A methodology to design a multi-functional device (MFD) with switchable absorption and polarization conversion (PC) modes is proposed in this research. The methodology combines graphene absorbing metasurface (GAM) and polarization conversion metasurface (PCM) in one component. The key point is that the absorbing metasurface (AM) should be made completely by conductivity tunable material, such as graphene. The PCM can be constructed by fine metal. A prototype is designed to verify the designing methodology by stacking a layer of gold pattern PCM and two layers of GAM with fishnet shape patterns. In the PC mode, the chemical potentials of the two GAMs are µc1 = µc2 = 0 eV, then, the graphene layers act as thin dielectric layers, which can be treated as transparency. Thus, the PCM plays a leading role in the performances of the whole structure. Therefore, a linear polarized incidence is rotated by 90° in the 2.10-3.13 THz (39.5% at 2.61 THz) with polarization conversion ratio (PCR) larger than 90%. In the absorption mode with µc1 = 0.5 eV and µc2 = 0.8 eV, the GAMs have strong surface plasmon-polaritons (SPPs), and they concentrate most of the incident power, which makes the GAMs dominate roles of the whole structure. Therefore, the incident power is dissipated in the GAMs, and the absorption rate is larger than 90% in the 1.30-3.01 THz (79.5% at 2.15 THz). The absorption band for absorption rate larger than 70% is 1.19-3.90 THz (106.7% at 2.54 THz). The design methodology is useful for the designing of components with switchable absorption and PC modes.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metamaterials have brought new possibilities in electromagnetic (EM) wave regime and advanced the functions of light controlling, propagation manipulating, radiation, localization and scattering [1–4]. The technology of metasurface, as a sub-category of metamaterial, is a hot topic as it can control electromagnetic (EM) waves with low profile and low loss [5–7]. In recent years, the metasurfaces are utilized to regulate the terahertz wave  as terahertz technologies have great potential in medical imaging and diagnostics, environmental monitoring and surveillance, chemical spectroscopy, high resolution radar and high-speed communication [9–11]. Especially, the metasurfaces are applied to design polarization converters and absorbers in the terahertz range. The polarization converters are able to change the polarization state of the incident waves, while the absorbers are capable of absorbing and dissipating the impinging EM waves.
Usually, fine metals are excellent materials to construct metasurface for polarization converting [12–15] as they are easily excited by EM wave. In , double L-shaped metasurface with two metallic gratings are utilized to construct a transmission-type polarization converter to rotate a linear polarization (LP) by 90°, and it achieves a bandwidth of 66.7%. In , coupled dielectric-metal grating achieves polarization conversion and asymmetric transmission from 0.2 to 1.2 THz (142.8% at 0.7 THz). Multi-layer rectangular patch metasurfaces are used to obtain a wide polarization conversion bandwidth, such as 116% in . In , a reflection-type linear to circular polarization converter was designed by single-buried-layer rectangular patches. Two bands of 28.4% and 42.1% are obtained at 2 THz and 10 THz, respectively. These polarization converters show good polarization conversion characteristics. However, their performances cannot be adjusted dynamically.
Polarization converters with tunable characteristics are interested in the terahertz community. The tunable ability can be achieved by using flexible substrate , liquid crystals  and VO2 film . The polarization state of incident terahertz beam can be controlled slightly by changing the structure of the two-dimensional array of metallic metasurface in . Though the metallic patches are printed on a flexible substrate, the polarization tuning is not conveniently and speedily. Liquid crystals drove by a voltage are utilized to adjust the phases of the orthogonal field components of a polarization converter in , then, the polarization state of the reflected terahertz wave can be tuned electrically. In , a vanadium oxide (VO2) film is used to tune the polarization conversion ratio (PCR) of a polarization converter by adjusting the temperature of the VO2 film. Thus, a temperature conditioner is required. The designs in  and  might lead to complicated system and be bulky.
The graphene had gained more interest in device designing owing to its characteristics of conductivity tunable ability, high carrier mobility, chemical stability and high mechanical strength [19–22]. For the graphene based tunable polarization converters, many of the researches are focused on frequency tuning [23–26]. In , a graphene sheet is etched with H-shaped slots to rotate an incident linear-polarized wave. By changing the bias voltage, the chemical potential was tuned, then, the operating frequency band can be adjusted from 26 THz to 38 THz. The researches in  and  also achieve polarization rotating and operating frequency tuning by cutting slots on graphene sheets. In , the research utilized 7-layer graphene as a ground plane and a graphene sheet with butterfly-shaped holes for polarization converting, and the operating band can be adjusted from 0.44 to 0.96 THz. Polarization state controlling is also achieved by using graphene [27–29]. In , two crossed graphene gratings separated by an insulator was designed to select specific polarization states. The hybrid all-dielectric-graphene metasurface in  can switch the reflected wave between right circular polarization to left circular polarization by using different bias voltages. The periodical graphene patterns in  are capable of dynamically tuning the operating frequency and polarization states by varying the Fermi energy. Therefore, the tunable ability of the graphene is very useful for performances adjusting of the polarization converters, such as frequency tuning and polarization state regulating.
On the other hand, absorbers made by metamaterials and metasurfaces are very interesting and they are comprehensively studied [30–32]. Fine metals, such as gold, are usually utilized to constructed metamaterials/metasurfaces based absorbers [33–38]. In , two gold layers are printed on a MgF2 dielectric layer (ZZL-U400H) while circular holes were etched on the top gold layer. This structure is polarization-independent to normal incidence and it obtains high absorption rate for large angle of incidence. In , two layers of metal metamaterials patterns are utilized for electromagnetic wave absorption with polarization-insensitive characteristics. Cross-shaped gold resonators were backed by an Al2O3 layer and a gold layer in  to form metamaterial perfect absorber with peak absorption of 0.97. An absorber with split ring resonators (SRR) backed by polyimide insulator and gold film is proposed in . TE mode incidence of the absorber is sensitive to incident angles while the TM mode incidence has good absorptivity over a wide range of incident angles. An absorber with gold particles and gold film separated by an Al2O3 layer is proposed in , while its operating wavelengths of TE and TM modes are different. Dual split ring resonators (SRRs) are utilized in  to obtain an absorber with quad-band. Therefore, metasurfaces constructed by metals are efficiency in EM wave absorption as they are easily excited by EM wave. However, their bandwidths are narrow owing to the strong plasmonic or photonic resonances . To realize wide bandwidth, three layers of cross-shaped gold resonators are constructed in . The cross-shaped gold resonators of different layers have differing dimensions to support several closely resonant frequencies for wide bandwidth of 37.2%. However, the bandwidth is valued by a moderate absorption rate of 60%. In , a wideband metasurface absorber was designed by combining four Mie dielectric resonators with different permittivity values into a single unit cell. The bandwidth of absorber is 2.7% by an absorption rate of 80%. Thus, the absorptivity performances of the wideband metal absorbers are limited.
Graphene was also used for absorber designing [41–46]. In , graphene micro-ribbon array backed by a thick dielectric spacer and a metallic plate form an absorber with complete absorption. In , grating strips with dual pairs of graphene-germanium array achieve polarization insensitive and wide incident angle absorption. Many of the graphene based absorbers realize frequency tuning [43–45]. Calcium fluoride grating and graphene plate in  form a TE-polarization absorber, and the operating frequency can be tuned by adjusting the chemical potential of the graphene. In , graphene ribbon, SiO2 substrate, graphene sheet and a gold layer form a dual-band absorber with operating frequencies tuning by adjusting the chemical potential. The above graphene based absorbers are narrow bandwidth [41–44]. Wideband graphene based absorbers are also studied [45,46]. In , by stacking four graphene ribbons of different widths on four dielectric layers, four absorption peaks are merged into a wideband. The bandwidth for absorptivity exceeds 90% is 12.8% at 9.4 THz, and the operating band can be adjusted by tuning the chemical potentials of the graphene ribbons. In , fishnet shape graphene obtains an absorption bandwidth (absorptivity larger than 90%) of 59.4% at 3.2 THz, and the absorption ratio can be controlled by tuning the chemical potential. It is found that, the graphene is highly efficient in absorber designing with wide bandwidth as graphene supports surface plasmons in the terahertz range. Besides, the performances of the graphene absorbers can be adjusted by bias voltage. In , it is found that a metal structure in closed to a graphene can enhance the absorption of the graphene. The research in  found that the interactions between graphene and metal metasurfaces can result in a wide absorption band, such as 134.88%.
The above researches presented good polarization conversion or power absorption performances. However, these characteristics are achieved in separating devices, and they are unable to realize all the characteristics in one component. Importantly, the polarization conversion and power absorption characteristics are greatly desired for components in electromagnetic (EM) measurement, antenna design and low Radar-Cross Section (RCS) applications. Therefore, it is significance to develop devices with switchable absorption and polarization conversion performances. In , we proposed a multi-functional device. This device can switch between PC mode and absorption mode. However, the research is an abecedarian analysis, and the designing methodology is not well discussed. Moreover, the design in  has a moderate absorptivity, which should be enhanced. The reason of limit absorptivity is discussed in Section 3.2 Analysis. Therefore, it is necessary to discuss the design methodology in detail, and it is significance to achieve absorptivity enhancement.
In this research, a methodology to design multi-functional device (MFD) that can switch between PC and absorption modes is proposed. The MFD operates as an absorber or a polarization converter by electric tuning. The designing methodology is discussed in Section 2, and it requires a polarization converter compounding with a graphene based absorber. A prototype that runs up to the MFD is proposed and analyzed in Section 3. The prototype demonstrates good performances for switching absorption and PC modes. The physical mechanisms and characteristics are investigated in Section 4.
In this research, the objective is to propose an approach to innovatively design MFD. The MFD can switch between absorption mode and PC mode. To realize the switching characteristic, tunable components or materials are necessary. Usually, diode, disjunctor and variode are used for adjustable designing. However, these components, especially for periodic designs, require complex biasing circuit, and it is difficult to obtain such components in the terahertz band. Fortunately, the graphene has tunable conductivity by adjusting its chemical potential (µc) , and its tunable conductivity characteristic can be used for adjustable designing. In this methodology, the graphene is utilized for switchable designing.
In this section, the characteristics of the graphene are investigated, and the methodology for MFD designing is discussed. According to , the conductivity of the graphene is complex and it has intraband and interband contributions that can be described by Kubo formula.
The conductivities of the graphene in terms of the chemical potential (µc) are plotted in Fig. 1. Figure 1(a) shows the real part and Fig. 1(b) demonstrates the imaginary part. As shown in the Figs., the σreal ≥ 0 and the σimag ≤ 0 in the band from 1 to 8 THz. It is mainly the intraband that contributes to the conductivity and the imaginary part plays an important role to the surface waves of the graphene sheet . It is also found that, the values of the imaginary part are larger than the real part. From the Figs., both the real and imaginary parts of the conductivity are increased with the chemical potential (µc) increasing. While from the perspective of frequency, the contrasting of the conductivities of different chemical potential (µc) become smaller with frequency increasing. The 1-4 THz band shows larger conductivity distinction than the higher frequencies and it might be good choice for some designing in this frequency range. The conductivities of the graphene are owing to the carrier density, which is controlled by the µc. The conductivity for µc = 0 eV is very small and close to zero due to low carrier density. In this case, the graphene acts as a thin dielectric media, which can be treated as transparency for EM wave. The conductivity of the graphene is large for large µc owing to larger carrier density, then, the graphene supports surface plasmon-polaritons (SPPs), which would be used to confine the incident EM wave absorption and lead to absorption.
Here, we consider a graphene sheet normally illuminated by a plane wave as shown in Fig. 2(a). The graphene sheet is laid in the xoy-plane, while the z > 0 space is filled with medium 1 characterized by ε1 and µ1, and the medium 2 in z < 0 space is ε2 and µ2. Then, the reflection coefficient, the transmission coefficient and the absorption are [46,51] as follow.
For the special case n1 = n2 = n0 = 1, where is the refraction coefficient of the air, the reflection coefficient, the transmission coefficient and the absorption can be written as
From Fig. 1, the conductivity σs of the graphene is almost zero for µc = 0 eV. Let’s suppose σs = 0 for simplicity, it is interesting to found from Eqs. (5) - (7) that, the transmission coefficient t = 1, the reflection coefficient r = 0, and the absorption A = 0. In this case, the graphene sheet is transparency to the electromagnetic wave. It is also clear from Eqs. (5) - (6) that, a large conductivity σs (generated by large µc) results in large reflection coefficient and small transmission coefficient. From Eq. (7), the absorbance would be very small as the imaginary part σreal is larger than the real part σimag, as also indicated in Fig. 1. Therefore, the graphene operates as semi-conductor with large µc, and it is necessary to form metamaterial/metasurface to obtain good absorption.
To verify the above discussion, the reflectance, transmission and absorption of the graphene sheet in terms of the chemical potential (µc) are calculated and plotted in Fig. 2(b), 2(c) and 2(d), respectively. As the conductivity of the graphene increased with µc increasing (Fig. 1), the graphene acts more like a metal, so that a larger reflectance is obtained with higher µc as demonstrated in Fig. 2(b). Correspondingly, the transmission decreased with µc increasing as shown in Fig. 2(c). It is also found that, the µc = 0 eV of the graphene sheet leads to a reflectance almost equals 0 and a transmission almost equals 1 in the 1-8 THz band, which verify the aforementioned conjecture that the graphene with µc = 0 eV acts as a very thin dielectric media. However, for the non-zero µc, the reflectance is increased while the transmission is decreased with µc increasing. The absorptions presented in Fig. 2 are very small for all the µc values. Therefore, to have perfect EM wave absorption, metamaterial/metasurface patterns should be etched on the graphene sheet to enhance plasmonic resonances.
According to the above discussion, we might design a MFD by combining a polarization converter and an absorber as hinted in Fig. 3. The Key point is that the absorber should be made totally by graphene metasurface. As the chemical potential (µc) of the graphene equals 0, the graphene metasurface can be handled as thin substrate and it is transparence to EM wave, thereafter, the polarization converter structure defines the characteristics of the whole structure. As the chemical potential (µc) is set as an appropriate value, the surface plasmon-polaritons (SPPs) resonances of the graphene metasurface confine and dissipate most of the power, which endows the absorber a dominant role. Then, the MFD is capable of switching between a polarization converter and an absorber, and its characteristics are managed by the chemical potential (µc) of the graphene.
3. Prototype design, results and analysis of the multi-functional device
3.1 Design and results
A polarization converter is designed first as shown in Fig. 4. The polarization conversion metasurface (PCM) is a periodic gold patterns as demonstrated in Fig. 4(a). The gold pattern is printed on a dielectric spacer backed by a gold layer. The thickness of the dielectric spacer is h = 10 µm, and the thickness of the gold is 200 nm. The gold layer is supported by a substrate, which can be Si. Note that, no incident wave can reach the supporting material, thus, the supporting material has little impact on the converter. In this research, the dielectric spacer has a relative permittivity of 3.46, which can be a suitable polymer material. The top view of a unit cell pattern of the PCM is plotted in Fig. 4(b). As exhibited in the Fig. 4(b), two L-shaped structures are diagonal placed , and a sandglass shaped structure is laid in the core with a 45° tilt. The parameters of the converter are W0 = 20 µm, L0 = 30 µm, d0 = 6 µm, W1 = 10 µm, L1 = 20 µm, d1 = 2 µm, W2 = 2 µm, L2 = 22 µm, d2 = 3 µm, and p = 50 µm. It is noted that, for the MFD designing, the pattern form of the PCM has little impact on the MFD designing according to the discussion in Section 2.
In the research, the structures were simulated by CST studio . As shown in Fig. 4(c), a unit pattern is built with unit cell boundaries around the four sides to imitate periodic structure. A floquet port on the top generates plane wave. The results of the polarization converter under normal incidence are plotted in Fig. 4(d) and 4(e). The reflections are the ratios of the reflected electric fields to the incident electric field [53,54]. Then, the rcc = Ecr/Eci and the roc = Eor/Eci. The subscripts i and r refer to the incident and reflected electromagnetic waves, respectively, while the subscripts c and o refer to the co-polarization and orthogonal (cross) polarization, respectively. From the reflection curves shown in Fig. 4(d), good polarization conversion performances are obtained at 2-THz band for both x- and y-polarized incidence. The electric field rotation ratio roc is greater than 0.9 in the 2.18-3.32 THz band. While the co-polarized electric field reflection ratio rcc is very small, and two poles are obtained at 2.32 and 3.10 THz. The polarization conversion ratio (PCR) PCRc = r2oc/(r2cc + r2oc) and the power absorption Ac = 1 - r2cc - r2oc are also calculated as demonstrated in Fig. 4(e). As exhibited in the Figs., the PCRs are larger than 90% in the 2.19-3.31 THz band (40.7% at 2.75 THz) and the maximum PCRs appear around 2.32 and 3.10 THz. On the other hand, the power absorption rate is very small, which ranges from 0.05 to 0.09, in the band. Therefore, the proposed polarization converter has good performances with high polarization conversion and low power loss.
To add absorption mode to the above polarization converter, graphene absorbing metasurface (GAM), such as graphene fishnet patterns, can be applied to the converter as shown in Fig. 5. The fishnet patterns , as demonstrated in Fig. 5(a), operate as an absorber and the absorptivity is controlled by the chemical potential (µc) of the graphene. Thereby, by regulating the µc, the structure (MFD) can switch between polarization conversion mode and absorption mode. Figure 5(b) and 5(c) demonstrate the 3D view and the side view of the proposed MFD, respectively. In this research, two layers of GAM are applied to obtained good absorption. In this design, the two identical layers of GAM are denoted as GAM 1 and GAM 2, respectively. The chemical potentials of the GAM 1 and GAM 2 are denoted as µc1 and µc2, respectively. The one graphene layer is placed above the gold pattern with a distance h1 and the other graphene layer is beneath the gold pattern with a distance h2. A graphene pattern is formed by four fishnets with central cross slots. The GAM 1 layer can be supported by Foam, such as Polymethacrylimide (PMI) . A 50 nm-thick Silicon layer and a 10 nm-thick aluminum oxide layer are used to support a graphene layer and served as an electrode [56,57]. As the electrode layers are extremely thin, their impacts are very small . Then, by tuning the bias voltage, the characteristics of the structure can be adjusted.
The results of polarization conversion and wave absorption of the MFD are plotted in Fig. 6. The Fig. 6(a) and 6(b) are the results of the PC mode, in which the chemical potentials of graphene are µc1 = µc2 = 0 eV. However, when the chemical potentials are changed to be µc1 = 0.5 eV and µc2 = 0.8 eV, the device is switched to an absorption mode, as shown in Fig. 6(c) and 6(d). In case of PC mode, the device can rotate a linear polarized incident wave to its cross-polarization wave, as shown in Fig. 6(a). The roc is larger than 90% in the 2.13-3.12 THz band, while the rcc is very small in the band. From Fig. 6(b), we see that the PCR is larger than 90% in the 2.10-3.13 THz while the absorption ranges from 0.065 to 0.12 in the band. The PCR bandwidth is 39.5% at 2.61 THz. It is found that the performances of the PC mode of the MFD are very similar to the polarization converter shown in Fig. 4. It means that the GAMs with µc1 = µc2 = 0 eV are almost transparent to the EM wave, and the PCM plays a major role. Thus, the MFD operates as a polarization converter.
For the absorption mode (µc1 = 0.5 eV and µc2 = 0.8 eV), Fig. 6(c) and 6(d) present the reflection and absorption curves of the device. As demonstrated in Fig. 6(c), both the roc and the rcc are small in the 1-4 THz band. That means an incident electromagnetic wave is little reflected, and most of the energies are dissipated in the structure. The absorption curve is calculated and plotted in Fig. 6(d). It is observed that the band for absorption larger than 90% is 1.30-3.01 THz (79.5% at 2.15 THz). The absorptions are larger than 70% in 1.19-3.90 THz band (106.7% at 2.54 THz). Moreover, the absorption band is much larger than those wideband absorbers as summarized in Table 1.
According to the interference theory , the out-of-phase radiations from metasurface and ground plane (metallic mirror) are important to realize cancellation for absorption. Similarly, in-phase superpositions from metasurface and ground plane are critical for polarization conversion. Consequently, the distances between the metasurface and the ground plane are different for absorber and polarization converter, and the distance for polarization converter is usually smaller than absorber. For example, without considering the reflection phase of the metasurface, it should be approximately a half-dielectric-wavelength (λd/2) for absorber, and about a quarter-dielectric-wavelength (λd/4) for polarization converter. Therefore, for the design in , it is impossible to coordinate the cancellation (absorption) and superposition (polarization conversion). To have good superposition (polarization conversion), the absorptivity of the absorption mode is mainly owing to the plasmonics resonances of the graphene metasurface, while the effects of the interference is small. Thus, only moderate absorptivity is obtained in .
To achieve nearly perfect absorption, two graphene layers are utilized in this research. To further interpret the operating principle of the proposed MFD, four structures by removing GAM or PCM from the proposed structure are discussed as demonstrated in Fig. 7 and Fig. 8. As shown in Fig. 7, the GAM 2 is removed from the structure. The schematic view and results of the structure with PCM are displayed in Fig. 7(a). In the polarization conversion mode (µc = 0 eV), the GAM is transparency, and good PCR is obtained in the 2.1-3.13 THz band as exhibited in Fig. 7(a). In the absorption mode (µc = 0.5 eV), some incident energies are absorbed as demonstrated in Fig. 7(a). The schematic view and absorption characteristics of the structure without PCM are plotted in Fig. 7(b). The absorptivity curve with PCM is also painted in Fig. 7(b) for comparison. As shown in the Fig., the structure without PCM has an absorption band from 1.34 to 2.59 THz with absorptivity larger than 0.9. It is interesting to found that the absorption for the structure with PCM is blemished in 1.88-3.1 THz, which is almost the polarization conversion band. It is because the disturbance of the PCM abates the cancellation according to the interference theory .
As demonstrated in Fig. 8, the GAM 1 is removed from the structure. Figure 8(a) exhibits the schematic view and results of the structure with PCM, and Fig. 8(b) presents the schematic view and results of the structure without PCM. As shown in Fig. 8(a), the structure still operates in PC mode as the chemical potential of the graphene µc = 0 eV. While tuning µc = 0.8 eV, the structure works at absorption mode. However, as the distance between the GAM and the ground plane is much smaller than λd/2, the absorptivity is low, typically 65% in the band. In this case, the absorption is mainly owing to the plasmonics resonance of the graphene metasurface. From Fig. 8(b), the absorptivity without PCM (typically 30%) is much lower than that with PCM. It is because the PCM made by gold enhances the surface plasmon-polaritons (SPPs) of the GAM [47,48]. Therefore, by placing a GAM close to the PCM, the absorptivity can be enhanced at absorption mode though the cancellation of interference is weak.
The absorptivity results of the structures are summarized in Table 2 for comparison. From the table and the above analysis, the structure with a GAM above the PCM (but without GAM 2) is difficult to obtain good absorption (at absorption mode) around the polarization conversion frequencies. While the structure with a GAM beneath the PCM (but without GAM 1) is difficult to obtain perfect absorption as cancellation of interferences is not tenable. However, by applying two GAMs, one above the PCM to achieve cancellation of interferences, one beneath and close to the PCM for SPPs enhancement, good absorptivity at absorption is obtained in a wide band.
4. Physical mechanisms and discussion
4.1 Physical mechanisms
To further reveal the physical mechanisms of switchable PC and absorption modes of the MFD, the current distributions of the two modes are discussed in this section. Only the fields of x-polarized illumination are demonstrated as the structure is symmetry. The current distributions of PC mode and absorption mode are exhibited in Figs. 9 and 10, respectively.
The frequencies of 2.3 THz and 2.8 THz are randomly chosen to demonstrate the current distributions of the PC mode (µc1 = µc2 = 0 eV). The Fig. 9(a) and 9(b) present the current density and phasor of 2.3 THz, respectively. The current density and phasor of 2.8 THz are demonstrated in Fig. 9(c) and 9(d), respectively. From Fig. 9(a) and 9(c), the currents for µc1 = µc2 = 0 eV are concentrated on the PCM. While the GAMs have very small currents, which can be negligible. The current distributions are accord with the discussions in Sections 2 and 3. It is because the conductivities of the graphene are almost zero for µc1 = µc2 = 0 eV (Fig. 1), then, the GAMs operate as a very thin dielectric layer and they are transparency to the terahertz wave (Fig. 2). Therefore, the PCM plays a major role, resulting in an operation mode of polarization conversion. The current at 2.3 THz is mainly distributed on the diagonal placed L-shaped structures as shown in Fig. 9(a), while the x-polarized incidence wave generates y-polarized moments on the L-shaped structures as exhibited in Fig. 9(b). From Fig. 9(c), the current of 2.8 THz is mostly distributed on the sandglass-shaped structure, and the x-polarized incidence wave also produces y-polarized moments as shown in Fig. 9(d). Therefore, the x-polarization illumination is rotated by 90°, and the MFD acts as a polarization converter for µc1 = µc2 = 0 eV.
For µc1 = 0.5 eV and µc2 = 0.8 eV, the GAMs have large carrier density, and strong plasmonics resonances are inspired. Therefore, the incident EM wave is confined at the graphene metasurfaces, which result in great wave dissipation. Therefore, the MFD operates at absorption mode in this case. To verify the above discussion, the current distributions of this mode are demonstrated in Fig. 10. Figure 10(a), 10(b) and 10(c) present the current densities of 1.5 THz, 2.3 THz and 2.8 THz, respectively. It is found that, the three frequencies have similar current distributions. Owing to the strong surface plasmon-polaritons (SPPs) of the graphene metasurfaces, the near-fields are tightly confined [27,60,61]. The currents of the absorption mode are concentrated on the GAMs as shown in Fig. 10(a), 10(b) and 10(c), while the currents on the gold pattern are negligible. That means the GAMs play a dominant role of the whole structure. Therefore, the electromagnetic wave is dissipated in the graphene metasurfaces, and the MFD operates as an absorber for µc1 = 0.5 eV and µc2 = 0.8 eV.
4.2 Oblique illuminations
The oblique illumination responses of the MFD are investigated and discussed in this section. Figure 11 demonstrates the responses of the PC mode under different incident angles (θ). The responses of the absorption mode under different incident angles (θ) are presented in Fig. 12. The incident angle θ is the angle between the z-axis and the incident ray. Here, both the s-polarized and p-polarized waves are illuminated on the structure.
For PC mode with µc1 = µc2 = 0 eV, the PCR curves for s-polarized and p-polarized incidences under different incident angles (θ) are plotted in Fig. 11(a) and 11(b), respectively. As shown in the Figs., the lower edge of the PCR band increased a bit with θ increasing, and the PCR of the middle frequencies of the operating band are also affected by the incident angle (θ). For θ ≤ 15°, the PCR curves are larger than 90% in the band for both s- and p-polarized illuminations, though a small drop is observed for p-polarized incidence with θ = 15°. It is also found that, the incident angle θ has more impacts on the p-polarized incidence than the s-polarized incidence. Thus, for s-polarized illumination, the PCR is almost larger than 80% in the band for θ = 30° as exhibited in Fig. 11(a). For p-polarized illumination, the PCR of a small range of frequencies in the middle of the band are smaller than 80% for θ = 30° as demonstrated in Fig. 11(b). Though the PCR of the middle frequencies of the band are quite sensitive to the θ, the PCR around 2.35 THz and 3.15 THz are stable to the incident angle θ.
For absorption mode with µc1 = 0.5 eV and µc2 = 0.8 eV, the absorption curves for s-polarized and p-polarized incidences under different incident angles (θ) are plotted in Fig. 12(a) and 12(b), respectively. As shown in the Figs., the lower edge of the absorption band moves to a higher frequency a bit with θ increasing for both s-polarized and p-polarized incidences. It is found that, the s-polarized and p-polarized incidences have different responses to the θ, and the incident angle θ has smaller impacts on the s-polarized incidence than the p-polarized incidence. With s-polarized illuminations as shown in Fig. 12(a), the absorption curves are larger than 90% in the band for θ ≤30°, the absorption rates are still larger than 80% for θ = 45° and θ = 60°, and the absorption rates are larger than 60% for θ = 75°. Thus, the s-polarized illumination is insensitive to the θ. The p-polarized illuminations are very sensitive to the θ as shown in Fig. 12(b). However, the absorption curves are still larger than 90% in the band for θ ≤15°, the absorption rates are larger than 80% for θ = 30°, the absorption rates are larger than 60% for θ = 45°, the absorption rates are almost larger than 60% in a narrower band with θ = 60°, and the absorption rates are small for θ = 75°. Therefore, the absorption rate of the p-polarization can be tuned by adjusting the incident angle.
4.3 The design approach for other PCM and polarizations
From the above, the methodology and a prototype for multi-functional device with switchable absorption and polarization conversion modes are discussed. To prove the universality of the approach, the PCM of the prototype is replaced for further discussion as demonstrated in Fig. 13. The pattern of the new PCM is a strip-loaded half elliptical ring (SHER), which was discussed in . Note that, the SHER has identical dimensions as in . As the new PCM is applied to the prototype, there are no needed to change the dimensions of the prototype as shown in Fig. 13(a). As discussed in , the PCM of SHERs can convert a linear polarization to its cross polarization. And, it also can convert a circular polarization to its cross-polarization.
Figure 13(b) demonstrates the results of linear polarization conversion, while Fig. 13(c) presents the results of circular polarization conversion. As shown in Fig. 13(b), in the polarization conversion mode (µc1 = µc2 = 0 eV), a linear polarization incident wave is rotated to its cross-polarization in 2.1-3.0 THz band with PCR ≥ 0.9, which is almost the same as . While the absorptivity for the polarization conversion mode is smaller than 0.15 in the band. In the absorption mode (µc1 = 0.5 eV and µc2 = 0.8 eV), the incident power of linear polarization is dissipated in 1.30-3.44 THz with absorptivity larger than 0.9, though the absorptivity at 2.34 THz is 0.88. Thus, the approach is valid for new PCM and linear polarization.
For incidence of circular polarization, as exhibited in Fig. 13(c), a circular polarization incident wave is converted to its cross-polarization at PC mode (µc1 = µc2 = 0 eV). The PCR is larger than 0.9 in the 2.16-3.01 THz band, while the absorptivity is smaller than 0.15 in the band. For the absorption mode (µc1 = 0.5 eV and µc2 = 0.8 eV), the incident power of circular polarization is dissipated in 1.31-3.50 THz with absorptivity larger than 0.9. Thus, the approach is also valid for circular polarization.
In this study, a methodology to design MFD with switchable PC mode and absorption mode is proposed. A prototype is also designed and discussed. The operating modes of the MFD can be controlled by adjusting the chemical potentials (µcs) of the graphene metasurfaces. The bandwidth of the absorption mode is much wider than the referred absorbers. The proposed methodology has the characteristic of universality, and different PCM and GAM can be combined to form a MFD. The prototype can be utilized in electromagnetic (EM) measurement, antenna design and low Radar-Cross Section (RCS) applications, etc.
National Natural Science Foundation of China (61661011, 61761010).
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