Metasurface with dynamic chiral meta‑atoms for spin multiplexing hologram and low observable reflection

Spin light manipulation based on chiral metasurfaces is a striking hotspot that has intrigued huge attention. Circular dichroism, a unique phenomenon of chiral atoms/ molecules, has been regarded as another auxiliary dimension for guiding electromagnetic waves, which has been explored in the field of artificial material sciences yet a challenging issue. Here, a generic strategy based on dynamic chiral meta-atom for revealing strong circular dichroism as well as applicable electromagnetic functionality is proposed in microwave regime. We demonstrate a dynamic metasurface that enables the fully independent holograms reconstruction for one circular polarization or the other at the active operating state. On the other hand, the electromagnetic scattering is realized for lowering observable backward reflection at the passive state.

chiroptical responses [19]. With the assistance of metasurfaces, the weak chiroptical effects in nature materials can be strikingly revealed and enhanced by artificially arranged meta-atoms [20][21][22]. Among them, the differential extinction for left-handed circular polarization (LHCP) state and right-handed circular polarization (RHCP) state can be achieved as circular dichroism by the meticulous design of geometric chirality [23][24][25][26][27][28]. Unlike the flat metallic sheets that reflect circularly polarized waves with reverse handedness, the chiral metasurfaces selectively reflect one of the circularly polarized waves without changing the handedness, while the other is absorbed [29]. On the other hand, chiral structures possess the potential to acquire circular polarization conversion, and thereafter governed by Pancharatam-Berry (PB) phase principle [30]. To date, introducing three-dimensional or cascaded structures are effective methods to achieve circular dichroitic responses in a large portfolio of designs and spanned the frequencies from visible to gigahertz (GHz) [31][32][33]. Nevertheless, this happens at the expense of complex fabrication schemes and limited reconfigurability.
With the increasing demands of modern encrypted communication and anti-counterfeiting technology, a tunable or dynamic strategy aiming to provide a platform for achieving more flourishing applications and functionalities is urgent. Although the explorations of metasurfaces are widely reported so far, most of them are passive with fixed functionality and suffer from limited flexibility [34]. Fortunately, the dynamic counterparts by loading active elements have been proposed to improve the limitation, which are demonstrated to offer effective manner to vary the electromagnetic response of meta-atoms, thereafter achieving dynamic functionality variations [35][36][37][38][39]. This approach presents great potential for anti-jam and capacity expansion, especially for the microwave regime. Therefore, it is enlightening to dynamically achieve circular dichroism as an auxiliary dimension of electromagnetic manipulation and simultaneously integrate multiple functionalities within a single aperture. However, the research of this academic issue still needs to be further explored.
Here, we propose a generic strategy to achieve dynamic control of electromagnetic waves for spin multiplexing hologram and low observable reflection by an active manner. By introducing varistor diode into the split-ring metallic resonator at mirrorsymmetrical/chiral positions, the proposed meta-atom can be used to dynamically achieve electromagnetic wave absorption of both spin states and circular dichroitic effect. Accordingly, the amplitude-based spin multiplexing hologram for reconstructing images for either left-handed or right-handed circularly polarized waves are presented by bias voltage driven. On the other hand, the amplitude distribution becomes uniform without the excitation of voltage. Accordingly, the phase difference of π of distinct meta-atoms can be correlated to the coding metasurface principle. Thereafter, the low observable reflection of beam scattering can be obtained. The fabricated prototype with 60 × 60 pixels demonstrates the feasibility of our proposed voltagedriven chiral metasurface. Encouragingly, the proposed paradigm offers additional dynamic strategy to push the circular dichroitic metasurface to application level, which possesses great potential in the fields of information encryption, anti-counterfeiting, and stealth. Figure 1 presents the conceptual illustration of the proposed voltage-driven chiral metasurface. A pair of electrodes are set at two sides of the metasurface to achieve series feed, which can efficiently achieve dynamic control without extra feed network. Based on the circular dichroitic effect of meta-atoms, the independent amplitude distributions (defined as L-code and R-code) can be obtained with left-handed and righthanded circularly polarized waves illuminating when the varistor diodes are operated at "On" states. Moreover, the proposed chiral metasurface becomes passive type when the varistor diodes are at "Off " states. Thus, the amplitude coding distributions become suppressed and the phase responses become the dominant effect to reveal low observable reflection.

Methods
The proposed chiral metasurface is fabricated by printed circuit board (PCB) technique which consists of 60 × 60 meta-atoms with the total size of 385 mm × 360 mm (including the feeding parts set at two sides of the fabricated prototype) that are fed in series in each row. The metallic structure of split-ring resonator and metallic back sheet are made of copper with the thickness of 0.02 mm and the conductivity of 5.8 × 10 7 S/m. The dielectric spacer F4B (the relative permittivity of 2.65 and the loss tangent of 0.001) with the thickness of 3.0 mm is applied to adapt the design. The varistor diodes (type of BAP70-02, with the total number of 3672) are loaded into the gaps of split-ring resonators, which can be controlled by the direct current (DC) bias voltage excitation. The inductances (type of LQW18AN27NGOOD, with the total number of 3600) are loaded into the gaps of series wires to introduce a radiofrequency choke, which blocks alternating current but allows direct current to pass through. In this way, the electromagnetic properties of the metasurface are relatively stable when driven by external voltage.

Results
To achieve circular dichroism, it is necessary to simultaneously break the n-fold rotational (n > 2) and mirror symmetries for a two-dimensional meta-atom [40]. However, the symmetry-breaking effect for circular dichroism in microwave frequency is relatively slight due to the inadequate losses of metal and substrate, especially for the meta-atoms of planar version. In order to reveal conspicuous circular dichroism and thereafter promote it to the application level of electromagnetic manipulation in microwave regime, we conduct the strategy of introducing Ohmic dissipation into the chiral position of meta-atom. In this design, four kinds of meta-atoms with the same framework but different to the varistor diode loading positions are implemented, which are shown in Fig. 2   According to the amplitude responses to the left-handed and right-handed circularly polarized waves, we use binary codes to define these meta-atoms as "00", "01", "10", and "11", respectively. Here, the first bit of "0" or "1" represents the absorption or reflection of left-handed circularly polarized waves. The second bit of "0" or "1" represents the absorption or reflection of right-handed circularly polarized waves. The electromagnetic properties of the meta-atoms are simulated by computer simulation technology (CST) Microwave Studio. The periodic boundary conditions of "Unit cell" are applied in both x-and y-directions, while two Flouquet ports are fixed at ±z directions.
The amplitude curves of the corresponding meta-atoms are depicted in Fig. 2 (e)-(h), in which the cases of varistor diode at "On" and "Off " states are compared. For the varistor diode, the resistance is 150 Ω when driven by DC bias voltage at "On" state, while it becomes 200 MΩ at "Off " state. It is observed that the "00" and "11" meta-atoms have the same amplitude responses to the incidence of circularly polarized waves with different spin states no matter the varistor diodes are at "On" or "Off " states. However, the amplitude responses of both spin states are sharply suppressed for "00" meta-atom when the loaded varistor diodes are at "On" states. On the contrary, the "01" and "10" metaatoms reveal conspicuous circular dichroitic effect when the varistor diodes are at "On" states. The "01" meta-atom achieves the absorption of left-handed circularly polarized reflection; the "10" meta-atom obtains the absorption of right-handed circularly polarized reflection. Nevertheless, the circular dichroitic effects disappear when the varistor diodes are at "Off " states. Instead, the reflected amplitude responses of both spin states are nearly 1 in 8-18 GHz. Noticed that the meta-atoms of "00" and "11" satisfy the requirements of simultaneously breaking the n-fold rotational (n > 2) and mirror symmetries within planar version (the positions of loading inductances break the symmetries), but the expected depression of circular dichroitic effects are acquired. These phenomena are caused by the ohmic dissipations of the used inductances are very small that can hardly enhance the chirality. The phase responses are shown in Fig. 2 (i)-(l), in which the varistor diodes are at "On" states. Obviously, the phase responses of "00" and "11" meta-atoms are insensitive to the spin states, and the phase responses of these two kinds of meta-atom are almost the same within 8-20 GHz. Moreover, it is noticed that there are phase differences between the spin states with the amplitude responses equal to 0 (φ LL for "01"; φ RR for "10") and the other of "01" and "10" meta-atoms. But for the amplitude holographic metasurface, these phase differences can hardly influence on the reconstructed images. Therefore, the phase deviations can be ignored for the spin multiplexing design.
On the other hand, the phase responses of left-handed and right-handed circular polarization with the varistor diodes at "Off " states are compared in Fig. 3. In this case, the metallic resonances become the dominant contribution to the electromagnetic characteristics of meta-atoms. It can be observed in Fig. 3(a) that the phase differences are about 180° between "00"/"01" and "10"/"11" meta-atoms in the frequency band of 8 GHz to 18 GHz for left-handed circular polarization. Similarly, the phase differences are about 180° between "00"/"10" and "01"/"11" meta-atoms within the same frequency band for right-handed circular polarization. Furthermore, the amplitude responses of the four kinds of meta-atom are larger than 0.75 in 13-17 GHz, thus the phase coding sequences can be theoretically implemented for "Off " states.
Considering that the resistance value of varistor diode is gradually changed when driven by different bias voltage. Here, the circular dichroism characteristics of the "01" and "10" meta-atoms under the different resistance value are explored, as shown in Fig. 4(a) and Fig. 4(b). Circular dichroism is the difference between the absorption of left-handed and right-handed circularly polarized waves, which can be calculated as [41]: where the corresponding absorption is governed by A LHCP(RHCP) = 1-|r LL ( RR )| 2 -|r RL ( LR )| 2 , r RL and r LR are the cross-polarized amplitude of left-handed and right-handed circularly polarized waves, respectively. It can be seen that the strong circular dichroitic effects are achieved when the varistor diodes are driven by bias voltage at "On" states (150 Ω), especially in 9-14 GHz the circular dichroism is nearly 1. With the increase of resistance value, the circular dichroism becomes gradually inconspicuous. When the varistor diodes are at "Off " states (200 MΩ), the circular dichroism almost disappears.
For the purpose of encoding spin-multiplexing holograms based on amplitude distributions, the Rayleigh-Sommerfeld diffraction formula as well as the point source   [42]. First, the 0/1 amplitude distributions of left-handed and right-handed circularly polarized waves are calculated, in which the phase distributions are considered as uniform. Subsequently, the amplitude distributions of each spin state are combined and each pixel is expressed by binary coding "00", "01", "10", and "11". According to the binary coding distribution, the corresponding meta-atoms are arranged at the predesigned positions. Here, the metasurface hologram contains 60 × 60 pixels with a period of 6.0 mm in x-and y-directions, which are in accordance with the periodic lengths of meta-atoms. As the proof of principles, the holographic images of "L" and "R" are calculated on the observing plane. The operating frequency is selected at 12 GHz, and the observing plane is set as 300 mm away from the proposed chiral metasurface. The reconstructed images are characterized by electric field scanning technique. The experimental setup is shown in Fig. 5(a), in which a standard horn antenna that can radiate circularly polarized waves and the fabricated metasurface prototype (connected with a DC source) are aligned in an anechoic chamber to reduce the electromagnetic interference. The horn antenna is set far enough with the metasurface prototype to ensure the quasi-plane wave incidence. The scanning probe is placed with a distance of 300 mm to the prototype that can move in the xoy-plane. The probe and the transmitter antenna were connected to two ports of an Agilent N5224A vector network analyzer, respectively. During the measurement, the scanning area is 360 × 360 mm 2 with a step of 5 mm. The simulated and experimental results are given in Fig. 5(b) and Fig. 5(c), respectively. Here, the bias voltage is applied as 52 V to ensure the varistor diodes in each row are operated at "On" states with the resistance of about 150 Ω. Both the simulated and experimental results manifest the conspicuous reconstructing images of "L" and "R" at the observing plane. The experimental results exhibit good performances with 57.93% and 65.43% imaging pattern efficiency (the energy concentrated in the image area referenced to the total energy on the measured plane), 28.43 and 31.61 peak signal-to-noise ratios (SNR, defined as the peak intensity in the image to the standard deviation of the background noise) for "L" and "R" images, respectively. However, compared with the simulated results, there is a slight deterioration in the experimental image quality, which is mainly caused by When the varistor diodes are operated at "Off " states, the amplitude distribution of the chiral metasurface become relatively uniform within the whole aperture. Instead, the phase distribution is similar to the coding metasurface principle with 0/π distribution according to aforementioned discussion of Fig. 3. Therefore, the far-field scattering patterns in the reflection sphere are governed by passive antenna array theory as [43]: where k 0 is the wave vector in free space, φ m,n is the reflection phase of each coding meta-atom, p is the periodic length of the meta-atom in x-and y-directions, θ i and φ i are the elevation and azimuth angles, respectively.
Accordingly, it is concluded that the beam scattering effect can be achieved. The farfield scattering patterns under the illumination of left-handed and right-handed circularly polarized waves at 10 GHz, 12 GHz, and 14 GHz are monitored and illustrated in Fig. 6(a)-(c) and Fig. 6(d)-(f ), in which the results of metal plate with the same size of proposed chiral metasurface, the results of "On" states and "Off " states are compared. Here, the results at xoz-plane are depicted. Obviously, the backward reflection is dramatically reduced when the varistor diodes are turned at "Off " states. Moreover, the side-lobes become larger than the metal plate. The reduction of 16.5 dB, 21.7 dB, and 15.5 dB is obtained for left-handed circularly polarized incidence at 10 GHz, 12 GHz, and 14 GHz. The reduction of 17.1 dB, 19.9 dB, and 14.3 dB is obtained for right-handed circularly polarized incidence at 10 GHz, 12 GHz, and 14 GHz. The differences between left-handed and right-handed circularly polarized reflection are owing to the non-identical coding sequences. It is noticed that the results of varistor diodes are at "On" states also have the slight effect of backward reflection reduction, which is attributed to the uneven distributions of amplitude responses.
To experimentally demonstrate the feasibility of low observable reflection, the reflection characteristics are investigated in anechoic chamber. The experimental setup is shown in Fig. 7(a), in which a pair of horn antennas that connect to the two ports of an  Fig. 7(b). The large RCS reduction effect is achieved for both left-handed and right-handed circularly polarized reflection in 8-16 GHz when the varistor diodes are at "Off " states. For further verification, we have measured the specular reflection of S 11 in the same frequency band. Compared with the curves of varistor diodes at "On" states, the S 11 curves of the passive states are lower. Especially in 10-14 GHz, the reduction of specular reflection is more than 15 dB than the metal plate with the same size, which verifies the good low observable reflection characteristics at "Off " states.

Conclusions
We have proposed a generic strategy that can tailor the magnitude of circular dichroism by manipulating different DC bias voltage. By meticulously arranging the proposed four kinds of meta-atoms, a dynamic chiral metasurface driven by bias voltage is fabricated to achieve the functionality integration of spin multiplexing hologram and low observable reflection in microwave regime. The good agreement between numerical simulations and experimental verifications manifests the feasibility of our proposed paradigm. Owing to the concise topological structures, it is inferred that the proposed strategy may be extended to the terahertz or optical frequency by replace the loaded varistor diodes with the phase change materials including Vanadium oxide, Ge 2 Sb 2 Te 5 (GST), and the like. Furthermore, by properly arranging the resistance value of varistor diode, the more levels of amplitude responses can be theoretically achieved and may apply to the practical fields requiring energy allocations of reradiation waves. In addition, the proposed paradigm is possible to provide spatially varying distributions by further developing the feed systems for independent control in each raw or each pixel with the support of field programmable gate array. In this way, it can further expand and explore the aspects of versatile functionalities/multiple meta-holograms integration. Encouragingly, the proposed strategy can be used to achieve more flourishing functionalities for dynamic control, which may find applications in information encryption, anti-counterfeiting, and other dynamic systems.