Active Metasurface-Based Reconfigurable Polarization Converter With Multiple and Simultaneous Functionalities

We introduce a new concept of three-states polarization converting metasurface (TS-PCM) based on p-i-n diodes. The proposed structure can offer three distinct wave manipulation functionalities: linear polarization to orthogonal linear polarization (LP-OLP), linear polarization to circular polarization (LP-CP), and linear polarization to full reflection (LP-FR) state. Unlike previous structures, the multiple conversion features are simultaneously achieved in different frequency ranges, depending on the operating state of diode (off or on). Finally, a sample prototype was fabricated and the experimental results were verified with the simulated ones.

the operating conversion states (i.e., LP-OLP or LP-CP) could not be selected after fabrication.
To mitigate these issues, switchable polarization converters were introduced [15], [16], [17], [18], [19], [20]. For example, in [15], the operating mode (either LP-OLP or LP-CP) is selected by using a MEMS switch. The design proposed in [16] offers a selection option between LP-CP and no conversion state. Similarly, switchable LP-OLP and LP-FR (FR: full reflection) state is achieved in [17]. In [18], by controlling the reverse bias voltage of varactor diodes, LP-OLP and LP-CP are achieved at 0 V and −19 V, respectively. However, both types of switchable conversion states (LP-OLP and LP-CP) are not achieved simultaneously. Very recently, in [19], by varying the properties of a solid-state plasma LP-OLP and LP-CP conversions have been demonstrated, but the used technology is rather complicated and costly.
In this letter, a three-state polarization converting metasurface (TS-PCM) is presented to achieve LP-OLP, LP-CP, and LP-FR functionalities. In the OFF state of p-i-n diodes, the structure offers two distinct operating modes, i.e., LP-CP and LP-FR. Conversely, when the diodes are switched ON, the device offers two distinct LP-OLP and LP-CP modes. Moreover, switchable LP-CP conversion is also achieved in two different modes at separate frequency bands to enhance the applicability of the device in real scenarios.

A. Geometrical Configuration
The geometry of the proposed TS-PCM consisting of slotloaded rotated rectangular ring is illustrated in Fig. 1, where p-i-n diodes are modeled according to their datasheet/characterization [21]. Two separate cut wires oriented +45 • w.r.t. the x-axis are purposefully included in the unit-cell to introduce an additional capacitive effect. The design of the unit-cell is carefully chosen so that it can also fit with an efficient biasing strategy, by incorporating the bias lines and vias.

B. Working Principle
The unit-cell (see Fig. 1), is simulated in ANSYS HFSS using master/slave boundary conditions and Floquet ports excitation.
It can be observed from Fig. 2(a) that, in the frequency range 5.20-6.10 GHz, the reflection magnitudes r yy and r xy are almost equal to −3 dB. Whereas, the magnitude of r yy is above −1 dB in the frequency range 2-3.66 GHz and 8.42-9.52 GHz, with the magnitude of r xy in the same both range are below −10 dB. These two results indicate that, when both diodes are in the OFF state, the designed TS-PCM generates a CP wave in a single frequency band while working as an LP wave reflector [22] in the two different frequency bands.
For the ON state of the diodes, a −10 dB reduction of r yy is observed [see Fig. 2(b)] in the range 3.77-6.20 GHz, where r xy is above −1 dB throughout the band. Additionally, it can be noted that, in the upper frequency band 7.75-8.60 GHz, the reflection magnitudes r yy and r xy are almost equal (−3 dB), resulting in the generation of a CP wave. This indicates that, when both the diodes are in the ON state, the designed TS-PCM simultaneously generates a cross-polarized wave (3.77-6.20 GHz) and left-handed CP wave (7.75-8.60 GHz) for normally incident y-polarized wave.

C. Performance Analysis
We start with the axial ratio (AR) parameter that is calculated as follows [23]: and Δϕ = ϕ xy − ϕ yy.
AR is reported in Fig. 3(a) and (b), during diodes are OFF and ON respectively. The AR is below 3 dB in the lower frequency band 5.20-6.10 GHz when the diodes are OFF, whereas it satisfies the same criteria in the band 7.75-8.60 GHz when the diodes are ON. These results confirm that the proposed TS-PCM behaves as an effective LP-CP converter within a frequency range that depends on the diode states. It can be further noted from Fig. 2 that the phase difference between the coand cross-polarization reflection phases are approximately equal to +270 • over the frequency band 5.20-6.10 GHz (when the diodes are OFF) and −90 • in the frequency band 7.75-8.60 GHz (when the diodes are ON). The PCR parameter is defined [17] as P CR y = |r xy | 2 / |r xy | 2 + |r yy | 2 . It is also calculated and presented in Fig. 4. It can be observed that, when the diodes are OFF, PCR is less than 0.6 within the operating frequency range, i.e., no cross-polarization conversion takes place, except CP and FR. However, when the diodes are switched to the ON state, PCR is larger than 0.9 within the frequency range 3.77-6.20 GHz, indicating highly efficient LP-OLP conversion. Further, when the structure is simulated by launching obliquely incident  EM PCR remains larger than 0.9 within the frequency range 3.80-6.17 GHz up to an incident angle of 10°.

III. PHYSICAL MECHANISM BEHIND THE TS-PCM OPERATION
This section aims to provide the physical insight on the operation of the TS-PCM. We provide the detailed analysis of the proposed structure from two different perspectives which are as follows.

A. Field Decomposition Along the U-V Axes
In particular, as shown in Fig. 5 (Inset), we define the uv axes along the ±45 • directions in the xy plane. Electric field ( E iy ) of the y-polarized incident wave in this system is given by [14] E iy = Accordingly, the electric field ( E r ) of the reflected wave is (3) When the diodes are in the OFF state, the reflection magnitudes exhibit similar values in the frequency ranges 5.20-6.10 GHz, 2-3.66 GHz, and 8.42-9.52 GHz in Fig. 5. The reflection phase difference [in Fig. 6(a)] is roughly +270 • in the frequency band 5.20-6.10 GHz (LP-CP conversion) whilst in the frequency bands 2-3.66 GHz, and 8.42-9.52 GHz, both ranges are neither ±180 • nor ±90 • , indicating that the waves are reflected without any polarization conversion. However, when the diodes are switched ON, the reflection magnitudes along the u-v axes exhibit similar values in the frequency ranges 3.80-6.17 GHz and 7.75-8.60 GHz in Fig. 5. The reflection phase difference  [in Fig. 6(b)] is nearly +180 • in the lower frequency range 3.80-6.17 GHz (LP-OLP conversion) and around −90 • in the higher frequency range 7.75-8.60 GHz (LP-CP conversion).

B. Surface Current Distribution
In the OFF state, shown in Fig. 7(a), the current flow is evaluated at 5.65 GHz for LP-CP conversion and can be divided in two parts, (i) current flow in the up-down direction ( J 1, total = J 1 + J 1 ) and (ii) current flow in the left-right direction ( J 2, total = J 2 + J 2 + J C + J C ), being J C the cut wire surface current density. Thus, the two flow directions are nearly orthogonal to each other. Next, the surface current distribution at the center frequency 8.17 GHz for the LP-CP polarization conversion when the diodes are in the ON state is presented. Again, as shown in Fig. 7(b), the net current flow consists of two orthogonal current paths, (J 1, total ) and (J 2, total ), (i) current flow in the up-down direction (J 1, total ) and (ii) current flow consists of two orthogonal current paths, (J 1, total ) and (J 2, total ), (i) current flow in the up-down direction (J 1, total ) and (ii) current flow in the left-right direction (J 2, total ). Similarly, the flow directions are nearly orthogonal to each other which gives LP-CP conversion [14].
Further, the surface current distribution on the top metal and ground layers are reported in Fig. 8, at two resonant frequencies for the LP-OLP conversion when the diodes are ON. From Fig. 8(a), it can be observed that the surface currents (J 1 ) flow along the two arms of the split rectangular shaped resonator along the v direction at 4.10 GHz. The surface current density in the ground layer (J ground ) is in the opposite direction at this frequency. Hence an incident y-polarized incident field is converted  into an x-polarized reflected field [14]. From Fig. 8(b), we observe that at 5.45 GHz, the current densities in the top and ground layers flow nearly in the same direction, which corresponds to an electric dipole resonance. Thus, the x-component of the induced electric field is rotated into a cross-polarized component. Again, the surface current distributions are evaluated for LP-FR mode at 2.83 and 8.97 GHz when diodes are OFF in Fig. 8

(c) and (d).
It is observed that the top layer is not excited and extremely low current is distributed which is almost negligible. Hence, no conversion takes place, except copolarized reflection.
The measured and simulated results of the r yy and r xy reflection magnitudes are compared in Fig. 10. When all the diodes are OFF state, the designed sample returns LP-CP conversion over the frequency range 5.05-6.17 GHz and LP-FR conversion over the frequency ranges 2-3.66 GHz and 8.30-9.92 GHz. Conversely, when all the diodes are switched to ON state, the device behaves as an LP-OLP converter over the frequency range 3.77-6.00 GHz and as an LP-CP converter over the frequency range 7.35-8.15 GHz.
Further, when the structure is measured under an obliquely incident EM (Fig. 11) wave, all functionalities are found to be stable up to an incident angle of 10°. It can be observed that the phase difference is approximately equals to +270 • ± 10 • (AR ≤ 3 dB) over the frequency band 5.17-5.99 GHz when  the diodes are OFF, and −90 • ± 10 • AR ≤ 3 dB) in the frequency band 7.75-8.47 GHz when the diodes are ON. Finally, a comparison with the state of the art is carried out and presented in Table I. Additionally, our proposed device is targeted to match the operating frequency bands for various application purposes [28], [29], [30], [31].

V. CONCLUSION
In this letter, a reconfigurable TS-PCM is designed, analyzed and experimentally tested. The proposed device is expected to find applications in reconfigurable devices, RCS reduction and also in polarization control devices.