Numerous studies have been made to design pattern reconfigurable THz antennas to achieve optimum performance for particular environmental conditions. However, it is still a challenge to achieve large angle beam steering for reconfigurable antenna in terahertz band. Here we propose a 360-degree beam steering THz antenna using active frequency selective surface (AFSS) based on hybrid graphene-gold structure. The proposed antenna consists of a THz omnidirectional monopole antenna coated with a hexagonal AFSS screen. By adjusting the chemical potential of graphene from 0 to 0.5eV, the AFSS unit cell can be switched from ON state (high transmission) to OFF state (total reflection) in terahertz, which can steer the beam direction as the monopole antenna is surrounded with six parts of AFSS screen with different ON/OFF states. In this way, the antenna can achieve beam scanning covering 360 degrees. Moreover, unlike the conventional AFSS with only two states, the reflection and transmission coefficient of the proposed AFSS are continuously variable due to the tunable chemical potential, which allows the radiation gain of antenna to be enlarged or suppressed. This antenna may serve the reconfigurable THz wireless system with flexible beam direction and gain level.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Graphene is a two-dimension structure built up by carbon atoms in honeycomb lattice. Since it was first isolated from graphite through mechanical exfoliation in 2004 , it has attracted much interests because of its unique electric properties. Therefore many graphene-based devices, such as filters [2,3], absorbers [4-11], modulator [12-14], frequency selective surface (FSS) [15-18] and antennas [19,20] have been proposed operating at microwave, THz and even optical frequencies. Among many properties of graphene, the possibility to dynamically tune both electronic and optical properties through the application of an external voltage has allowed the design of reconfigurable devices, especially graphene-based reconfigurable THz antennas.
Pattern reconfigurable antennas have been widely exploited as a cost-effective substitute for conventional phased arrays that consist of phased shifters and amplifiers. Electronic beam-switching antennas employing active frequency selective surfaces (AFSS) have been investigated since it is feasible to achieve multi-reconfigurability . Most of the AFSS for beam-switchable antenna is tuned by PIN or varactor diodes, which restricts its applications in the millimeter and terahertz [22,23]. In the last years, we have witnessed the exponential development of terahertz (THz) antennas by means of cutting-edge materials like graphene, which is significantly correlated with the unique properties shown by graphene in this range. In order to meet the developed antenna demands with specific capabilities in terms of beam steering and directivity, many graphene-based THz reconfigurable antennas have been designed. Among the proposed antennas, the majority of them use graphene as the main part of the antenna. For instance, a reconfigurable MIMO system based on graphene antenna have been investigated in , whose beam width and direction can be controlled by the states of each graphene patch in the antenna. In addition, the proposed antenna in  is a novel graphene-based leaky-wave THz antenna with beam scanning capabilities at fixed frequencies, which is composed of a graphene sheet transferred onto a back-metallized substrate and a set of polysilicon DC gating pads locked beneath it. The direct use of graphene as an antenna radiator, after all, has some limitations. Therefore, a graphene-based switchable high-impedance surface (HIS) is designed to realize the beam reconfigurable of antenna . By applying different voltages for different rows of HIS units, the antenna beam can vary in a range of ± 30◦. However, although all of these antennas can achieve pattern reconfiguration, the adjustment range of the graphene chemical potential is relatively large, and the beam scanning range of the antenna is limited.
In this paper, we present a large angle beam scanning THz antenna with AFSS based on hybrid graphene-gold structure. The radiation source is an omnidirectional monopole antenna, which is surrounded by a hexagonal AFSS screen dividing the azimuth plane into six equal parts of 60°. By changing the graphene chemical potential of each part of the AFSS, the proposed antenna not only realizes beam steering with six radiation beams covering the entire azimuth plane, but also allows for flexible control of the gain level in each direction.
2. Structure of the active FSS unit cell
Generally speaking, patch-type FSS exhibit bandstop property, whereas aperture-type FSS exhibit bandpass property. However, for the aperture-type FSSs, we cannot separately apply bias voltage to each unit cell due to the metallic connections between adjacent cells. In order to solve this problem, we propose the AFSS structure unit cell as shown in the Fig. 1, which consists of six layers: gold pattern layer, monolayer graphene later, silicon dioxide layer, p-type doped silicon layer, gold feed line layer, and foam layer. As we can see from Fig. 1(b), the proposed AFSS unit cell consists of a Minkowski fractal gold ring and a square single layer graphene sheet. This structure allows us to build a passband between two stopbands, and the tunable properties are achieved by controlling the surface conductivity of graphene.
Graphene can be modeled as an ultra-thin conducting surface whose surface conductivity is σ(ω, μc, τ, T) , where ω is the angular frequency, μc is chemical potential, τ is the relaxation time, and T is the temperature in Kelvin. Here we assume T = 300K and τ = 100fs, which is based on the theoretical estimation of maximum mobility in graphene . In HFSS, the graphene sheet can be modeled as the impedance boundary with complex impedance as Zs = 1/σ. Monolayer graphene film is transferred on the silicon dioxide layer and etched to square pattern. The silicon dioxide, with thickness of 0.3μm and permittivity ɛ = 4, is chosen as insulating layer between graphene and p-type silicon. The relative dielectric constant of p-type silicon is chosen as 11.7, and foam substrate with thickness of 6μm acts as a support.
By applying a DC bias voltage via the gated structure, the chemical potential of graphene can be changed expediently; this allows dynamically tuning of the surface conductivity. An approximate closed-form expression to relate μc and Vdc is given by Fig. 3(a) shows the relationship between applied DC voltage and chemical potential. In order to apply the bias voltage to the graphene, as Fig. 1(c) shows, gold feed lines are designed for the hybrid graphene-gold structure based AFSS unit cell.
The AFSS unit cell is simulated using the ANSYS High Frequency Structural Simulator (HFSS) with master-slave boundary conditions applied along the x- and y-axes and two Floquet ports located along the z-direction. From the current and electric field distributions shown in Fig. 2, we can obtain the equivalent circuit of the AFSS unit cell. At 0.89 THz and 2.11 THz, it is equivalent to an LC series resonant circuit and exhibits a bandstop characteristic. At 1.44 THz, it can be equivalent to an LC parallel resonant circuit, showing the bandpass characteristics. As we all know, the patch-type FSSs have bandstop property near the resonant frequency, and we use its fundamental resonance frequency and third harmonic frequency to form a passband at 1.44 THz. In addition, graphene as a lossy film that electrically connects the gold ring, which is equivalent to an inductance, changes the resonance of the LC circuit.
Figure 3(a) and (b) show the simulated reflection coefficients and transmission coefficient when the chemical potential ranges of graphene is from 0eV to 0.5eV, respectively. It can be seen that by changing the chemical potential of graphene, the resonant frequency of the AFSS has changed. This allows the designed AFSS to achieve the conversion between high transmission and almost total reflection at a fixed frequency. When the chemical potential is 0eV, the transmission coefficient of AFSS is as high as −1.89dB with a small reflection coefficient of −10.6dB, which is named ON state. As the chemical potential is 0.5eV, the transmission coefficient of AFSS is only −11.1 dB with a large reflection coefficient of −1.9dB, which is represented as OFF state. From Fig. 3 (b) we also find that the AFSS exhibits transmission property at 1.44THz with a loss of 1.89dB, which is determined by the properties of graphene itself and is often inevitable.
We have also investigated the robustness of reflection and transmission coefficients under oblique incidence. The incident wave was modeled as a Floquet port so that TE polarization can be easily obtained. For clarity, TE polarization is defined as follow. The wave vector of the incident light is in the yoz plane and the electric field is in the y-direction. The simulated reflection and transmission spectra for incident angles up to 40° of the designed structure are plotted in Fig. 3(c) for the TE polarization. For the change of the incident angle, the reflection and transmission curves show good stability in the operating band.
3. Performance and mechanism of the Proposed THz antenna
In Fig. 4, the schematic of the proposed beam steering THz antenna is given. As can be seen from Fig. 4(a), an omnidirectional monopole antenna is designed as a radiating source and surrounded by a hexagonal AFSS screen. Each side of the hexagonal AFSS screen consists of an array of 2 × 2 which is composed of unit cells described in Section II, and the feed line is derived to facilitate the application of the DC voltage. Figure 4(b) is the front view of the proposed antenna, where six independent parts numbered 1-6 are employed and placed as a regular hexagonal prism with an angular periodicity of 60°.
The geometry of the proposed radiation source and its reflection coefficient are shown in Fig. 4(c). The antenna printed on the polyamide (with a relative permittivity of 4.3 and a thickness of 1.6μm) consists of two parts, a rectangular radiation source located on the front side of the substrate and a metal ground on the back. We use this kind of antenna because its omnidirectional radiation characteristic in the azimuth plane is subject to beam scanning, as shown in Fig. 4(d). Moreover, this monopole antenna structure is simple, which makes it easy to fabricate. The operating frequency range of antenna with reflection lower than −10dB is from 1.38 to 1.56 THz, which is within the frequency range of the designed AFSS.
As illustrated in Fig. 5(a), the proposed beam steering THz antenna achieves three typical operating states. In each state, the AFSS is divided into six parts. By adjusting the chemical potential of graphene, each part can be turned into ON state standing for high transmission, or OFF state standing for total reflection. In the following section, we will describe in detail how the antenna works under these conditions.
Case A: The schematic of this case is shown in Fig. 5(a) and the numbers from 1 to 6 represent the graphene-based AFSS screen on each side. As shown, three of them are given zero voltage (the chemical potential of graphene is 0eV, and the AFSS is in the ON state) and the others are given positive DC voltage (the chemical potential of graphene is 0.5eV, and the AFSS is in the OFF state). The parts with zero voltage have a high transmission coefficient and almost transparent for incident electromagnetic waves radiated from the monopole antenna in the center, while the rest parts with positive DC voltage provide a very large reflection coefficient acting as a metallic reflector. This means three parts exhibit ON state and the other three parts are in OFF state as to the propagation of electromagnetic waves. Therefore, by changing the voltage applied on the graphene of each side, the radiation pattern has the ability to scan the entire azimuth plane in six steps at 1.44THz. In Fig. 5(a), Case A shows the 3D radiation pattern of this kind of condition, indicating that the omnidirectional pattern of the radiation source turns into directional radiation due to the loading of the proposed hexagonal AFSS screen. And the simulated radiation patterns in the azimuth plane are shown in Fig. 5(b). It is clear that six different directional radiation patterns in the azimuth plane are obtained. In addition, we can also adjust the gain of the antenna by controlling the applied voltage. As an example, different voltages are applied to 6, 1 and 2 to tune the chemical potential of the graphene as 0eV, 0.1eV, and 0.3eV, respectively, and the others are still 0.5eV. From Fig. 5(c) we can clearly see that when the chemical potential is 0eV, 0.1eV, 0.3eV, the corresponding gains are 3.1dBi, 0dBi, −2dBi, respectively.
Case B: One of the major advantages of this antenna is its ability to control the antenna's radiation direction flexibly. The operating mechanism of Case B is similar to Case A. In this case, the ON state of the AFSS on each side is different between every two adjacent faces. As Fig. 5(a) shows, when the positive DC voltage is applied to the three sides numbered 1, 3 and 5, and zero voltage is applied to the other three sides, the antenna can radiate in three different directions at the same time. Figure 5(d) shows the simulated radiation patterns in the azimuth plane of Case B.
Case C: This case intends to show that the designed FSS can control the full angle radiation gain of the antenna with the ON/OFF state. In this operation, a zero voltage or a positive DC voltage will be applied to all the six parts of the hexagonal AFSS screen. Figure 5(e) shows the simulated radiation pattern. When no voltage is applied, the gain of the antenna has little difference with that of the original monopole antenna and it can still be used as an omnidirectional radiation antenna. When the chemical potential increases with a positive DC voltage applied, the gain of antenna is highly depressed which tends to a non-radiative state.
The simulation results of the model are consistent with the expectation. However, there are still many issues need to be considered during processing. For example, the relaxation time of chemical vapour deposition (CVD) graphene tends to be short, although we have used the relaxation time as small as possible in calculating the surface conductivity of graphene, it is still not easy to produce graphene that fully meets the requirements. In addition, during the etching process of AFSS, graphene sheets may crack, and the process of transferring graphene may also have PMMA residues, which are problems we need to overcome.
In summary, by loading a hexagonal AFSS screen based on hybrid graphene-gold structure around an omnidirectional monopole antenna, beam steering property of the proposed terahertz antenna have been investigated and discussed. The chemical potential of the graphene can be controlled through altering external bias voltage. When the chemical potential varies from 0eV to 0.5eV, we are able to switch the wave propagation though AFSS between high transmission (ON) state and total reflection (OFF) state. Simulation results indicate the graphene-based terahertz antenna can achieve six radiation beams covering 360 degrees in the azimuth plane, and the gain in each direction is continuously adjustable. In addition, the antenna can also realize multi-directional and omnidirectional ON/OFF radiation. The characteristics of this proposed terahertz antenna make it possible to implement graphene-based reconfigurable transceivers and sensors, and it can be used in the THz communication to reduce the complexity and cost of the communication system as well as to suppress multipath fading and increase the channel capacity in the future.
National Natural Science Foundation of China (61771360, 61671150, 61671147, 61601360); Key Industry Chain Project of Shaanxi Province; the Shaanxi Youth Science and Technology Star Project (2017KJXX-32); National Defense Science and Technology Innovation Fund of Chinese Academy of Sciences (Y723412261).
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