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Abstract
With the rapid development of wireless technology, the revolution of tailoring transmission window in dynamic way for the next generation communication systems is urgently required. However, the degree-of-freedom for switching transmission spectra of an effective medium still needs further investigation. Here, we propose a paradigm of solving this difficult academic issue via the method of bias-voltage-driven. Leveraging PIN diodes and varactor diodes into the predesigned positions of plasmonic meta-structures, the macro-control of transmission windows switch and the detailed dispersion manipulation can be separately achieved by synergy modulation of feed networks. Both the numerical simulations and experimental verifications are conducted to support the effectiveness of the proposed method. Significantly, the proposed paradigm presents great potential for applications in intelligent radome, adaptive communication systems, and other EM scenarios with multi-degree-of-freedom.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
During the past few decades, metamaterials have gained enormous attention due to their remarkable electromagnetic (EM) properties [1–3], which are mainly introduced by their subwavelength structures and functional arrangements. As forms of planar metamaterials, metasurfaces not only overcome the challenges presented in bulk metamaterials [4] but also impose strong manipulations of EM waves by wavefront shaping [5,6], radiation control [7–9], and polarization conversion [10,11]. Due to this versatility, many metasurfaces have been implemented for various devices, such as optical cloaks [12,13], beam formers [14,15], flat lens [16,17]. With the increase demand of modern wireless communication systems, the passive devices with fixed electromagnetic functionalities cannot meet the requirements. Accordingly, more attention has been given to active metasurfaces which can achieve tunable functions controlled by electric bias [18–20], mechanical actuation [21], temperature [22,23], and pump light [24,25].
Unfortunately, most of the existed active metasurfaces are designed as reflected types to prevent the unexpected interactive between metallic resonators and underneath feed network, especially for microwave regimes. Nevertheless, tailoring transmission fields is also an important issue that possess great potentials in various application fields, including stealth radome [26,27], multi-functional radar [28], and multi-standard antenna systems [29]. To date, there are two main methods for tailoring the transmission window. One is to implement switching between transmission and reflection states [30–32]. The other is to tune the dispersion of the transmission window [33–35]. However, little researches have been reported to achieve both simultaneously. The freedom to switch the transmission spectrum of the effective medium is limited and needs further investigation.
To push the development of this challenging task, we propose a dynamic method to tailor transmission window with multi-degree-of-freedom. PIN diodes and varactor diodes are embedded into the plasmonic meta-atom. The transmission windows switch and the dispersion manipulation can be separately achieved by synergy modulation of feed networks. As a proof-of-principle, an active meta-device loading PIN diodes and varactor diodes is proposed. The varactor diodes and PIN diodes are mounted on the top and bottom layers, respectively. When the PIN diodes are in the ON state, the transmission window appears in the C-band. The dispersion is modulated from 5.98 GHz to 5.09 GHz as the bias voltage of the varactor diodes is adjusted. The transmission window switches to S-band in the OFF state of PIN diode. The tunable range of dispersion is from 3.95 GHz to 3.40 GHz with the change of varactor diode voltage. In addition, the angular stability of the transmission window respectively reaches 45° and 75° in ON and OFF states. The proposed meta-device provides an alternative method to simultaneously realize the transmission windows switch and the dispersion manipulation, which possesses the potential in versatile applications, including intelligent radome, adaptive communication systems, and other EM scenarios with multi-degree-of-freedom.
2. Design and analysis of the active meta-device
The dimensions and layouts of a meta-atom are depicted in Fig. 1. The proposed meta-atom consists of two metallic layers that are separated by a dielectric layer. The dielectric layer is PTFE ceramic composite substrate TFA-2 whose relative dielectric constant and loss tangent are 3.0 and 0.001, respectively. The metal structure on the top layer consists of bent metal strips arranged symmetrically. The varactor diodes are mounted in the gap between the bent metal strips. Metallic strips and thin metal lines for bias network comprise the structure on the bottom layer. The metal strips in the middle part are connected by PIN diodes. Due to the arranged structure and bias network, the meta-device can independently achieve the transmission windows switch and the dispersion manipulation by the method of bias-voltage-driven. The geometric parameters are fixed as: the periodical length is p = 12 mm, the dimensions of the bent metal strip are a = 10 mm and b = 11 mm, the gap for loading varactor diodes and PIN diodes is d = 0.8 mm, the width of the bent metal strip is w1 = 0.2 mm, the length of the center metal strip is l = 5.2 mm and the width of the metal strip on the bottom layer is w2 = 0.2 mm.
Fig. 1. The geometrical structure of proposed meta-atom: (a) Schematic view of unit cell; (b) top layer; (c) bottom layer.
Varactor diode and PIN diode have different functions in electromagnetic regulation. In order to clearly reveal the effect of diodes, the surface current distributions of the proposed meta-atom at different operating states are monitored as shown in Fig. 2. Figures 2 (a), (b) and (e) indicate the surface current distribution in the ON state of the PIN diode. And the situations in the OFF state are depicted in Figs. 2 (c), (d) and (f). It can be clearly seen that the ON and OFF state of the PIN diode change the direction of surface current. In the ON state of PIN diode, Fig. 2 (a) and (b) indicate the situations when the capacitance Cv of the varactor diode is 0.23 pF and 2.10 pF, respectively. The corresponding cases in the OFF state are shown in Figs. 2 (c) and (d). It can be seen from the figures that the overall surface current intensity of the structure is altered, as the capacitance (Cv) of the varactor diode changes. The arrangement of PIN diodes and varactor diodes achieves regulation on the direction and intensity of surface currents. Therefore, the transmission window of this meta-device can be switched macroscopically by controlling the PIN diode and the dispersion is tuned as the capacitance Cv of the varactor diode alters.
Fig. 2. Surface current distribution of the proposed meta-atom under different states of the PIN diode and the varactor diode: The top layer: (a) ON state, Cv = 0.23 pF. (b) ON state, Cv = 2.10 pF; (c) OFF state, Cv = 0.23 pF; (d) OFF state, Cv = 2.10 pF; The bottom layer:(e) ON state; (f) OFF state.
The working mechanism of the proposed method is further analyzed and revealed by the equivalent circuit model (ECM) shown in Fig. 3. On the top side, the varactor diode [36] is equivalent to the serial connection of the constant resistance Rv = 5.41 Ω, inductance Lv = 0.45 nH, and adjustable capacitance Cv. It is connected in series with the inductor L1 of the metal strip. The dielectric substrate is regarded as a transmission line. Since the length is much shorter than the wavelength of operating frequency in the low-frequency region, it is almost negligible. On the back of the meta-atom, metal strips on both sides are equivalent to the inductor L2, in parallel with the PIN diodes Zp and the inductor L3 of the connected metal strip.
The equivalent inductance L and capacitance C of the metal structure can be calculated by the following equations [37]:
The PIN diode is modeled according to its datasheet [38]. When the PIN diode is in the ON state, Zp consists of small resistance RON = 1.5 Ω, and small inductance Lp = 1.5 nH; And in the OFF state, the PIN diode is equal to high capacitance COFF = 0.1 pF and small inductance Lp = 1.5 nH. The impedance of the equivalent circuit can be expressed as follow:
By substituting ECM of the PIN diode into Eq. (4), the equivalent circuit of the active meta-device loaded with PIN and varactor diode can be obtained. And the equivalent impedance in the two states can be expressed as follows:
In the ON and OFF state of the PIN diode on the bottom layer, the resonant frequencies (fON, fOFF) can be calculated as follows:
Full-wave simulations are implemented by CST Microwave Studio. And the boundaries in the x- and y-direction are set as unit cell and the boundary in the z-direction is set as open (add space). The “Lumped Element” is used to simulate the PIN diode and the varactor diode, and the “RLC” is set to the corresponding value. In addition, the frequency domain solver is chosen to calculate.
The simulated results are depicted in Fig. 4. When the PIN diode is in the ON state and the varactor diode change Cv from 0.23 pF to 2.10 pF, the resonance frequency (fON) moves to low frequency from 5.98 GHz to 5.09 GHz for TE polarization. The insertion losses are from 0.54 dB to 0.81 dB. In the OFF state of the PIN diode, the resonance frequency (fOFF) moves from 3.95 GHz to 3.40 GHz with the change in capacitance Cv of the varactor diode. The insertion losses are from 0.29 dB to 0.23 dB. The ON and OFF states of the PIN diode cause the transmission window to switch between the S- and C-band. The dispersion is dynamically modulated by controlling the capacitance Cv of the varactor diode.
Fig. 4. The simulated results of the transmission coefficient with Cv from 0.23 pF to 2.10 pF: (a) ON state; (b) OFF state.
In various working states, the angle range of stable electromagnetic performance is different. According to Fig. 2, the operating state of the PIN diode directly determines the flow of surface current. In addition, the concentration site of the surface current is changed. This leads to different angular stability in the ON and OFF states. The angular stability of the transmission window is simulated and depicted in Fig. 5. The x-coordinate represents the frequency and the y-coordinate denotes the incidence angle. It can be observed from the Fig. 5 that in the ON state, the resonance frequency and bandwidth of the passband remain constant as the incidence angle increases from 0° to 45°. When the PIN diode is switched to the OFF state, the bandwidth of the transmission window decreases with increasing incidence angle. However, the resonant frequency remains stable. The high-efficiency transmissivity over 0.9 is obtained as the incidence angle is less than 75°.
Fig. 5. The transmission coefficient as the incidence angle increases: (a) ON state and Cv = 0.23 pF. (b) ON state and Cv = 2.10 pF. (c) OFF state and Cv = 0.23 pF. (d) OFF state and Cv = 2.10 pF.
3. Fabrication and measurement
As shown in Figs. 6 (a) and (b), a prototype is fabricated to experimentally validate the performance of proposed meta-device. The overall size of prototype is 384 × 384 mm2 and it consists of 32 × 32 meta-atoms. The varactor diodes SMV2201-040LF are mounted on the top layer and are in same direction. The PIN diodes BAP70-03 are embedded on the bottom layer of the prototype. They are in opposite direction within the cell and between adjacent columns. A schematic diagram of the bias network is shown in Figs. 6 (c) and (d). When the positive and negative terminals of the DC power supply are connected to the prototype, the varactor diodes on the top layer are driven by the flowing direct current. The bias network and structure on the bottom layer constitute a metal grid. Direct current flows from the positive terminal to the negative terminal through the metal grid and PIN diodes. In addition, the measured setup is shown in Fig. 7. The Agilent N5224A vector network analyzer and two horn antennas are used to measure the transmission response. The entire test process is conducted in a microwave anechoic chamber to reduce EMI from the external environment.
Fig. 6. Actual version of fabricated meta-device: (a) The top side and (b) the bottom side. Schematic view of the biasing network: (c) Top layer and (d) bottom layer.
The measurement results for the prototype are presented in Fig. 8. To demonstrate the performance of the proposed meta-device more clearly, the corresponding simulation and test results are shown as straight lines and scribed lines in the same color, respectively. When the bias voltage U1 of the PIN diode reaches 1.1 V, it is in the ON state. As shown in Fig. 8 (a), the dispersion of transmission window can be regulated from 5.14 GHz to 6.05 GHz. When the varactor diode is not connected to a DC voltage source (the voltage of the varactor diode U2 = 0 V), the resonant frequency of the transmission window is 5.14 GHz. As U2 increases to 4.9 V, 10.4 V and 20.3 V, the resonant frequency shifts to 5.45 GHz, 5.78 GHz, and 6.05 GHz respectively. The PIN diode is switched to the OFF state by removing the external power supply (the voltage of the PIN diode U1= 0 V). Adjusting the voltage U2 of the varactor diode, the dispersion of the transmission window is moved from 3.95 GHz to 3.40 GHz. When U2 are 0 V,6.2 V,10.4 V and 20.3 V, the resonant frequencies of transmission window are 3.40 GHz, 3.65 GHz, 3.84 GHz and 3.95 GHz respectively.
Fig. 8. The measurement transmission coefficients under different operating status and incident angles: ON state (a) 0°; (b) 15°; (c) 30°; (d) 45°; OFF state (e) 0°; (f)30°; (g) 60°; (h) 75°.
In addition, the angular stability of the proposed structure in the ON and OFF states is further verified by the measurements. From Figs. 8 (a), (b), (c) and (d), it can be seen that the resonant frequency and bandwidth of the transmission window remain stable as the incident angle changes from 0° to 45°. This indicates that the proposed structure stably exhibits electromagnetic performance in the ON state. Comparing and analyzing Figs. 8 (e), (f), (g) and (h), the resonant frequency in the OFF state remains constant as the incident angle increases from 0° to 75°.However, the bandwidth of the transmission window becomes narrow.
When the incidence angle in the ON state is larger than 30° and the applied voltage U2 = 20.3 V, the transmission curve fluctuates at 7.53 GHz. And the transmission curve also fluctuates at 6.64 GHz in the OFF state with the incident angle above 60° and the applied voltage U2 = 10.4 V. These are the grating lobes due to electromagnetic wave scattering. As the angle of incidence increases, the equivalent spacing between meta-atoms decreases and this phenomenon inevitably occurs. However, the proposed structure appears with grating lobes at about 7.53 GHz and 6.64 GHz. This does not have an effect on the transmission windows switch and the dispersion manipulation.
Compared with the simulation results, there are some fluctuations and deviations in the measurement results. The testing process and prototype fabrication lead to deviations. The output of the external power supply is not stable. The electromagnetic waves transmitted by the horn are not strictly planar waves. All these result in fluctuations. However, the test results are in overall agreement with the simulation results. They demonstrate the effectiveness and feasibility of the proposed method. This approach can simultaneously achieve the transmission windows switch and the dispersion manipulation. This also shows that introducing both varactor diode and PIN diode into plasmonic meta-structures leads to more diverse electromagnetic regulation.
From the comparison of previous researches in Table 1, most of the current findings use a single kind of active components for the transmission windows switch or the dispersion manipulation [30–35]. The regulation range is limited. In [39], two types of diodes are implemented together. However, the transmission windows switch is only effective at 4.0 GHz. The specific active components used and the experiments are not stated. Dispersion regulation, transmission window switching and polarization selection functions are achieved in [40]. But the switching of reflection and transmission states is only valid for a single transmission window. It is not available to mutually switch between dual transmission windows. In this paper, all the above limitations are investigated and addressed. The underlying mechanism of active components is revealed through surface current modulation. The relationship between the electrical parameters of the active component and the resonant frequency is constructed by an equivalent circuit model. Based on the above analysis, an active meta-device with the transmission windows switch and the dispersion manipulation is designed. In addition, the simulation and test results illustrate the advantages of the proposed meta-device in angular stability and tuning range.
Table 1. Comparison with previous researchesa
4. Conclusion
In this paper, a paradigm to tailor transmission window with multi-degree-of-freedom is proposed in a dynamically controllable method. The ON/OFF state of the PIN diode change the direction of surface current. The capacitance of the varactor diode influences the surface current intensity. The macro-control of transmission windows switch and the detailed dispersion manipulation are achieved by tuning the PIN diode and varactor diode. A meta-atom is designed as a proof-of-principle. In the ON state, the dispersion of the transmission window is modulated from 5.98 GHz to 5.09 GHz as the bias voltage of the varactor diode is changed. However, the transmission window is switched to S-band by tuning the PIN diode to the OFF state. The dispersion is regulated from 3.95 GHz to 3.40 GHz as the voltage of the varactor diode changes. In addition, the angular stability of the transmission window reaches 45° and 75° in ON and OFF states, respectively. The proposed paradigm provides a dynamic method for diversified modulation of transmission windows, which has applications in intelligent radome, adaptive communication systems, and other EM scenarios with multi-degree-of-freedom.
Funding
National Key Research and Development Program China (2017YFA0700201); National Natural Science Foundation of China (61971435, 61971437); Natural Science Foundation of Shaanxi Province (2020JM-342, 2022JM-352, 2022JMQ-630).
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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