of ITO is the fatal flaw in this implementation. We are working on a project to design conductive metallic meshes to shield radio-frequency/microwave interference for optical windows to optimise mesh patterns and parameters to improve optical transmission and to minimise the high-order optical diffraction effect, as well as enhance electromagnetic shielding effectiveness. Challenges will be to further optimise the structure of the mesh-coated FSS to improve the performance, specifically the sheet resistance, bandwidth and the transmission stability of incidence angles and polarisations. The FSS has high sheet resistance which will impact Wi-Fi transmission, however the FSS has adjustable thickness to enhance its conductivity and improve Wi-Fi transmission performance while having no impact on optical transparency. Additional challenges include developing ways to integrate the mesh-coated FSS with optical windows of existing buildings. Fortunately we can fabricate the FSS on flexible PET films and add the composite films to each side of existing optical windows. Over the next few years novel periodic electromagnetic structures will integrate metamaterials, active circuit elements and other new techniques to develop security wallpapers to protect communications security. The trend of periodic electromagnetic structures for electromagnetic filtering and shielding will evolve from expensive, heavy and passive to low-cost, lightweight, active and self-tuning. We would like to see increasing numbers of new periodic electromagnetic structures being developed throughout numerous frequency ranges, perhaps even the entire electromagnetic spectrum. We believe all these beautiful periodic electromagnetic structures will be brought into real-world use and will make our lives better!

Sandwich structures with frequency selective surfaces have been widely used to fabricate low-observable radomes. Traditional frequency selective surfaces were made of metals, such as copper and aluminum. There were problems when used the frequency selective surfaces with the sandwich structures, such as a bonding layer and thermal mismatch. Because of the thermal mismatch between the sandwich structures and the metals, mechanical properties of the sandwich structures usually decreased. To avoid these disadvantages, a new type of composite frequency selective surface was put forward in this study. A four-legged slot-array frequency selective surface and a square-aperture array frequency selective surface were fabricated by cutting carbon fiber/epoxy composites. Free-space method and finite-element method were carried out to evaluate the electromagnetic transmission characteristics of the frequency selective surface specimens, respectively. Results show that the composite frequency selective surface with four-legged slot array can realize the function of frequency selection, and its minimum transmission loss can be decreased by increasing the electrical conductivities of the composite material, adjusting the thickness, and increasing the aperture-to-cell ratio of it. For the composite frequency selective surface with square-aperture array, grating lobes come closely after its resonant frequency, although its minimum transmission loss is very close to zero.

A triband antenna with high gain and low profile is proposed. This antenna is based on the concept of Fabry–Perot cavity antenna (FPCA) and three resonances are obtained by employing two frequency selective surface (FSS) layers. Two FSS layers lead to two resonant frequencies by making each FSS layer satisfy the resonant condition of Fabry–Perot cavity. The third resonant frequency is obtained by exploiting the combined effects of the two FSS layers together. The operating principle of this proposed antenna is explained and the achievable frequency range is investigated. It is shown that the proposed method enables all three resonant frequencies to be independently tuned within certain frequency range. Because of the 0° reflection phase of the combined-FSSs, the present antenna has a low profile. To verify the design concept, one C-/X-/Ku-band FPCA is designed, fabricated and tested. Experimental results agree well with the simulated results. High gain with good impedance matching in three bands is obtained, which reaches a peak gain of 13.4 dBi at 5.2 GHz, 18.9 dBi at 9.7 GHz, and 20 dBi at 14.6 GHz. The overall height of the proposed antenna is approximately 0.36 wavelength at its lowest operating frequency.

A patch-slot-patch frequency selective surface (FSS) exhibiting bandpass response is proposed in this paper. The FSS unit cell is composed of two cross-dipole shaped patches and a cross-dipole shaped slot in between. The proposed FSS exhibits bandpass response with two closely spaced transmission poles around 3 GHz and 3.5 GHz. A 3-dB transmission bandwidth of upto 20% is achieved for the FSS. The proposed design has a thickness on the order of 0.032 λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . The paper presents with parametric studies how the poles can be tuned individually. The transmission phase of the FSS varies linearly towards zero with frequency that makes the FSS useful in designing Filter-Antenna when placed as a superstrate above a broadside radiator.

A Fabry–Perot antenna using a frequency selective surface layer with wideband and Low RCS for Mm-wave applications

A new bandpass frequency selective surfaces (FSS) is designed and the composite system (patch antenna covered by an FSS) is analyzed using the finite difference time domain (FDTD) method. Far-field patterns of the patch antenna, with and without FSS radomes, are obtained and compared with each other. The feed is modeled by using a coaxial line extension, which enables us to compute the return loss of the patch and the composite system accurately. Numerical results show a frequency shift in the return loss response of the patch in the presence of the FSS cover.

In this letter, a novel bandpass frequency selective surface (FSS) with miniaturized periodic element is presented. The proposed FSS is printed on one face of a single-layer substrate with a relative permittivity of 2.65. The novel FSS has promising miniaturization characteristics with the unit-cell dimension 0.058λ×0.058λ, where λ represents the free-space wavelength corresponding to the resonant frequency. The FSS exhibits excellent stability under different polarizations and incident angles. Both the simulation and measurement results validate the stable performance of this FSS.

Single-layer dual-band frequency selective surface for coverage-enhancing applications in millimeter-wave 5G

In order to analyze the effects of changing the value of PIN diodes and variodes on the resonant frequency and the depth of the resonance, we firstly use electromagnetic simulation solver to simulate frequency selective surface (FSS) loaded with PIN diodes and variodes respectively in this paper. Then, the equivalent circuit method (ECM) is used to validate correctness and prove rule of influence. Finally, active FSS loaded with DC bias line is also analyzed as a whole, and the influences of the DC bias line on the active FSS's electromagnetic property are illustrated.

In this paper, a complementary split ring resonator (CSRR) loaded compact CPW-fed circular shaped monopole UWB antenna with dual band notch characteristics is designed on an FR-4 substrate. The propounded band-notched UWB antenna is integrated with two frequency selective surface layers referred as bandpass frequency selective surface (FSS-1) and reflector (FSS-2) to offer enhanced radiation performance. Bandpass FSS-1 enhances the gain of the antenna by improving directivity and reflector FSS-2 reflects back the backside radiation of the proposed antenna towards the forward direction thereby increasing gain and directivity of the proposed antenna. Furthermore, a two-element orthogonal MIMO antenna is proposed with improved gain using two FSS layers. The complete configuration of the proposed two-element MIMO antenna consists of CSRR-based radiating patches, monopole ground planes and two integrated FSS layers. The proposed MIMO antenna operates in UWB range from 3.46[Formula: see text]GHz to 16.02[Formula: see text]GHz with dual notches at 3.9[Formula: see text]GHz and 7.2[Formula: see text]GHz. Isolation between MIMO elements is greater than 23 dB. Both antennas are fabricated and investigated at 5.9[Formula: see text]GHz for vehicular communications. Maximum gain improvements of 8 dB and 6.4 dB are observed in case of the presented single element and MIMO antenna within the entire operating band due to the integration of dual FSS layers. The diversity performance of the suggested MIMO antenna is investigated by analyzing mean effective gain, envelope correlation coefficient, channel capacity loss and diversity gain parameters. The proposed MIMO antenna is a suitable candidate for vehicular and UWB wireless communication applications.

A highly selective tri-band bandpass frequency-selective surface (FSS) is presented by cascading three layers of periodic arrays. The middle layer is composed of double square loops (DSLs) while the two exterior layers are composed of gridded-double square loops (G-DSLs) structure. The proposed FSS can provide multitransmission zeros which lead to a wide out-of-band rejection between each two adjacent passbands and sharp rejection behavior on both sides of each passband. Furthermore, the FSS exhibits stable response over a wide range of incident angles for both TE and TM polarizations. The design procedure, simulation, and experiment of the FSS are presented. The measured results are in good agreement with the simulations.

In this paper, a multifunctional switchable frequency selective rasorber (FSR) incorporating water-based active thermal control devices is proposed. Compared with other FSRs, the proposed design, on the basis of reconfigurability, exploits the thermochromic dispersion and fluidity characteristics of water-based materials to achieve enhanced functional integration. The proposed design consists of a water-filled chamber absorber, a bandpass frequency selective surface (FSS), and integrated micropumps and thermal control devices. The proposed absorber is fabricated using 3D printing technology, whereas the bandpass FSS is manufactured using printed circular board (PCB) technology. Experimental results demonstrate that, under the FSS transmission mode, the proposed design achieves a -3 dB bandwidth of 10.3-11.6 GHz with a minimum insertion loss of 0.74 dB within the passband. Under the integrated absorption-attenuation mode, the in-band attenuation exceeds 5.0 dB, while the absorption rate is greater than 80% in the out-of-band frequency ranges of 11.9-13.6 GHz and 20.9-21.4 GHz. The two operating modes can be reversibly switched by loading and unloading water via the micropump. Additionally, the proposed design incorporates a water-cooling circulation system with thermal control devices to enable effective thermal management and tunable absorption performance. In the cooling mode, the temperature can be rapidly reduced from 53.2 °C to 32.3 °C within 60 s. By activating the temperature control mechanism, adjustable absorption levels can be achieved. The proposed multifunctional switchable FSR not only reduces manufacturing costs but also satisfies the requirements for thermal stability in complex communication and detection environments, demonstrating significant potential for radome applications.

A second-order bandpass frequency selective surface (FSS) for terahertz applications is experimentally demonstrated in this paper. The proposed FSS is designed using miniaturized element unit cells that enable analysis of their frequency by an equivalent circuit model. A transmission zero is introduced in the upper stopband of the transmission response for improving the out-of-band performance. Numerical and experimental results show a harmonic-free response up to three times the passband center frequency.

In this paper, a technique for enhancing the gain of a direct microstrip line fed dielectric resonator antenna (DRA) with a bandpass frequency selective surface (FSS) as superstrate has been proposed. A 3.6 dBi increase in gain is observed when the proposed multi-layered FSS has been placed at a distance of half of the resonating wave length from the ground plane of the microstrip line fed DRA. The antenna with FSS offers a peak gain of 9.1 dBi at the resonating frequency and 5% bandwidth covering the frequency band from 6.42 GHz to 6.75 GHz.

In this paper, a new geometrical shape of the bandpass modified circular ring frequency selective surface (FSS) structure with significant electrical response such as angular stability up to $$50^{\circ }$$50? of the angle-of-incidence (AOI) for both the perpendicular (TE) and parallel (TM) polarized waves has been discussed. The geometrical parameters of the proposed bandpass FSS is obtained by the transformation of bandpass single square loop FSS structure parameters. To achieve the frequency response of the proposed bandpass structure, commercially available 3D electromagnetic simulators such as CST Microwave Studio and Ansoft HFSS are used. The proposed bandpass FSS structure provides significant angular (i.e., up to $$50^{\circ })$$50?) and polarization (i.e., perpendicular and parallel) stability as compared to that of various other reported literatures. In addition to this, one of the important characteristics of the resonant structure is the quality factor (Q-factor), which has also been discussed for the proposed bandpass FSS structure at different AOI.