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Abstract
Owing to the intriguing capability of manipulating electromagnetic (EM) properties, the metasurface has aroused great attention of researchers and promoted its applications in EM invisibility. However, there are strong demands to provide an efficient transparent window for signals transmitting in EM invisibility devices. Here, we propose a scheme of a circular dichroism assisted metadevice to provide efficient transmission and broadband absorption in microwave frequencies. By employing chiral meta-atoms to introduce a strong asymmetric response for circularly polarized waves, a chiral metadevice for spin-selective absorption with an efficient transmission is presented. Then, we couple four chiral atoms into a polarization-insensitive atom pair, thus the achiral metadevice presents an identical high-efficiency absorption for both the x- and y-polarized wave. Here, both the chiral and achiral metadevices are realized by loading the metasurface-based absorber on a bandpass frequency selective surface. A proof-of-prototype is fabricated to verify the achiral design. The simulated and experimental results have demonstrated wideband, high-efficiency, polarization-insensitive absorption and high in-band transmission. Interestingly, the proposed paradigm can not only provide the potential for chirality-enhanced absorber design but also may trigger applications in spin-dependent systems, stealth antenna systems, and EM camouflage devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, metasurface, the two-dimensional counterpart of metamaterial, has presented promising potentials in electromagnetic (EM) properties manipulation and miniaturized integration. By properly arranging artificial subwavelength structures on the plane, the amplitude, phase, and polarization of incident EM waves can be sophisticatedly engineered [1–3]. Compared with metamaterial, the planar profile of metasurface bypasses the problem of large bulk and complicated fabrication, exhibiting characteristics of low power consumption and deep subwavelength thickness for chip integration. To date, metasurface has been widely investigated in research fields, boosting its applications in beam-forming [4–6], holographic display [7–9], perfect absorber [10–12], and so on. Due to the attractive features of metasurface, chiral metasurface is gaining increasing attention for chiroptical effects and phenomena, such as chirality-enhanced optical activity, circular dichroism (CD), and asymmetric lens imagining. With the help of multidimensional EM characteristics modulation, a lot of interesting metasurface-based optical applications are realized, including asymmetric transmitter [13–15], chiral lens imaging [16–19], circular polarizer [20–22], and CD absorber [23–25].
CD, the differential absorption for left-handed and right-handed circularly polarized waves, has been widely applied to molecular biology, analytical chemistry, and physical electronics [26–29]. Accordingly, CD metasurface (CDM) is of great significance to realize efficient spin-dependent absorption for circularly polarized (CP) waves in a planar structure. By optimizing the design and configuration of chiral patterns, researchers have proposed high CD value [30] and broadband absorption [31] CDMs in microwave frequencies. Compared with traditional absorbers [32–33], these CDMs have better frequency selectivity and good absorptive efficiency in less thickness, which is highly suitable for practical application. However, efficient transmission is also essential for EM invisibility devices to exchange information, especially communication systems. Therefore, the CDM and frequency selective surface (FSS) combination seems to be a valid way to provide a spin-selective absorption with a transparent window. Moreover, owing to the excellent performance of CDM, it is an alternative method for perfect absorber design by breaking the asymmetry of CDM to realize identical absorption for different polarization states.
In this paper, we propose a method of designing metadevice based on CD to realize broadband absorption with an efficient transmission. Firstly, a chiral metadevice composed of a bandpass FSS, an ultra-thin CDM comprising of CD meta-atoms and a dielectric spacer is designed. The ultra-thin CDM employs split rings inserted with resistive film to provide efficient spin-selective absorption for CP waves. And the bandpass FSS is served as a metal ground for the chiral absorber outside the passband and transmitting window at low frequency with low insertion loss. Consequently, the chiral metadevice presents broadband absorption for left-handed circularly polarized (LCP) waves with high CD value and efficient transmission. Then we change the CD meta-atoms of CDM with C4 2×2 CD meta-atom pairs to switch the chiral metadevice to achiral metadevice. Strong coupling between the CD meta-atoms has been motivated and makes a great contribution to the absorption of right-handed circularly polarized (RCP)waves. In this case, a polarization-independent absorption with a high-efficiency transmission is realized. Here, the achiral metadevice is demonstrated by theory analysis, simulation, and experimental measurement. Encouragingly, the proposed CD-assisted metadevice opens a new way of absorber design and promotes applications in EM shielding and invisibility.
2. Results
The construction of the proposed chiral metadevice is illustrated in Fig. 1(a)-(c). It is composed of three parts: an ultra-thin CDM, an FSS, and a PMI foam spacer between them. The ultra-thin CDM consisted of a 0.1 mm FR4 dielectric substrate (ɛr=4.3, tanδ=0.025) and the CD meta-atom arrays, which are dual-split ring resonators inserted with resistive film. The square-shaped metallic chip and a 0.5 mm thick F4B (ɛr = 2.65, tanδ=0.001) dielectric substrate constituted the FSS, which is designed for high rejection and low-pass characteristics. The optimized geometrical structure parameters of the chiral metadevice are depicted in the caption of Fig. 1. The simulated reflectivity and transmissivity of the FSS under the normal incidence of CP waves are given in Fig. 1(d). Obviously, the reflectivity under CP wave incidence is larger than -1dB above 7.42 GHz, and the transmissivity is greater than -1dB below 1.97 GHz. Figure 1(e)-(g) illustrates the simulated reflections, transmissions, and absorptions for CP waves normally illuminating onto the designed structure. It is shown that both the normally incident LCP and RCP waves are highly transmitted below 1.5GHz with insertion loss less than 1 dB. The absorbance for normally incident LCP wave exceeds 0.9 from 8.83 to 16.58 GHz and 0.8 spans from 7.78 to 20.00 GHz. However, the normal incidence RCP wave is efficiently reflected and converted into LCP wave with the circular cross-polarization reflection rLR greater than 0.9 over a wide frequency range from 9.0 to 15.5 GHz.
Fig. 1. (a) Perspective view of the chiral metadevice (The repetition period 2a is twofold of the repetition period of the CD meta-atoms, a=6.5 mm, the edge length of the metallic square patch b=11.5 mm, the thickness of the PMI foam d=5 mm). (b) Front view of the FSS. (c) Front view of the CD meta-atoms (The outer diameter of the dual-split ring is r=2.9 mm and the metal width is 0.25 mm, the split width s=2.2 mm, the loaded resistive film width s0=3 mm, the square resistance of the resistive film R0=60Ω/sq, and the angle between the two splits γ=90°). (d) Simulated reflectivity and transmissivity of the FSS under CP wave normal incidence. (e-g) Simulated reflections, transmissions, and absorptions for CP wave normal incidence onto the designed metadevice.
As for the ultra-thin CDM, it is well known that spin-selective must be implemented by chiral structures which can simultaneously break the n-fold rotational (n>2) and mirror symmetries. And for the reciprocal, lossless, and passive infinitesimally-thin metasurfaces, we can derive the nonlinear equation as follows,
where S represents the complex reflection or transmission coefficient (R or T). From above formula, the amplitudes of the circular cross-polarization coupling coefficients |SRL(LR)| are determined only by the circular co-polarization coupling coefficients SRR(LL). The maximum circular cross-polarization coupling efficiency is |SRL(LR)|2=0.25 as the amplitude of the circular co-polarization coupling coefficient sLL(RR)=0.5. But for the ultra-thin lossy metasurfaces, the linear cross-polarization coupling coefficients are identical, i.e., Sxy=Syx. Thus the circular co-polarization coupling coefficients are identical for LCP and RCP incident waves, but the circular cross-polarization coupling coefficients can be different. This will lead to differential absorption of the LCP and RCP waves. According to the above analysis, a resistive film-loaded chiral meta-atom is designed to achieve spin-sensitive absorption.The reflections, transmissions, and absorptions of the ultra-thin CDM under CP wave illumination are simulated, and the results are depicted in Fig. 2. Figure 2(a)-(d) give the simulated results of the ultra-thin CDM illuminated from the front direction, and Fig. 2(e)-(h) depicts the results for CP wave incidence from the back side. The simulated results reveal inverse CD of the ultra-thin CDM for CP wave incidence from opposite directions. As the CP wave is incident from the front side, the cross-polarization reflection and transmission are identical and greater than 0.4 in a frequency region 9.6-23.2 GHz for RCP incident wave, but less than 0.4 below 20.8 GHz for LCP incident wave. The absorption of LCP wave is over 0.4 from 9.3 to 23.8 GHz, but the absorption is less than 0.1 under the illumination of RCP wave over a wide frequency range from 9.5 to 21.1 GHz. Thus it can be considered as a left-handed CD meta-atom as illuminated from the front side, while it is a right-handed CD meta-atom as CP wave incidents from the backside. In this case, anti-symmetrical reflections and transmissions are realized, and the absorption of RCP wave is much greater than that of LCP wave. Compared with previous chiral absorbers, the proposed chiral metadevice achieves broadband chiral absorption with high efficiency and efficient transmission with an ultra-thin thickness, as shown in Table 1.
Fig. 2. Simulations of the ultra-thin CDM normally illuminated by CP wave from both sides. (a) Schematic view of the ultra-thin CDM illuminated by CP wave from the front side, (b) Amplitudes of the simulated reflection and transmission coefficients for CP wave illuminating from the front side, (c) Phases, and (d) Absorptions. (e) Schematic view of the ultra-thin CDM illuminated by CP wave from the backside, (f) Amplitudes of the simulated reflection and transmission coefficients for CP wave illuminating from the backside, (g) Phases, and (h) Absorptions.
Table 1. Comparison between previous works and our work
To reveal the underlying physical mechanism of the chiral metadevice, we introduce the interference theory to analyze the broadband absorption and polarization conversion quantitatively. The ultra-thin CDM is assumed to be a surface with zero thickness. The proposed absorber can be treated as a decoupled system, in which the near-field coupling between the CDM and the FSS back-sheet can be neglected. As illustrated in Fig. 3, the chiral metadevice is composed of two interfaces: a CDM and an FSS back-sheet. The two interfaces are separated by the PMI foam. The CDM acts as a part of the reflection surface which can reflect/transmit part of the incident wave as presented in Fig. 2. $R_{LL(RR)}^ +{=} |{R_{LL(RR)}^ + } |{e^{i\varphi _{LL(RR)}^{r + }}}$ and $T_{LL(RR)}^ +{=} |{T_{LL(RR)}^ + } |{e^{i\varphi _{LL(RR)}^{t + }}}$ are circular co-polarization coupling coefficients of CDM illuminated from the front side, while $R_{LL(RR)}^ -{=} |{R_{LL(RR)}^ - } |{e^{i\varphi _{LL(RR)}^{r - }}}$ and $T_{LL(RR)}^ -{=} |{T_{LL(RR)}^ - } |{e^{i\varphi _{LL(RR)}^{t - }}}$ are circular co-polarization coupling coefficients of CDM illuminated from the backside. The FSS back-sheet is worked as a perfect reflector in the absorption band of the chiral metadevice. Thus the transmission in this frequency region is approximately zero. According to the interference theory, the overall reflection is then the superposition of the multiple reflections: [34]
Fig. 3. (a) Interference model of the chiral metadevice. (b) Calculated absorption and polarization conversion reflectance under the normal incidence of CP wave according to interference theory compared with the simulated results.
According to interference theory, the sum of absorption and polarization conversion reflectance in the decoupled system is calculated using the simulated reflection and transmission coefficients of the ultra-thin CDM presented in Fig. 2. The calculated results compared with the simulated results are depicted in Fig. 3. The theoretical results reveal excellent agreement with the simulations, and both validate the interference model. It is observed that the designed chiral metadevice can achieve wideband, high-efficiency absorption, and polarization conversion reflection for LCP and RCP incident wave, respectively. The absorption is dominative under the illumination of LCP wave, and the polarization conversion reflectance is much larger than absorption for RCP incident wave.
Using the designed chiral metadevice, coupling-induced polarization-insensitive metadevice is achieved, as illustrated in Fig. 4(a). It consists of an ultra-thin metasurface comprising of C4 2×2 CD meta-atom pair, the FSS with high rejection low-pass characteristics, and a foam dielectric substrate between them. The FSS is the same as that designed in the chiral metadevice. The ultra-thin metasurface is composed of C4 CD-meta-atom pairs printed on a 0.1 mm thick FR4-dielectric substrate, and four identical CD meta-atoms with different in-plane azimuth angles (β=0°, 90°, 180°, and 270°) constituted the C4 CD-meta-atom pair as shown in Fig. 4(a). Although the C4 CD-meta-atom pair consists of four left-handed CD meta-atoms, it presents polarization insensibility. That is, all the reflections, transmissions, and absorptions have no concern with the polarization of both linearly polarized (LP) and CP waves. This is demonstrated by the simulated reflections, transmissions, and absorptions of the designed polarization-insensitive metadevice under normal incidence of LP wave depicted in Fig. 4(d)-(f). It is observed that both y- and x-polarized waves are strongly absorbed with an absorption greater than 0.9 over a wide frequency range from 8.25 GHz to 17.36 GHz. Below the frequency 1.5 GHz, the y- and x-polarized incident waves efficiently transmit the polarized-insensitive metadevice with insertion loss less than 1 dB. To verify the simulated results, a prototype is fabricated for further demonstration, as shown in Fig. 4(b)-(c). The ultra-thin C4 CD-meta-atom pair metasurface and FSS back-sheet are firstly fabricated using print circuit board (PCB) technique, and then the final sample is derived via printing the resistive film at the splits using screen-printing technology. The metadevice prototype is measured by the free-space method in an anechoic chamber. The measured performance under the normal incidence of LP wave is illustrated in Fig. 4(g). Experimental results and simulations are in good agreement. In addition, the deviation between the measured and simulated results may be attributed to the fabrication accuracy and measurement errors.
Fig. 4. (a) Front view of the C4 CD-meta-atom pair, (b) Photograph of the fabricated FSS, foam and ultra-thin C4 metasurface prototypes, (c) Photograph of the fabricated achiral metadevice prototype, (d-f) Simulated reflections, transmissions, and absorptions of the achiral metadevice under the normal incidence of LP waves, (g) Measured transmissivity and reflectivity.
Moreover, the incidence angle dependence of the transmissivity and absorption for LP wave incidence onto the designed achiral metadevice is considered. The transmissivity and absorption spectrums are simulated and depicted in Fig. 5. In the figures, the x-coordinate represents the frequency, and the y-coordinate denotes the incidence angle. It can be observed from the figures that under y-polarized wave incidence, as the incidence angle increases from 0° to 60°, the absorption and its bandwidth decrease slightly except for some obvious oscillations, but the absorption decreases sharply as the incidence angle is greater than 60°. The transmissivity in the passband decreases with increasing incidence angle. Under incidence of the x-polarized wave, the absorption band has a blue-shift with increasing incidence angle, and high-efficiency absorption over 0.9 is obtained as the incidence angle is less than 45°. In the passband, the transmissivity is nearly independent of the incidence angle, and the passband is broadened with increasing incidence angle.
Fig. 5. Incidence angle dependence of the absorption and transmission for the achiral metadevice. (a) Simulated absorption spectrum under the incidence of x-polarized wave A(f, θ) and (b) y-polarized wave A(f, θ), (c) Transmittivity spectrum for x-polarized incident wave txx2(f, θ) and (d) y-polarized wave tyy2(f, θ).
Obviously, the reflections, transmissions, and absorptions under the normal incidence of LP wave are absolutely independent of the polarization angle. The absorption over 0.8 is achieved from 9.5 GHz to 18.5 GHz for both y- and x-polarized waves. As is well known, the LP incident wave can be decomposed into two beams of CP wave, one beam is LCP, the other is RCP. The C4 CD-meta-atom pair is composed of four left-handed CD meta-atoms, which efficiently absorb LCP waves and convert RCP waves into LCP waves. According to the theoretical prediction, the decomposed RCP wave can't be efficiently absorbed by the C4 CD-meta-atom pair. But in fact, both LCP and RCP waves are absorbed by the C4 unit cell. This is attributed to the couplings between adjacent CD meta-atoms. To visualize the coupling effect, the surface current distributions on the CD meta-atom and C4 unit cell under the normal incidence of CP waves are simulated and presented in Fig. 6. It is observed that under LCP wave incidence onto the CD meta-atom, the surface currents are enhanced at the left part of the dual-split ring, where the resistive film is located. Thus strong absorption is introduced, which originates mainly from the ohmic loss of the resistive film. Under RCP wave incidence, the surface currents are primarily gathered at the right part of the dual-split ring. Dipolar resonance resulted in high-efficiency circular polarization conversion. For LCP waves normally illuminating onto the C4 unit cell, the surface currents on the CD meta-atoms I and III are opposite, and which are also opposite on the CD meta-atoms II and IV. Between the surface currents on the meta-atoms II and III, the phase difference is about π/2. Under RCP wave incidence, the phase differences between the surface currents on meta-atoms in the diagonal directions are also π, and that between the surface currents on the meta-atoms in the horizontal or vertical directions are π/2. The difference is that the surface current on meta-atom III lags behind that on meta-atom II for LCP incident wave, but the surface current on meta-atom II lags behind III under RCP wave incidence. For both LCP and RCP incident waves, the coupling between the horizontal counters resulted in a symmetric mode, and the coupling between the vertical counters led to an anti-symmetric mode.
Fig. 6. Surface current distributions of the ultra-thin CD metasurface and C4 CD-meta-atom pair metasurface under the normal illumination of CP waves at 10 GHz.
To further analyze the polarization-insensitive absorption mechanism, the interference theory is used to calculate the absorption of the designed achiral metadevice. For the designed achiral metadevice, the ultra-thin C4 metasurface is employed as a partially reflecting surface with the complex reflection and transmission coefficients R± and T±, where ‘+’ is for LP wave incidence from + z direction, and ‘-’ for wave illuminating from –z-direction. The FSS functions as a perfect reflector in the rejection band. As an interference model, the C4 metasurface and FSS located at the two sides of the PMI foam are only linked by multiple reflections, while any near-field interaction or magnetic resonance is neglected. According to the interference theory, the overall reflection is the superposition of the multiple reflections:
where β=kd is propagation phase accumulation of the wave between the C4 metasurface and FSS. k is the wavenumber in foam spacer, and d is the propagation distance of wave between the two surfaces. The absorption can be derived through A=1-|R|2. Accordingly, the absorptions of the designed AFSS under the normal incidence of y- and x-polarized waves are calculated using the simulated reflection and transmission coefficients of the ultra-thin C4 metasurface given in Fig. 7. The calculated results compared with the simulated results are also depicted in Fig. 7. It can be found that the calculated results reveal excellent agreement with the simulations. The discrepancy at the side frequency is owing to the non-perfect reflection of the FSS back-sheet.Fig. 7. (a) Amplitudes of the simulated reflection and transmission coefficients of the ultra-thin C4 metasurface, (b) The corresponding phases, (c) Absorptions of the ultra-thin C4 metasurface under normal incidence of LP waves, (d) Calculated absorptions of the designed achiral metadevice compared with the simulated results.
3. Conclusion
In summary, CD-assisted metadevices are proposed by placing the ultra-thin metasurface-based absorber above a bandpass FSS. Ultra-thin, wideband absorbers are implemented via loading resistive film into chiral resonators. The chiral absorber is realized by CD meta-atoms, while the achiral absorber is accomplished by C4 CD-meta-atom pairs made of a 2×2 array of CD meta-atoms with diverse in-plane orientation angles. The polarization-insensitive absorption of the C4 unit cell is attributed to the couplings between adjacent CD meta-atoms. Both the chiral metadevice and achiral metadevice are analyzed using interference theory. Finally, the proposed achiral metadevice is demonstrated by numerical simulation and experimental measurement. Our scheme provided a solid method for efficient absorber design and may push applications in microwave camouflage and EM invisibility.
Funding
China Postdoctoral Science Foundation (2019M651644); National Natural Science Foundation of China (61971435, 61971437).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. D. Lin, P. Fan, E. Hasman, and M. L. Brongersma, “Dielectric gradient metasurface optical elements,” Science 345(6194), 298–302 (2014). [CrossRef]
2. J. P. B. Mueller, N. A. Rubin, R. C. Devlin, B. Groever, and F. Capasso, “Metasurface Polarization Optics: Independent Phase Control of Arbitrary Orthogonal States of Polarization,” Phys. Rev. Lett. 118(11), 113901 (2017). [CrossRef]
3. L. Jiang, Y. Jing, Y. Li, W. Wan, Y. Cheng, L. Zheng, J. Wang, Z. Zhu, H. Wang, M. Feng, J. Zhang, and S. Qu, “Multidimensionally Manipulated Active Coding Metasurface by Merging Pancharatnam–Berry Phase and Dynamic Phase,” Adv. Opt. Mater. 9(13), 2100484 (2021). [CrossRef]
4. Z. Shen, B. Jin, J. Zhao, Y. Feng, L. Kang, W. Xu, J. Chen, and P. Wu, “Design of transmission-type coding metasurface and its application of beam forming,” Appl. Phys. Lett. 109(12), 121103 (2016). [CrossRef]
5. X. Wan, T. Y. Chen, X. Q. Chen, L. Zhang, and T. J. Cui, “Beam Forming of Leaky Waves at Fixed Frequency Using Binary Programmable Metasurface,” IEEE Trans. Antennas Propag. 66(9), 4942–4947 (2018). [CrossRef]
6. V. Popov, B. Ratni, S. N. Burokur, and F. Boust, “Non-Local Reconfigurable Sparse Metasurface: Efficient Near-Field and Far-Field Wavefront Manipulations,” Adv. Opt. Mater. 9(4), 2001316 (2021). [CrossRef]
7. H. Ren, G. Briere, X. Fang, P. Ni, R. Sawant, S. Héron, S. Chenot, S. Vézian, B. Damilano, V. Brändli, and S. A. Maier Genevet, “Metasurface orbital angular momentum holography,” Nat. Commun. 10(1), 2986 (2019). [CrossRef]
8. R. Zhao, L. Huang, and Y. Wang, “Recent advances in multidimensional metasurfaces holographic technologies,” PhotoniX 1(1), 20 (2020). [CrossRef]
9. Y. Cheng, Y. Li, H. Wang, H. Chen, W. Wan, J. Wang, L. Zheng, J. Zhang, and S. Qu, “Ohmic Dissipation-Assisted Complex Amplitude Hologram with High Quality,” Adv. Opt. Mater. 9(13), 2002242 (2021). [CrossRef]
10. G. M. Akselrod, J. Huang, T. B. Hoang, P. T. Bowen, L. Su, D. R. Smith, and M. H. Mikkelsen, “Large-Area Metasurface Perfect Absorbers from Visible to Near-Infrared,” Adv. Mater. 27(48), 8028–8034 (2015). [CrossRef]
11. Y. C. Chang, A. V. Kildishev, E. E. Narimanov, and T. B. Norris, “Metasurface perfect absorber based on guided resonance of a photonic hypercrystal,” Phys. Rev. B 94(15), 155430 (2016). [CrossRef]
12. F. B. Barho, F. G. Posada, M. Bomers, A. Mezy, L. Cerutti, and T. Taliercio, “Surface-Enhanced Thermal Emission Spectroscopy with Perfect Absorber Metasurfaces,” ACS Photonics 6(6), 1506–1514 (2019). [CrossRef]
13. C. Pfeiffer, C. Zhang, V. Ray, L. J. Guo, and A. Grbic, “High Performance Bianisotropic Metasurfaces: Asymmetric Transmission of Light,” Phys. Rev. Lett. 113(2), 023902 (2014). [CrossRef]
14. F. Zhang, M. Pu, X. Li, P. Gao, X. Ma, J. Luo, H. Yu, and X. Luo, “All-Dielectric Metasurfaces for Simultaneous Giant Circular Asymmetric Transmission and Wavefront Shaping Based on Asymmetric Photonic Spin–Orbit Interactions,” Adv. Funct. Mater. 27(47), 1704295 (2017). [CrossRef]
15. J. Sung, G. Y. Lee, C. Choi, J. Hong, and B. Lee, “Polarization-dependent asymmetric transmission using a bifacial metasurface,” Nanoscale Horiz. 5(11), 1487–1495 (2020). [CrossRef]
16. M. Khorasaninejad, W. T. Chen, A. Y. Zhu, J. Oh, R. C. Devlin, D. Rousso, and F. Capasso, “Multispectral Chiral Imaging with a Meta-lens,” Nano Lett. 16(7), 4595–4600 (2016). [CrossRef]
17. Q. Fan, P. Huo, D. Wang, Y. Liang, F. Yan, and T. Xu, “Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays,” Sci. Rep. 7(1), 45044 (2017). [CrossRef]
18. Y. Chen, J. Gao, and X. Yang, “Chiral Grayscale Imaging with Plasmonic Metasurfaces of Stepped Nanoapertures,” Adv. Opt. Mater. 7(6), 1801467 (2019). [CrossRef]
19. M. A. Ansari, I. Kim, D. Lee, M. H. Waseem, M. Zubair, N. Mahmood, T. Badloe, S. Yerci, T. Tauqeer, M. Q. Mehmood, and J. Rho, “A Spin-Encoded All-Dielectric Metahologram for Visible Light,” Laser Photonics Rev. 13(5), 1900065 (2019). [CrossRef]
20. S. Yan and G. A. E. Vandenbosch, “Compact circular polarizer based on chiral twisted double split-ring resonator,” Appl. Phys. Lett. 102(10), 103503 (2013). [CrossRef]
21. Y. Chen, J. Gao, and X. Yang, “Direction-Controlled Bifunctional Metasurface Polarizers,” Laser Photonics Rev. 12(12), 1800198 (2018). [CrossRef]
22. H. Lv, Z. Mou, C. Zhou, S. Wang, X. He, Z. Han, and S. Teng, “Metasurface circular polarizer based on rotational symmetric nanoholes,” Nanotechnology 32(31), 315203 (2021). [CrossRef]
23. E. Plum and N. I. Zheludev, “Chiral mirrors,” Appl. Phys. Lett. 106(22), 221901 (2015). [CrossRef]
24. L. Jing, Z. Wang, Y. Yang, B. Zheng, Y. Liu, and H. Chen, “Chiral metamirrors for broadband spin-selective absorption,” Appl. Phys. Lett. 110(23), 231103 (2017). [CrossRef]
25. C. Wang, J. Liang, Y. Xiao, T. Cai, H. Hou, and H. Li, “High-performance and broadband chirality-dependent absorber based on planar spiral metasurface,” Opt. Express 27(10), 14942–14950 (2019). [CrossRef]
26. S. P. Rodrigues, S. Lan, L. Kang, Y. Cui, and W. Cai, “Nonlinear Imaging and Spectroscopy of Chiral Metamaterials,” Adv. Mater. 26(35), 6157–6162 (2014). [CrossRef]
27. G. Rui, H. Hu, M. Singer, Y. J. Jen, Q. Zhan, and Q. Gan, “Symmetric Meta-Absorber-Induced Superchirality,” Adv. Opt. Mater. 7(21), 1901038 (2019). [CrossRef]
28. J. Mun, M. Kim, Y. Yang, T. Badlo, J. Ni, Y. Chen, C. W. Qiu, and J. Rho, “Electromagnetic chirality: from fundamentals to nontraditional chiroptical phenomena,” Light: Sci. Appl. 9(1), 139 (2020). [CrossRef]
29. J. Kim, A. S. Rana, Y. Kim, I. Kim, T. Badloe, M. Zubair, M. Q. Mehmood, and J. Rho, “Chiroptical Metasurfaces: Principles, Classification, and Applications,” Sensors 21(13), 4381 (2021). [CrossRef]
30. H. Wang, Y. Jing, Y. Li, L. Huang, M. Feng, Q. Yuan, H. Bai, J. Wang, J. Zhang, and S. Qu, “Tailoring Circular Dichroism for Simultaneous Control of Amplitude and Phase via Ohmic Dissipation Metasurface,” Adv. Opt. Mater. 9(12), 2100140 (2021). [CrossRef]
31. X. Kong, Z. Wang, L. Du, C. Niu, C. Sun, J. Zhao, and X. Li, “Optically transparent metamirror with broadband chiral absorption in the microwave region,” Opt. Express 27(26), 38029–38038 (2019). [CrossRef]
32. F. Ding, Y. Cui, X. Ge, Y. Jin, and S. He, “Ultra-broadband microwave metamaterial absorber,” Appl. Phys. Lett. 100(10), 103506 (2012). [CrossRef]
33. M. D. Banadaki, A. A. Heidari, and M. Nakhkash, “A Metamaterial Absorber With a New Compact Unit Cell,” IEEE Antennas Wireless Propag. Lett. 17(2), 205–208 (2018). [CrossRef]
34. H. T. Chen, “Interference theory of metamaterial perfect absorbers,” Opt. Express 20(7), 7165–7172 (2012). [CrossRef]
