Introduction

Metasurfaces are specially designed two-dimensional surfaces that can manipulate the electromagnetic waves in terms of their phase, amplitude, polarization, and frequency [1]. This is achieved by utilizing an array of resonant elements, such as metallic patches or dielectric particles, which interact with the incoming waves to produce the desired effects. By leveraging this technology, metasurfaces can be utilized to create a wide range of devices, including filters, antennas, and lenses [2,3,4]. On the other hand, a reconfigurable metasurface (RMS) possesses the ability to dynamically adjust its properties based on its surrounding environment. This is made possible through the incorporation of sensors and actuators that monitor and control the resonating components of the metasurface [5]. The potential applications for reconfigurable metasurfaces are extensive and encompass adaptive beamforming, dynamic cloaking, multi-functionality, reconfigurable reflectarrays, smart surfaces, and self-complementary metasurfaces (SCMs) [6]. SCMs possess unique characteristics in which they exhibit self-mirroring properties by maintaining symmetrical arrangements of resonating elements on both sides. Moreover, this property remains unchanged even when the metasurface is rotated by 180° [7]. Consequently, SCMs offer numerous advantages over traditional metasurfaces, including simplified complexity, expanded bandwidth, improved impedance matching, and the capability to alter polarization. A polarization converter modifies the polarization of an electromagnetic wave. Polarization converters find applications in various fields such as optics, telecommunications, and radar [8,9,10].

A genus surface antenna is a type of antenna that utilizes a metasurface, which is a two-dimensional engineered surface composed of subwavelength resonating elements called meta-atoms or unit cells. These resonating elements are designed to manipulate the reflection or transmission of electromagnetic waves allowing genus surface antennas to achieve functionalities such as beamforming, focusing, and polarization control. To improve the re-configurability of genus surface antennas, you can introduce a polarization converter without changing the structure by using mechanisms such as phase shifters, varactors, MEMS switches, or reconfigurable materials like water, graphene, and plasma material [11]. The research field of genus surface antennas is rapidly progressing, with continuous exploration of new applications in telecommunications, aerospace, and healthcare [12, 13]. Plasma material is an ionized gas, which means that some of its atoms have lost electrons and become positively charged ions. The free electrons in plasma give it unique properties such as high electrical conductivity, the ability to emit light, and the ability to interact with electromagnetic fields [14]. Plasma antennas are a unique type of antenna that utilizes ionized gas to emit electromagnetic waves. They have several advantages over traditional antennas, including compactness, Tunable refractive index. Plasma exhibits a unique property where their refractive index can be dynamically adjusted by manipulating the electron density. This allows for real-time control of the antenna’s radiation characteristics, enabling features like beam steering, frequency switching, and adaptive modulation, Ultra-thin and lightweight. Compared to traditional metal antennas, plasmonic metasurfaces are incredibly thin and lightweight, making them ideal for applications where size and weight are critical factors, High efficiency. Plasmonic antennas can achieve high radiation efficiency by effectively converting optical energy into electromagnetic waves. This is particularly advantageous for energy-harvesting or wireless communication applications, efficiency, and tenability. A reconfigurable metasurface reflectarray (RMRA) is an antenna that utilizes an array of metasurface unit cells, each of which can be designed to manipulate the reflection of an electromagnetic wave [15]. By carefully determining the size, shape, and spacing of the unit cells, the behavior of the electromagnetic wave can be effectively controlled. RMRAs offer several advantages over traditional antennas such as a compact size and higher gain compared to conventional antennas, making them well-suited for long-range communication requirements. Furthermore, RMRAs can operate over a wide bandwidth of frequencies, making them versatile antennas that can be utilized in a variety of applications [16].

This paper presents the design of a genus IRS utilizing plasma self-complementary elements for polarization conversion at Ka-band. The polarization conversion properties of the unit-cell are examined through the analysis. LP horn antenna with 8 × 8 genus reconfigurable IRS is investigated. The gain is enhanced and the radar-cross section radiation characteristics of the 8 × 8 genus reconfigurable IRS are discussed. A full-wave simulation based on the finite integral technique is used.

Numerical Results

Plasma-Based Unit Cell Design

The proposed unit-cell element comprises of 2 × 2 sequential rotated self-complementary sub-cells A and B, as illustrated in Fig. 1. The sub-cell configuration comprises of three glass containers in a triangle-rectangular-triangle arrangement. These containers have a relative permittivity εrg = 3.4 and a height, Hc, and are filled with argon gas. The containers are supported by a 1 mm thick dielectric substrate with relative permittivity εrs = 1.1 and a loss tangent tanδ = 0.01. The inert gas within the self-complementary sub-cells undergoes ionization and is transforming into a plasma state as depicted in Fig. 1b. The dimensions of the polarization converter unit-cell are listed in Table 1. To model the dispersive nature of the ionized plasma, the Durde model [14] is employed,

$${\varepsilon }_{r}={\varepsilon }_{o}\left[1-\frac{{\omega }_{p}^{2}}{\omega \left(\omega -j{v}_{p}\right)}\right]$$
(1)

where \({\varepsilon }_{o}\) is free space dielectric constant, \(\omega\) is the angular operating frequency, \({\upsilon }_{p}\) is the angular collision frequency, and \({\omega }_{p}\) is the angular plasma frequency and is given by:

$${\omega }_{p}=\sqrt{\frac{{n}_{e}{e}^{2}}{{m}_{e}{\varepsilon }_{o}}}$$
(2)

where \({m}_{e}\) is the electron mass and e is the electron charge. The Floquet mode simulator is used to estimate the polarization converter characteristics. It assumes an infinite extended array of identical unit-cell elements and employs perfect electric and perfect magnetic boundary conditions along the x- and y-axis. A LP plane wave illuminates the unit-cell along the z-axis. The incident electric field is decomposed into two orthogonal wave components, the vertical (Rxy), and horizontal (Rxx) components as plotted in Fig. 2. The AR of CP bandwidth is calculated from the normalized magnitude (Rxy, Rxx), and their phase difference, using the following equation [10]:

$$AR={\left[\frac{1+{\beta }^{2}+\sqrt{{\left(1-{\beta }^{2}\right)}^{2}+4{\beta }^{2}{{\text{cos}}}^{2}\delta }}{1+{\beta }^{2}-\sqrt{{\left(1-{\beta }^{2}\right)}^{2}+4{\beta }^{2}{{\text{cos}}}^{2}\delta }}\right]}^{1/2}$$
(3)

where \(\beta =\left|Rxx\right|/\left|Rxy\right|\) and \(\delta =\mathsf{a}\mathsf{n}\mathsf{g}\left(Rxx\right)-\mathsf{a}\mathsf{n}\mathsf{g}\left(Rxy\right)\). The CP radiated field is achieved when \(\delta =\frac{n\pi }{2 }\) and β = 1 (n =  ± 1, ± 3, ± 5 …). The relative component magnitude, and phase difference, over the frequency band determine the operating CP bandwidth. In terms of CP, the proposed self-complementary plasma-based unit-cell element has two resonant frequencies. The first resonant frequency is 7.3 GHz, with an AR bandwidth of 200 MHz. The second resonant frequency is at 8.6 GHz, with an AR bandwidth of 1.1GHz, as depicted in Fig. 2c.

Fig. 1
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The detailed structure of the self-complementary polarization converter unit-cell element

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Table 1 Dimensions of self-complementary polarization converter unit-cell element
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Fig. 2
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The frequency response of proposed self-complementary plasma based unit-cell element, a Magnitude of Rxy, and Rxx. b Phase difference between Rxy, and Rxx, c The axial ratio

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Parametric Study of Self-Complementary Plasma-Based Unit-Cell Element

The frequency response axial ratio is observed through a parametric study on the designed parameters of the self-complementary plasma-based unit-cell. The width, W of the rectangular container is changed from 5.5 mm to 7.5 mm while keeping the other designed parameters fixed. At W = 5.5 mm, the AR bandwidth is 1 GHz, and four bandwidths with ARs less than 3 dB at 7.3 GHz, 8.56 GHz, 8.7 GHz and 14.5 GHz, respectively can be obtained when W = 7.5 mm as shown in Fig. 3. The optimal AR bandwidth is 1.15 GHz occurs at W = 6.5 mm. The AR versus the frequency at different E-field polarization angles, Ψ are plotted in Fig. 4a. The AR bandwidth is varying when, Ψ is changed. The AR bandwidth for Ψ = 0° and Ψ = 90° are similar. Figure 4b explores the effect of plasma frequency, fp, on the AR response at a collision frequency υp = 9 GHz. By increasing the plasma frequency, the plasma conductivity is increased and the CP at 9 GHz is lost. For fp = 400 GHz, the minimum AR value is 2.5 dB at 9 GHz, and for fp = 500 GHz, the minimum AR value is 1.34 dB at 9 GHz. The 3-dB AR bandwidth is reduced from 8.1 to 9.25 GHz. The LHCP radiation is achieved when the ionization of the sub-cells A and B are reversed as shown in Fig. 5a. The frequency response of the AR bandwidth for the LHCP is shown in Fig. 5b. The AR bandwidth for the LHCP self-complementary plasma-based unit-cell is 1.2 GHz like the AR bandwidth for the RHCP.

Fig. 3
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The AR bandwidth versus the width (W) of the rectangular container filled with plasma of self-complementary unit-cell

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Fig. 4
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a AR versus the frequency at different E-field polarization angle, and b AR versus the frequency at different plasma frequency

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Fig. 5
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The detailed structure of the PC self-complementary unit-cell for LHCP

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Parametric Study of the Array Size

Different sizes of the array have been considered, as depicted in Fig. 6. The AR response of array sizes 2 × 2, 4 × 4, 8 × 8, and 16 × 16 have been computed and presented in Fig. 7. In the case of the 2 × 2 array, multiple polarization conversion frequencies were observed at 10.8 GHz, 12.3 GHz, and 14 GHz. The 4 × 4 array size exhibited three resonant AR frequencies at 10.3 GHz, 11.9 GHz, and 14 GHz. In case of 8 × 8 array, it displayed two distinct AR bandwidths. The first bandwidth had a width of 2.1 GHz, while the second one resonated at 14 GHz with a bandwidth of 2.5 MHz The 16 × 16 array showcased three resonant AR frequencies at 10.1 GHz, 10.9 GHz, and 14 GHz. The impact of the incident wave angle, θ, on the variation of AR with frequency is graphically represented in Fig. 8. Deviating the incident wave angle from the 0° direction resulted in a reduction of the 3-dB AR bandwidth due to the violation of the CP requirement of the unit-cell element.

Fig. 6
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Different size of arrays made of self-complementary plasma-based unit-cell

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Fig. 7
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Axial ratio of complementary genus intelligent surface versus frequency when changing array size

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Fig. 8
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AR versus frequency at different incident angle of the wave

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Genus Plasma-Based Reconfigurable Intelligent Surface

The 8 × 8 genus reconfigurable intelligent surface is designed to generate LHCP and RHCP signals from its upper and lower sides. Sub-cells A and B are strategically placed on each surface to convert incident waves into LHCP or RHCP electric fields. When illuminated with an LP-plane wave, the scattered field is transformed into CP-waves. At 11 GHz, the 3D RHCP radar cross section of the 8 × 8 genus reconfigurable intelligent surface is depicted in Fig. 9a from the top side, while the LHCP radar cross section is shown in Fig. 9b from the bottom side. Both sides of the genus RIS scatter nearly identical beams. Figure 10 displays the circularly polarized electric field components ER and EL scattered from the array at 11 GHz. The ratio of co-polar to cross-polar level is 20 dB, with a 3 dB half-power beam width of 10°. The frequency responses of the AR of the scattered field from the upper and lower sides of the genus RIS are illustrated in Fig. 11. The AR bandwidth remains consistent at 2.1 GHz for both surfaces.

Fig. 9
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3D radar cross section of 8 × 8 genus intelligent surface at 11 GHz

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Fig. 10
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The RCS from normal incident plane illuminated on 8 × 8 array

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Fig. 11
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The AR versus frequency of 8 × 8 genus intelligent surface

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Characteristic of Horn Loaded by Genus Plasma IRS

A LP horn antenna with dimensions 24 × 12 × 60 mm3 is designed at the Ka-band and loaded with 8 × 8 genus plasma IRS as shown in Fig. 12a. The 3D gain pattern of the loaded horn at 11 GHz is shown in Fig. 12b. A peak gain of 13.2 dBi is obtained with SLL of 6 dB. The back-radiation occurs due to the limited size of the ground plane of the genus surface. The distance between the horn aperture and the IRS is optimized for the large AR bandwidth as shown in Fig. 12c. The genus surface converts the radiated waves from LP-to-CP with AR bandwidth of 500 MHz. However, the radiated beam gain is reduced due to the losses in the plasma-based unit-cell element as appeared in Fig. 12d. The horn antenna shows higher gain compared with the loaded one.

Fig. 12
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The radiation characteristics of horn antenna loaded with 8 × 8 genus IRS

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Characteristic of Horn Antenna Loaded with Genus Reflectarray IRS

To improve the gain with CP conversion characteristics, the plasma IRS unit-cell element is reconfigured to focus the radiated beam into a single directive beam. The frequency response of the reflected coefficient phase of the unit-cell is calculated using a waveguide simulator, employing the image-theory as depicted in Fig. 13a. Figure 13b illustrates the variations of the reflection coefficient phase from 0 to 313° with magnitude variations from − 3 to 0 dB as the plasma frequency changes from 125 to 500 rad/s. Three phases 60°, 180°, and 300° are used to construct quantized IRS reflectarray. Figure 14 illustrates the configuration of a 13 × 13 IRS reflectarray, which was designed using continuous phase values, compared with the arrangement utilizing quantized three phases at 11 GHz. By employing phase quantization, the complexity of the array arrangement is significantly reduced by 98%. Figure 15a shows the 3D gain pattern at 11 GHz, for both continuous and quantized phases. In comparison, the continuous IRS reflectarray exhibits a higher radiated gain of 19.4 dBi, whereas the quantized version achieves 18.5 dBi. Figure 16 plots the frequency response of the AR and gain for the two arrangements. Interestingly, the AR bandwidth remains unaffected, while the gain experiences a reduction of 0.9 dB due to the phase approximation.

Fig. 13
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The characteristics of the plasma reflectarray unit-cell at 11 GHz

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Fig. 14
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The arrangements of the 13 × 13 plasma IRS reflectarray antenna at 11 GHz

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Fig. 15
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The 3D gain patterns of the 13 × 13 plasma IRS reflectarray antenna at 11 GHz

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Fig. 16
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The radiation characteristics of the 13 × 13 plasma IRS reflectarray antenna

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Conclusion

This paper investigates the genus reconfigurable intelligent reflectarray antenna plasma-based self-complementary metasurface use of self-complementary metasurface. The effect of the direction of the electric field and the incident wave angle is introduced. Genus IRS consists of 64-elements designed to convert LP wave into CP with LHCP from upper surface and RHCP from the lower surface. The AR bandwidth for the both polarization is 1.2 GHz. Different IRS sizes are studied. The far-field radiation characteristics gain and RCS of 64-elements IRS are depicted. A peak gain of 13.2 dBi is obtained with SLL of 6 dB. The ratio of co-polar to cross-polar level is 20 dB, with a 3 dB half-power beam width of 10 degrees. The horn antenna shows higher gain compared with the loaded one. The genus IRS gain is enhanced by controlling the plasma ionization inside each element, when a reflectarray is designed. The reflection coefficient phase from 0° to 313° degrees with magnitude variations from -3 dB to 0 dB as the plasma frequency changes from 125 to 500 rad/sec. Three phases 60°, 180°, and 300° degrees are used to quantized IRS reflectarray. In comparison, the continuous IRS reflectarray exhibits a higher radiated gain of 19.4 dBi, whereas the quantized version achieves 18.5 dBi.