Polarization Conversion Metasurface Research Articles

Graphene-Based Active Tunable Metasurfaces for Dynamic Terahertz Absorption and Polarization Conversion

Simultaneous broadband absorption and polarization conversion are crucial in many practical applications, especially in terahertz communications. Thus, actively tunable metamaterial systems can exploit the graphene-based nanomaterials derived from renewable resources because of the flexible surface conductivity and selective permeability of the nanomaterials at terahertz frequencies. In this paper, we propose a graphene-based active tunable bifunctional metasurface for dynamic terahertz absorption and polarization conversion. The graphene ring presents a certain opening angle (A) along the diagonal of the xoy plane. When A = 0°, the proposed metasurface behaves as a broadband absorber. Numerical results show the feasibility of achieving this polarization-insensitive absorber with nearly 100% absorptance, and the bandwidth of its 90% absorptance is 1.22 THz under normal incidence. Alternatively, when A = 40° after optimization, the proposed metasurface serves as a broadband polarization convertor, resulting in robust broadband polarization conversion ratio (PCR) curves with a bandwidth surpassing 0.5 THz in the reflection spectrum. To tune the PCR response or the broad absorption spectrum of graphene, we change the Fermi energy of graphene dynamically from 0 to 0.9 eV. Furthermore, both the broadband absorption and the linear polarization conversion spectra of the proposed metasurface exhibit insensitivity to the incident angle, allowing large incident angles within 40° under high-performance operating conditions. To demonstrate the physical process, we present the impedance matching theory and measure electric field distributions. This architecture in the THz frequency range has several applications, such as in modulators, sensors, stealth, and optoelectronic switches. THz wave polarization and beam steering also have broad application prospects in the field of intelligent systems.

Ultrabroadband chiral metasurface for linear polarization conversion and asymmetric transmission based on enhanced interference theory

Ultrabroadband chiral metasurface for linear polarization conversion and asymmetric transmission based on enhanced interference theory

Low RCS, Broadband Polarization Conversion Metasurface Antenna Based on Reactance Loading

A reactance-loaded polarization conversion metasurface (PCMS) is presented and exploited to realize a metasurface antenna (MTA) with both low radar cross section (RCS) and broadband radiation. Distinct from the conventional low-RCS PCMS, a metallic surface with etched open and closed slits is integrated into a PCMS consisting of double L-shaped particle pairs. Due to the edge capacitance induced by open slits and the surface inductance imposed by closed slits, the PCMS is endowed with reactance features that act as a perturbation to modulate the near-field interactions, thereby expanding the polarization conversion bandwidth. Besides, the reactance-loaded metallic surface, which has the same period as L-shaped particle pairs, combined with the grounded substrate, constitutes a broadband metasurface radiator. With the help of characteristic mode analysis, a linearly polarized aperture-coupled PCMS antenna is proposed, which effectively stimulates two resonant modes with broadside radiation to help bring about a broad operating bandwidth of 45.52%. Subsequently, the checkerboard-layout PCMS array is coupled to a sequentially feeding network through apertures on the ground plane, implementing a circularly polarized MTA with a broad 3 dB axial ratio bandwidth of 20.11%. Relying on phase cancellation and outstanding polarization conversion behaviors, an ultrawideband RCS reduction bandwidth of 7–39 GHz (139.13%) is also available.

A low scattering tightly coupled dipole array based on frequency-selective polarization converter

In this paper, a low radar cross section (RCS) tightly coupled dipole array (TCDA) based on novel dual-polarized frequency-selective polarization converter (FSPC) has been proposed. The proposed FSPC structure is comprised of a polarization conversion metasurface and a band-pass frequency selective surface, and the coding sequence of FSPC can be derived by simulated annealing algorithm to achieve simulated mono-static RCS reduction. The process of the co-design of the TCDA with FSPC is presented, and the radiation performance of the antenna array meets the design requirements. A TCDA sample and 20 × 20 FSPC elements were designed and fabricated. The measured results indicated that a significant reflection reduction from 15 GHz to 21 GHz compared with a metallic plane of the same size at the boresight, and the simulated and measured normalized radiation patterns are in good agreement.

Frequency multiplexed metasurface for polarization conversion in full-space

In order to solve the problem of a single function in passive metasurface, a frequency multiplexed metasurface using full-space to realize different polarization conversion modes is proposed. On the basis of the fundamental theory of metasurface polarization transformation, a spliced coupling of multilayer structures was utilized. A compact architecture of five metallic layers and four dielectric layers was fabricated, and the principle analysis of polarization transformation was also performed. In the metallic part, two layers of oblique arrows and one layer of oblique rectangular patches constitute a low-frequency transmission structure, one layer of cross-shaped patches are constituted a high-frequency reflection structure, and one layer of rectangular patches constitutes an intermediate filter structure. Thus, it achieves a variable frequency polarization multiplexing function for transmission on one side and reflection on the opposite one. When the x- and y-polarized waves in the 6.3 GHz–8 GHz band are incident on the low-frequency unit side, they are converted into left- and right-hand circularly polarized transmitted waves, respectively. The axial ratio is less than 3 dB, and the transmission amplitude is up to 0.84 and 0.8. When 45° linearly polarization incident waves in 16.4 GHz–16.8 GHz and 16.9 GHz–18 GHz bands impinge on the high-frequency unit side, the cross-polarized wave reflection is realized. The measured and simulated results are in good agreement. The metasurface proposed in this paper has potential applications in communication frequency multiplexing and multifunctional integration.

Full-polarization radar target feature modulation based on active polarization conversion metasurface

Electromagnetic (EM) metasurfaces comprising artificially designed subwavelength unit cells have drawn considerable attention due to the EM properties beyond the limits of natural materials. As one of the representative structures, active polarization conversion metasurface (APCM) is switchable by loading active components. It provides great freedom to manipulate the polarization state of EM waves. However, the current research mainly focused on the application of communication and paid less attention to the radar effect of APCM. APCM redistributes electromagnetic wave energy in multi-polarization channels, so it will have great application potential in polarimetric radar. Herein, based on the fully polarimetric radar one-dimensional high resolution range profile, the radar effect of time-modulated metasurface is studied. For this purpose, a method of target scattering mechanisms manipulation and a polarization-insensitive structure of APCM are proposed. The amplitude-phase joint modulation method is specifically analyzed in detail. The distance transformation and virtual multi-target phenomena are further discovered. Virtual targets along the distance dimension are generated in multi-polarization channels, while the scattering mechanisms of k-order targets are effectively manipulated. The relationship between the target scattering matrix and the modulation parameters is obtained. It may provide an effective method for the application of active metasurface in fully polarimetric radars.

A high efficiency ultra-wideband circular-to-linear polarization conversion metasurface

In this work, an 8-shaped metasurface is proposed for circular-to-linear (CTL) polarization conversion, which is an orthotropic anisotropic structure composed of an array of 8-shaped metal patches printed on a grounded dielectric substrate and covered by a top dielectric layer. Because the 8-shaped metasurface has an appropriate anisotropy, which can make the reflection phase difference under u- and v-polarized incidences close to +90∘ in an ultra-wide frequency range, it can be used as an ultra-wideband CTL polarization conversion metasurface. Numerical simulated results show that the 8-shaped metasurface can achieve high-efficiency ultra-wideband CTL polarization conversion, it can convert right-handed circular-polarized (RHCP) wave to a y-polarized one and convert left-handed circular-polarized (LHCP) wave to a x-polarized one in the ultra-wide frequency band from 8.34 to 26.06 GHz with a relative bandwidth of 103.0%, and the polarization conversion rate (PCR) can be kept larger than 99.6% in the total working band. Finally, an experimental verification is carried out, and a reasonable agreement is observed between the simulated and experimental results.

Low Profile Reflective Polarization Conversion Metasurface With High Frequency Selectivity

In this article, a multilayer reflective polarization conversion metasurface with both high-frequency selectivity and low profile is proposed. The frequency selective polarization conversion metasurface (FSPCM) is composed of four metal layers separated by three substrates. The cross-shaped patch on the top layer is connected to the folded microstrip line on the bottom layer with two via holes through the circle slots inside two ground layers, which converts the linearly polarized electromagnetic incident wave into cross-polarized wave after reflection. The second-order response can extend to the third-order response by introducing a coupled half-wavelength microstrip resonator on the bottom metal layer, which achieves high-frequency selectivity with a transmission zero and high polarization conversion rate (PCR) without increasing the thickness. The equivalent circuit analysis, structure simulation, and experimental demonstration are conducted in detail; the frequency selectivity of the fabricated FSPCM at the lower and upper band edge is 94 and 192 dB/GHz, respectively; PCR is more than 97% from 4.85 to 5.12 GHz, and the thickness is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.028\lambda _{0}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free space wavelength at the center operating frequency. Our findings pave a new way for enhancing the anti-interference capability and electromagnetic compatibility in applications of communication system, microwave imaging, and stealth technology.

High-temperature metasurface for polarization conversion and RCS reduction

In this study, we present a high temperature-resistant linear-to-linear polarization conversion metasurface for wideband RCS reduction in the microwave regime. The linear-to-linear polarization conversion metasurface is composed of a single layer high-temperature-resistant metallic pattern array. When linearly polarized waves impinge on the metasurface, the polarization conversion rate is greater than 90% from 9 GHz to 16.8 GHz at 800°C. The polarization conversion unit arrangement forms a checkerboard configuration metasurface, which can effectively reduce RCS reflection. The wideband RCS reduction mechanism is scattering cancellation rather than absorption. The metasurfaces were designed, fabricated, and measured, which can realize linear-to-linear polarization conversion and wideband RCS reduction. The simulation results and measurement results are all in good agreement. Meanwhile, the measurement provides a solid verification for our design. The results provide an alternative method to reduce the RCS at high temperatures.

Topology optimization of the azimuth-rotation-independent polarization conversion metasurface for bandwidth enhancement.

Metasurfaces offer an unprecedented opportunity for flexible manipulation of electromagnetic wave. The azimuth-rotation-independent (ARI) polarization conversion metasurface (PCM) is an ultrathin device, which could convert an arbitrary linearly-polarized incident wave to its cross-polarized state. However, the bandwidth of an ARI PCM with a high cross-polarized transmission is usually limited. Here, a topology optimization method of multi-feature points based on the differential evolution (DE) algorithm is adopted to enhance the bandwidth of the traditional ARI PCM while maintaining a high transmission and polarization conversion ratio. The simulated results of the optimized structure indicate a 2.08 times bandwidth expansion in the cross-polarization conversion compared with the original structure. In addition, the measured results are consistent with the simulated ones and the ARI characteristic is validated. The proposed method provides a promising route for efficient high-performance metasurface designs.

Low-profile and wide-band RCS-reduction antenna array based on the metasurface polarization converter.

Antenna elements with a low profile and high front-to-back (FB) ratio mean that no additional reflective cavity is required when forming the array, which greatly helps to simplify and lighten the entire array system. In this paper, we enhance the FB ratio of the antenna to 35 dB while maintaining an ultra-low profile of 0.05 λ0 by attaching the broadband polarization conversion metasurfaces (PCMs) as the parasitic patches to the surface of the radiating patch. Meanwhile, the array formed by the proposed antenna is arranged in a checkerboard form, which makes it have a lower radar cross section (RCS) in the X- and Ku- bands. Even with PCMs loaded, the antenna element maintains a small size of 0.58 λ0 × 0.58 λ0, which ensures the proposed array can achieve the ± 45° beam scanning, making it suitable for the phased array. For verification, we propose a low-sidelobe array composed of the proposed antenna elements, which exhibits a low profile, high FB ratio, and broadband RCS reduction through simulation and measurement.

A Triple-Band Reflective Polarization Conversion Metasurface with High Polarization Conversion Ratio for Ism and X-Band Applications.

A compact and triple-band polarization converting reflective type metasurface (PCRM) with a high polarization conversion ratio (PCR) is proposed for strategic wireless antenna-integrated applications. The unit cell of the metasurface is composed of S- and G-shaped patches separated with a parasitic gap and the grounded via is connected to the full ground plane. The unit cell is etched on an FR4 substrate (dielectric constant, εr = 4.4, loss tangent, tan δ = 0.02), with compact dimensions of 10 mm3 × 10 mm3 × 1.6 mm3. This structure provides a resonance at 5.2 (ISM), 6.9, and 8.05 GHz (X-band) frequencies. The designed unit cell structure is studied for Transverse Electric (TE)/Transverse Magnetic (TM) incident waves and their responses to the various incident angles. The corresponding PCR is calculated, which shows 92% in the lower frequency band (5.2 GHz), 93% in the second frequency band (6.9 GHz), and 94% in the high-frequency band (8.05 GHz). The total efficiency of the structure shows 83.2%, 62.95%, and 64.6% at the respective resonance bands. A prototype of the proposed PCRM with 3 × 3 unit cells is fabricated to validate the simulated results. The experimental data agrees with the simulation results. The compactness, triple-band operation with a high PCR value of more than 92% makes use of the designed metasurface in wireless antenna-integrated applications at ISM and X-bands.

A novel circularly polarized patch antenna based on polarization conversion metasurface for radiation and scattering performance improvement

A new design based on sequential-rotation arranged polarization conversion metasurfaces (PCMs) is proposed for reducing backscattering and improving radiation performance of circularly polarized (CP) patch antennas. The proposed antenna is composed of a conventional corner-truncated patch antenna loaded with a PCMs. The configuration of the PCMs increases the operating bandwidth and realized gain, as well as reduce radar cross section (RCS). The relative CP bandwidth (both |S11| < −10 dB and AR <3 dB) is improved to 13.65% due to achieving an additional mode by the PCMs. Also, the peak gain is enhanced by 6.6 dB. Moreover, an in-band and out-of-band RCS reduction is achieved compared with the conventional CP patch antenna due to the polarization conversion characteristic of the PCMs. The maximum RCS reduction is 27.5 dB within the band. This design enables an effective strategy for wideband, high-gain, and low-RCS CP patch antennas. A sample operating at C-band was fabricated and measured, demonstrating a satisfactory match between simulation and measurement results, so verifying the proposed idea.

Low‐profile high‐gain broadband circularly polarized slot antenna with RCS reduction using polarization conversion metasurface

AbstractA novel design of circularly polarized broadband slot antenna with high gain, low profile, and low radar cross‐section (RCS) is proposed by employing polarization conversion metasurface (MS). By introducing sawtooth corner‐truncated square MS with diagonal gaps, a compact circularly polarized antenna composed of 4 × 4 unit cells is presented. It is found that a smaller MS slot antenna can achieve an impedance bandwidth of 42.9% (3.7–5.72 GHz) and a 3‐dB axial ratio (AR) bandwidth of 22.8% (4.55–5.72 GHz). Simultaneously, the MS can realize polarization conversion. The MS and its orthogonal units were arranged in a chessboard‐type pattern and applied as the radiator of a larger MS slot antenna, which was fed by a sequentially rotated feeding network. In addition, owing to the anti‐phase cancellation and scattering, the 5‐dB average RCS reduction can be obtained within the frequency range from 3.8 to 6.9 GHz. For verification, the proposed larger circularly polarized antenna was fabricated and tested. The operating bandwidth was measured as 44% (3.78–5.91 GHz) with a 3‐dB AR bandwidth of 39.6% and a high gain of 10.3 dBic. Furthermore, the larger antenna produces a useful RCS reduction from 3.7 to 6.7 GHz. The simulated and measured results are in good agreement.

A dual-broadband dual-function polarization converter based on reflective metasurface

In this paper, we introduce a dual-broadband dual-function polarization conversion metasurface, which can convert a linearly polarized (LP) incident electromagnetic wave into a cross-polarized LP wave and a circularly polarized wave, effectively, in different bands. The numerical simulations and experiments show that the metasurface can be used as a high-efficiency cross-polarization converter from 5.50 to 12.4 GHz, where the polarization conversion ratio is greater than 0.9 with a fractional bandwidth of 77.1%. Simultaneously, it can convert the LP incident wave into a right-handed circularly polarized wave from 13.50 to 19.75 GHz, where the axis ratio is lower than 3 dB and the fractional bandwidth is 37.4%. A honeycomb is used as a support layer instead of air gaps, which improves the efficiency of cross-polarization conversion and broadens the bandwidth of linear to circular conversion. To reveal the conversion mechanism of the proposed polarization converter, the surface current is analyzed. Finally, we fabricated and tested it, and the experimental results are in line with the simulation results.

Dual-Linear-to-Circular Polarization Converter Based Polarization-Twisting Metasurface Antenna for Generating Dual Band Dual Circularly Polarized Radiation in Ku-Band

We propose a polarization-twisting metasurface antenna (PTMA) to generate the dual-band dual-circularly polarized (CP) radiation by integrating a dual-linear-to-circular polarization converter (DLCPC) with a linear polarization conversion metasurface (LPCM) in the Ku-band. The DLCPC possesses the functionality of polarization selection and is capable of transforming the linearly polarized (LP) waves into right hand CP (RHCP) waves in low-frequency (LF) and left hand CP (LHCP) waves in high frequency (HF). Based on the DLCPC, the dual-band dual-CP PTMA is designed and fabricated, and then, the measured results indicate that the PTMA is capable of realizing the RHCP radiation in LF and the LHCP radiation in HF. In the meanwhile, the common bandwidths of −1.5 dB gains and 3 dB axial ratios are 12.25–12.95 and 14.00–14.55 GHz, respectively. Our design, by integrating the DLCPC and LPCM to construct the dual-band dual-CP PTMA, should, thus, pave the way for building up more efficient wireless communication systems with the requirement of different CPs as the receiver and the transmitter at downlink and uplink in the Ku-band.

A broadband and low‐profile magnetoelectric dipole antenna with low radar cross section

AbstractIn this study, a novel broadband and low profile magnetoelectric dipole (MED) antenna with a low radar cross section (RCS) is proposed. The 10 dB RCS reduction is achieved by shifting the reflected energy to six directions, which is an efficient optimization method based on convolutional operations. Specifically, the polarization conversion metasurface (PCM) unit with high polarization conversion rate is constructed and employed in the MED antenna for RCS reduction. The proposed antenna is fabricated and measured. Measured results depict that the low RCS antenna offers good impedance matching in 12–15.6 GHz (relative bandwidth 26%). Compared with the reference antenna, the RCS of the proposed antenna is reduced by more than 10 dB from 9.9 to 16 GHz. It is proved that the proposed antenna has broadband and stable radiation performance and outstanding specular reflecting suppression capabilities.

High-Resolution Range Profile Feature Transformation Method Based on Amplitude-Phase Modulation Metasurface

Time-modulated metasurfaces have attracted much attention due to their flexible manipulation of electromagnetic waves. Time-varying response leads to spread-spectrum and reallocation of energy. Thus, an amplitude-phase joint modulation (TAJM) method is proposed based on a time-modulated active polarization conversion metasurface (APCM). Taking radar high resolution range profile (HRRP) as the object, the distance transformation and multi-target phenomena are further discovered. The proposed method achieves a flexible manipulation of target amplitude and phase. The relationship between the modulation parameters and HRRP feature is studied. It is revealed that the absorption depth is the dominant factor for the energy distribution of TAJM. The method can be applied to multi-target generation and target feature manipulation.

An X-Band Reflective Active Polarization Conversion Metasurface

In this paper, an active polarization conversion metasurface (APCM) operating in the X-band (8–12 GHz) is presented. Active unit cells consist of three metal layers and two substrate layers. Anisotropic metal patches, positive-intrinsic-negative (PIN) diodes, and bias lines are integrated into the surface layer to form a chiral structure. A simple and efficient polarization conversion function is realized in this designed structure. The operating mode is switched between linear-to-linear (LTL) polarization conversion and linear-to-circular (LTC) polarization conversion by switching the loaded PIN diode. In the LTL polarization conversion state, the polarization conversion ratio of APCM is over 90% in X-band. Reflected wave circular polarization axis ratio &lt;3 dB in 8–12 GHz when in LTC state. The proposed structure is verified by experimental measurements.

Optically-Controlled Terahertz Multifunctional Polarization Conversion Metasurface with Reflection and Transmission Modes.

In the terahertz band, how integrating multiple functions into a device with a tiny unit structure is a challenge. In this paper, an optically-controlled multifunctional linear polarization conversion metasurface working in the terahertz band is proposed. The reflection and transmission polarization conversion functions can be realized by irradiating the metasurface with pump light with different wavelengths. The metasurface is designed with a multilayer structure, and a photosensitive semiconductor alone is used to control multiple functions, which makes the manipulation of multifunctional devices easy. When the photosensitive semiconductor germanium (Ge) and silicon (Si) are in different states, the metasurface can realize broadband reflection and transmission polarization conversion functions, the corresponding relative bandwidth are 102.4% and 98.9%, respectively, and the work efficiency can be regulated by pump light with different intensity and wavelength. In addition, the working principle of the metasurface is analyzed by eigenmode theory and surface current distributions. The stability of the metasurface to structural parameters and incident angles are discussed.