Full-polarization reconfigurable metasurface for broadband and versatile modulation of electromagnetic waves

Zuntian Chu; Xinqi Cai; Jie Yang; Tiefu Li; Huiting Sun; Fan Wu; Ruichao Zhu; Tonghao Liu; Yajuan Han; Jiafu Wang

doi:10.1364/PRJ.533473

1. INTRODUCTION

Controlling the multi-dimensional electromagnetic (EM) fields on demand, including amplitude, phase, polarization, and transmission mode of EM waves [1], while combining various functional microwave devices, such as amplifiers, attenuators, modulators, and polarization converters/multiplexers, into an electronic integrated circuit is of great significance to achieve versatile microwave chips [2,3]. Generally, mature microwave devices are based on numerous and redundant radio frequency (RF) modules and multilayer electronic configurations, which have limitations in bandwidth and functionalities and suffer from low efficiency and lack of complete regulation of spatial propagating wave. Fortunately, with the rapid development of micro–nano fabrication technology, metasurfaces [4], ultrathin metamaterials composed of planar periodic or aperiodic arrangements of artificially designed subwavelength elements, have attracted in-depth attention and appeared as a powerful and flexible platform for manipulating EM waves. Given the merits of low profile, easy fabrication, and exotic EM properties, metasurfaces have engineered kaleidoscopic functionalities and shown fascinating applications, containing achromatic lenses [5,6], invisibility cloaks [7], beam deflectors [8,9], vectorial holograms [10 –12], and orbital angular momentum generators [13,14]. More interestingly, by introducing the concept of chirality-assisted phase and phase-corrected gradient and synthesizing with propagation phase and geometric phase, metasurfaces can be designed to achieve extreme beam diffraction management and full-polarimetric hologram encryption [15 –17]. To date, the metasurfaces have experienced the evolution procedure from passive metasurfaces to active (reconfigurable) metasurfaces. Compared with passive metasurfaces, reconfigurable metasurfaces enable real-time EM waves modulation and digital information processing. More encouragingly, the emergence of active alteration mode tremendously enhances the extensibility in practical applications and the perception of environment for metasurfaces, meeting the urgent needs of the current highly compact and controllable system.

According to different external factors applied, the reconfigurable metasurfaces can be roughly divided into electric control, mechanical control, temperature control, and multi-physical control, on the basis of which some researchers have gotten down to tuning versatile EM waves. For electric control, graphene [18], varactor diodes [19,20], and liquid crystals [21] can be loaded into the meta-particles, and the absorption efficiency, operation frequency, and resonant phase can be adjusted via changing the voltage. With regard to mechanical control, the interval between basic meta-particles can be dynamically transformed through axial stress to convert the polarization and prune the wavefronts [22,23]. In light of multi-physical regulation, multispectral information detection and conversion and microwave transmission are dexterously compounded to establish hybrid wireless communication systems [24,25]. Among them, the metasurfaces loaded with diodes driven by electric supply are widely used attributed to simplicity and celerity and also accompanied by increasement of degrees of freedom (DoFs) [26]. However, the control DoFs of reconfigurable metasurfaces are intimately related to the number and type of active components. As the control DoFs of the reconfigurable metasurfaces increase, the structure complexity and the number of devices are also increasing. Therefore, the trade-off between them directly determines further development of reconfigurable metasurfaces [27]. In other words, multi-mode and full-polarization EM manipulation empowered by single controllable components and meta-particles is of great interest for the miniaturization and integration of systems.

In 2014, the concept of digital coding and reprogrammable metasurfaces was proposed [28], which constructs a bridge linking the physical world and digital world. Subsequently, considerable reconfigurable metasurfaces based on amplitude-coding [29] or phase-coding modulation [30] have sprung out to control the energy distributions or wavefront shapes. Making efficient energy management with low longitudinal profile, some amplitude-controlled reconfigurable metasurfaces provide an effective prescription [31] to adjust the reflected/refracted waves or endow energy conversion between transmission, reflection, and absorption [32] but have difficulty switching the polarization states of EM waves. By contrast, phase-controlled reconfigurable metasurfaces depend on altering the phase delay between orthogonally polarized waves to achieve polarization conversions with multiple inputs and multiple outputs [33], yet rarely take account of energy, suffering from the risk of radiation pollution. In addition, the amplitude-phase-joint-coding reconfigurable information metasurfaces are successively excavated for polarization-modulated encryption wireless communications [34,35], and time-varying metasurfaces are also fabricated to introduce additional amplitude ratio and phase differences between anisotropic meta-particles to transform full Poincaré spheres [36,37]. It should be noted that the aforementioned meta-devices are operated in the form of orthogonal linearly polarized (LP) and not circularly polarized (CP) combinations as attempts, narrowing the intrinsic bandwidth and requiring complicated modulators and networks. Actually, amplitude-controlled reconfigurable metasurfaces facilitate energy harvest and continuous polarization switching and are consistent with phase-modulated polarization while maintaining other parameters unchanged from an equivalent perspective, which ameliorates polarization agility and multi-scenario adaptability [38]. Therefore, rendering simultaneously continuous full-polarization conversion and mode transition in amplitude-controlled operational modality with simplistic framework is worthy of expectation, but also challenging.

To address this problem, herein, we propose an electronically controlled full-polarization reconfigurable metasurface (FPRM) for broadband and versatile modulation of EM waves. Circumventing the harsh phase-modulated reconfigurable metasurfaces, dual-mode routing ingeniously utilizing amplitude modulation is employed to alter the energy distributions of co-polarized reflected waves, showcasing the reconfigurability of the continuous state range and the rapid real-time responses of multiple bias states. Two identical positive–intrinsic–negative (PIN) diodes are individually embedded into split-ring metallic resonators at mirror-symmetrical positions, and the proposed meta-particles can dynamically produce co-polarized waves the same as both CP incident states with different energy values via the alterable external voltages controlled through field-programmable gate array (FPGA) and further introduce electric dipole loss on the structural surface while nearly keeping phase shift consistency. Accordingly, capitalizing on Stokes formula analysis, an arbitrarily polarized wave is regarded as the linear superposition of this orthogonal basis vectors; thus, the diversified EM modulation modes under full-polarization states can be modulated, as schematically illustrated in Fig. 1. As verification, the prototype with $0.1 λ_{0}$ thickness is fabricated and measured to demonstrate our design. The measured results indicate that the FPRM can continuously switch from total reflection to polarization conversion and to total absorption in the frequency range of 6 to 8.6 GHz. Such a robust and integrated reconfigurable metasurface with a single configuration in controlling versatile EM waves with unprecedented availability and DoFs is expected to open a new perspective for constructing multifunctional stealthy devices and intelligent detection systems.

Figure 1.Conceptual illustration of the FPRM and its function demonstrations under different operating types. The bias voltages of the PIN diodes embedded into the reconfigurable meta-particle are controlled via FPGA, which can realize synchronous amplitude modulation with almost fixed phase in orthogonal CP channels. First, in both the ON and OFF states of two PIN diodes, arbitrarily polarized incident waves can be completely reflected and absorbed, respectively. In addition, when the PIN diodes are in transition states, the incident polarized waves can be converted to other polarized waves. The built-in zoom-in-view inset illustrates the detailed structure of the reconfigurable meta-particle in the dashed box. The height of the cuboid represents the amplitude of reflection, and the orange cuboids represent the invariant phase of $θ$ .

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2. META-PARTICLE DESIGN AND PRINCIPAL ANALYSIS

As a practical implementation in the microwave frequency band, an anisotropic spin-decoupled reconfigurable meta-particle was delicately designed for constructing the FPRM to achieve anticipatory EM property modulation. A schematic of the meta-particle is illustrated in Fig. 2(a), which can simultaneously and independently tailor the responses of two orthogonal CP incident waves. The meta-particle is arrayed in a square periodic lattice spaced at $P = 12 mm$ and consists of three metallic copper layers (electric conductivity $σ = 5.8 \times 10^{7} S / m$ , thickness $t = 0.02 mm$ ) separated by two F4B dielectric layers (loss tangent $\tan σ = 0.001$ , dielectric constant $ε_{r} = 2.65$ , upper thickness $h_{1} = 4 mm$ , lower thickness $h_{2} = 0.25 mm$ ). To clearly describe the design of the meta-particle structure, the topological layouts are disintegrated and the sectional views are shown in Fig. 2(b). The top metallic layer [top right panel of Fig. 2(b)] comprising a split-ring resonator and three quadrate patches is symmetrically patterned to exhibit mirror symmetry with respect to the $y$ axis and loaded with two identical electrically driven PIN diodes (BAP70-02, NXP, Holland) into the same position of left and right arms, such that the LCP and RCP responses can be tuned independently by changing the resistance of the PIN diodes in corresponding rotational directions. Note that in one meta-particle, three 36 nH chip-inductors are elaborately welded between the direct current (DC) power lines and metallic pattern, the role of which is taken as radio frequency chokes for the sake of blocking alternating current (AC) but allowing DC paths and then reducing coupling between each other. With two subtracted circular areas, the second metallic pattern [lower left panel of Fig. 2(b)] is connected to the top metallic quadrate patch $P_{I I}$ by a metallized blind hole Via II, working as the negative electrode to provide a DC bias voltage. This layer is also utilized as the grounded plane to block transmission. The third metallic layer [lower right panel of Fig. 2(b)] contains two quadrate patches and two bias lines. The bottom two quadrate patches embedded on two bias lines are connected to the top metallic quadrate patches $P_{I}$ and $P_{I I I}$ by two metallized through holes Via I and Via III isolated from the second metallic layer and used as the positive electrode to provide a DC bias voltage. Thereinto, the outer and inner radii of the split-ring resonator are separately $r_{1} = 5 mm$ and $r_{2} = 4 mm$ , and the other parameters shown in the meta-particle are $r_{3} = 0.7 mm$ , $r_{4} = 0.4 mm$ , $w_{1} = 1.1 mm$ , $w_{2} = 1.6 mm$ , $w_{3} = 0.6 mm$ , $w_{4} = 0.75 mm$ , $w_{5} = 1.1 mm$ , $w_{6} = 1.1 mm$ , $l_{1} = 1.4 mm$ , $l_{2} = 1.1 mm$ , $l_{3} = 1.1 mm$ , $g_{1} = 0.2 mm$ , and $α = 90 °$ . The design of the square metallic patches is to facilitate the soldering and feeding of the PIN diodes and meet the needs of prototype preparation. The equivalent circuit of the adopted PIN diodes is shown in Fig. 2(c); it is a series connection of a fixed inductor ( $L = 1.2 nH$ ) and the parallel connection of a fixed capacitor ( $C = 0.2 pF$ ) and a variable resistor ( $R_{D} = 210 - 1 M Ω$ ), in which the resistance of the variable resistor in the simulation procedure is controlled by the external bias voltage in the measurement process. Herein, we define the variable resistance of PIN diodes I and II as $R_{D 1}$ and $R_{D 2}$ .

Figure 2.Schematic diagram of the meta-particle and its amplitude and phase responses. (a) Perspective view of the meta-particle. (b) Sectional view of the meta-particle with geometric parameters. (c) Equivalent circuits of the PIN diode. Simulated (d) co-polarized reflection amplitude $r_{L L}$ , (e) co-polarized reflection phase $φ_{L L}$ , (f) cross-polarized reflection amplitude $r_{R L}$ , and (g) absorption $A_{L}$ when $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ at LCP normal incidence. Simulated (h) co-polarized reflection amplitude $r_{R R}$ , (i) co-polarized reflection phase $φ_{R R}$ , (j) cross-polarized reflection amplitude $r_{L R}$ , and (k) absorption $A_{R}$ when $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ at RCP normal incidence. Simulated co-polarized (l) reflection amplitude $r_{L L}$ and (m) phase $φ_{L L}$ when $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ at LCP normal incidence. Simulated co-polarized (n) reflection amplitude $r_{R R}$ and (o) phase $φ_{R R}$ when $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ at RCP normal incidence.

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To better understand the physical mechanism of the amplitude control, we made a theoretical analysis for the incidence and reflection of EM waves passing through the proposed reconfigurable meta-particle based on a complex Jones matrix. Subsequently, the absorption for both incident waves is deduced, which provides the basis and guidance for the following polarization conversion design. The thickness of the meta-particle is much smaller than the working wavelength, which can be neglected in theoretical analysis. Thus, when two orthogonal CP waves are vertically incident, taking accounting of the opposite propagation direction between the incident and reflected waves, the relationship between the complex amplitude profiles of the reflected electric field and the incident electric field can be written via Jones matrix $R_{cir}$ representing CP basis as $(\begin{matrix} E_{L}^{r} \\ E_{R}^{r} \end{matrix}) = R_{cir} (\begin{matrix} E_{R}^{i} \\ E_{L}^{i} \end{matrix}) = (\begin{matrix} R_{L R} & R_{L L} \\ R_{R R} & R_{R L} \end{matrix}) (\begin{matrix} E_{R}^{i} \\ E_{L}^{i} \end{matrix}),$ (1)where $E_{L / R}^{i}$ ( $E_{L / R}^{r}$ ) denote input (output) electric field of LCP and RCP waves and $R_{L L} = r_{L L} \cdot e^{i φ_{L L}}$ ( $R_{R R} = r_{R R} \cdot e^{i φ_{R R}}$ ) and $R_{R L} = r_{R L} \cdot e^{i φ_{R L}}$ ( $R_{L R} = r_{L R} \cdot e^{i φ_{L R}}$ ) are co- and cross-polarized complex reflection coefficients of LCP (RCP) waves, respectively. The absorption for LCP and RCP incident waves can be calculated as follows: $A_{L} = 1 - R_{R L} - R_{L L} = 1 - {| r_{R L} |}^{2} - {| r_{L L} |}^{2},$ (2) $A_{R} = 1 - R_{L R} - R_{R R} = 1 - {| r_{L R} |}^{2} - {| r_{R R} |}^{2} .$ (3)

In light of Eqs. (2) and (3), the absorption of LCP and RCP incident waves is respectively related to $r_{R L}$ , $r_{L L}$ and $r_{L R}$ , $r_{R R}$ , which means that the more the cross- and co-polarized reflectivity is reduced, the more the absorption is enhanced.

By means of employing field monitors in the simulation software, the surface current and energy loss on the top metallic pattern surface are recorded at 7 GHz under different $R_{D}$ under the illumination of LCP and RCP waves, respectively, which reveal the interaction between the incident waves and the loaded PIN diodes. From Figs. 3(a)–3(d), it can be observed that under the irradiation of LCP waves, when the value of $R_{D 1}$ gradually decreases from $1 M Ω$ to $210 Ω$ while the value of $R_{D 2}$ remains fixed at $1 M Ω$ , the oscillating surface current is enhanced significantly on the left arm and is immensely weak on the right arm. Likewise, under the irradiation of RCP waves, the surface current on the right arm also strengthens as the value of $R_{D 2}$ gradually decreases from $1 M Ω$ to $210 Ω$ when keeping the value of $R_{D 1}$ fixed at $1 M Ω$ , as depicted in Figs. 3(e)–3(h). Remarkably, the surface currents on both arms oscillate in the same direction as the handedness of the corresponding incident waves. Therefore, surface currents on each arm of the split-ring resonator can be equivalent to single electric dipole, which effectively change the equivalent permittivity and permeability of the meta-particle. In this case, the transformation of surface current from weak to strong results in improved impedance matching between the meta-particle and free space. Meanwhile, the introduced ohmic loss of the incident waves will also be reinforced. As verification, the energy loss distributions are provided in Figs. 3(i)–3(p), which demonstrate the incident waves are both aggravatedly dissipated as the values of $R_{D}$ decrease and also basically validate the tendency of absorption shown in Fig. 2.

Figure 3.Simulated surface current and energy loss distributions of the proposed meta-particle. Surface current distributions of the meta-particle under the normal incidence of (a)–(d) LCP waves at 7 GHz when $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ and (e)–(h) RCP waves when $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ , respectively. Energy loss distributions of the meta-particle under the normal incidence of (i)–(l) LCP waves when $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ and (m)–(p) RCP waves at 7 GHz when $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ , respectively. The color depth and direction of the red arrows represent the surface current intensity and flow direction, respectively.

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3. VERSATILE EM MODULATION UNDER FULL-POLARIZATION WAVE EXCITATION

The above part of the presentation only illustrates the simultaneous and independent control of LCP and RCP illuminated waves via the meta-particle. For the intention of verifying the universality and multiplicity of the designed meta-particle, in this part, we proceed to exhibit EM modulation under full-polarization wave excitation, including functionality switching among perfect reflection, polarization conversion, and perfect absorption. Aiming at arbitrary polarization state $| n ⟩$ of the incident EM waves, we can decompose that into the superposition of the orthogonal polarization states (in this paper, i.e., LCP and RCP states) with different amplitude and phase, as described as $| n ⟩ = η_{L} e^{i δ_{L}} | L ⟩ + η_{R} e^{i δ_{R}} | R ⟩,$ (4)where $| L ⟩$ and $| R ⟩$ represent LCP and RCP states, respectively; $η_{L}, η_{R}$ and $δ_{L}, δ_{R}$ are the amplitude and phase coefficients of two CP waves. Consequently, the ellipticity angle $χ$ and azimuth angle $ψ$ of polarization ellipse corresponding to the polarization state $| n ⟩$ can be derived based on vectorial decomposition and Stokes–Cartesian coordinate transformation as $χ = \frac{1}{2} \arcsin (\frac{η_{R}^{2} - η_{L}^{2}}{η_{R}^{2} + η_{L}^{2}}), ψ = \frac{δ_{R} - δ_{L}}{2} .$ (5)

In this way, an arbitrary full-polarization DOF, including the orientation, ellipticity, and chirality of the reconstructed field, is determined via deducting the amplitude ratio $η_{R} / η_{L}$ and phase retardation $δ_{R} - δ_{L}$ . On the basis of adopting co-polarized reflection components of two CP basis vectors, the solution to generate an output wave with a polarization state of ( $χ$ , $ψ$ ) can be obtained: $\frac{| η_{R} |}{| η_{L} |} = \frac{| r_{R R} |}{| r_{L L} |}, δ_{R} - δ_{L} = φ_{R R} - φ_{L L} .$ (6)

Herein, without loss of generality, we then engineer $x$ - and $y$ -polarized incident waves as examples, which can be equivalent to excitation sources of equal-amplitude LCP and RCP waves with in-phase and out-of-phase superposition. Then, via altering the values of both $R_{D 1}$ and $R_{D 2}$ to $1 M Ω$ and $210 Ω$ , i.e., the working states of PIN diode II and PIN diode I are controlled from OFF to ON state, two LP incident waves separately achieve high co-polarized reflection and absorption, both peaks of which exceed 0.98, as shown in Figs. 4(a)–4(d). Compared with the simulated results of the reflection, the simulated absorption of two LP incident waves is not identical, simultaneously accompanied by the different co-polarized reflection amplitudes. The main reason might be that the cross-polarized reflection amplitudes are almost identical with the co-polarized reflection amplitudes of CP basis vectors for the designed meta-particle and cannot be neglected in this case, making inconsistency of the synthetized output LP waves. In fact, for arbitrarily polarized incident waves, the designed meta-particle in this study can achieve reflection and absorption to some extent through synchronous amplitude control of orthogonal CP basis vectors.

Figure 4.Versatile functional demonstration of the proposed meta-particle under full-polarization wave excitation. Simulated co-polarized, cross-polarized reflection amplitudes and absorption of the meta-particle under (a) $x$ -polarized or (b) $y$ -polarized excitation when $R_{D 1} = R_{D 2} = 1 M Ω$ . Simulated co-polarized, cross-polarized reflection amplitudes and absorption of the meta-particle under (c) $x$ -polarized or (d) $y$ -polarized excitation when $R_{D 1} = R_{D 2} = 210 Ω$ . (e) Sketch of the Poincaré sphere representing some exemplary states of linear, circular, and elliptical polarizations and (f) mapping of the ellipticity angle $χ$ and the polarization azimuth angle $ψ$ on corresponding polarization ellipse. (g) Simulated amplitude ratio $r_{L L} / r_{R R}$ and phase difference $φ_{L L} - φ_{R R}$ when $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ under CP normal incidence at 7 GHz. (h) Simulated amplitude ratio $r_{R R} / r_{L L}$ and phase difference $φ_{R R} - φ_{L L}$ when $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ under CP normal incidence at 7 GHz. Calculated ellipticity angle $χ$ and polarization azimuth angle $ψ$ when (i) $R_{D 1}$ is varied and $R_{D 2}$ is fixed at $1 M Ω$ and (j) $R_{D 2}$ is varied and $R_{D 1}$ is fixed at $1 M Ω$ under CP normal incidence at 7 GHz. (k) Polarization ellipses of the reflected waves at different values of $R_{D 1}$ when $R_{D 2}$ is fixed at $1 M Ω$ under $x$ -polarized excitation.

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Then, in order to provide an intuitive illustration of the polarization states of EM waves and the polarization evolution between different states, the polarization space (Poincaré sphere) is plotted and displayed in Fig. 4(e). Each point on the surface of the sphere represents a full-polarization state, whereas the point inside the sphere represents a partial polarization state. In most cases, an arbitrary point on the sphere surface can be represented by its latitude $2 χ$ and longitude $2 ψ$ , corresponding to the polarization state with an ellipticity angle $χ$ and azimuth angle $ψ$ in real space, as exhibited in Fig. 4(f). The two axes represent the two orthogonal components of the electric field. Figures 4(g) and 4(h) separately show the variation of amplitude ratio $η_{L} / η_{R}$ and phase retardation $δ_{L} - δ_{R}$ when the value of $R_{D 1}$ gradually increases from $210 Ω$ to $1 M Ω$ while the value of $R_{D 2}$ is fixed at $1 M Ω$ and the variation of amplitude ratio $η_{R} / η_{L}$ and phase retardation $δ_{R} - δ_{L}$ when the value of $R_{D 2}$ gradually increases from $210 Ω$ to $1 M Ω$ while the value of $R_{D 1}$ is fixed at $1 M Ω$ both under CP normal incidence. According to Eq. (5), the results shown in Figs. 4(i) and 4(j) can be obtained. The above derivation shows that when the resistance values of PIN diodes on the arms in line with spin direction change, the calculated ellipticity angle $χ$ and azimuth angle $ψ$ at 7 GHz can be separately tuned from 39.5° and 26.7° to 0° and from $- 39.5 °$ and $- 26.7 °$ to 0°. Furthermore, the large-scale change of ellipticity angle $χ$ and the small-scale change of azimuth angle $ψ$ also verify the wide range of variable amplitude and the almost constant phase under orthogonal CP incidence. Hence, when an $x$ -polarized wave is vertically incident onto the meta-particle, the polarization ellipses of the reflected waves at different values of $R_{D 1}$ when $R_{D 2}$ is fixed at $1 M Ω$ can be extracted, as denoted in Fig. 4(k). In particular, under the above condition, when the value of $R_{D 1}$ is $210 Ω$ , the reflected wave is approximately a CP wave. When the value of $R_{D 1}$ is $1 M Ω$ , the reflected wave is an LP wave. When the value of $R_{D 1}$ varies between $210 Ω$ and $1 M Ω$ , the reflected wave turns into a mutable elliptical wave.

4. FABRICATION AND MEASUREMENT

Eventually, an elaborate proof-of-concept prototype including $36 \times 36$ meta-particles of the same size as the simulation is fabricated to experimentally demonstrate the versatile EM modulation under full-polarization wave excitation, as illustrated in Figs. 5(a) and 5(b). The prototype proposed in this study is fabricated via utilizing the printed circuit board (PCB) etching technique. In the fabrication process, the reconfigurable metasurface is integrated with three pieces of F4B dielectric slabs and two metallic copper layers, in which the required PIN diodes and chip-inductors are welded at the corresponding positions of the prototype. Subsequently, the meta-particles are in parallel along the $y$ direction, and each adjacent meta-particle in a column is connected to the bias lines on the bottom layer via metallized through holes. Then, both the upper and lower sides of the prototype leave a space of 20 mm, respectively, to inset pin headers for connecting the bias lines to positive and negative electrodes of the external power supply. Hence, the entire sample size is approximately $432 mm \times 472 mm$ . As presented in Fig. 5(c), the reflection coefficient experiments of the prototype are implemented in the microwave anechoic chamber, and the experimental setups consist of the vector network analyzer (VNA), the C-band CP horn antennas, the C-band LP horn antennas, the DC voltage supply, and the fabricated prototype. In one pair of antennas, one is used as the transmitting antenna and the other is used as the receiving antenna. Simultaneously, the two horn antennas are connected to the two ports of the VNA to obtain visualized analysis results. When setting up the antennas, the transmitting antennas and receiving antennas are symmetrically placed deviating about 6° from the normal direction of the prototype and put at the same height as the prototype center, so that the reflected waves are more effectively received.

Figure 5.Experimental characterization of the FPRM. (a) Front view and (b) bottom view of the fabricated FPRM’s photographs, where the insets exhibit $3 \times 3$ local meta-particles. (c) Experimental setup in microwave anechoic chamber. Measured (d) co-polarized reflection amplitude $r_{L L}$ , (e) co-polarized reflection phase $φ_{L L}$ , (f) cross-polarized reflection amplitude $r_{R L}$ , and (g) absorption $A_{L}$ when $V_{D 1}$ is varied and $V_{D 2}$ is fixed at 0 V at LCP normal incidence. Measured (h) co-polarized reflection amplitude $r_{R R}$ , (i) co-polarized reflection phase $φ_{R R}$ , (j) cross-polarized reflection amplitude $r_{L R}$ , and (k) absorption $A_{R}$ when $V_{D 2}$ is varied and $V_{D 1}$ is fixed at 0 V at RCP normal incidence. Measured CP reflection amplitudes when (l) $V_{D 1} = 0 V$ and $V_{D 2} = 0 V$ and (m) $V_{D 1} = 0 V$ and $V_{D 2} = 0.86 V$ under $x$ -polarized excitation or when (n) $V_{D 1} = 0 V$ and $V_{D 2} = 0 V$ and (o) $V_{D 1} = 0.86 V$ and $V_{D 2} = 0 V$ under $y$ -polarized excitation.

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Figures 5(d)–5(k) show the measured results under orthogonal CP incidence, in which the applied controlled voltages $V_{D 1}$ and $V_{D 2}$ are utilized to control the working states of PIN diodes I and II. It can be observed that when $V_{D 2}$ is fixed at 0 V (PIN diode II works in OFF state) and $V_{D 1}$ gradually increases from 0 V to 0.86 V (PIN diode I works from OFF state to ON state) in the frequency band of 6 to 8.6 GHz under LCP incidence, the co-polarized reflection amplitude $r_{L L}$ continuously decreases with almost invariant co-polarized reflection phase $φ_{L L}$ , which is basically consistent with the simulated result displayed in Fig. 2. In particular, the co-polarized reflection amplitude $r_{L L}$ reaches maximum of 0.95 at 7 GHz when $V_{D 1} = 0 V$ , but the co-polarized reflection phase $φ_{L L}$ appears with fluctuation of about 70° when $V_{D 1}$ changes from 0 V to 0.86 V. In addition, the cross-polarized reflection amplitude $r_{R L}$ remains at a low level; thereby, the calculated absorption of LCP incident waves increases with the enhanced $V_{D 1}$ with the maximum of 0.92 at 7.9 GHz. Similar measured results can be noticed under RCP incidence when $V_{D 1}$ is fixed at 0 V (PIN diode I works in OFF state) and $V_{D 2}$ gradually increases from 0 V to 0.86 V (PIN diode II works from OFF state to ON state) due to spin-insensitivity of the meta-particle.

Combined with the theoretical analysis of Eqs. (4 )–(6), such amplitude and phase control performance of the meta-particle for orthogonal CP waves can be utilized to tune the output polarization states of the reflected waves under other polarizations such as LP states, as depicted in Figs. 5(l)–5(o). Thereinto, under the normal incidence of the $x$ -polarized wave, when the bias voltages $V_{D 1} = 0 V$ and $V_{D 2} = 0 V$ are employed, the equal-amplitude LCP and RCP reflected waves are achieved. On the contrary, when the bias voltages $V_{D 1} = 0 V$ and $V_{D 2} = 0.86 V$ are employed, the nearly total LCP and zero RCP reflected waves are achieved. Moreover, under normal incidence of the $y$ -polarized wave, when the bias voltages $V_{D 1} = 0 V$ and $V_{D 2} = 0 V$ are employed, the equal-amplitude LCP and RCP reflected waves are also achieved. By contrast, when the bias voltages $V_{D 1} = 0.86 V$ and $V_{D 2} = 0 V$ are employed, the nearly zero LCP and total RCP reflected waves are realized. Thus, in these four cases, LP to LP preservation and LP to CP conversion can be attained, respectively. It is observed that some deviations including frequency shift and imperfect amplitude and phase emerge in the experiment, which might be caused through the imprecise alignment of the experimental setup, the actual volume of the PIN diodes, and inevitable fabrication errors in PCB processing and PIN diode welding. Such issues can be further improved through applying laser collimation and location to establish the experimental platform or utilizing high-precision sample fabrication and multiple experimental measurements to enhance the meta-particle control accuracy. Nevertheless, overall, the experimental results are basically in agreement with the theoretical predictions and indeed verify the diversified EM modulation for arbitrarily polarized waves.

5. CONCLUSION

To sum up, we propose a simplistic framework of full-polarization reconfigurable metasurface with the ability of versatile real-time EM modulation in the broadband frequency band of 6 to 8.6 GHz. By altering the external bias voltage applied to the PIN diodes of the reconfigurable meta-particle, the meta-particle is able to reflect LCP waves and RCP waves into corresponding co-polarized waves attributed to the opposite propagation direction with controllable amplitudes in large scale and almost invariant phase. In addition, the meta-particle can flexibly and individually tailor the energy proportion of orthogonal CP components, which enables the established metasurface to dynamically and continuously switch the functions including reflection, polarization conversion, and absorption at arbitrarily polarized wave incidence. A series of simulations and experiments collectively corroborates the effectiveness of our proposed metasurface. More significantly, our proposed strategy could be scaled up to the entire spectrum, spanning from low frequencies up to optical ones via selecting other appropriate alternative materials, which opens the door for the practical implementations of full-polarization reconfigurable metasurfaces in advanced and highly integrated multifunctional systems. With the ongoing development of science and technology, the proposed full-polarization reconfigurable metasurface might be useful for emerging intersectional domains in information encryption, biomedical science, and chiral discrimination.

Category: Surface Optics and Plasmonics

Received: Jun. 20, 2024

Accepted: Jul. 31, 2024

Published Online: Nov. 27, 2024

The Author Email: Ruichao Zhu (zhuruichao1996@163.com), Tonghao Liu (liutonghaor@163.com), Yajuan Han (mshyj_mail@126.com), Jiafu Wang (wangjiafu1981@126.com)

DOI:10.1364/PRJ.533473

CSTR:32188.14.PRJ.533473