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Abstract
Broadband communication with high data rates is a dire need for state-of-the-art wireless technologies. For achieving efficient wireless communication (particularly in an indoor environment), the electromagnetic (EM) waves should maintain their state of polarization despite encountering multiple reflections. Metasurfaces provide a unique platform to design subwavelength-featured meta-reflectarrays which enable the desired retention of the polarization state of an EM wave upon reflection. We present a single-layered broadband meta-reflectarray, simultaneously breaking n-fold (n > 2) rotational and mirror symmetry, which exhibits an unprecedented control over the phase, amplitude, and polarization of a reflected EM wave. This unique control enables the retention of polarization state and recording of spin-encrypted information for the reflected EM waves. Such novel multifunctional meta-reflectarray can be crucial to building an indoor setup for high data rate wireless communications. Meanwhile, the meta-array’s ability to encode phase information provides an extra degree of freedom to structure and control (via incident spin) the reflected EM beam in the desired way. For the proof of concept, we have experimentally demonstrated a spin-encrypted holographic display which reconstructs the recorded holographic image at an image plane for the left circularly polarized (LCP) illumination and exhibits circular dichroism for the right circularly polarized (RCP) incident waves. The proposed meta-array can find applications in 5G indoor wireless communication, chiral sensing, spin-selective imaging, holography, and encryption.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Achieving full control of the propagation of electromagnetic (EM) waves has been the focus of fundamental and applied research in wireless communication technologies. To attain such power, there is a need to build devices that are capable of achieving high efficiency, full phase (0–2π) modulation and polarization retention in the modern wireless era. Conventionally used methods cannot provide complete control on polarization with high efficiency. Owing to this, metamaterials and metasurfaces has been proved more effective and hold promise for the development of advanced wireless technologies [1].
Metamaterials (artificially engineered subwavelength structures exhibits electromagnetic properties that are not found in natural media) received a lot of interest in past owing to the exceptional capabilities of manipulating the flow of light in novel light-matter interactions [2–4]. Such abilities lead to a wide variety of applications in absorbers [5–7], enhancement of antenna performance [8,9], scattering reduction [10], super-lenses [11,12], energy harvesting [13–15], clocking [16–18] and many others in optical and microwave regime. However, 3D metamaterials are not deemed suitable for applications due to the high loss and fabrication complexities.
Metasurfaces (artificially engineered thick surfaces of subwavelength featured resonators) are the planar version of metamaterials that can provide solutions for on-chip applications and photonic integrated circuits (PIC) [19–24]. Metasurfaces can be easily fabricated using planar fabrication tools [25,26] and the resonance behaviour of subwavelength featured resonators can be engineered carefully to exhibit a wide range of exciting applications such as meta-lensing [27], meta-holography [28–30], light-structuring [31–37], absorbers [38], etc.
Moreover, meta-reflectarrays (reflective metasurfaces) are highly useful for applications involving polarization selectivity. Some conventional meta-reflectarrays share the property of converting the polarization of incident EM waves into its opposite handedness upon reflection [1] [39–41]. This happens due to the zero tangential electric field at the surface, so the phase of the incident electric field is reversed in reflection to satisfy the energy conservation. The n-fold (n > 2) rotational symmetry breaking meta-reflectarrays have also been reported in the literature, which holds the property of polarization retention upon reflection [42]. Additionally, the use of a metal layer in the bottom of the meta-reflectarrays can radically boost the efficiency of EM waves in reflection [43,44]. Such meta-reflectarrays go beyond the conventional meta-reflectarrays and fulfil the property of polarization retention. Still, the phase modulation has not been discussed, which is essential for full control of wavefront [45–47]. Meta-reflectarrays fulfilling the property of polarization retention and spin- encryption has been employed using multilayered structures which have fabrication complexities [48]. To avoid complex fabrication procedures, single-layered meta-device introduced previously [49] but still there is a need to introduce highly efficient meta-reflectarrays for high data rate wireless communication and imaging at broadband microwave regime.
As there is an increasing demand in the coming era for high data rate communications, the Federal Communications Commission (FCC) of United States issues 5G spectrum for next-generation wireless technologies [50]. The major drawback of wireless communication is high attenuation which interrupts the high data rates due to non-line of sight (N-LOS) and changing polarization. There can be many solutions to this problem like increasing the power of transmitters, use of highly sensitive receivers or deployment of multiple repeaters or access points. However, these solutions have health, and economic disadvantages; that’s why we cannot increase the power of transmitters beyond a specific limit and deployment of repeaters is not economically feasible. Therefore, the more convenient solution to this problem is to use highly efficient meta-reflectarrays. To the best of author’s knowledge, no one has demonstrated a single-layered compact meta-reflectarray in a broadband microwave regime holding the functionality of polarization retention and spin-encrypted phase-encoding.
Here, we have proposed a novel single-layered multifunctional meta-reflectarray to achieve polarization retention and spin encrypted phase encoding for the reflected beam. The proposed structure gives high reflection from the co-polarized components and fully suppresses the cross-polarized reflections for the circularly polarized (CP) incident light. Moreover, it allows spin-controlled phase encoding and can be tailored to achieve circular dichroism. To attain these functionalities, we have broken the required n-fold (n > 2) rotational and mirror symmetries of the structure. Eight unit-elements are optimized to achieve full control over the phase of the reflected EM. These optimized elements can be arranged in a two-dimensional array, forming a meta-array, to encode the desired phase information. Such multifunctional meta-reflectarray can be useful for highly efficient indoor wireless communication where perseverance of the polarization state is crucial. They can also be vital for other intriguing applications like chiral sensing, spin-selective imaging, and encryption.
2. Design and optimization
Ordinary meta-reflectarray exhibits the conversion of handedness of incident EM spin to its opposite spin upon reflection from its surface at normal incidence. In contrast, the proposed meta-reflectarray reflects a specific spin of the circularly polarized wave while preserving its handedness and fully absorbing the opposite handedness. It can be an optimal bi-functional planar device for highly efficient selective absorption and reflection. However, to design such meta-arrays for the aforementioned applications and any phase-encoded phenomena, the first step is to design a meta-atom by keeping in mind to acquire complete phase control $({0-2\pi } )$ and fulfill the necessary symmetry conditions.
In this regard, we optimize an asymmetric split ring resonator (ASSR) as a meta-atom for broadband applications by 1) breaking n-fold (n >2) rotational symmetry for polarization retention and 2) breaking mirror symmetry for circular dichroism (CD) and 3) full phase $({0-2\pi } )$ modulation. The rotation of meta-atom from 0–180 degrees covers the complete (0–2π) phase modulation.
Figure 1(a) illustrates the schematic of the working principle of designed system explaining the conversion from conventional reflectarray to a meta-reflectarray with circular dichroism along with encrypted-holography phenomenon with polarization retention. The designed system contains copper based meta-atoms patterned on Roger’s (RO4350B) substrate of thickness 1.524 mm with the copper ground plane. The commercially available full-wave EM simulator by Computer Simulation Technology (CST) is used for optimizing the structure, where r represents the inner radius and w is the difference between the outer and inner radius of the meta-atom. The parameters s and a are the important geometrical parameters to break the mirror symmetry for spin-selective absorption. At the same time, g describe the gap to break the n-fold rotational symmetry for the polarization retention phenomenon. The building block of conventional reflectarray with n-fold rotational symmetry depicted in Fig. 1(b) which shows maximum reflectance for cross-polarized components for CP (circularly polarized) waves while utterly absorbing the co-polarized reflectance parameters in Fig. 1(c). Unlike conventional reflectarray, the meta-atom for meta-reflectarray [Fig. 1(d)] shows maximum reflectance for co-polarized parameters and maximum absorption for cross-polarized parameters from 20 GHz to 30 GHz which proves the concept of polarization retention for broadband spectrum as depicted in Fig. 1(e).
Fig. 1. Conversion from a conventional reflectarray to a meta-reflectarray with circular dichroism along with structural parameters (p=6 mm, w=0.69 mm, r=1.02 mm, g=0.39 mm, s=1.56 mm, a=0.49 mm). (a) The working principle of the designed system depicts polarization retention phenomenon with circular dichroism at resonance frequency while polarization retention phenomenon at off-resonance frequencies. (b) and (c) Depicts the top view of the geometry of building block for conventional reflectarray with n-fold rotational symmetry and the reflectance parameters for LCP and RCP incidences, respectively. The conventional meta-reflectarray shows maximum reflectance for cross-polarized parameters. (d) and (e) The top view of the geometry of the building block of meta-reflectarray breaking n-fold (n > 2) rotational symmetry and its reflectance parameters depicted, respectively. It shows maximum reflectance along with polarization retention at broadband range of 20–30 GHz for LCP and RCP incident waves. (f) and (g) The top view of the meta-atom for designing the broadband meta-reflectarray with circular dichroism at 20 GHz achieved by simultaneously breaking n-fold (n > 2) rotational and mirror symmetries and representing the reflectance parameters at broadband range of frequencies with circular dichroism of the co-polarized component of RCP at the frequency of 20 GHz and maximum reflectance at all other frequencies, respectively.
In the coming era of high data rate communication, we need multifunctional devices for security applications. Therefore, the structure with handedness preserving and handedness dependent properties can be useful in next-generation wireless communication for encryption. The structure depicted in Fig. 1(f) is opted to break simultaneously mirror and n-fold (n >2) rotational symmetries. It is illustrating maximum absorption for RCP at the frequency of 20 GHz and maximum reflectance for the co-polarized component of LCP and RCP at a broadband (20–30 GHz) EM spectrum as shown in Fig. 1(g). It proves the concept of meta-reflectarray with polarization retention and spin-encryption, which will be suitable for broadband high data rate wireless communication in next-generation networks and polarization-dependent encryption for security applications.
It is noticeable in Fig. 1(f) that LCP is reflecting with high efficiency and RCP is fully absorbing for LCP and RCP incident EM waves, respectively. The reflectance parameters here defined as
To prove the concept of selective reflection and absorption for CP EM waves mathematically [41], we use the formulation based on Jones calculus. The reflected fields we can write as
In Eq. (5), the ‘+’ and ‘−’ in subscripts are defined as the notation for the right (clockwise) and left (counter-clockwise) circularly polarized waves respectively, and these notation holds when viewed along + z direction. As the wavevectors are in the opposite direction for transmitted and reflected EM waves, so each reflection coefficient has a different meaning. For example, in our case, for LCP incident EM waves ${r_{ -{-} }}$ and ${r_{ +{-} }}$ are the cross-polarized and co-polarized components respectively. To prove our results mathematically for the proposed structure, we put the following values ${r_{ +{+} }} = \; {r_{ -{+} }}\; = \; \; {r_{ -{-} }} = 0\; and\; {r_{ +{-} }} = 1\; \; $ in Eq. (5) and we get the values for linear reflection matrix after solving it simultaneously
For RCP incidence:
For LCP incidence:
From Eqs. (8) and (9), we proved mathematically that our proposed geometry is completely reflecting the LCP incident waves with polarization retention and utterly absorbing the RCP incident waves.
3. Results and discussions
For wavefront shaping, it is necessary to cover the full phase (0–2π) along with amplitude and full polarization control. Owing to this, we achieve full phase modulation using the geometric phase mechanism. As stated in Pancharatnam-Berry Phase theory, rotating a meta-atom can give complete phase coverage [51]. Optimization of the meta-atom shown in Fig. 1(f) discloses that turning it from 0 to 180 (degrees) can achieve complete $({0-2\pi } )$ phase control with maximum possible reflectance for the designed frequency of 20 GHz as depicted in Fig. 2(a).
Fig. 2. Design and optimization of the proposed meta-atom. (a) Full phase (0–2π) coverage of the designed meta-atom of meta-reflectarray depicted in Fig. 1(f). It is representing that the complete phase coverage achieved by rotating the structure from 0–180 (degrees) for the co-polarized component of LCP which have a maximum reflectance for the designed structure. Reflectance parameters concerning the rotation of subwavelength unit element for full phase coverage at 20 GHz. (b) The reflectance parameter of the co-polarized component of LCP with maximum reflectance for the frequency of circular dichroism (20 GHz) and above, for the rotation of unit element from 0–180 degrees. (c) Proves the circular dichroism for the co-polarized component of RCP at 20 GHz by describing the maximum absorption for rotating meta-atom. (d) The extinction ratio ER = ${R_{LL}}/{R_{RR}}$ (the ratio of the co-polarized component of LCP and RCP) which maintains the maximum amplitude at 20 GHz for the whole rotation cycle. (e) and (f) Shows the maximum absorption for the cross-polarized parameters of LCP and RCP for all the rotations of the meta-atom.
Simulated reflectance parameters of the optimized meta-atom for full phase coverage, high reflection and absorption depicted in a frequency range of 18–22 GHz. Reflectance parameters plotted against frequency and rotation to prove that the resonance of the meta-atom not shifting by rotating it to cover the full $({0-2\pi } )$ phase. Figure 2(b) and 2(c) depicts the co-polarized reflectance parameters for LCP and RCP incident wave, respectively. It clearly shows that the resonance point maintains at 20 GHz by rotating the meta-atom. The maximum reflectance is around 84% for LCP without reversing its handedness and less than 10% for RCP. The white dotted line shows the resonance point of optimized meta-atom. The cross-polarized reflectance parameters for both LCP and RCP are presented in Fig. 2(e) and 2(f) with minimum amplitude. The extinction ratio defined as $ER = \; {R_{LL}}/{R_{RR}}$ (the ratio of the two co-polarized components) depicted in Fig. 2(d).
When an observer looks along the direction of wave propagation, the rotation of the wavevector defines weather its LCP or RCP. For normal incidence, the reflected far-field pattern achieved at a specific angle which is difficult for practical applications. For high data rate communications, it is necessary to optimize the metasurface for a wide range of incident angles. Hence, our proposed system fulfils the criteria for indoor wireless communications. However, oblique incidences change the polarization of EM waves from circular to elliptical to make the characterization complicated but the proposed system efficient enough to ultimately reflect the CP waves with preserving handedness. It’s a notable performance of our system at a wide range of angle of incidences as depicted in Fig. 3. In xz-plane, it completely absorbs the RCP and reflects the LCP while in yz-plane it maintains excellent performance up to 40 degrees.
Fig. 3. Absorption for the co-polarized component of LCP and RCP vs incident angle (0–80 degrees) of EM waves. It is representing the absorption for a wide range of incident angle (0°−80°) in xz-plane for co-polarized component of RCP and (b) LCP incident waves. The absorption for a wide range of incident angle (0°−80°) in yz-plane for co-polarized component of (c) RCP and (d) LCP. At resonance frequency, it shows maximum and minimum absorption for RCP and LCP waves, respectively.
In Fig. 4 the simulated co-polarized reflectance parameters presented to check the sensitiveness of designed system by changing different optimization parameters. Figures 4(a)–4(c) depicts the change in reflectance of co-polarized parameter for LCP incident waves by tuning optimization parameters a, s and g, respectively. Likewise, the change in reflectance of co-polarized parameter of RCP incident waves for changing optimization parameters a, s and g, respectively. One can clearly see that there is a little shift in the absorption peak for RCP incident waves but for LCP incidence the efficiency almost remains same irrespective of the peak shifts.
Fig. 4. Co-polarized reflectance parameters for LCP and RCP incidences with varying optimization parameters of the meta-atom. Representing the co-polarized reflectance of LCP incidence by changing parameters (a) a (0.47–0.51 mm) (b) s (1.36–1.76 mm) (c) g (0.37–0.41 mm). There is a minor shift in reflectance by changing all the parameters but overall maintains the maximum efficiency. Similarly, the co-polarized reflectance of RCP incidence by changing parameters (a) a (0.47–0.51 mm) (b) s (1.36–1.76 mm) (c) g (0.37–0.41 mm). We can notice a small shift in dichroism peak by changing all the parameters around 20 GHz.
As the proposed structure achieved full phase $({0-2\pi } )$ modulation for meta-atom, thus it can be used for any phase dictated phenomenon. For the proof of concept, we have simulated a spin-encrypted holographic display by reconstructing the recorded holographic image. It is necessary to generate a phase map to implement any hologram based on meta-atom. By applying the Gerchberg–Saxton (GS) algorithm on the desired pattern at the focal plane, the required phase map achieved [52]. GS algorithm implemented on a surface of ∼ (600 × 600) mm2 contains a matrix of 100 × 100 elements to generate a meta-hologram of letter ‘C’. The computer-generated hologram (CGH) of desired image ‘C’ using the phase map obtained from the GS algorithm as depicted in Fig. 5(a). Due to limited computational resources, we have implemented a phase map of the desired image on a small portion of the surface. Meta-atoms patterned on a surface using the phase map generated by using the GS algorithm. Figures 5(b) and 5(c) depict the simulated meta-hologram for LCP and RCP EM waves in terms of normalized electric field intensity. Numerically simulated results prove the concept of our designed meta-reflectarray with spin-encryption phase-encoding. It shows the recorded holographic image at a focal plane for LCP illumination and exhibits absorption for RCP incident waves.
Fig. 5. Simulated polarization retention and spin-encrypted reflectance in the form of holographic images (a) Computer-generated hologram using GS algorithm for letter “C” consist of 100 × 100 pixels and each pixel’s size is (6 × 6) mm2. (b) Reconstructing meta-hologram for meta-reflectarray of (620 × 620) mm2 for LCP illumination at 20 GHz. (c) Exhibiting full absorption/no holographic image for meta-reflectarray of 620 × 620 mm2 for RCP incident waves at 20 GHz. These results prove the concept of polarization retention and spin-encryption at microwave frequencies. The same meta-reflectarray works well with high reflectance for both LCP and RCP above 20 GHz.
4. Experimental verification of spin-encrypted hologram
The proposed design fabricated using standard circuit board technique for the confirmation of simulated results. The fabricated meta-reflectarray contains a Rogers RO4350B dielectric substrate layer of thickness 1.524 mm along with 50 × 50 copper elements at the top and a copper layer at the bottom. The overall fabricated area of meta-reflectarray is (310 × 310) mm2. Figure 6(a) depicts the experimental setup of the printed meta-reflectarray for characterization. An antenna with linear polarization was used to illuminate the fabricated meta-reflectarray. The reflected EM waves from the sample received at the receiver antenna. Then the magnitudes and phases of the four reflection parameters ${r_{xx}}$, ${r_{yx}}$, ${r_{xy}}$, and ${r_{yy}}$ of the reflection matrix were recorded at a vector network analyzer which was connected to the receiver.
Fig. 6. Measurement setup for fabricated meta-reflectarray and measured results. (a) Representing the measurement setup for fabricated meta-reflectarray of (310 × 310) mm2 consists of 50 × 50 meta-atoms. (b) Describing the measured electric field intensity of the holographic image of letter “C” for LCP incident EM waves at 20 GHz. (c) Representing the full absorption of electric field intensity of the holographic image of letter “C” for RCP incident EM waves at 20 GHz. The measured results prove the concept of handedness preserving reflection and polarization encryption.
As we have simulated meta-reflectarray for CP EM waves, we can achieve the circularly polarized reflection parameters using the circular basis in Eq. (10).
After applying the circular basis, we have obtained co-polarized and cross-polarized reflection parameters for CP incident waves. The results in Figs. 6(b) and 6(c) depicts the measured spin-encrypted meta-hologram. The measured results confirm the simulated results as it displays recorded hologram in reflection for LCP illumination and a blank screen for the RCP incident waves. The variations in the simulated and measured results were due to the unavailability of an ideal incident source in the experiment, and the fabricated sample size was almost half of the simulated one. After comparing the measured and simulated results, we can notice that the proposed meta-reflectarray works well for the polarization retention and spin-encrypted phase encoding.
5. Conclusion
We have demonstrated a single layer meta-reflectarray for polarization retention based on symmetry breaking subwavelength meta-atoms for several applications in the microwave regime. The meta-atom optimized in such a way that it provides maximum reflectance for co-polarized parameters at the broadband spectrum from 20 GHz-30 GHz, regardless if it is LCP or RCP. The vital application of such a system can be in high data rate wireless networks like in 5G and next-generation networks to avoid a lot of attenuation. Due to breaking n-fold rotational and mirror symmetries simultaneously, the demonstrated system exhibits absorption for RCP incident spin while reflecting the handedness preserving waves for LCP incident. Consequently, the system can be a valuable addition to polarization retention and spin-encryption based applications. Meanwhile, the full phase ($0-2\pi $) modulation demonstrated for the meta-atom lead us to use this system for any phase dictated phenomenon. For the proof of concept, we have implemented spin-encrypted hologram at 20GHz and compared simulated and experimental results to verify the idea of meta-reflectarray with circular dichroism. The proposed system can have a lot of simultaneous application like high data rate 5G communication, polarimetric imaging, encryption and molecular spectroscopy.
Disclosures
The authors declare no conflicts of interest.
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