0.2 λ0 Thick Adaptive Retroreflector Made of Spin‐Locked ... · Dr. H. Cai, Dr. A. Y. D. Gu, Dr....

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COMMUNICATION 1802721 (1 of 8) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de 0.2 λ 0 Thick Adaptive Retroreflector Made of Spin-Locked Metasurface Libin Yan, Weiming Zhu,* Muhammad Faeyz Karim,* Hong Cai, Alex Yuandong Gu, Zhongxiang Shen, Peter Han Joo Chong, Dim-Lee Kwong, Cheng-Wei Qiu,* and Ai Qun Liu* L. B. Yan, Dr. M. F. Karim, Prof. Z. X. Shen, Prof. A. Q. Liu School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798, Singapore E-mail: [email protected]; [email protected] Prof. W. M. Zhu School of Optoelectronic Science and Engineering University of Electronic Science and Technology Chengdu 610051, China E-mail: [email protected] Dr. H. Cai, Dr. A. Y. D. Gu, Dr. D.-L. Kwong Institute of Microelectronics A*STAR Singapore 117686, Singapore Prof. P. H. J. Chong Department of Electrical and Electronic Engineering Auckland University of Technology Auckland 1142, New Zealand Prof. C.-W. Qiu Department of Electrical and Computer Engineering National University of Singapore Singapore 117576, Singapore E-mail: [email protected] Prof. C.-W. Qiu NUS Suzhou Research Institute (NUSRI) Suzhou Industrial Park, Suzhou 215123, China DOI: 10.1002/adma.201802721 Metasurfaces are artificially designed ultrathin 2D materials composed of sub- wavelength resonators, which outperforms natural materials with extraordinary elec- tromagnetic properties. Metasurfaces with subwavelength thickness are capable to tailor the wavefront of the electromag- netic (EM) wave, which has led to various intriguing applications, [1,2] such as beam deflection, [3–8] beam splitting, [9–12] flat lens, [13–17] waveplates, [18–26] holograms, [27–32] cloaking, [33–36] and so forth. Further- more, metasurface with spin-based applications [37–44] has drawn consider- able attention due to the cutting-edge research field of spin optics. [45,46] The spin of light refers to the right- and left-handed circular polarization (RHCP and LHCP) of a light beam, whereby the handedness is dependent on the direction of propaga- tion. Recently, functionality of retroreflec- tion has been demonstrated using a doubly stacked metasurface, which is the first demonstration of retrore- flection using planar metasurfaces. Retroreflection is defined as the reflection of the EM wave propagating back along its incident direction. Devices like corner reflector and Luneburg lens are widely used for retroreflection. The schematic representation of corner reflector composed of mutually perpendicular flat surface is shown in Figure 1a. Clearly, they are bulky and nonplanar, and thus not natively compatible for integration and miniaturiza- tion. Subwavelength-thick planar metasurface with the capability to tailor the EM wave is a good candidate to realize flat ultrathin retroreflectors. Single metasurface has been used to demon- strate retroreflection. [47] The modified metallic square loops with designed geometry are used as unit cells to introduce phase shift to the oblique incident EM wave with TE-polarized incidence. Its static phase gradient can only give rise to retroreflection at a single angle of incidence. In addition to the gradient metasurface, a different design of metasurface called metagratings is proposed to realize retroreflection with unitary efficiency. [48] The split-ring wire loops and bianisotropic omega particles are proposed to form a grating instead of a phase gradient for wavefront manipulation. Nevertheless, retroreflection for spin-polarized EM waves is not demonstrated. Furthermore, this metasurface is only discussed theoretically without any experimental demonstration. A pio- neer work of flat retroreflectors accommodating various incident angles is demonstrated using cascaded metasurface composed of The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength-thick reconfigurable C-shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ 0 down to only 0.2 λ 0 . The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)-polarized and transverse electric (TE)-polarized incidences. With the phase gradient being engineered, an in-plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro-reflected wave identical to that of the incident light. Such spin-locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin-locked metasurface could find promising applications in spin-based optical devices, communication systems, remote sensing, RCS enhancement, and so on. Retroreflective Metasurfaces Adv. Mater. 2018, 30, 1802721

Transcript of 0.2 λ0 Thick Adaptive Retroreflector Made of Spin‐Locked ... · Dr. H. Cai, Dr. A. Y. D. Gu, Dr....

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CommuniCation

1802721 (1 of 8) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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0.2 λ0 Thick Adaptive Retroreflector Made of Spin-Locked Metasurface

Libin Yan, Weiming Zhu,* Muhammad Faeyz Karim,* Hong Cai, Alex Yuandong Gu, Zhongxiang Shen, Peter Han Joo Chong, Dim-Lee Kwong, Cheng-Wei Qiu,* and Ai Qun Liu*

L. B. Yan, Dr. M. F. Karim, Prof. Z. X. Shen, Prof. A. Q. LiuSchool of Electrical and Electronic EngineeringNanyang Technological UniversitySingapore 639798, SingaporeE-mail: [email protected]; [email protected]. W. M. ZhuSchool of Optoelectronic Science and EngineeringUniversity of Electronic Science and TechnologyChengdu 610051, ChinaE-mail: [email protected]. H. Cai, Dr. A. Y. D. Gu, Dr. D.-L. KwongInstitute of MicroelectronicsA*STARSingapore 117686, SingaporeProf. P. H. J. ChongDepartment of Electrical and Electronic EngineeringAuckland University of TechnologyAuckland 1142, New ZealandProf. C.-W. QiuDepartment of Electrical and Computer EngineeringNational University of SingaporeSingapore 117576, SingaporeE-mail: [email protected]. C.-W. QiuNUS Suzhou Research Institute (NUSRI)Suzhou Industrial Park, Suzhou 215123, China

DOI: 10.1002/adma.201802721

Metasurfaces are artificially designed ultrathin 2D materials composed of sub-wavelength resonators, which outperforms natural materials with extraordinary elec-tromagnetic properties. Metasurfaces with subwavelength thickness are capable to tailor the wavefront of the electromag-netic (EM) wave, which has led to various intriguing applications,[1,2] such as beam deflection,[3–8] beam splitting,[9–12] flat lens,[13–17] waveplates,[18–26] holograms,[27–32] cloaking,[33–36] and so forth. Further-more, metasurface with spin-based applications[37–44] has drawn consider-able attention due to the cutting-edge research field of spin optics.[45,46] The spin of light refers to the right- and left-handed circular polarization (RHCP and LHCP) of a light beam, whereby the handedness is dependent on the direction of propaga-tion. Recently, functionality of retroreflec-tion has been demonstrated using a doubly

stacked metasurface, which is the first demonstration of retrore-flection using planar metasurfaces. Retroreflection is defined as the reflection of the EM wave propagating back along its incident direction. Devices like corner reflector and Luneburg lens are widely used for retroreflection. The schematic representation of corner reflector composed of mutually perpendicular flat surface is shown in Figure 1a. Clearly, they are bulky and nonplanar, and thus not natively compatible for integration and miniaturiza-tion. Subwavelength-thick planar metasurface with the capability to tailor the EM wave is a good candidate to realize flat ultrathin retroreflectors. Single metasurface has been used to demon-strate retroreflection.[47] The modified metallic square loops with designed geometry are used as unit cells to introduce phase shift to the oblique incident EM wave with TE-polarized incidence. Its static phase gradient can only give rise to retroreflection at a single angle of incidence. In addition to the gradient metasurface, a different design of metasurface called metagratings is proposed to realize retroreflection with unitary efficiency.[48] The split-ring wire loops and bianisotropic omega particles are proposed to form a grating instead of a phase gradient for wavefront manipulation. Nevertheless, retroreflection for spin-polarized EM waves is not demonstrated. Furthermore, this metasurface is only discussed theoretically without any experimental demonstration. A pio-neer work of flat retroreflectors accommodating various incident angles is demonstrated using cascaded metasurface composed of

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength-thick reconfigurable C-shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ0 down to only 0.2 λ0. The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)-polarized and transverse electric (TE)-polarized incidences. With the phase gradient being engineered, an in-plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro-reflected wave identical to that of the incident light. Such spin-locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin-locked metasurface could find promising applications in spin-based optical devices, communication systems, remote sensing, RCS enhancement, and so on.

Retroreflective Metasurfaces

Adv. Mater. 2018, 30, 1802721

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one transmissive metasurface and a reflective metasurface.[49] The design is sketched in Figure 1b, where the transmissive metas-urface functions as a spatial Fourier transform to focus EM wave at various incident angles to the corresponding locations of the reflective metasurface. Different phase gradients are pre-engi-neered at different locations of the reflective metasurface, in order to bestow corresponding momentum to the reflective wave. How-ever, such cascaded metasurface relies on its spacing distance between those two constituting metasurfaces. This will drastically increase the overall thickness (nearly 590 λ0).[49] It is quite chal-lenging to reduce this thickness, because a decently longer prop-agation distance will result in a larger footprint for every phase gradient to be well engineered. Furthermore, in their design, the retroreflection efficiency under TM-polarized incidence is much lower than TE-polarized incidence, which is because that TM-polarized incidence excites an additional axial magnetic reso-nance mode besides the in-plane electric resonance mode, while TE-polarized incidence only excites the in-plane electric resonance mode.[49] This polarization dependence makes it impossible to handle spin-polarized waves. Here, we demonstrate a spin-locked retroreflection at various angles based on a single adaptive metasurface with 0.2 λ0 thickness, whereby the handedness of the retroreflection is kept the same as the incidence (Figure 1c). The reconfigurable C-shaped resonators (RCRs) are specifically engi-neered to become building blocks of the metasurface. The orien-tation of the RCRs is designed to be 45° or 315°. In such cases, both the amplitude and phase of cross-polarized reflection are approximately equal for TE- and TM-polarized incidences at dif-ferent incident angles.[50] Assuming that the RHCP incident EM wave with incident angle of a is propagating along a, where a is the position vector, the incident E-field can be expressed as

, ,0In TE TM( )eE iE E i k a ta

( )= ω⋅ − (1)

where ω is the angular frequency,

k a is wave-vector along a, and i is the imaginary unit (i2 = −1).

Then, the E-field of retroreflection can be expressed as

, ,0RT TM TE( )p,s s,pE A iE e E e ei i i k a ta

( )= ϕ ϕ ω⋅ ′−′ (2)

where A is amplitude modulation and 0 < A < 1; ϕp,s and ϕs,p are the introduced phase by the metasurface to the cross-polarized reflection for the incidence of ETE and ETM, respectively; a′ is

the position vector of retroreflection with propagation direction that is opposite to a, and k a

′ is the wavevector of retroreflection. Because of the symmetry of RCR,[50] ϕp,s and ϕs,p are approxi-mately identical within the range of oblique incident angles studied in this work. Therefore, we can assume ϕp,s = ϕs,p = ϕ. Then, Equation (2) can be simplified as

, ,0RT TM TE( )E iE E ei k a ta

( )= ω ϕ⋅ ′− +′ (3)

It is clear from Equations (1) and (3) that the reflection and incidence carry the same optical spin. Therefore, spin-locked retroreflection can be realized using RCR-based metasurface. Such adaptive metasurface is designed to meet the condition

θ θ λ= = Γ−sin ( /2 )RT In1 (4)

where θRT is the angle of retroreflection equal to the incident angle θIn, λ is the wavelength of EM wave, and Γ is the period of the metasurface covering the 2π phase range. Retroreflec-tion at various angles can be achieved by tuning the length of Γ.

Figure 2a presents the schematic representation of spin-locked retroreflection realized by RCR-based adaptive meta-surface. The top layer is composed of subwavelength RCRs to interact with incident EM wave. The middle is dielectric layer. The bottom is a perfect electric conductor (PEC) layer to enhance retroreflection. The RCRs are designed with orienta-tions along either 45° or 315° and different gap opening. Two π phase modulation of the EM wave can be realized by changing the gap opening and orientation of the RCR. In Figure 2a, eight RCRs (highlighted in red) form one period of the metas-urface. Each RCR is realized in a microfluidic ring channel by injecting liquid metal through the respective inlet. The gap of each RCR is generated by inputting air through the air inlet, as shown in Figure 2b. The orientation of each RCR along either 45° (Figure 2b) or 315° (Figure 2c) is controlled by switching the air inlet located at the direction of 45° and 315° via pneu-matic valves. Each time, only one of the air inlets is selected and pumped based on the desired gap orientation and opening, which can be tuned by a pneumatic control system.[51–55] There-fore, the period of the metasurface can be actively adjusted. As an example, Figure 2d,e shows the period of metasur-face composed of eight and six RCRs, respectively. Based on Equation (4), our spin-locked metasurface could adapt to dif-ferent incident angles for optimized retroreflection by adjusting

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Figure 1. Comparison among bulky device, cascaded metasurface, and adaptive metasurface for retroreflection. Schematic representation of: a) bulky corner reflector, b) cascaded metasurface, and c) adaptive metasurface.

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the period Γ of the metasurface via tuning the geometry of each RCR.

The optical property of RCR for TE- and TM-polarized inci-dence with various incident angles at 15 GHz is analyzed using the microwave studio module of Computer Simulation Tech-nology software. Figure 3a,b shows the simulated colormaps of cross-polarized reflection amplitude as a function of the inci-dent angle of EM wave and gap openings of RCR under TE- and TM-polarized incidence, respectively. The orientation of RCR is set along 45°. When the incident angle of EM changes from 0° to 20° and the gap opening of RCR varies from 30° to 180°, the reflection amplitude of both TE and TM incidences is approximately identical. Under normal incidence, there is no difference between the reflection amplitude for TE- and TM-polarized incidence with various gap opening of RCR (Figure 3e). There is negligible deviation of the reflection ampli-tude between these two perpendicular polarized incidences

with oblique incidence, the largest deviation of the amplitude is around 1.3% with incident angle of 20° (Figure 3f). Further, Figure 3c,d shows the phase shift of the scattered EM wave as a function of incident angle and gap opening of RCR for TE- and TM-polarized incidence, respectively. With a fixed incident angle, the phase shift of the reflected EM wave changes with the gap opening of RCR. Nearly π phase shift coverage can be realized by changing the gap opening from 30° to 180° with various incident angles from 0° to 20°. The phase shift is iden-tical under normal incidence of both TE- and TM-polarization (Figure 3e), while it is approximately identical under oblique incidence. The biggest deviation of phase shifts between TE- and TM-polarized incidence is around 0.056 π with incident angle of 20° (Figure 3f), which has trivial effect to the perfor-mance of the metasurace. There is negligible difference of both amplitude and phase between TE- and TM-polarized incidence because the additional magnetic resonance mode excited by TM-polarized incidence is weak for the working frequency. To realize retroreflection, 2 π phase shift is needed to fully con-trol the wavefront of the EM wave. With orientation of 45°, phase shift coverage reaches π by changing the gap opening of RCR. Another π-range of phase shift can be realized using RCR orienting along 315° with various gap opening as shown in Figure S1c,d in the Supporting Information, while the ampli-tude of reflection is identical to the case when RCR’s orientation is along 45° (see Figure S1a,b in the Supporting Informa-tion). Figure 3e,f shows the examples that with either normal incidence or oblique incidence at 20°, 2 π phase shift can be realized by changing the gap opening and orientation of RCR, while the amplitude and phase shift are approximately equal for TE- and TM-polarized incidence.

The metasurface is adaptive to incidence with various angles to realize retroreflection while locking the spin characteristics of the EM wave. Based on Equation (4), spin-locked retroreflec-tion with various angles is realized by changing the length Γ of the period of our adaptive metasurface. As a proof of prin-ciple, the spin-locked retroreflection is demonstrated at 15 GHz (λ = 20 mm) with various incident angles at 10°, 12°, 15°, and 20°. The period Γ of the adaptive metasurface is tuned to be 3λ, 2.5λ, 2λ, and 1.5λ, respectively. The geometries of the RCRs to compose these periods of the adaptive metasurface are listed in Table S1 in the Supporting Information, which are realized by tuning the gap opening and orientation of each RCR of the metasurface. Figure 4a,b shows the simulated results of cross-polarized scattered E-fields for TE- and TM-polarized incidence with incident angle of 10°, respectively. In this case, the period Γ of the metasurface is 3λ, which is composed of twelve RCRs. The EM-waves are reflected to the incident direction of 10° for both TE- and TM-polarized incidence as retroreflections. By tuning the geometries of the RCRs to change the period Γ of metasurface into six RCRs with length of 1.5λ, retroreflection is realized for TE- (Figure 4c) and TM-polarized (Figure 4d) incidence with incident angle of 20°. Similarly, retroreflection at 12° and 15° are achieved as shown by the scattered E-fields in Figure S2 in the Supporting Information.

The fabricated adaptive metasurface consists of RCRs with period of 5 mm, as shown in Figure 5a. The radius, width, and thickness of the RCR are 2, 0.4, and 0.1 mm respec-tively. RCR is formed by injecting liquid metal Galinstan

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Figure 2. a) Schematic representation of spin-locked adaptive meta-surface, where RCRs are the building blocks and retroreflection can be realized in various angles. RCR formed by liquid metal (depicted in yellow) and air gap (depicted in blue) with orientation along b) 45° and c) 315°. Period of metasurface is tuned with d) eight RCRs and e) six RCRs.

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(σ = 3.46 × 106 S m−1) into the conformal microfluidic ring channel. The ring channel is fabricated in 2 mm thick polydi-methylsiloxane (PDMS, εr = 2.69) using soft lithography. The gap opening of the RCR is formed by partially expelling out Galinstan in the microfluidic-ring channel using hydrochloric vapor. The gap opening of RCR can be tuned from 5° to 180° by injecting the hydrochloric vapor with different pressure. The orientation of RCR is controlled by injecting the hydrochloric vapor through the selected air inlet along either 45° or 315°. The pneumatic control system is applied to control the geom-etry of each RCR. The middle dielectric layer is designed with 1.25 mm thick PDMS mounted on 1 mm thick poly(methyl methacrylate) (εr = 2.57). A 23 cm × 23 cm copper plate is designed at the bottom as the PEC layer. Figure 5b,c shows the

zoom-in view of RCR with different gap opening, controlled by the pneumatic system.

The spin-locked retroreflection is verified by experimental measurements in an anechoic chamber with a setup of two wideband double-ridged horn antennas (HD-20180DRHA10SK) connected with a vector network analyzer (Agilent N5230A). The metasurface is put at the center of an arc track with radius of 1.2 m. The two horn antennas are mounted on the arc track. One of the horn antenna serves as the source to illuminate oblique incidence to the adaptive metasurface. By rotating the antenna, TE- and TM-polarized EM wave can be applied. Another horn antenna works as receiver moving along the arc track to measure the far-field cross-polarized reflection at various angles. Figure 6a,b shows the simulated

Adv. Mater. 2018, 30, 1802721

Figure 3. Simulated colormaps of cross-polarized reflection amplitude for: a) TE- and b)TM-polarized incidence with respect to the incident angle and gap opening, while the orientation is 45°. Simulated colormaps of cross-polarized reflection phase for: c) TE- and d) TM-polarized incidence with respect to the incident angle and gap opening, while the orientation is 45°. Simulation results of reflected amplitude and phase as a function of gap opening and the orientation of the RCR with incident angle of: e) 0° and f) 20°.

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Figure 4. Simulated E-field of spin-locked retroreflection at 10° for: a) TE- and b) TM-polarized incidence with period of metasurface composed of twelve RCRs. Simulated E-field of spin-locked retroreflection at 20° for: c) TE- and d) TM-polarized incidence when the period is tuned to be six RCRs.

Figure 5. a) An overview photograph of the fabricated adaptive metasurface. b,c) Zoom-in view of array of RCRs with different gap opening and orientation, which both can be actively controlled using the pneumatic control system.

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and experimental results of far-field cross-polarized scattered intensity distribution for retroreflection at various angles of 10°, 12°, 15°, and 20°, under TM- and TE-polarized incidence, respectively. The scattered intensity is normalized against the efficiency of normal retroreflection when the adaptive meta-surface is without phase gradient with RCR’s gap opening of 60°. Adaptive to the incident angle, the period Γ of the meta-surface is tuned to be 60, 50, 40,and 30 mm, corresponding to the incident angles at 10°, 12°, 15°, and 20°, respectively. When the adaptive metasurface is tuned at a certain configuration, retroreflection at the same angle is realized for both TM- and TE-polarized incidence. The colored solid lines are the simu-lated results while the colored dash lines are the experimental results, which are in good agreement with each other for both TM- and TE-polarized incidence. The relation between the angle of retroreflection and the period Γ of adaptive metasurface is further illustrated in Figure S3 in the Supporting Information. The solid line represents the calculation based on Equation (4), while the symbol of blue circle and red star represent meas-ured retroreflection for TM- and TE-polarized incidence, respectively. The measured results fit well with the calculated results. In Figure 6c, the measured difference of efficiency between retroreflections at various angle for TM-(red sphere) and TE-polarized (blue cube) incidence is less than 1%, which agrees well with simulated results (solid lines). In addition, the measured phase difference (blue star) between retroreflections under TE- and TM-polarized incidence is less than 0.033 π, as

shown in Figure 6d. These results solidly verify the feasibility of this metasurface for circular polarized EM waves and the spin-locked retroreflection for different incident angles.

In conclusion, we demonstrated, for the first time, a spin-locked retroreflection using adaptive metasurface composed of RCRs with subwavelength thickness working at 15 GHz. The symmetry of RCR enables the adaptive metasurface to realize spin lock. By tuning the period Γ of the adaptive metasurface, spin-locked retroreflection at various angles has been demon-strated. With the advancement of microfluidic technology,[56,57] the adaptive metasurface can be implemented in THz or even optical region. The adaptability, spin-lock, and subwavelength-thick features of the adaptive metasurface offer an additional degree of freedom in manipulating vortex EM waves with practical applications in compact system of communication, sensing, information processing, and so on.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThe work was supported by Ministry of Education (MOE) AcRF Tier 3 Funding, Singapore (Grant No. MOE 2017-T3-1-001), MOE, Singapore

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Figure 6. Cross-polarized scattered intensity versus reflection angle for: a) TM- and b) TE-polarized incidence with various incident angles at 10°, 12°, 15°, and 20°. c) The efficiency for retroreflection at various angles for TE- and TM-polarized incidence. d) The phase difference between retroreflections under TE- and TM-polarized incidence at various angles.

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(Grant No. RG 89/13), and Singapore National Research Foundation under the Incentive for Research & Innovation Scheme (1102-IRIS-05-04) administered by PUB. C.-W.Q. acknowledges the financial support from the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP award NRFCRP15-2015-03). This work was supported by National Natural Science Foundation of China (NSFC) (61731010).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsadaptive metasurfaces, retroreflection, spin-lock, subwavelength-thickness

Received: April 28, 2018Revised: June 29, 2018

Published online: August 20, 2018

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