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Abstract
Reconfigurable intelligent surfaces (RISs) that dynamically manipulate scattered waves have attracted much attention regarding accommodating coverage holes in wireless communication systems using radio wave frequencies higher than millimeter waves. RISs generally actualized through metasurface technologies must be visually unaffected so that they can be installed in various locations such as existing walls and glass windows in environments where propagation should be controlled. We propose a novel method that dynamically controls scattering characteristics of metasurfaces while achieving a large area and high optical transparency. For transparency in the visible light range, we use transparent glass as a substrate and meshed metal patterns. Furthermore, by stacking a metasurface substrate onto another transparent substrate and controlling the interlayer distance, we achieve dynamic control of the scattered waves over a large area in the 28-GHz band. Fabricated prototypes successfully operate when switching transmission and reflection modes, exhibiting extremely low loss of less than -1 dB. In metasurface lenses that can be attached to glass windows in outdoor-to-indoor scenarios, the lens gain of 25.4 dB is achieved for a static lens, and dynamic switching operation between single focus and dual focus is also successfully verified for a dynamic lens.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The 5th generation mobile communication system (5G) has been deployed in numerous countries, and expectations are rising for the application of 5G technology in various fields such as cross-reality technology for virtual reality/augmented reality/mixed reality, and industrial/infrastructure upgrading using Internet of things devices [1]. 5G uses millimeter waves (mmWs) that were not used until the 4th generation mobile communication system (4G) and introduces novel technologies, including beamforming using a massive multiple-input and multiple-output (MIMO) method, in which a very large number of antennas are employed. With the introduction of these new technologies, the potential of 5G in terms of ultra-high speed, high capacity, ultra-low latency, and high reliability has been experimentally demonstrated [2–4]. With the launch of 5G, further development towards 5G evolution and the 6th generation mobile communication system (6G) is being investigated by industry, academia, and government, in which even higher frequencies, including terahertz wavebands, are under discussion [5–7]. However, through these investigations, it has become clear that there are some issues that must be addressed in order to effectively utilize mmWs and higher frequencies in cellular wireless communication systems. Since mmWs have strong linearity of propagation and their behavior is close to that of light, they do not diffract behind obstacles. Therefore, determining how to prevent the formation of coverage holes due to obstacles is important in utilizing the mmW band for cellular wireless communications.
In recent years, the concept of an intelligent radio environment (IRE) has received much attention in coverage improvement toward 5G evolution and 6G. In an IRE, not only the transmitter (Tx) and receiver (Rx) are controlled, but also the radio environment is considered as a controllable element to significantly improve the performance of wireless networks. This approach is referred to as the IRE or smart radio environment to emphasize the conceptual difference from that for conventional wireless network systems [8]. To realize the IRE, transmitarray/reflectarray technologies have been investigated for many years, and since 2019, under reconfigurable intelligent surface (RIS) name, they were actively studied as an enabler technology [9–11]. RISs are electromagnetic waves scatterers comprising a large number of passive elements, and metasurface technology is commonly used to design and control the two-dimensional distribution of the scattering characteristics. Depending on the substrate material, metasurfaces can be fabricated as thin flexible sheets to control the scattering characteristics while maintaining the shape of the existing structures. This means that the propagation channel can be controlled. RISs are supposed to control adaptively the wireless environment by periodically repeating the procedure of estimating the propagation-channel information and controlling the scattering characteristic distribution of the RIS based on the obtained information. Various approaches have been studied for the implementation of this procedure [12]. One reason why RISs have attracted attention is that their placement in a real environment has become more practical since the frequency of the used radio waves has increased [13]. In the case of a square RIS, when the length of one side of the RIS is greater than the radius of the 1st Fresnel zone, the propagation loss of the path through the RIS is equivalent to the free-space path loss (FSPL) of the total path length of the Tx to the RIS and the RIS to the Rx. For example, when the path length of the Tx to the RIS is 100 m and that of the RIS to the Rx is 100 m, the radius of the 1st Fresnel zone is approximately 70 cm, which is a reasonable size to place in an actual environment.
In order to actualize the IRE concept, it is assumed that multiple RISs comprising metasurfaces are scattered in an environment where the propagation channel is to be controlled. This means that RISs must be visually unaffected. There are many reports regarding dynamic control of the scattering characteristics in various approaches using optically [14] or electrically [15] controlled carriers, micro-electro-mechanical systems [16,17], graphene [18,19], vanadium dioxide [20–22], or semiconductor-based devices [23–25]. Based on these approaches, many promising studies have been conducted in recent years. The manipulation of the wavefront to focus, collimate, or deflect scattered waves was achieved with reflectarrays [19,26–28] and transmitarrays [24,25,29], in which the same elements are uniformly arrayed and the control signal is distributed in the array plane to produce desired intensity or phase profile. In addition, static metasurfaces based on indium-tin-oxide (ITO) for optical transparency were also reported [30–32]. However, it is difficult for these approaches to achieve both a radius larger than that of the 1st Fresnel zone for typical scenario and transparency so that the environment is not affected visually.
In this paper, we propose a novel method to dynamically control scattering characteristics of metasurfaces while achieving a large radius and transparency. To actualize optically transparent metasurfaces, transparent glass is used as the base substrate and meshed metasurface metal patterns are fabricated. Furthermore, by stacking a metasurface substrate onto another transparent substrate and controlling the interlayer distance between these substrates, dynamic switching between transmission and reflection modes over a large area while maintaining its optical transparency is achieved for 5G networks in the 28-GHz band (Fig. 1). In addition, in the mmW band, coverage of an indoor area using outdoor base stations is an issue due to the large FSPL and weak diffraction capability. To address this problem, we propose the use of metasurfaces that enable glass windows to function as radio lenses by developing the design concept of transparent dynamic metasurfaces. In the presented lens, multiple types of unit cells are arranged so that the intensity profile can be switched even with a uniform control signal (in our case, the interlayer distance varies uniformly in the substrate plane). Previously, we have demonstrated a proof of concept [33]; however, detailed experimental and analytical data and design methodology have not been discussed yet. In this paper, the influence of the unit-cell structure on the controllability of the proposed method is examined and the design guideline is shown. Moreover, the design methodology with detailed experimental and analytical results for optically transparent dynamic metasurface lens is reported for the first time.
Fig. 1. A concept for transparent dynamic metasurface toward new radio network topology. Channel state of the wireless environment is adaptively controlled according to the position of user terminals, obstacles, vehicles, etc. Proposed RIS prototypes control transmission/reflection modes or focus points of waves passing through metasurface lenses.
2. Transparent dynamic metasurface
The proposed method to actualize dynamic control of the scattering characteristics of large-area metasurfaces while maintaining high optical transparency is shown in Fig. 2. In the proposed method, a metasurface pattern is formed on a transparent substrate (metasurface sub.), and another transparent dielectric substrate (movable sub.) is stacked on the top of the metasurface sub. to control the scattering characteristics of the metasurface by varying the interlayer distance between these two substrates. The resonant frequencies of the elements that constitute the metasurface vary depending on the permittivity of the material around the elements. When the interlayer distance is short (contact state), the effective permittivity around the resonant elements is predominantly determined by that of the metasurface and movable substrates, resulting in relatively low resonant frequencies. On the other hand, when the interlayer distance is long (separate state), the permittivity around the element is dominated by the permittivity of the metasurface sub. and the air, leading to relatively high resonant frequencies. In other words, in band A in Fig. 2, the incoming waves are reflected during the contact state and are transmitted during the separate state. In band B, the relations of transmission and reflection are opposite to those in band A. The split-ring resonator (SRR) based unit cell used as a resonant element, which is described later, has a frequency characteristic where the transmittance decreases gradually as it approaches the resonant frequency on the lower frequency side of the resonant frequencies and increases steeply on the higher frequency side. This means that the required change in resonant frequencies to achieve the contact state with high transmittance in band B can be smaller than that to achieve the separate state with high transmittance in band A. Therefore, band B is mainly used in this study. In addition, to prevent visible light from being blocked by the metasurface metal pattern, the metal pattern is meshed as in [34], which results in an increase of visible light transmission rate to 79.5% or higher.
In this study, a lead zirconate titanate (PZT) based piezoelectric element, whose dimension varies with the applied voltage, or an electric motor is used as the actuator to control the interlayer distance between the metasurface and movable subs.
Figure 3(a) shows the filled pattern of the unit cell used as resonant elements. In this study, we adopt a unit cell structure based on the SRRs to dynamically control the transmission/reflection state and to create a two-dimensional intensity profile for a lens function. The SRRs have a resonant mode of LC-resonance in which the circulating current is excited by the incident waves. This enables us to actualize a unit-cell smaller than the wavelength [35,36]. Four gaps are constructed at the corners of the SRRs to yield a 4-fold rotational symmetry to support vertical (V-) and horizontal (H-) polarizations. In addition, since the electric fields are concentrated at the gap of the SRR in the LC-resonant mode, it is possible to achieve highly sensitive scattering characteristics with respect to the position of the movable subs., which makes the required change in the interlayer distance small.
Fig. 3. Geometry and dimensions of unit-cell used in (a) filled and (b) meshed patterns. (c) Schematic of layer structure of glass substrate.
To achieve the transparency of the meshed pattern, the linewidth (M) = 20 μm and the pitch (P) = 300 μm were used, as in our previous work [34], and are indicated in Fig. 3(b). For the beforementioned mesh parameters, the optical transparency is approximately 79.5% even in the worst case where the entire glass is covered with metal mesh. When the metasurface pattern is designed with these parameters, the area covered by the metal is much smaller than the worst-case above, so the expected optical transparency of our meshed metasurfaces is greater than 79.5%. This indicates, for example, our metasurfaces have the potential to satisfy the regulations of almost all countries for the front windshield of a vehicle, which generally requires the transparency of at least 70–80%.
Figure 3(c) shows a cross-section of the unit cell used in the transparent dynamic metasurface. The metasurface sub. is fabricated by attaching a polyethylene terephthalate (PET) film with a copper patterned metasurface to a glass substrate. The PET film is attached in such a way that the metal pattern is on the movable sub. side in order to improve the controllability of the scattering characteristics using the movable sub. position. The scattering characteristics change as the interlayer distance, d, changes with the actuator.
3. Design and discussion
To confirm the feasibility of i) controlling the scattering characteristics of the metasurface using the movable sub. and ii) improving the controllability of the scattering characteristics by concentrating the electric field at the gaps in the SRRs, full 3D electromagnetic (EM) simulations are carried out using the model shown in Fig. 3(c) in which the filled pattern is used. In the EM simulations, scattering parameters are extracted by assuming a two-dimensional infinite periodic structure.
Fig. 4 shows the verification results of the effect of gap size G, where the electric field is concentrated, on the transmittance. The gap is varied from 50 μm to 400 μm while adjusting the length of the SRR edges (Ax = Ay) so that the resonant peaks in all cases are localized at 28 GHz when there is no movable sub., as shown in Fig. 4(a). Based on the electric-field intensity distribution in the cross-section of the gap at 28 GHz, the figure shows that the smaller the gap is, the stronger the local concentration of the electric field exists. This tendency of the electric-field spreading is not only in the substrate plane but also in the direction perpendicular to the substrate. When the movable sub. overlaps the space where the electric field is concentrated, the capacitance component of the SRR causing the resonant frequency shifts. That is, when the interlayer distance is short, the sensitivity of the scattering characteristics, in relation to the position of the movable sub., should be higher for the SRRs with a narrower gap where the electric field is more concentrated. Figure 4(c) shows the simulation results of the controllability of the resonant frequency when varying the interlayer distance for each gap. When the gap is as large as 400 μm, the resonant frequency gradually approaches 28 GHz as the interlayer distance increases from the contact state. On the other hand, in the case of a narrow gap of 50 μm, the resonant frequency changes more rapidly when the movable sub. moves away from the contact state, and converges to 28 GHz when the distance is 500 μm. Based on the above, we confirm that the sensitivity of the movable substrate can be precisely designed by the gap size optimization.
Fig. 4. Simulated (a) transmittance and (b) E-field distribution at the gap for each unit cell with different gaps. (c) Gap dependency of resonant-frequency controllability on the movable substrate position. Here, G is varied from 50–400 μm, while adjusting Ax = Ay from 1.55 mm to 1.9 mm to maintain the resonant frequency 28 GHz, in which linewidth W is 200 μm, and thicknesses of substrates and relative permittivity are 500 μm and 5.4, respectively.
Next, the influence of the relative permittivity of the movable and metasurface subs. on the controllability of the resonant frequency is investigated. The capacitance component of the SRRs can be considered as a parallel connection of Csub that is not affected by the position of the movable sub. but determined by the relative permittivity of the substrate on which the metasurface is fabricated, and variable capacitance Cmov varied using the interlayer distance, which is predominantly determined by the relative permittivity of the air and the movable sub. as shown in Fig. 5(a). Assuming the inductance component of the SRRs as L, the resonant frequency, fres, of the SRRs can be estimated as
When controlling fres using the interlayer distance variation, i.e., using the change in Cmov, the controllability should be better when Cmov is greater than Csub in the contact state. Then, the effect of the relative permittivity of the metasurface and movable substrates on the controllability of the resonant frequency using the interlayer distance is confirmed by the EM simulation. In this simulation, the metasurface sub. is not a two-layer structure comprising PET film and a glass substrate as shown in Fig. 3(c), but a simple single-layer structure as shown in Fig. 5(a) to facilitate understanding. Figure 5(b) shows the dependency of the resonant frequency on the interlayer distance when the relative permittivity of the movable sub., εmov, is 3.8 and that of the metasurface sub., εsub, is varied from 2.7 to 5.4. For comparison, the resonant frequencies for each εsub are normalized by that for only the metasurface sub. For εsub = 2.7 and εsub = 5.4, the normalized resonant frequencies in the contact state are approximately 0.75 and 0.84, respectively, which indicates that the smaller the εsub, the smaller the change in the resonant frequencies. This is due to the fact that Csub in (1) becomes larger as εsub becomes larger, making it more difficult to shift fres using the change in Cmov. On the other hand, when εsub is fixed at 3.8 and εmov is changed, the larger εmov becomes, the larger the change in the resonance frequencies, since a large εmov increases Cmov at each interlayer distance.
Fig. 5. (a) Cross-section of SRR gap and corresponding capacitances. Dependence of relative permittivities (b) εsub and (c) εmov of resonant-frequency controllability on the position of the movable sub. Thicknesses of metasurface sub. and movable sub. are 500 μm, and the relative permittivity of one substrate is changed in the range of 2.7 - 5.4, while that of the other substrate is fixed at 3.8.
Based on the above, to increase a change in the resonant frequency and to improve the sensitivity to the interlayer distance, the gap should be reduced, the relative permittivity of the metasurface sub. should be small, and that of the movable sub. should be large.
4. Results and discussion
In this section, fabricated prototypes of the transparent dynamic metasurface that can control the transmission/reflection mode while maintaining transparency to visible light are evaluated. Figure 6(a) shows photos of the fabricated transparent dynamic metasurface. Following the discussion in Section 3, a metasurface-patterned PET film is attached to synthetic fused quartz (εr= ∼3.8) [34] while the alkali-free glass (εr= ∼5.4) is used as the movable sub. to improve the controllability of the resonant frequencies. As shown in Fig. 6(a), the meshed pattern appears almost transparent to visible light. In this prototype, the metasurface area is 20 cm x 20 cm, and the interlayer distance can be adjusted manually or controlled by applying a bias voltage to the PZT actuator shown in Fig. 6(b).
The prototypes are evaluated using a configuration comprising a vector network analyzer (VNA) and horn antennas for the Tx and Rx to measure the transmittance. In the experiment, the prototypes are placed between the Tx and Rx antennas, and the centers of the antennas and the prototypes are aligned along the same straight line. Figure 7 shows the measured transmittance for the filled and meshed patterns without a movable sub. For comparison, the filled pattern and meshed pattern have the same outline. All the measurement results are normalized to the measured value for the frame jig without the substrates. The spectra of the filled and meshed patterns are in good agreement with that in the EM-simulation. The resonant frequency of the meshed pattern is shifted to the lower frequency side by approximately 5%, as compared to that for the filled pattern. This is due to the increase in the SRR inductance component in the meshed metal pattern.
Fig. 7. Simulated and measured transmittance for filled and meshed patterns without movable substrate. The solid line represents measured results, and the dotted line represents the simulated result where measured results are normalized to the value measured with only the frame without the metasurface sub.
Next, we verify that transmission/reflection modes can be controlled by varying the interlayer distance between the metasurface and the movable subs. using the PZT actuator. Figures 8(a) and 8(b) show the measured transmittance while varying the dimensions of the PZT actuator from 0 μm to 300 μm for the filled and meshed patterns, respectively. Note that the frame jig is constructed so that the interlayer is decreased to the contact state when the PZT actuator is set to 0 μm. As expected, the resonant frequencies increase as the interlayer distance increases. When the value of the PZT is set to 300 μm, the resonant frequencies are almost identical to that of only the metasurface substrate for the filled and meshed patterns. When the PZT actuator is set to 0 μm, the resonant frequencies of the filled and meshed patterns are higher than those for the contact state in the EM simulation. This should be due to the manufacturing accuracy of the frame jig, which does not allow the ideal contact state as in the simulation. In this prototype, the transmission and reflection modes can be controlled by defining 0 μm for the PZT actuator as the contact state and 300 μm for the separate state at approximately 27 GHz. For the separate state, the transmittance is less than -10 dB (corresponding reflectance > -1 dB) in the 25.8–28 GHz band for the filled pattern, and in the 24.7 - 26.7 GHz band for the meshed pattern, which means that most of the incident waves are reflected. Furthermore, the transmittance in the contact state is > -2 dB for the filled pattern and > -2.5 dB for the meshed pattern. For example, considering a channel bandwidth of 400-MHz which is supported in the 28-GHz band for 5G, the transmission and reflection modes can be controlled with an extremely low loss of less than -1 dB. If the PZT actuator is set to a value between the separate state and contact state, the ratio of transmission and reflection can be controlled continuously.
Fig. 8. Simulated and measured transmittance dependencies on the distance between metasurface and movable substrate for (a) filled and (b) meshed patterns. Measured results are normalized to the value measured with only the frame without the metasurface.
5. Application to metasurface lens for glass window
The mmWs band is subject to high attenuation over long distances and its feature of low diffraction makes it difficult to establish wireless indoor communication links using outdoor Txs. For that reason, we investigate appending a lens function to the glass window to concentrate the energy received on the broad surface of a window at a specific focal point inside a building. As shown in Fig. 9, by installing a repeater or reflector at the focal point, it is possible to guide efficiently the radio waves coming from outdoors to indoors and generate an mmW area. Considering the use of multiple repeaters and reflectors, or with a view to tracking the user terminal in the future, a dynamic metasurface lens that can dynamically switch the focal point is also investigated. In the verification of the metasurface lens, 4-mm thick soda-lime glass (εr: ∼6.8), which is commonly used as window glass, is used as the base material to reproduce an actual implementation. On a static metasurface lens, a metasurface film is attached to the soda-lime glass, in which the metasurface pattern is embedded in the PET film to protect the metal pattern. On the other hand, for the dynamic metasurface lens, 0.7-mm-thick alkali-free glass is used as the metasurface sub. and soda-lime glass is used as the movable sub. so that the metasurface sub. has lower permittivity than the movable sub., as discussed in Section 3. In an actual implementation, the movable sub. should be fixed and the metasurface sub. moves relatively instead.
The metasurface lens is designed based on the Fresnel-zone-plate (FZP) theory. A zone plate consists of several radially symmetric rings, which are called zones. When compared with the same aperture area and wavelength, the efficiency of the metasurface lens is mainly determined by the deviation of its phase distribution from the ideal 2π-range continuous phase distribution. Therefore, quantized phase distribution with a higher quantization bit number is preferred to obtain the lens with a larger gain. Since the maximum abrupt phase change obtained by single resonant mode is theoretically π rad., multiple metal layers are needed to obtain a designability or controllability of the 2π-range phase with low transmission loss. Recently, some new wavefront control mechanisms are reported, in which the different metasurfaces with particular phase profiles are stacked and one of the metasurfaces is rotated to vary the total phase profile of the devices [37,38]. However, considering an easy implementation in a real environment, the window with lens function should be realized by easily attaching PET film with a single metal layer. In this work, the lens function is made with binary intensity distribution. Even though its efficiency is lower than that with phase distribution, a significant improvement in the link budget in wireless systems can be expected, given that the window size is much larger than the wavelength. The zones alternate between transparent and reflective areas, and are designed so that waves passing through the transparent areas constructively interfere at the desired focal point. The boundaries between the zones can be expressed as
where n represents the zone number, which is a positive integer from 1 to n, and f, rn, and λ are the focal length, radii of the boundaries, and wavelength of an incident wave, respectively.In a static metasurface lens, the odd-numbered zones are filled with air to be transparent. The even-numbered zones are filled with the SRRs (Fig. 3) so that only the 28-GHz band is focused and it behaves as normal transparent glass for lower frequency waves including LTE and sub-6-GHz bands.
As for the dynamic metasurface lens, switching between two FZP patterns with different focal points is considered. When two FZP patterns (pattern 1 for focus 1 and pattern 2 for focus 2) are designed in the same plane, the following four overlap combinations of a)–d) between the two patterns occur:
- a) Transparent area of pattern 1 and transparent area of pattern 2
- b) Reflective area of pattern 1 and reflective area of pattern 2
- c) Transparent area of pattern 1 and reflective area of pattern 2
- d) Reflective area of pattern 1 and transparent area of pattern 2.
In each of these four types, unit-cell structures with different combinations of transmission and reflection for the contact and separate states are arranged to actualize a dynamic metasurface lens that can switch the FZP pattern by controlling the interlayer distance. The unit-cell structure, in which the metal and air portions of the SRRs shown in Fig. 3 are reversed, is called a complementary SRR (CSRR), and it exhibits a transmission/reflection ratio that is opposite that for SRRs according to Babinet’s principle [39]. In the area of d), the unit cell structure based on CSRRs is used in band B in Fig. 2, as shown in Fig. 10(a). The areas of a), b), and c), are designed to be filled by air, a metal layer, and SRRs, respectively. As a result, a focus is formed at focus 1 in the contact state and focus 2 in the separate state. Furthermore, in an intermediate state that exists between the contact and separate states, radio waves can be focused on both focus 1 and focus 2. Similar to the previous section, this approach allows the dynamic metasurface lens to be transparent in the visible light range and to be a large area. In addition, the proposed method of dynamic control has the advantage of being able to control the distribution of scattering characteristics with a uniform control signal as opposed to the conventional way that requires the use of a control signal with a two-dimensional profile to control the distribution of scattering characteristics such as that for an FZP lens. If transparency in the visible light range is not necessary, it is possible to switch the distribution of the scattering characteristics using a simple uniform signal even for the common method using semiconductor devices such as diodes, by designing the metasurface pattern in the same way as the proposed method for a) - d) cases above. Here, in order to focus on the verification of the lens function, a filled pattern is used as the metasurface pattern and optical transparency is not concerned for the fabricated samples.
Fig. 10. (a) Design concept for FZP theory-based dynamic metasurface lens. (b) Fabricated static metasurface lens and (c) dynamic metasurface lens. Focus for the static metasurface lens is designed to be 800 mm away from the lens. Foci for dynamic metasurface lens are designed to be 300 mm away from each other in the x-direction at a distance of 300 mm from the lens. Here, the filled pattern is used to verify its lens function and optical transparency is not concerned for the samples in (b) and (c).
The near-field characteristics of metasurface lenses are evaluated with a VNA and horn antennas for the Tx and dipole antenna for the Rx as shown in Fig. 11. In the static metasurface lens, the Rx antenna moves along a straight line in the x and y directions around the focal point of focus 0, which is designed to be 800 mm away from the lens. The measurement for the dynamic metasurface lens in the y-direction is conducted at each focal point of focus 1 and focus 2, which are designed to be 300 mm away from each other in the x-direction at a distance of 300 mm from the lens.
Fig. 11. Experimental configuration of near-field intensity profile measurement for (a) static metasurface lens and (b) dynamic metasurface lens.
Figure 12 shows the measurement results for the static metasurface lens. Here, represented values are normalized by measured values for only the soda-lime glass, without the metasurface pattern. We observe that the lens gain of 25.4 dB is achieved at 27.6 GHz at the position of (x, y) = (400 mm, 800 mm) as shown in Figs. 12(a) and 12(b). From the measurement in the y-direction, it is verified that the focal point is formed at almost the same position as designed. Figure 12(c) shows the frequency characteristics of the lens gain at the focal position of (x, y) = (400 mm, 800 mm), representing a 3-dB bandwidth of approximately 1 GHz, which is sufficiently wide compared to the bandwidth allocated to each telecom operator for the 5G 28-GHz band.
Fig. 12. The measured near-field distribution of relative received power on (a) x- and (b) y-axis for static metasurface lens. Values are normalized by measured values for 4-mm thick soda-lime glass without the metasurface pattern. (c) Frequency characteristics of lens gain.
In the prototype of the dynamic metasurface lens, the ideal contact state is not achieved due to manufacturing limitations in the frame jig, so the intermediate state for dual focus at focus 1 and focus 2, and the separate state for single focus at focus 2 are evaluated. Based on Figs. 13(a)–13(c), dual focus points of approximately 300 mm away from each other in the x-direction, at a distance of 300 mm from the lens, are successfully observed in the intermediate state. The x-direction separation between the two foci is slightly greater than 300 mm, which is considered to be due to the fact that the Tx antenna is not placed in front of the centers of FZP pattern 1 and pattern 2. In the separate state, the received power in the vicinity of focus 1 is decreased by 6 dB and that of focus 2 is increased by 4 dB compared to the intermediate state indicating that the dynamic switching operation between single focus and dual focus is successfully demonstrated.
Fig. 13. Measured near-field relative received power distribution for dynamic metasurface lens (a) on the x-axis and (b) on the y-axis for intermediate and (c) separate states. All measured values are normalized by measured values for 4-mm thick soda-lime glass without the metasurface pattern.
In this paper, we use a simple LC-resonant mode in which the circulating current is excited in the SRRs to switch the single and dual focus modes. However, it should be possible to increase the variation in the switchable focus modes using a complex spectrum of hybridized resonant modes with coupled resonators reported for the electromagnetically induced transparency (EIT) study [40–42]. Further, there are some reports in which coupled modes between multiple layers are designed based on coupled-mode theory [43–45]. The interlayer distance strongly affects the coupling of resonant modes, which in turn affects the transmission amplitude and phase. This indicates the possibility of controlling the shape of the complex transmission spectrum as well as the polarization by changing the interlayer distance with the proposed method.
6. Conclusions
In this paper, we presented glass-based metasurfaces in which scattering characteristics are dynamically controlled while achieving both a large area and transparency for higher frequencies than millimeter waves. Meshed metal patterns and another transparent substrate were stacked where the interlayer distance between the substrates was varied to shift the resonant frequency of an SRR-based unit cell. Fabricated prototypes exhibited a 10-dB bandwidth of greater than 7.5% in the 28-GHz band and successful operation of controlling transmission and reflection modes with an extremely low loss of less than -1 dB considering the 400-MHz channel bandwidth for the 28-GHz band. Moreover, the development of the method for controlling the scattering characteristics and metasurface lenses that can be attached to a glass window in an outdoor-to-indoor scenario was also investigated. For the static metasurface lens, the lens gain of 25.4 dB at the designed focus can be observed at 27.6 GHz. As for the dynamic metasurface lens, we confirmed that the waves passing through the lens are concentrated at a single focus in the separate state and dual foci in the intermediate state. This indicates that dynamic switching operation between the single focus and dual foci was successfully performed. The proposed method for dynamic control of the metasurfaces has the advantages of a large scale, transparency, and control of the distribution of scattering characteristics with a uniform control signal, as opposed to the conventional way that requires the use of a control signal with a two-dimensional profile. For further development, increasing variation of controllable modes, in terms of the propagation direction and additional foci should be possible using coupled resonators with a complex spectrum of hybridized resonant modes.
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. NTT DOCOMO INC., “DOCOMO 5G White Paper 5G Radio Access: Requirements, Concept and Technologies,” https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_5 g/DOCOMO_5G_White_Paper.pdf.
2. T. Nakamura, A. Benjebbour, Y. Kishiyama, S. Suyama, and T. Imai, “5G Radio access: Requirements, concept and experimental trials,” IEICE Trans. Commun. E98.B(8), 1397–1406 (2015). [CrossRef]
3. D. Kitayama, K. Tateishi, D. Kurita, A. Harada, M. Inomata, T. Imai, Y. Kishiyama, H. Murai, S. Itoh, A. Simonsson, and P. Ökvist, “High speed mobility experiments on distributed MIMO beamforming for 5G radio access in 28-GHz band,” IEICE Trans. Commun. E102B(8), 1418–1426 (2019). [CrossRef]
4. D. Kurita, K. Tateishi, D. Kitayama, A. Harada, Y. Kishiyama, H. Murai, S. Itoh, A. Simonsson, and P. Ökvist, “Indoor and field experiments on 5G radio access for 28-GHz band using distributed MIMO and beamforming,” IEICE Trans. Commun. E102B(8), 1427–1436 (2019). [CrossRef]
5. Ericsson AB., “A research outlook towards 6G,” https://www.ericsson.com/4ab9e4/assets/local/reports-papers/white-papers/ericsson-white-paper-research-outlook-towards-6 g.pdf.
6. University of Oulu, “Key drivers and research challenges for 6G ubiquitous wireless intelligence,” http://jultika.oulu.fi/files/isbn9789526223544.pdf.
7. NTT DOCOMO INC., “5G Evolution and 6G,” https://www.nttdocomo.co.jp/english/binary/pdf/corporate/technology/whitepaper_6 g/DOCOMO_6G_White_PaperEN_v3.0.pdf.
8. M. Di Renzo, M. Debbah, D. T. Phan-Huy, A. Zappone, M. S. Alouini, C. Yuen, V. Sciancalepore, G. C. Alexandropoulos, J. Hoydis, H. Gacanin, J. de Rosny, A. Bounceur, G. Lerosey, and M. Fink, “Smart radio environments,” IEEE J. Select. Areas Commun 2019(129), 2450–2525 (2019).
9. Q. Wu and R. Zhang, “Intelligent Reflecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming,” IEEE J. Select. Areas Commun 18(11), 5394–5409 (2019). [CrossRef]
10. E. Basar, M. Di Renzo, J. De Rosny, M. Debbah, M. S. Alouini, and R. Zhang, “Wireless communications through reconfigurable intelligent surfaces,” IEEE Access 7, 116753–116773 (2019). [CrossRef]
11. V. Arun, H. Balakrishnan, and I. Nsdi, “RFocus: Beamforming Using Thousands of Passive Antennas,” in Proceedings of 17th USENIX Symposium on Networked Systems Design and Implementation (USENIX Association, 2020), pp. 1047–1061.
12. M. Di Renzo, A. Zappone, M. Debbah, M. S. Alouini, C. Yuen, J. De Rosny, and S. Tretyakov, “Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How It Works, State of Research, and the Road Ahead,” IEEE J. Sel. Areas Commun. 38(11), 2450–2525 (2020). [CrossRef]
13. D. Kitayama, D. Kurita, K. Miyachi, Y. Kishiyama, S. Itoh, and T. Tachizawa, “5G radio access experiments on coverage expansion using metasurface reflector at 28 GHz,” in Asia-Pacific Microwave Conference Proceedings (IEEE, 2019), pp. 435–437.
14. W. J. Padilla, A. J. Taylor, C. Highstrete, M. Lee, and R. D. Averitt, “Dynamical electric and magnetic metamaterial response at terahertz frequencies,” Phys. Rev. Lett. 96(10), 107401 (2006). [CrossRef]
15. H. T. Chen, W. J. Padilla, M. J. Cich, A. K. Azad, R. D. Averitt, and A. J. Taylor, “A metamaterial solid-state terahertz phase modulator,” Nat. Photonics 3(3), 148–151 (2009). [CrossRef]
16. Z. Han, K. Kohno, H. Fujita, K. Hirakawa, and H. Toshiyoshi, “MEMS reconfigurable metamaterial for terahertz switchable filter and modulator,” Opt. Express 22(18), 21326 (2014). [CrossRef]
17. F. Ma, Y. Qian, Y. S. Lin, H. Liu, X. Zhang, Z. Liu, J. Ming-Lin Tsai, and C. Lee, “Polarization-sensitive microelectromechanical systems based tunable terahertz metamaterials using three dimensional electric split-ring resonator arrays,” Appl. Phys. Lett. 102(16), 161912 (2013). [CrossRef]
18. C. Argyropoulos, “Enhanced transmission modulation based on dielectric metasurfaces loaded with graphene,” Opt. Express 23(18), 23787 (2015). [CrossRef]
19. B. Xiao, Y. Zhang, S. Tong, J. Yu, and L. Xiao, “Novel tunable graphene-encoded metasurfaces on an uneven substrate for beam-steering in far-field at the terahertz frequencies,” Opt. Express 28(5), 7125 (2020). [CrossRef]
20. T. Driscoll, S. Palit, M. M. Qazilbash, M. Brehm, F. Keilmann, B. G. Chae, S. J. Yun, H. T. Kim, S. Y. Cho, N. M. Jokerst, D. R. Smith, and D. N. Basov, “Dynamic tuning of an infrared hybrid-metamaterial resonance using vanadium dioxide,” Appl. Phys. Lett. 93(2), 024101–4 (2008). [CrossRef]
21. M. D. Goldflam, M. K. Liu, B. C. Chapler, H. T. Stinson, A. J. Sternbach, A. S. McLeod, J. D. Zhang, K. Geng, M. Royal, B. J. Kim, R. D. Averitt, N. M. Jokerst, D. R. Smith, H. T. Kim, and D. N. Basov, “Voltage switching of a VO2 memory metasurface using ionic gel,” Appl. Phys. Lett. 105(4), 041117 (2014). [CrossRef]
22. S. Wang, L. Kang, and D. H. Werner, “Hybrid Resonators and Highly Tunable Terahertz Metamaterials Enabled by Vanadium Dioxide (VO2),” Sci. Rep. 7(1), 1–8 (2017). [CrossRef]
23. D. Shrekenhamer, S. Rout, A. C. Strikwerda, C. Bingham, R. D. Averitt, S. Sonkusale, and W. J. Padilla, “High speed terahertz modulation from metamaterials with embedded high electron mobility transistors,” Opt. Express 19(10), 9968 (2011). [CrossRef]
24. W. Pan, C. Huang, P. Chen, M. Pu, X. Ma, and X. Luo, “A beam steering horn antenna using active frequency selective surface,” IEEE Trans. Antennas Propag. 61(12), 6218–6223 (2013). [CrossRef]
25. J. Reis, R. Z. Al-Daher, N. Copner, R. Caldeirinha, and T. Fernandes, “Two-dimensional antenna beamsteering using metamaterial transmitarray,” in Proceedings of 9th European Conference on Antennas and Propagation (EurAAP, 2015), 1(3), pp. 3–7.
26. H. Yang, X. Cao, F. Yang, J. Gao, S. Xu, M. Li, X. Chen, Y. Zhao, Y. Zheng, and S. Li, “A programmable metasurface with dynamic polarization, scattering and focusing control,” Sci. Rep. 6(1), 35692 (2016). [CrossRef]
27. C. Huang, S. Hu, G. C. Alexandropoulos, A. Zappone, C. Yuen, R. Zhang, M. Di Renzo, and M. Debbah, “Holographic MIMO Surfaces for 6G Wireless Networks: Opportunities, Challenges, and Trends,” IEEE Wireless Commun. 27(5), 118–125 (2020). [CrossRef]
28. W. Yao, L. Tang, J. Wang, C. Ji, X. Wei, and Y. Jiang, “Spectrally and spatially tunable terahertz metasurface lens based on graphene surface plasmons,” IEEE Photonics J. 10(4), 1–8 (2018). [CrossRef]
29. J. R. Reis, R. F. S. Caldeirinha, A. Hammoudeh, and N. Copner, “Electronically Reconfigurable FSS-Inspired Transmitarray for 2-D Beamsteering,” IEEE Trans. Antennas Propag. 65(9), 4880–4885 (2017). [CrossRef]
30. Z. Gao, C. Xu, X. Tian, J. Wang, C. Tian, B. Yang, S. Qu, and Q. Fan, “Ultra-wideband flexible transparent metamaterial with wide-angle microwave absorption and low infrared emissivity,” Opt. Express 29(14), 22108 (2021). [CrossRef]
31. T. Li, K. Chen, G. Ding, J. Zhao, T. Jiang, and Y. Feng, “Optically transparent metasurface Salisbury screen with wideband microwave absorption,” Opt. Express 26(26), 34384 (2018). [CrossRef]
32. K. Chen, L. Cui, Y. Feng, J. Zhao, T. Jiang, and B. Zhu, “Coding metasurface for broadband microwave scattering reduction with optical transparency,” Opt. Express 25(5), 5571 (2017). [CrossRef]
33. NTT DOCOMO INC. and AGC Inc., “Press Releases: DOCOMO and AGC Use Metasurface Lens to Enhance Radio Signal Reception Indoors,” https://www.nttdocomo.co.jp/english/info/media_center/pr/2021/0126_00.html.
34. O. Kagaya, Y. Morimoto, T. Motegi, and M. Inomata, “Transparent glass quartz antennas on the windows of 5G-millimeter-wave-connected cars,” IEICE Trans. Commun. E104.B(1), 64–72 (2021). [CrossRef]
35. D. Kitayama, M. Yaita, and H.-J. Song, “Laminated metamaterial flat lens at millimeter-wave frequencies,” Opt. Express 23(18), 23348 (2015). [CrossRef]
36. W. J. Padilla, M. T. Aronsson, C. Highstrete, M. Lee, A. J. Taylor, and R. D. Averitt, “Electrically resonant terahertz metamaterials: Theoretical and experimental investigations,” Phys. Rev. B 75(4), 041102 (2007). [CrossRef]
37. Y. Luo, C. Hung Chu, S. Vyas, H. Yu Kuo, Y. Hsin Chia, M. Ku Chen, X. Shi, T. Tanaka, H. Misawa, Y.-Y. Huang, and D. Ping Tsai, “Varifocal Metalens for Optical Sectioning Fluorescence Microscopy,” Nano Lett. 21(12), 5133–5142 (2021). [CrossRef]
38. X. Cai, R. Tang, H. Zhou, Q. Li, S. Ma, D. Wang, T. Liu, X. Ling, W. Tan, Q. He, S. Xiao, and L. Zhou, “Dynamically controlling terahertz wavefronts with cascaded metasurfaces,” Adv. Photonics 3(3), 036003 (2021). [CrossRef]
39. T. Zentgraf, T. P. Meyrath, A. Seidel, S. Kaiser, H. Giessen, C. Rockstuhl, and F. Lederer, “Babinet’s principle for optical frequency metamaterials and nanoantennas,” Phys. Rev. B 76(3), 033407 (2007). [CrossRef]
40. Z. Shen, H. Yang, X. Huang, T. Xiang, J. Wu, Y. Zhou, and Z. Yu, “Electromagnetically induced transparency metamaterial with strong toroidal dipole response,” Mater. Res. Express 7(3), 035802 (2020). [CrossRef]
41. J. Gu, R. Singh, X. Liu, X. Zhang, Y. Ma, S. Zhang, S. A. Maier, Z. Tian, A. K. Azad, H. T. Chen, A. J. Taylor, J. Han, and W. Zhang, “Active control of electromagnetically induced transparency analogue in terahertz metamaterials,” Nat. Commun. 3, 1151 (2012). [CrossRef]
42. N. Xu, M. Manjappa, R. Singh, and W. Zhang, “Tailoring the Electromagnetically Induced Transparency and Absorbance in Coupled Fano–Lorentzian Metasurfaces: A Classical Analog of a Four-Level Tripod Quantum System,” Adv. Opt. Mater. 4(8), 1179–1185 (2016). [CrossRef]
43. B. Yang, T. Liu, H. Guo, S. Xiao, and L. Zhou, “High-performance meta-devices based on multilayer meta-atoms: interplay between the number of layers and phase coverage,” Sci. Bull. 64(12), 823–835 (2019). [CrossRef]
44. Y. Li, J. Lin, H. Guo, W. Sun, S. Xiao, and L. Zhou, “A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption,” Adv. Opt. Mater. 8(6), 1901548 (2020). [CrossRef]
45. H. Guo, J. Lin, M. Qiu, J. Tian, Q. Wang, Y. Li, S. Sun, Q. He, S. Xiao, and L. Zhou, “Flat optical transparent window: mechanism and realization based on metasurfaces,” J. Phys. D: Appl. Phys. 51(7), 074001 (2018). [CrossRef]
