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Abstract
Active metasurfaces with dynamically switchable functionalities are highly in demands in various practical applications. In this paper, we experimentally present an active metasurface based on PIN diodes which can realize nearly perfect reflection, transmission and absorption in a single design. Such switchable functionalities are accomplished by controlling the PIN diodes integrated in both layers of the metasurface. A transmission line model is employed to further investigate the underlying mechanism of the metasurface. This proposal is confirmed by numerical simulations and experiments. As a novel metasurface with multiple switchable functionalities, our design may find some practical applications such as smart radomes.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metasurfaces are two-dimensional planar metamaterials with unique and novel electromagnetic properties beyond the limitations of natural materials, which have gained huge interests since last decade [1–4]. Many metasurfaces have been successfully implemented for a variety of intriguing devices, such as optical cloaks [5,6], beam formers [7,8], flat lens [9,10], and so on. Recently, in order to satisfy the growing demand of versatile devices, much attention have been paid on active metasurfaces which can present switchable functionalities controlled by electric bias [11–13], mechanical actuation [14], temperature [15,16], as well as pump light [17,18].
In microwave regime, active components such as PIN diodes and varactors are widely used in active metasurfaces [19–26], and some of them have been implemented for practical applications such as programmable imagers [27,28], reconfigurable antennas [29,30], and polarization convertors [31,32]. By tuning the bias voltages applied to varactors, the electromagnetic characteristics of metasurfaces can be tuned accordingly. However, such designs with varactors require accurate tuning of capacitances, which tend to be affected by the parasitic capacitances and the fluctuation of bias voltages. Another common scheme is using PIN diodes as tunable resistances, which are as unstable as varactors since PIN diodes are working at the nonlinear region. The PIN diode working at this narrow region is very sensitive to the bias voltage, and its resistance rapidly decreases with the increase of the bias voltage. Therefore, considering the practical applications of the metasurface, utilizing the ON and OFF states of the PIN diodes is a better choice.
Moreover, by suitably designing the structures and arranging the active components, active metasurfaces can achieve desired dynamic manipulation of the magnitude [33], phase [34] and/or polarization [35] of the electromagnetic waves. Magnitude is one of the most fundamental and important properties of the electromagnetic waves, it is always desirable to have full control in reflection, transmission, and absorption of electromagnetic waves in one active metasurface. However, the sophisticated relations among these three channels which should be carefully balanced make the design very challenging.
Most recently, significant efforts have been devoted on the realization of switchable metasurfaces which can realize two of three functionalities (i.e. reflection/transmission, reflection/absorption, or transmission/absorption) using PIN diodes [36–38] or varactors [39]. The functionalities of such metasurfaces can be switched by applying different voltages on the diodes. It is worth noting that only the work by Lustrac et al. [40] has made a step toward the possibility of switching among reflection, transmission, and absorption in a metasurface screen. However, they make the PIN diode resistive to achieve high absorption state, which requires extremely accurate electrical biases, resulting relatively low absorptivity. Moreover, their design is composed of two metasurfaces, two glass layers, and one thick spacer, which lose the merit of low profile.
In this paper, a novel active metasurface with three modes is proposed, which presents dynamically switchable functionalities of nearly perfect reflection, transmission and absorption. Two sets of PIN diodes are employed as switches, and thus three modes can be achieved without resistive state of the PIN diodes. This metasurface is composed of a dielectric layer and two metallic layers. Two metallic layers with different patterns make the metasurface asymmetric, bringing more distinctive characteristics. The transmission line model is employed to analyze the electromagnetic performance of the metasurface, providing insight into the physical characteristics. Subsequently, the metasurface is fabricated and experimentally measured. The simulated and experimental results demonstrate that the proposed metasurface can realize three switchable functionalities by switching the bias voltages. The proposed switchable metasurface may find great potential in practical applications as a highly versatile device, such as smart radomes for radars, antennas, and sensors.
2. Unit cell and simulation
A tri-layer metasurface composed of one dielectric layer sandwiched by two metallic layers incorporated with active components can be used as a meta-device with switchable functionalities. The top and bottom views of the unit cell are depicted in Figs. 1(a) and 1(b), respectively. Here, different metallic patterns are chosen for the two metallic layers since the inversion symmetry imposes strict constrains on the system [36]. The dielectric substrate is made of Rogers RO4350B with a relative permittivity of 3.48 and a loss tangent of 0.0037. The thickness of the substrate is 1.524 mm. Two kinds of square copper rings are placed alternately on the top of the substrate, while the bottom layer is composed of another kind of square rings. All the gaps between adjacent rings are $d$ = 2 mm, where the PIN diodes are loaded so that they are in series connection. To design the unit cell of the metasurface for operations around 7.5 GHz, a parametric study is performed on the dimensions of the unit cell and the square rings. The optimized parameters are as follows: $x$ = 14.56 mm, $y$ = 11.96 mm, $l_1$ = 4.43 mm, $l_2$ = 1.76 mm, $w_1$ = $w_2$ = 10.40 mm, $l_3$ = 4.98 mm, $w_3$ = 6.24 mm, $a_1$ = 2.80 mm, $b_1$ = 0.50 mm, $a_2$ = 7.00 mm, $b_2$ = 1.17 mm, $a_3$ = 3.40 mm, and $b_3$ = 1.00 mm. The commercial PIN diode BAP50-03 from NPX is used as switchable elements. According to [37], the PIN diode without DC voltage can be simply modeled as a series of a resistor $R_{\textrm {OFF}} = 40~\Omega$, and a capacitor $C_{\textrm {OFF}}$ = 0.19 pF. With forward DC voltages, the PIN diode performs as a series of a resistor $R_{\textrm {ON}} = 2~\Omega$, and an inductor $L_{\textrm {ON}} = 1.5$ nH. The equivalent circuits of the PIN diode under two states are exhibited in Fig. 1(c).
Fig. 1. (a) Top view and (b) bottom view of the designed unit cell with PIN diodes incorporated. (c) Equivalent circuits of the PIN diode.
By controlling the DC bias voltages separately applied to the PIN diodes on the two metallic layers, the proposed metasurface can realize different functionalities including nearly perfect reflection, transmission, and absorption. The electromagnetic performance of the metasurface is simulated using the commercial software CST Microwave Studio, with unit cell boundary conditions in the $x$ and $y$ directions, and open (add space) boundary in the $z$ direction.
As shown in Fig. 2(a), when both PIN 1 and PIN 2 work at OFF-state, the metasurface exhibits nearly perfect absorption and the absorptivity reaches its maximum of 94.7% at the working frequency 7.5 GHz. The relative bandwidth of over 90% absorption is 6.4%. As shown in Fig. 2(b), with PIN 1 working at ON-state and PIN 2 at OFF-state, nearly perfect reflection can be achieved in a relatively wide band. As shown in Fig. 2(c), with PIN 1 and PIN 2 both biased by forward voltages, the metasurface exhibits nearly perfect transmission and the transmission coefficient is 0.9 at 7.5 GHz. Our simulation results clearly demonstrate that the proposed metasurface can achieve nearly complete absorption, reflection, and transmission through switching states of PIN diodes.
Fig. 2. Magnitudes of reflection coefficients (red) and transmission coefficients (blue) with different states of PIN diodes, obtained by simulation (symbols) and calculation (curves). (a) Absorption mode, (b) reflection mode, and (c) transmission mode.
Further numerical simulations have been performed on the unit cell to assess the performance of the metasurface under off-normal incidence (15$^{\circ }$, 30$^{\circ }$, and 45$^{\circ }$), which is given in Fig. 3. When the metasurface works at absorption mode, its transmission coefficients increase along with the incident angle, while its reflection coefficients almost remain constant, as shown in Fig. 3(a). At reflection mode, the simulation results keep unchanged under different incident angles, as shown in Fig. 3(b). At transmission mode, bigger angle of incidence leads to a blue shift of working frequency and the reduction of transmissivity, as shown in Fig. 3(c). Overall, though the oblique incidence causes the performance of absorption mode and transmission mode to degrade slightly, three functionalities of the metasurface can still be realized.
Fig. 3. Magnitudes of reflection coefficients (solid curves) and transmission coefficients (dashed curves) with different states of PIN diodes, obtained by simulation under normal and oblique incidences. (a) Absorption mode, (b) reflection mode, and (c) transmission mode.
3. Model and discussions
The equivalent transmission line model is used to analyze the mechanism of the proposed metasurface in detail [41]. To better understand the relationship between the lumped circuit elements and the metallic structures of the metasurface, we show the electric filed and the surface current distributions on both metallic layers, as shown in Fig. 4. The locations where the electric fields appear imply the existence of capacitors, whiles the positions of surface currents are treated as inductors along the directions of the currents. The PIN diodes loaded in the metallic layers can be modeled by its equivalent circuit corresponding to its working state, i.e. ON state or OFF state, as we mentioned before.
Fig. 4. (a) Electric field and (b) surface current distributions on the top layer. (c) Electric field and (d) surface current distributions on the bottom layer.
Therefore, the top layer is equal to a circuit formed by lumped elements $C_1, L_1, L_2$ and two PIN diodes, and the bottom layer is equal to a circuit formed by lumped elements $C_2, C_3, C_4, L_3$ and one PIN diode. The substrate layer can be modeled as transmission line of specific length (thickness of the substrate), with the characteristic impedance $Z_{\textrm {sub}} = Z_0/\sqrt {\varepsilon _r}$ and phase constant $\beta = 2\pi /\lambda \cdot \varepsilon _r$, where $Z_0$ is the free space impedance, $\varepsilon _r$ is the relative permittivity of the substrate, $\lambda$ is the wavelength in the free space. Three layers of the metasurface are cascades in order and the ports are terminated with $Z_0$. Finally, the equivalent circuit of the metasurface is obtained, as shown in Fig. 5. Thus, we can derive the transmission matrix of the metasurface from this equivalent circuit,
In order to validate the equivalent circuit model, the parameters of the lumped elements are optimized by non-linear least squares in the frequency range of 6.5-8.5 GHz. The detailed parameters are: $C_1 = 0.26$ pF, $L_1 = 0.98$ nH, $L_2 = 0.15$ nH, $C_2 = 0.26$ pF, $C_3 = 0.08$ pF, $C_4 = 0.31$ pF, and $L_3 = 0.84$ nH. One can analyze the metasurface easier with the help of the equivalent circuit, whose external responses are equivalent to the metasurface. The reflection coefficients and transmission coefficients from model calculation are shown in Fig. 2, which reasonably agree with those from simulations, demonstrating the validity of the extracted circuit.
4. Experimental results
To verify the performance of the proposed metasurface, a prototype, which measures 291.20 mm $\times$ 263.12 mm, with $20\times 20$ unit cells and peripheral bias circuit, is fabricated using standard printed circuit board (PCB) technology. Figure 6 presents photographs of the fabricated prototype. The PIN diodes are soldered on the metallic layers by the surface mounted technology (SMT) and the wires are connecting to the bias lines by welding. The experiments are carried out twice to measure the reflection coefficient and the transmission coefficient of the metasurface, respectively. The experimental surroundings and instruments are not identical. The photographs of the experimental setup are shown in Fig. 7. The measured reflection coefficient is normalized with that of the metal plane in same dimensions, and the measured transmission coefficient is normalized with that of without metasurface. Two DC powers supply the bias voltages to the PIN diodes through the bias wires. We bias the top layer with volatges of 40 V, and the bottom layer with 20 V, so each PIN diode is biased with approximate 0.96 V, considering the resistances of the bias lines and metallic structures.
Fig. 7. Experimental setup for measuring (a) reflection coefficients and (b) transmission coefficients.
The measured results of the three different modes are shown in Fig. 8. By switching the states of diodes, the metasurface can work at absorption mode, reflection mode, and transmission mode accordingly. As shown in Fig. 8(a), when all the PIN diodes are unbiased, the reflection coefficient reaches its minimum of $0.364$ at 7.1 GHz and the transmission coefficient is less than 0.2, so the metasurface absorbs the incoming energy with the absorptivity about 85%. As shown in Fig. 8(b), when PIN 1 is switched to ON-state and PIN 2 is still unbiased, the reflection coefficient keeps relatively high and the transmission coefficient is very low within the frequency range. The reflection coefficient is $0.893$ at 7.1 GHz. As shown in Fig. 8(c), by biasing both PIN 1 and PIN 2 on, the reflection coefficient of the metasurface becomes below 0.2 and the transmission coefficient is larger than 0.9 between 6.9–7.2 GHz. Compared with our simulation result, the transmission curves generally blue shift and the reflection curves generally red shift, resulting in the working frequency of the metasurface shifts toward lower frequency near 7.1 GHz. Also, the spiking cavity-like behavior only occurs to the reflection coefficients, but not the transmission coefficient. Therefore, it is quite possible that two different experiment setups lead to the deviation of measured results. Nevertheless, the experimental results are reasonably in good agreement with the simulated results, considering the influence of the experimental surroundings and measurement errors. The switchable functionalities of the proposed metasurface are successfully achieved, revealing that our design can provide a possibility for versatile devices.
Fig. 8. Magnitudes of reflection coefficients (red) and transmission coefficients (blue) with different states of PIN diodes, obtained by simulation (dashed curves) and measurement (solid curves). (a) Absorption mode, (b) reflection mode, and (c) transmission mode.
5. Conclusion
In summary, we have proposed a switchable metasurface, which exhibit high absorption, reflection, and transmission. By integrating PIN diodes in the top layer and bottom layer, the functionality of the proposed metasurface can be switched by controlling the bias voltages. Also, we employed the transmission line model to analyze the metasurface, revealing its intrinsic characteristics. Furthermore, we demonstrate the experimental results which validate the functionalities of our design. Our work not only provides an approach to realize dynamically switchable devices with multiple modes, but also has potential prospects for applications in practice.
Funding
National Natural Science Foundation of China (51777168, 62071291).
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.
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