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Frequency-Domain and Spatial-Domain Reconfigurable Metasurface

Guang Yao Liu, ‡ Long Li,* Jia Qi Han, ‡ Hai Xia Liu, ‡ Xiao He Gao, Yan Shi, and Tie Jun Cui*

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ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: The recently proposed digital reconfigurable metasurfaces make it possible to manipulate electromagnetic (EM) waves flexibly. However, most existing reconfigurable metasurfaces can only exhibit a relatively single performance in the spatial domain. Here, we propose a general frequency- and spatial-domain reconfigurable metasurface (FSRM) that can manipulate the EM waves and realize reconfigurable functions in multifrequency bands. In the frequency domain, FSRM can convert different linearly polarized (LP) incident waves into left- and right-hand circularly polarized reflected waves, in which PIN diodes are used to switch the polarization conversions in different frequency bands. When the polarization direction of the incident LP wave is 45° from the +x-axis, the FSRM modulates the incident waves as a 1-bit programmable metasurface in the spatial domain. Two-dimensional beam scanning, vortex beams with orbital angular momentums, and specific beams with desired transmission directions are demonstrated via real-time adjustment of the digital coding state. To validate the modulation methodology, an FSRM prototype is fabricated and measured, which could respond to different functions for different polarization incidences. The measured results agree well with the theoretical analyses. The proposed FSRM will provide new opportunities for smart material designs. KEYWORDS: beam dynamic scanning, frequency and spatial domains, orbital angular momentums, polarization conversion, programmable metasurface

1. INTRODUCTION

Different from the bulky metamaterial composed of a threedimensional (3D) subwavelength structure, a metasurface is a kind of planar topology array. The metasurface is considered an artificial functional material, 1−5 due to its advantages of flexible phase control ability, 6−8 optional conversion modes in wideband or multiple bands, 9,10 ultrasmall thickness, 11−13 and other finely tailorable characteristics. 14−17 There have been many metasurface devices applied to manipulate electromagnetic (EM) waves such as reflective and transmissive polarization converters, 18−21 beam deflection, 22,23 a flat lens, 24 low-scattering materials, 25 etc. However, most previously mentioned devices have fixed structures and are generally difficult to adjust in real time, which greatly limits the scope of applications for metasurfaces.

With the demand for a miniaturized apparatus, reconfigurable metasurfaces have been gradually developed. The reconfigurable metasurfaces are typically implemented by polarized modulation and mechanical and electronic (PIN diode, varactor, and MEMS) techniques. 26−30 On the other hand, digital coding and programmable metasurfaces have rapidly evolved since 2014. 23,30−32 Cui et al. developed the first 1-bit and higher-bit information metasurfaces in the microwave frequency, 30 from which they further proposed the space-time-coding digital metasurfaces. 23 A metasurface proposed by Luo and co-workers © 2020 American Chemical Society can change its local phase distribution to achieve dynamical beam deflection and polarization transformation in the spatial domain. 33 In the field of optics, Maguid et al. reported a metasurface with multifunctional spin-dependent dielectrics, 34 which demonstrates the possibility to achieve complete control of the fundamental properties of light in different frequency bands. The phase response of the metasurfaces plays a key role in realizing the above unnatural physical phenomena and interesting functions, 32 whereas the existing theories and methods are mainly focused on the performance of metasurfaces with a phase gradient in the spatial domain, and the frequency domain properties are rarely explored. 35−38 Recently, Ratni et al. 39 loaded varactor on the unit cell to form an active metasurface, which can convert linearly polarized (LP) waves into circularly polarized (CP) waves in different frequency bands by adjusting the bias voltage. However, the performance of the

by adjusting the bias voltage. However, the performance of the frequency domain modulation of this device is limited, and Received: February 13, 2020 Accepted: April 27, 2020 Published: April 27, 2020

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furthermore, the varactor shows higher loss and a more complicated bias circuit. Due to the dramatic increase in mobile traffic and the demand for a high-performance communication network with low cost, the modulation of EM waves in the spatial and frequency domain radiation characteristics on a single device has become a research hotspot.4 0 So far, the reconfigurable multifunctional metasurfaces are made in many forms, and a few achievements in realizing multiple functions on the same device in the frequency and spatial domains have been reported. At the same time, the design of a multiple-function active metasurface with multi- frequency bands, a wideband, and simple bias circuit has become a challenging problem.4 1 Here, we propose a general frequency- and spatial-domain reconfigurable metasurface (FSRM) for multifunctional control of EM waves. In the frequency domain, FSRM can achieve polarization selection, frequency reconfigurable polarization conversion, and polarization reconfiguration by switching the states of the PIN diodes. At the same time, the polarization inversion modulation technology is introduced in the metasur- face, which makes the conversions from LP waves to the left- hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves according to different polarization directions of the incident waves. In the spatial domain, by encoding the reflection phase of the FSRM units, three typical EM wave regulations are analyzed and realized, such as the dynamical two-dimensional (2D) beam scanning, vortex-beam generations with orbital angular momentum (OAM) modes, and specific beam transmission with predesigned direction. The efficiency and reliability of the proposed reconfigurable metasurface are proved by measurement results. This metasur- face has tremendous potential applications for smart and reconfigurable devices and systems. 2. DESIGN AND ANALYSES OF FSRM 2.1. Frequency-Domain and Spatial-Domain Modu- lation. In the EM environment, FSRM is made of groups of elements with different EM responses in the frequency and spatial domains. Here, we mainly propose a method to design FSRM using scattering parameters that are related to the frequency and spatial azimuth. The scattering matrix S of an interface relates the incident and reflected electric fields on the same side of the interface = Ä Ç Å Å ÅÅ ÅÅ ÅÅ ÅÅ ÅÅ ÅÅ ÅÅ ÅÅ ÅÅ Å É Ö ÑÑÑ Ñ ÑÑ Ñ ÑÑÑ ÑÑ ÑÑ Ñ ÑÑÑ ÑÑ Ñ Ä Ç Å Å ÅÅ ÅÅ ÅÅ ÅÅÅÅ ÅÅ ÅÅ ÅÅ ÅÅ Å É Ö ÑÑÑ Ñ ÑÑ Ñ ÑÑÑ ÑÑ ÑÑ Ñ ÑÑÑ ÑÑ Ñ E E E E E E S x r y r u r x i y i u i ( ) ( ) ( ) ( ) ( ) ( ) (1) where the superscripts (i) and (r) indicate the incident and reflected directions and the subscripts x, y, and u indicate the polarization directions of the field, respectively. In order to achieve the unit cell adjustable in the frequency domain and spatial domain simultaneously according to the polarization direction of the incident wave, the scattering matrix of the pt h element is given by θ φ = Ä Ç Å Å ÅÅ ÅÅ ÅÅ ÅÅ ÅÅÅ ÅÅ ÅÅ ÅÅÅ É Ö Ñ Ñ ÑÑ ÑÑ ÑÑ ÑÑ ÑÑ ÑÑ ÑÑÑÑ ÑÑ S f S f S S ( ) ( ) ( , ) p f p f p s p (2) Figure 1. FSRM with frequency-domain and spatial-domain modulations and its unit cell structure. (a) The frequency-domain and spatial-domain modulations of FSRM. (b) The equivalent circuit of the PIN diode under different working conditions. (c) The 3D topology expanded view of the unit cell. (d) The top view of the unit cell. ACS Applied Materials & Interfaces www.acsami.org Research Article ht tps://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 23555

ACS Applied Materials & Interfaces www.acsami.org

where S f p and S sp are the scattering parameters of the p th unit cell in the frequency domain and spatial domain, f is the working frequency, and θ and φ are the elevation and azimuth angles corresponding to the scattered beam of the element, respectively. The scattering parameter matrix is a Jordan standard type, 42 so that the frequency- and spatial-domain scattering modulation functions of the unit do not interfere with each other. According to the modulated field E (r) p of each element, the scattering E-field of the metasurface can be induced current movement, so that the element can provide a 1- bit phase modulation in the spatial domain and have a wide working frequency bandwidth. The rectangular groove also brings variable factors to the scattering matrix S of the FSRM element, which may cause the frequency domain resonance of the unit to further increase f 5 . Each PIN diode is individually controlled to give the unit cell modulation scattering parameters greater freedom. The biasing mechanism of the bias circuit layer is deliberately designed to minimize the biasing circuit loss, and two layers are connected by metallized holes to simplify the structure; the active devices connect various portions of the driven layer to form a unitary body. 43 Furthermore, in order to suppress the influence of the high-frequency signal introduced by the power supply, two 27 nH inductors are placed between the bias line and the patch symmetrically.

A pair of PIN diodes are connected to the patches on both sides of the slots. The PIN diode (Skyworks SMP1340-040LF), which can work within 10 GHz, is employed in this paper to show low insertion loss and excellent electrical performance. 44 Moreover, the equivalent circuit of the PIN diode under different working conditions in the aimed frequency band is shown in Figure 1b. For the “ON” or “OFF” state, the PIN diode is modeled as a series of lumped resistance (R) and inductance (L) or resistance (R), capacitance (C), and inductance (L), respectively. To verify the functionalities and the reflection characteristics, the commercial software, HFSS 15.0 is used to analyze and optimize the proposed metasurface element. The element is simulated by a periodic boundary condition combined with Floquet port. Moreover, the PIN diodes and inductors are replaced by equivalent circuit models composed of lumped elements in the corresponding cases under the “ON” and “OFF” operating states.

2.3. Frequency-Domain-Controllable Polarization Converter. When the incident EM wave is LP with the electric field E (i) along the y direction or x direction, the metasurface can modulate the LP wave in the frequency domain. Accordingly, the FSRM can be regulated as a multifunctional polarization converter (MPC). The y-polarized incident wave is illustrated as an example, and the incident E-field radiated on the p th unit can be written as two orthogonal linear components

element, the scattering E-field of the metasurface can be expanded as ∑ ∑ = = = = i k j j j jjj j y { z z zzzz z E E S E p p r p p p i 1 ( ) 1 ( ) (3)

= = k { p p 1 1 (3) The scattering parameters in eqs 2 and 3 can be expressed as2 3 ∑ = ! − + ! − + ! − + ! − + = Ä Ç Å Å ÅÅ ÅÅÅÅ ÅÅ É Ö Ñ Ñ ÑÑÑÑ ÑÑ ÑÑ S f a n f f b n f f c n f f d n f f L ( ) ( ) ( ) ( ) ( ) f p n p n n p n n p n n p n n 0 1 2 3 4 (4)

S sp ( θ , φ ) = α pn · exp [− jk ( x sin θ cos φ + y sin θ sin φ ) + j Φ p ]

(5) Scattering parameters are expanded in the frequency domain according to the Taylor series, where f 1 , f 2 , ... are different resonant frequencies, a pn , b pn , ... are nth-order resonance coefficients at the corresponding resonant frequencies, α n p is a set of spatial modulation coefficients, k is the wavenumber, and Φ p is the phase factor.

It is difficult for the fixed-structure particles to switch states in different frequency bands. In this design method, we introduce the frequency modulation technology and use PIN diodes to freely change the resonance frequency of the unit. The scattering parameters can also modulate EM waves in multiple directions, which will be analyzed in a later section. Figure 1a illustrates the modulation method mentioned above.

2.2. Design of FSRM Element. We present a metasurface to validate the FSRM design concept and the scattering parameter adjustment method. The geometrical configuration and detailed dimensions of the FSRM element are shown in Figure 1c,d. More specifically, the unit cell structure is composed of a driven patch layer, a metal ground (GND), and a bias circuit layer. These layers are spaced by two F4B substrates whose relative permittivity is 2.65 and loss angle tangent is 0.001. The driven patch layer is a square patch, which is divided into three blocks with different sizes by the gaps parallel to the diagonal direction of the patch. The structure and specific parameters of the unit are described in detail in the Supporting Information.

It is worth mentioning that the size ratio and shapes of the three blocks of the driven layer enable the unit cell to evidence reconfigurable characteristics in the frequency domain, and the resonant frequencies are f 1 , f 2 , f 3 , and f 4 in this configuration. By adjusting the positions of two parallel slots in the u direction, the central response frequency of each polarization conversion state will be changed. In order to connect the working frequency bands of the polarization conversion states smoothly, the area ratio of the three blocks is about 1:13:35. As the interconnection of the patches of the driven layer is changed, the drive path of the induced current and the resonant state of the driven layer are changed accordingly. Another novelty of the element is the introduction of a rectangular groove, which increases the path of Research Article

= + = + − E E E E E u v ( ) e p i p u p v p u p v j kz ( ) (6)

where E pu and E pv are components of E (i) p in the u and v directions, respectively; u and v indicate the unit vectors of the corresponding direction; k is the wavenumber in the free space. The vector direction of the incident E-field and its components are illustrated in Figure 1d. According to the scattering

parameters, the macroscopic reflected E-field of the pt h unit in the frequency domain is expressed as Ep( r) = Ep( i) ·Sf p( f). In

practice, the incident E-field excites induced current (related to the scattering parameter) on the unit metal topology, and the reflected E-field can be expressed as the sum of two orthogonal components of the current 45 23556 https://dx.doi.org/10.1021/acsami.0c02467

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Figure 2. Polarization working mechanism and polarization transition conditions of MPC. (a) Polarization working mechanism of the incident and reflected waves. (b) Reflected wave axis ratio and (c) phase difference of the components in the two vertical directions in four operating states. (d) Surface current distributions of the unit cell in state “10” at 5.67 GHz and (e) state “11” at 7.19 GHz. ACS Applied Materials & Interfaces www.acsami.org Research Article

∫ ∫ ∫ ∫ ∫ ∫ ω μ ω μ π μ μ π μ π ε χ π μ π ε χ π = ′ + ′ = · ′ + ′ = · ′ · · · ′ · + ′ · · · ′ · Ä Ç Å ÅÅÅÅ Å Å É Ö Ñ ÑÑÑÑ Ñ Ñ ( ) j G j G S f j G j G S f j G j S f S f j G j S f S f E r J u r J v r J u r J v r E r u r E r v d d ( ) 2 d d ( ) 2 d 2 ( ) ( , 2 ( )) d 2 ( ) ( , 2 ( )) p r u cw u v cw v f p u cw u v cw v f p u f p cw u u f p v f p cw v v f p ( ) 0 , 0 , 0 , 0 , 0 0 , 0 0 , (7) = + = + = | | + | | ϕ ϕ − E E E R E E R E R E R u R v ( ) ( e e ) e p r p u v pu u pv v u pu j v pv j j kz ( ) u v (8) where Rp u = −μ0 4 π2 ·Sf p 2 (f)ε0 e j ϕu u and Rp v = −μ0 4 π2 ·Sf p 2 (f)- ε0 e j ϕv v are reflection coefficients for the u and v directions, respectively. Rp u and Rp v have the same magnitude |Rp u| and |Rp v| , and the corresponding phase shifts are ϕu and ϕv . Meanwhile, ϕu and ϕv have a Δϕ phase difference, which can be expressed as ϕ ϕ ϕ Δ = − u v (9) If Δϕ = ±90° and Eu | Rp u| = |Rp v| Ev , the incident LP wave can be

converted into the CP wave. 46 The AR of a reflected wave can be calculated by the principle of the transmission line in which G u and G v are the Green’s functions and J cw,u and J cw,v i j

converted into the CP wave. The AR of a reflected wave can be calculated by the principle of the transmission li ne = | | + | | + | | + | | − i k j j j jjj y { z zz zz z t t R R R R AR (dB) 10 log pu pv pu pv 10 2 2 2 2 (10) where ϕ = | | + | | + | | | | Δ t R R R R 2 cos(2 ) pu pv pu pv 4 4 2 2 (11)

where with the E-field and can be exempted from integration. Equation 7 can be further simplified to 23557 https://dx.doi.org/10.1021/acsami.0c02467

ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 ACS Applied Materials & Interfaces www.acsami.org

According to eq 9, it can be inferred that the incident LP wave is converted to the LHCP wave when the phase difference Δϕ = 90°. Similarly, the LP wave can be converted to the RHCP wave as the incident wave has Δϕ = −90°. The polarization modulation scheme of the incident and reflected waves of MPC is depicted in Figure 2a.

Here, four states of 2-bit (“00” “01” “10” “11”) digital coding are used to describe the working states of the two PIN diodes in the unit cell, in which state “01” means that PIN diode #1 is “OFF” and PIN diode #2 is “ON”. As the interconnection of the patches of the driven layer is changed, the drive path of the induced current and the resonant state of the driven layer are changed accordingly, so that the polarization conversion states and the response frequency bands of the element are switched. As shown in Figure 2b, the 3-dB AR bandwidth and the corresponding center resonant frequency of the reflected waves are different along with the states of PIN diodes. In state “11”, two resonance points appear, making the AR bandwidth wider. This phenomenon as a consequence of an extra resonant mode is introduced by the two oblique slits when the patch structures in the unit cell are all connected. Figure 2c shows the phase difference of components in the u and v directions in different states. State “10” has a phase difference of −90° (RHCP) between 5 and 6.5 GHz, which is significantly different from other states. The result is the same as the preset, and the LHCP and RHCP waves can be realized in different states when the excitation is along the y-polarization direction. Therefore, MPC has the frequency-domain polarization selective function. The specific parameters are listed in Table 1. In particular, the surface as listed in Table 1. This is because the metasurface unit is optimized to a structure (symmetric in the diagonal direction) that is compatible with the polarization inversion technique. In summary, these results show that more functions can be introduced by more scattering resonances, such as crosspolarization conversion, ultra-wideband regulation, and so on. When more PIN diodes or active devices on the unit are introduced, on the one hand, the unit can be operated in more resonance frequency bands, and on the other hand, it can adjust the various states of the device more flexibly for different functions.

2.4. 1-Bit Programmable Metasurface in the Spatial Domain. When the polarization direction of the incident LP waves is 45° (counterclockwise rotation) from the +x-axis, the FSRM can modulate the incident waves in the spatial domain on the basis of the scattering parameters and convert itself into a 1- bit programmable metasurface, as shown in Table 2. In fact, Table 2. Index of the 1-Bit Reflected State of the Metasurface under the Case of 45° from the +x-Axis LP Incidence polarization state fractional bandwidth

Table 2 · 6 columns × 6 rows · p.5–5 · CSV
3dB ARfractionalcenter
bandwidthbandwidthfrequencyx-axis LPy-axis LP
state[GHz][%][GHz]incidenceincidence
105.18−6.1517.125.67LHCPRHCP
015.95−6.305.726.13RHCPLHCP
006.21−6.859.806.53RHCPLHCP
116.81−7.5710.577.19RHCPLHCP

[deg.] 1-bit quantization phase

[deg.] bandwidth [GHz] 0 1 54.6 to 105.6 −106.5 to −56.3 90 −90 6.1−6.6 when the phase difference is within the 180° ± 20° range, it meets the requirements of 1-bit phase change. 27,47 As shown in Figure 3, when the PIN diode #1 is “ON” and the PIN diode #2 is “OFF”, the reflection phase of the unit is around −90° (digital state 1) from 6.1 to 6.6 GHz. As the states of the two diodes are inverted simultaneously, the reflection phase of the unit cell is around 90° (digital state 0) in the same frequency band. In addition, the amplitudes of S 11 in two states are larger than −0.15 dB, indicating that the units can achieve nearly perfect 1- bit reflection characteristics, as shown in Figure 3. The proposed element is periodically arranged to construct a programmable metasurface (M × Q). The reflected E-field of the mq th unit in the spatial domain is given by Table 1. Parameters of the Reflected CP Waves under Different States in the Cases of LP Incidences along x-Axis and y-Axis 3dB AR bandwidth [GHz] Research Article

θ φ α = · = · · S E E E ( , ) m q r m q i s mq m q i m q n ( ) ( ) ( )

exp [− jk ( x sin θ cos φ + y sin θ sin φ ) + j Φ mq ] current distributions of the patch at states “10” and “11” are shown in Figure 2d,e. In addition, the amplitude and phase characteristics of the reflection coefficients under different polarization conversion states are shown in Figure S4.

It is worth pointing out that the 3-dB AR bandwidths in the four states are continuous, as illustrated in Figure 2b; hence, the polarization converter can convert the LP wave into the CP wave between 5.18 and 7.57 GHz, and the 3-dB AR relative bandwidth is up to 37.5%. Much more interestingly, the CP waves can be realized in different frequency bands under different states, and this function of MPC is called the frequency reconfigurable polarization conversion. The frequency reconfi- gurability allows the element to achieve different polarizations at the same frequency, and this feature is called polarization reconfigurable.

The functions of the frequency-domain-controllable polarization converter are described in detail in the Supporting Information. Furthermore, when an excitation is along the xpolarized direction, the polarization direction (LHCP or RHCP) of the reflected wave in different states will be reversed, (12) The subscript mq is a 2D representation of the p th unit of Section 2.1. Correspondingly, α n mq ≈ cos q e (θ) is the spatial modulation parameter of the mq th unit. As the reflected E-field is manipulated, the reflection phase of the metasurface element is compensated according to the continuous phase compensation formula. 48,49 Then, the continuous compensation phase

tion formula.4 8,49 Then, the continuous compensation phase ϕm q is discretized as ϕ ϕ ϕ = − ° − ° ≤ < ° ° ° ≤ < ° l m o o o n o o o 9 0 1 80 0 90 0 180 cmq mq mq (13)

The discrete phases in eq 13 comprise the phase quantization error in the entire operating band. In the same situation, if the quantization phases of the unit are set to other values, the beam deflection error will increase and the metasurface radiation efficiency will decrease. According to the discrete compensation phase, the working states (state 0 or state 1) of the elements are determined. The scattering far-field of the programmable metasurface at the n-order harmonic frequency is written as 23558 https://dx.doi.org/10.1021/acsami.0c02467

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∑ ∑ θ φ ϕ = | | · = = E j E ( , ) e xp( ) m M q Q m q r cmq 1 1 ( ) (14) On the basis of eq 14, several functions such as 2D beam scanning, vortex beam carrying orbital angular momentum (OAM), and beam splitting of reflected beams can be implemented by encoding the metasurface at the zero-order harmonic frequency. The specific functions are shown in Figure 3. In this work, an active metasurface containing 256 (16 × 16) unit cells is encoded to verify the functions, as shown in Figure 3. The main beam can be scanned between ±45° in the θ and φ directions under different coding patterns. Since the scanning conditions in two dimensions are homologous, only the θ direction is explained. As shown in Figure 4a, the beam pointing error is less than 2°. At the same time, the difference between the gain of the main lobe and side lobes is greater than 10 dB. In addition, with the increase of scanning angle, the main beam gradually widens, and the gain of the main beam becomes smaller. In this case, the reflected waves of units are oriented in the same direction as the main beam is deflected. Furthermore, when the metasurface is encoded to make the reflected beam of each metasurface unit converge toward a point, the microwave energy of the reflected wave is focused on a minuscule area. Here, as shown in Figure 4b, four cases of (0, 0, 0.8) m, (0, 0.3, 0.8) m, (0.5, 0.5, 0.8) m, and (0.3, 0.7, 0.8) m are taken as examples. In simulations, the perpendicular distance between the metasurface and the observation plane is 0.8 m, and the power provided by feeding is transferred to the designed focusing position, which reveals the validity of the theoretical design. As the deflection angle of the focusing position increases, the metasurface focusing ability becomes weak, and the maximum energy intensity becomes smaller in the fixed size area. This feature brings new ideas to wireless energy transfer technology. In addition, the OAM vortex waves can be generated by encoding the metasurface. In order to introduce the spiral phase in the wavefront, the phase factor in eq 12 is set to Φp = l·φm q. 5 0 Here, φm q is the azimuth of the mqt h unit. Figure 4c illustrates the 1-bit phase quantization results of different reflective OAM vortex beams, in which the main beam pointing directions are (0°, 0°). According to the quantified results, the phase distributions in the middle of the array are consistent with the calculations, but phase discontinuity appears in some parts of the array edge. The reflected electric-field amplitudes and phases of mode l = 1, mode l = 2, and mode l = 4 are obtained by full-wave simulations, as illustrated in Figure 4c. Compared to the low- order mode, the beam of higher-order mode vortex wave is more divergent, causing energy to be further dispersed. On the fixed viewing plane, the phase discontinuity of mode l = 1 that is introduced by 1-bit phase quantization results in a wider Figure 3. Multiple functions of the 1-bit programmable reflective FSRM. Function I: 2D beam dynamic scanning. Function II: vortex-beam generation of OAM. Function III: specific splitting beams with desired transmission directions. ACS Applied Materials & Interfaces www.acsami.org Research Article ht tps://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 23559

reflection beam and energy diffusion at the edge portion of the array. The reflected vortex waves with higher-order modes have similar situations, which are consistent with the theoretical expectations. If the size of the metasurface is further increased, the aforementioned errors can be reduced. The method to realize different scanning beams and the OAM vortex at the same time is similar to the above method. It should be noted that, since the continuous compensation phases of the two functions affect the 1-bit phase quantization result of the units, the gain of the main beams in this case will decrease, and the beam pointing error will increase. The proposed FSRM has further capabilities to reshape the scattering distribution to generate the beam splitting effect. In order to verify the beam splitting performance, the metasurface elements are periodically encoded, such as 00001111..., and the specific pattern is shown in Figure 4d. When the metasurface coding is changed, the two main beams of the pattern can be symmetrically deflected in opposite directions. It can also be seen from the figure that the metasurface can realize different wavefronts of splitting beams in different coding situations. Similarly, other specially scattered beams can be generated by the metasurface, as shown in Figure 4d. The beam scattering effect is often used to reduce the radar cross section (RCS). When the above aspects are combined, the FSRM can achieve multiple functions in the spatial domain under the same aperture. In the Supporting Information, the simulation patterns of the metasurface in different spatial-domain modes are described in detail. We remark that the metasurface is excited by the horn antenna in Figure 4a−c, while the beams shown in Figure 4d are excited by plane waves. 3. EXPERIMENTAL VERIFICATION To experimentally verify the performance of the proposed FSRM, a metasurface sample with 12 × 12 elements was fabricated using a standard printed circuit board technology, as illustrated in Figure 5b. The operation and measurement process of the real-time reconfigurable system is described in the Supporting Information. The metasurface DC bias network and the bias PIN settings of each unit are shown in Figure S6. As illustrated in Figure 5a, two wideband horn antennas (transmitter #1 and receiver #2) are used to measure the polarization conversion performance of the metasurface in the frequency domain. The antennas are connected to a vector network analyzer, and the sample is placed in the center of the turn table. In operation, the transmitter horn antenna radiates the y-polarized incident wave onto the sample and the receiving horn antenna is used to receive x- and y- polarized reflected waves, respectively. The measured and simulated AR and phase difference Δϕ of the reflected E-field are compared in Figure 5c−f. The experimental results indicate good agreement with the simulated results, which demonstrates that the proposed device can complete three typical functions (frequency domain polarization Figure 4. Simulation analysis of the beam performance for the proposed 1-bit programmable metasurface in the spatial domain. (a) 2D scattering pattern in the θ direction and (b) electric-field intensity distributions of 2D energy focusing in four states. (c) The 1-bit coding patterns, reflected electric-field amplitude, and phase distributions of the OAM vortex waves with modes l = 1, l = 2, and l = 4, respectively. (d) The beam-direction control strategy in scattering patterns and special scattering patterns in different coding states. ACS Applied Materials & Interfaces www.acsami.org Research Article ht tps://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 23560

selection, frequency reconfigurable polarization conversion, and polarization reconfiguration) in different frequency bands. In addition, the measured AR illustrated in Figure 5 reveals that its 3-dB bandwidth is slightly less than the simulated result. The overall 3-dB relative bandwidth of the measured AR is approximately 34.2%. At the same time, the measured 3-dB AR frequency bands are slightly offset from the corresponding simulated frequency bands. These differences are caused by tolerances in the fabrication and measurement processes. On the other hand, a 2D scanning technique to evaluate the 1-bit programmable feature of the FSRM in the spatial domain is shown in Figure 6a. The scanning plane is 1.2 m × 1.2 m. For the beam scanning function, seven scanning angles (with step of 15°) are measured in xoz- and yoz-planes. It can be seen from the measured 2D scattering patterns at 6.3 GHz in Figure 6b (yoz-plane) that the actual main beam pointing directions in the seven states are −44.7°, −31.5°, −14.3°, 0°, 14.5°, 31.8°, and 45°, respectively. The steering angles are in good agreement with the theoretical predictions, which proves the single-beam deflection capability of the proposed metasurface. The measured beam scanning results in the xoz-plane are demonstrated in Figure S8. Certainly, more beam-deflection angles are expected through the ingenious design of the programming state imparted on the metasurface. Next, the experimental system shown in Figure 6a was performed to measure the reflected OAM vortex wavefront generated by the FSRM. The measured near-field amplitude and phase distributions of the vortex beams at different modes are shown in Figure 6c, respectively. These measured results indicate that different OAM modes of the vortex beams with l = 1 and l = 2 are consistent with the theoretical analysis and simulations. It was demonstrated that the continuous controls of the vortex beams with different transmission modes can be successfully realized using the metasurface. Meanwhile, the scattering patterns of FSRM in the spatial domain were also verified. As shown in Figure 6d, the two symmetrical scattered beams are modulated to direct in different deflection angles on the basis of three different periodic coding states (11110000..., 111000..., 1010...). Since the incident wave radiated by the horn antenna is not a perfect plane wave and the size of the metasurface prototype is relatively small, the main lobes of the scattering patterns are wider and the side lobes are a little larger. Accordingly, as the deflection angle increases, the symmetry of the main beams deteriorates. In addition, due to the blocking effect of the feed horn, the main beam’s gain of state 2 is smaller than that of state 1, but these errors are also useful for the RCS reduction function. 4. CONCLUSION In summary, a general FSRM modulation method and a novel FSRM have been proposed, theoretically and experimentally demonstrating the excellent performance of the reconfigurable metasurface in multifunctional EM manipulations within a shared aperture. When one controls the PIN diodes loaded on each metaparticle, the metasurface can implement the conversions from the LP waves to LHCP and RHCP waves in Figure 5. Experimental verification to the polarization conversion performance of FSRM. (a) The far-field experimental setup and (b) fabricated metasurface. The comparison of measured and simulated results of AR and Δϕ under the conversion (c) state 10, (d) state 01, (e) state 00, and (f) state 11. ACS Applied Materials & Interfaces www.acsami.org Research Article ht tps://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 23561

different frequency bands, respectively. It is worth mentioning that the polarization inversion technique is introduced in the metasurface, which makes the conversion vary with the polarization direction of the incident waves. Additionally, in the spatial domain, when the reflection phase of the metasurface units is encoded, the general cases of flexible-beam scanning, generation and continuous switch of different-mode OAM beams, and the beam split of reflected waves can be realized. The design strategies presented in this paper can also allow one to tailor the EM waves in many other intriguing ways for further exploration. The proposed metasurface can be extended to the optical and acoustic regimes and provides possibilities to perfectly manipulate EM waves in other applications such as satellite communications, MIMO (Multiple Input Multiple Output) scenarios, and holographic imaging. ■ ASSOCIATED CONTENT *sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c02467. Details on the FSRM elements, features of frequency- domain-controllable polarization converter, simulated 3D radiation pattern of FSRM in the spatial domain, the operating and measurement process of the real-time reconfigurable system, top view of the FSRM element, electric field distributions, functional characteristics of the frequency-domain reconfigurable polarization converter, amplitude and phase characteristics of the reflection coefficients, coding patterns and far-field simulation results, operating and measurement process of the real- time reconfigurable system, FSRM DC bias network, and measured beam scanning results (PDF) ■ AUTHOR INFORMATION Corresponding Authors Long Li − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China; orcid.org/0000- 0003-0472-7314; Email: lilong@mail.xidian.edu.cn Tie Jun Cui − State Key Laboratory of Millimeter Waves, Southeast University, Nanjing, Jiangsu 210096, China; orcid.org/0000-0002-5862-1497; Email: tjcui@seu.edu.cn Authors Guang Yao Liu − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China Jia Qi Han − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China Hai Xia Liu − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China Xiao He Gao − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China Yan Shi − Key Laboratory of High Speed Circuit Design and EMC of Ministry of Education, School of Electronic Engineering, Xidian University, Xi’an, Shaanxi 710071, China Complete contact information is available at: https://pubs.acs.org/10.1021/acsami.0c02467 Author Contributions ‡ G.Y.L., J.Q.H., and H.X.L. equally contributed to the present work. Figure 6. Experimental verification to the 1-bit programmable performance of FSRM. (a) The experimental setup and (b) the measured beam scanning results in the yoz-plane at 6.3 GHz. (c) The measured near-field amplitude and phase distributions of the vortex beams at modes l = 1 and l = 2. (d)The measured 2D scattering patterns with the different periodic coding sta tes. ACS Applied Materials & Interfaces www.acsami.org Research Article ht tps://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 23562

ACS Applied Materials & Interfaces www.acsami.org

Notes Simultaneous Control of Phase and Amplitude. Adv. Mater. 2014, 26,

5031−5036. (16) Xu, H. X.; Hu, G.; Han, L.; Jiang, M.; Huang, Y.; Li, Y.; Yang, X.; Ling, X.; Chen, L.; Zhao, J.; et al. Chirality-Assisted High-Efficiency Metasurfaces with Independent Control of Phase, Amplitude, and Polarization. Adv. Opt. Mater. 2019, 7, 1801479. (17) Xu, H.-X.; Sun, S.; Tang, S.; Ma, S.; He, Q.; Wang, G.-M.; Cai, T.; Li, H.-P.; Zhou, L. Dynamical control on helicity of electromagnetic waves by tunable metasurfaces. Sci. Rep. 2016, 6, 27503. (18) Black, L.-J.; Wang, Y.; de Groot, C. H.; Arbouet, A.; Muskens, O.

L. Optimal Polarization Conversion in Coupled Dimer Plasmonic

Nanoantennas for Metasurfaces. ACS Nano 2014, 8, 6390−6399. (19) Guo, Y.; Pu, M.; Zhao, Z.; Wang, Y.; Jin, J.; Gao, P.; Li, X.; Ma, X.; Luo, X. Merging Geometric Phase and Plasmon Retardation Phase in Continuously Shaped Metasurfaces for Arbitrary Orbital Angular Momentum Generation. ACS Photonics 2016, 3, 2022−2029. (20) Zhang, K.; Yuan, Y.; Ding, X.; Ratni, B.; Burokur, S. N.; Wu, Q. High-Efficiency Metalenses with Switchable Functionalities in Microwave Region. ACS Appl. Mater. Interfaces 2019, 11, 28423−28430. (21) Ma, X.; Pan, W.; Huang, C.; Pu, M.; Wang, Y.; Zhao, B.; Cui, J.; Wang, C.; Luo, X. An Active Metamaterial for Polarization Manipulating. Adv. Opt. Mater. 2014, 2, 945−949. (22) Wei, Z.; Cao, Y.; Su, X.; Gong, Z.; Long, Y.; Li, H. Highly efficient beam steering with a transparent metasurface. Opt. Express 2013, 21, 10739−10745. (23) Zhang, L.; Chen, X. Q.; Liu, S.; Zhang, Q.; Zhao, J.; Dai, J. Y.; Bai, G. D.; Wan, X.; Cheng, Q.; Castaldi, G.; Galdi, V.; Cui, T. J. Space-timecoding digital metasurfaces. Nat. Commun. 2018, 9, 4334. (24) Wang, Q.; Zhang, X.; Xu, Y.; Tian, Z.; Gu, J.; Yue, W.; Zhang, S.; Han, J.; Zhang, W. A Broadband Metasurface-Based Terahertz FlatLens Array. Adv. Opt. Mater. 2015, 3, 779−785. (25) Zhao, J.; Sima, B.; Jia, N.; Wang, C.; Zhu, B.; Jiang, T.; Feng, Y. Achieving flexible low-scattering metasurface based on randomly distribution of meta-elements. Opt. Express 2016, 24, 27849−27857. (26) Li, L.; Li, Y.; Wu, Z.; Huo, F.; Zhang, Y.; Zhao, C. Novel Polarization-Reconfigurable Converter Based on Multilayer Frequency-Selective Surfaces. Proc. IEEE 2015, 103, 1057−1070. (27) Yang, H.; Yang, F.; Xu, S.; Mao, Y.; Li, M.; Cao, X.; Gao, J. A 1-Bit 10 × 10 Reconfigurable Reflectarray Antenna: Design, Optimization, and Experiment. IEEE Trans. Antennas Propag. 2016, 64, 2246−2254. (28) Han, J.; Li, L.; Yi, H.; Xue, W. Versatile orbital angular momentum vortex beam generator based on reconfigurable reflective metasurface. Jpn. J. Appl. Phys. 2018, 57, 120303. (29) Arbabi, E.; Arbabi, A.; Kamali, S. M.; Horie, Y.; Faraji-Dana, M.; Faraon, A. MEMS-tunable dielectric metasurface lens. Nat. Commun. 2018, 9, 812. (30) Cui, T. J.; Qi, M. Q.; Wan, X.; Zhao, J.; Cheng, Q. Coding metamaterials, digital metamaterials and programmable metamaterials. Light: Sci. Appl. 2014, 3, e218. (31) Li, L.; Jun Cui, T.; Ji, W.; Liu, S.; Ding, J.; Wan, X.; Bo Li, Y.; Jiang, M.; Qiu, C.-W.; Zhang, S. Electromagnetic reprogrammable coding-metasurface holograms. Nat. Commun. 2017, 8, 197. (32) Zhang, X. G.; Tang, W. X.; Jiang, W. X.; Bai, G. D.; Tang, J.; Bai, L.; Qiu, C.-W.; Cui, T. J. Light-Controllable Digital Coding Metasurfaces. Adv. Sci. 2018, 5, 1801028. (33) Huang, C.; Zhang, C.; Yang, J.; Sun, B.; Zhao, B.; Luo, X. Reconfigurable Metasurface for Multifunctional Control of Electromagnetic Waves. Adv. Opt. Mater. 2017, 5, 1700485. (34) Maguid, E.; Yulevich, I.; Yannai, M.; Kleiner, V.; L Brongersma, M.; Hasman, E. Multifunctional interleaved geometric-phase dielectric metasurfaces. Light: Sci. Appl. 2017, 6, e17027. (35) Estakhri, N. M.; Alù , A. Recent progress in gradient metasurfaces. J. Opt. Soc. Am. B 2016, 33, A21−A30. (36) Albooyeh, M.; Tretyakov, S.; Simovski, C. Electromagnetic characterization of bianisotropic metasurfaces on refractive substrates: General theoretical framework. Ann. Phys. 2016, 528, 721−737. (37) Wu, H. T.; Wang, D.; Fu, X.; Liu, S.; Zhang, L.; Bai, G. D.; Zhang, C.; Jiang, W. X.; Jiang, H. L.; Wu, R. Y.; Wu, J. W.; Cui, T. J. SpaceThe authors declare no competing financial interest. ■

ACKNOWLEDGMENTS This work is supported by the Outstanding Youth Science Foundation of Shaanxi Province (2019JC-15) and the National Key Research and Development Program of China. T.J.C. acknowledges the support from the National Key Research and Development Program of China (2017YFA0700201, 2017YFA0700202, and 2017YFA0700203), the National Science Foundation of China (61631007 and 61571117), and the 111 Project (111-2-05). ■

REFERENCES (1) Ni, X.; Kildishev, A. V.; Shalaev, V. M. Metasurface holograms for visible light. Nat. Commun. 2013, 4, 2807. (2) Huang, L.; Chen, X.; Bai, B.; Tan, Q.; Jin, G.; Zentgraf, T.; Zhang, S. Helicity dependent directional surface plasmon polariton excitation using a metasurface with interfacial phase discontinuity. Light: Sci. Appl. 2013, 2, e70. (3) Falcone, F.; Lopetegi, T.; Laso, M. A. G.; Baena, J. D.; Bonache, J.; Beruete, M.; Marqué s, R.; Martín, F.; Sorolla, M. Babinet Principle Applied to the Design of Metasurfaces and Metamaterials. Phys. Rev. Lett. 2004, 93, 197401. (4) Yao, Y.; Shankar, R.; Kats, M. A.; Song, Y.; Kong, J.; Loncar, M.; Capasso, F. Electrically Tunable Metasurface Perfect Absorbers for Ultrathin Mid-Infrared Optical Modulators. Nano Lett. 2014, 14, 6526−6532. (5) High, A. A.; Devlin, R. C.; Dibos, A.; Polking, M.; Wild, D. S.; Perczel, J.; de Leon, N. P.; Lukin, M. D.; Park, H. Visible-frequency hyperbolic metasurface. Nature 2015, 522, 192−196. (6) Chen, K.; Feng, Y.; Monticone, F.; Zhao, J.; Zhu, B.; Jiang, T.; Zhang, L.; Kim, Y.; Ding, X.; Zhang, S.; Alù , A.; Qiu, C.-W. A Reconfigurable Active Huygens’ Metalens. Adv. Mater. 2017, 29, 1606422. (7) Zheng, G.; Mü hlenbernd, H.; Kenney, M.; Li, G.; Zentgraf, T.; Zhang, S. Metasurface holograms reaching 80% efficiency. Nat. Nanotechnol. 2015, 10, 308−312. (8) Yu, S.; Liu, H.; Li, L. Design of Near-Field Focused Metasurface for High-Efficient Wireless Power Transfer with Multifocus Characteristics. IEEE Trans. Ind. Electron. 2019, 66, 3993−4002. (9) Wu, P. C.; Zhu, W.; Shen, Z. X.; Chong, P. H. J.; Ser, W.; Tsai, D. P.; Liu, A.-Q. Broadband Wide-Angle Multifunctional Polarization Converter via Liquid-Metal-Based Metasurface. Adv. Opt. Mater. 2017, 5, 1600938. (10) Liu, Z.; Li, Z.; Liu, Z.; Cheng, H.; Liu, W.; Tang, C.; Gu, C.; Li, J.; Chen, H.-T.; Chen, S.; Tian, J. Single-Layer Plasmonic Metasurface Half-Wave Plates with Wavelength-Independent Polarization Conversion Angle. ACS Photonics 2017, 4, 2061−2069. (11) Luo, W.; Sun, S.; Xu, H.-X.; He, Q.; Zhou, L. Transmissive Ultrathin Pancharatnam-Berry Metasurfaces with nearly 100% Efficiency. Phys. Rev. Appl. 2017, 7, 044033. (12) Ding, X.; Monticone, F.; Zhang, K.; Zhang, L.; Gao, D.; Burokur, S. N.; de Lustrac, A.; Wu, Q.; Qiu, C.-W.; Alù , A. Ultrathin Pancharatnam−Berry Metasurface with Maximal Cross-Polarization Efficiency. Adv. Mater. 2015, 27, 1195−1200. (13) Yu, N.; Capasso, F. Flat optics with designer metasurfaces. Nat. Mater. 2014, 13, 139−150. (14) Gollub, J. N.; Yurduseven, O.; Trofatter, K. P.; Arnitz, D.; Imani, M. F.; Sleasman, T.; Boyarsky, M.; Rose, A.; Pedross-Engel, A.; Odabasi, H.; Zvolensky, T.; Lipworth, G.; Brady, D.; Marks, D. L.; Reynolds, M. S.; Smith, D. R. Large Metasurface Aperture for Millimeter Wave Computational Imaging at the Human-Scale. Sci. Rep. 2017, 7, 42650. (15) Liu, L.; Zhang, X.; Kenney, M.; Su, X.; Xu, N.; Ouyang, C.; Shi, Y.; Han, J.; Zhang, W.; Zhang, S. Broadband Metasurfaces with 23563 Research Article https://dx.doi.org/10.1021/acsami.0c02467 ACS Appl. Mater. Interfaces 2020, 12, 23554−23564 ACS Applied Materials & Interfaces www.acsami.org Research Article

Frequency-Domain Gradient Metamaterials. Adv. Opt. Mater. 2018, 6, 1801086. (38) Wu, H.; Liu, S.; Wan, X.; Zhang, L.; Wang, D.; Li, L.; Cui, T. J. Controlling Energy Radiations of Electromagnetic Waves via Frequency Coding Metamaterials. Adv. Sci. 2017, 4, 1700098. (39) Ratni, B.; de Lustrac, A.; Piau, G.-P.; Burokur, S. N. Electronic control of linear-to-circular polarization conversion using a reconfigurable metasurface. Appl. Phys. Lett. 2017, 111, 214101. (40) Xu, H. X.; Hu, G.; Jiang, M.; Tang, S.; Wang, Y.; Wang, C.; Huang, Y.; Ling, X.; Liu, H.; Zhou, J. Multiplexed Metasurfaces: Wavevector and Frequency Multiplexing Performed by a SpinDecoupled Multichannel Metasurface. Adv. Mater. Technol. 2020, 5, 2070005. (41) Zhang, N.; Chen, K.; Zheng, Y.; Hu, Q.; Qu, K.; Zhao, J.; Wang, J.; Feng, Y. Programmable Coding Metasurface for Dual-Band Independent Real-Time Beam Control. IEEE J. Emerg. Sel. Topic Circuits Syst. 2020, 10, 20−28. (42) Chen, K.; Ding, G.; Hu, G.; Jin, Z.; Zhao, J.; Feng, Y.; Jiang, T.; Alù , A.; Qiu, C. W. Directional Janus Metasurface. Adv. Mater. 2020, 32, 1906352. (43) Kamoda, H.; Iwasaki, T.; Tsumochi, J.; Kuki, T.; Hashimoto, O. 60-GHz Electronically Reconfigurable Large Reflectarray Using SingleBit Phase Shifters. IEEE Trans. Antennas Propag. 2011, 59, 2524−2531. (44) Skyworks Solutions, SMP1340-040LF Plastic Packaged PIN Diodes, https://www.rfmw.com/products/detail/smp1340040lfskyworks/415034/ (accessed 2019-05). (45) Grady, N. K.; Heyes, J. E.; Chowdhury, D. R.; Zeng, Y.; Reiten, M. T.; Azad, A. K.; Taylor, A. J.; Dalvit, D. A. R.; Chen, H.-T. Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction. Science 2013, 340, 1304. (46) Arbabi, A.; Horie, Y.; Bagheri, M.; Faraon, A. Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission. Nat. Nanotechnol. 2015, 10, 937−943. (47) Hum, S. V.; Perruisseau-Carrier, J. Reconfigurable Reflectarrays and Array Lenses for Dynamic Antenna Beam Control: A Review. IEEE Trans. Antennas Propag. 2014, 62, 183−198. (48) Yang, H.; Cao, X.; Yang, F.; Gao, J.; Xu, S.; Li, M.; Chen, X.; Zhao, Y.; Zheng, Y.; Li, S. A programmable metasurface with dynamic polarization, scattering and focusing control. Sci. Rep. 2016, 6, 35692. (49) Yuan, Y.; Zhang, K.; Ding, X.; Ratni, B.; Burokur, S. N.; Wu, Q. Complementary transmissive ultra-thin meta-deflectors for broadband polarization-independent refractions in the microwave region. Photonics Res. 2019, 7, 80−88. (50) Xu, H.-X.; Hu, G.; Li, Y.; Han, L.; Zhao, J.; Sun, Y.; Yuan, F.; Wang, G.-M.; Jiang, Z. H.; Ling, X.; et al. Interference-assisted kaleidoscopic meta-plexer for arbitrary spin-wavefront manipulation. Light: Sci. Appl. 2019, 8, 1−10.

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