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Abstract
An actively reconfigurable broadband terahertz (THz) metamaterial functional device based on the phase-change material vanadium dioxide (VO2) and two-dimensional graphene material is theoretically proposed and demonstrated. The device has excellent tolerance under oblique incidence. When the VO2 is in the metallic state, and the Fermi energy of graphene is fixed at 0.1 eV, the designed device acts as a broadband THz absorber in the transverse magnetic (TM) polarization mode. The absorptance bandwidth exceeds 0.55 THz with a complete absorption intensity of more than 90%. In this state, the absorber operates as a broadband modulator with the total modulation depth exceeding 91.5% as the continually decreased conductivity of VO2 from 200000 S/m to 10 S/m. In the transverse electric (TE) polarization process, the structure behaves as a dual-band absorber with two perfect absorption peaks. When the conductivity of VO2 is changed, the tunable absorber can also be regarded as an absorptance modulator, with a maximum modulation intensity of 92.1%. Alternatively, when VO2 behaves as an insulator at room temperature in the TE polarization mode, a strong broadband electromagnetically induced transparency (EIT) window is obtained, with a bandwidth exceeding 0.42 THz in the transmittance spectrum. By varying the Fermi energy of graphene from 0 to 0.9 eV, the EIT-like window or broadband transmission spectrum (in TM mode) can be switched. The results indicate that the device can also be operated as a modulator in the transmission mode. The impedance matching theory is used, and electric field distributions are analyzed to quantify the physical mechanism. An advantage of the manipulation of the polarization angle is that the modulation performance of the proposed multi-functional THz device can be regulated after fabricated.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials are artificial composite materials with a periodic unit cell structure; they have many extraordinary physical characteristics that differ from conventional materials occurring naturally [1–3]. Recently, metamaterials have attracted significant attention as terahertz (THz) functional devices due to their unparalleled properties in controlling light waves [4–8]. Besides, THz wavebands are widely used in spectroscopy, imaging, detection, and tunable sensing applications [9–12]. Several researchers integrated active materials with multiple resonances in each unit cell by adjusting the position of the model in the plane to reconfigure the properties; examples include, liquid crystals [13–15], Dirac semimetals [16–17], graphene [18–20], and phase-change materials [21–23]. Vanadium dioxide (VO2) exhibits transition behavior from an insulator to metal at a temperature of around 340 K [24]. Several studies have shown that the phase transition process can be induced by thermal [25–26], electrical [27–28], or optical excitation [29]. Moreover, the conductivity of VO2 changes by five orders of magnitude during this process. Cao et al. proposed a new broadband tunable metamaterial absorber that used VO2 rings with different radii located on the spacer; the maximum tunable range of the device was 2.3% to nearly 100% [30]. Song et al. presented a tunable meta-modulator operating in the THz range that exploited the dependency of toroidal dipolar resonance on the phase change of VO2 [31]. Huang et al. numerically demonstrated an actively tunable broadband THz absorber; by changing the conductivity of VO2, perfect amplitude modulation was achieved from 4% to 100% in the absorption mode [32].
On the other hand, graphene has a two-dimensional (2D) honeycomb structure composed of one monolayer of carbon atoms [33]. From the perspective of condensed matter physics, the conduction band and valence band of graphene are in close contact. Currently, graphene has been used for metamaterial devices due to its flexible surface conductivity and strong electric field confinement at THz frequencies [18–20,34–36]. Zhao et al. numerically proposed a switchable THz device as an amplitude modulator during the absorption process. The state of the device could be switched from absorption to reflection in the entire operating bandwidth when the Fermi energy was changed [34]. Mou et al. designed a broadband tunable THz absorber based on a graphene-based metasurface. Plasmonic hybridization between the two graphene rings increased the device’s performance [35]. Fang et al. experimentally demonstrated that nanopatterning a graphene layer into an array of closely packed graphene nano-disks could increase the absorptance from less than 3% to 30% in the infrared region [36]. These results indicate that graphene can significantly improve the device’s performance.
This discussion shows that these types of devices have to become more flexible and smarter in the future. In this work, we propose a polarization-sensitive THz functional device that is tunable in both absorption and modulation and has hybrid sandwich unit cells. The phase-change material VO2 acts as a switch for the two modulation modes. When VO2 is in the metallic state, the meta-device can be regarded as a THz absorber. The absorption rate is always more than 90% in the range of 0.4-1.0 THz for transverse magnetic (TM) polarization mode, and it is above 60% in the range of 0.3-1.0 THz for transverse electric (TE) polarization mode. When the Fermi energy of graphene is fixed at 0.1 eV, by triggering the phase change process of VO2, the maximum modulation depth in the TM and TE polarization modes are 91.5% and 92.1% as the conductivity increases from 10 S/m to 200000 S/m, respectively. Alternatively, when VO2 reaches its insulating state at room temperature, the proposed device can be treated as a transmission modulator, and total modulation intensities of 70.5% and 71.1% are achieved in the TE and TM polarization modes as the Fermi energy of graphene increases from 0 to 0.9 eV, respectively. Our design has the advantages of reconfigurable dual modulation modes in a single metamaterial, demonstrating superiority over previously reported tunable THz devices with only one function.
The performance of the designed device is better than that of reconfigurable systems reported in recent studies; both Ref. [31] and Ref. [37] presented a polarization-dependent sandwich unit cell structure with reconfigurable behavior in the low THz range. Although, in Ref. [31], they reported a switchable metamaterial with bi-functionality of absorption and electromagnetically induced transparency (EIT) based on the phase change material. It is a pity that the proposed device just realized narrow band absorption at the frequency of 0.498 THz. And when VO2 is in an insulating state, the full width at half maximum (FWHM) of the obtained EIT transmission window is only reach around 0.08 THz. However, our proposed unit cell structure achieves a wider range of dynamically tunable functions in the transmission and absorption states in the target THz range simultaneously. The most distinctive broad absorption band is observed that 90% absorptance reaches 0.6 THz from 0.4 to 1 THz. Besides, in the TE polarization mode, the FWHM bandwidth of the strong transparent window exceeds 0.42 THz. When this device is in the broadband absorption or transmission state, the amplitude of the absorption spectrum can be adjusted by changing the conductivity of VO2 or the Fermi energy of graphene. Furthermore, in this work, maximum modulation depths of 92.1% and 71.1% are realized in the absorption as well as transmission modes, respectively. Additionally, the proposed device exhibits a high tolerance to different incident angles and practical operability.
2. Design and analysis of device
The unit cell of the proposed reconfigurable metamaterial THz absorber/modulator is schematically presented in Fig. 1. One can clearly see that the device consists of five layers, consisting from top to bottom of a layer of the hybrid gold-VO2 resonator, a dielectric spacer, a layer of VO2 film, a monolayer graphene sheet, and a dielectric layer. A circular VO2 array coupled with metal resonators is located on the top of the unit cell; the resonators are arranged in a square-ring configuration, with VO2 nanorods inserted into the two arms. Besides, the dielectric spacer consists of lossless SiO2, with a relative permittivity of 3.9 [35]. And the thickness of the upper and lower substrates is ${t_s} = 60\; \mu m$ and ${t_b} = 20\; \mu m$, respectively. The gold resonator is considered a high-loss metal, with a conductivity of $4.56 \times {10^7}\; S/m$ [23] and a thickness of $0.5\; \mu m$. In the simulation, when VO2 is in the metallic state, the thickness of the VO2 films in this structure is $0.35\; \mu m$, which is greater than skin depth to suppress any THz waves [37]. Furthermore, the optimized dimension parameters of the unit cell are listed as follows: $P\; = \; 130\; \mu m,L\; = \; 90\; \mu m,\; R\; = \; 20\; \mu m,\; r\; = \; 17\; \mu m,\; W\; = \; 3\; \mu m$.
VO2 is a thermal or optical control material, and a Drude-like model is used to define the optical characteristics of VO2 in CST Microwave Studio software. In the target THz range, it used to be written as [38,39]
Furthermore, in the modeled process, the thickness of the graphene film (${t_g}$) is assumed to be a typical value of 1 nm [20]. The dynamic surface conductivity of graphene ${\sigma _g}(\omega )$ in the low THz range can be calculated using to the Kubo formula and can be described as ${\sigma _g}(\omega )= {\sigma _{inter}}(\omega )+ {\sigma _{intra}}(\omega )$, consisting of intra-band ${\sigma _{intra}}(\omega )$ and inter-band ${\sigma _{inter}}(\omega )$ contributions [34–36]:
In the calculation and modeling process, the target THz range is 0.1 to 1.2 THz, and the frequency domain finite element method (FEM) solver of the CST Microwave Studio software is employed to obtain the transmission coefficient $|{{S_{21}}} |$ and the reflection coefficient $|{{S_{11}}} |$. Additionally, the characteristics of the designed unit cell structure are improved by using the adaptive mesh refinement. Unit cell boundary conditions are set in the x-direction and the y-direction, and open boundary conditions are set in the z-direction. When VO2 is in the metallic state, the absorption intensity of the device can be described as follows: $A(\omega )= 1 - R(\omega )- T(\omega )= 1 - {|{{S_{11}}} |^2} - {|{{S_{21}}} |^2}$ [42], where $R(\omega )= {|{{S_{11}}} |^2}$ and $T(\omega )= {|{{S_{21}}} |^2}$ represent transmittance and reflectance, respectively. Moreover, the thickness of the middle VO2 film is large enough to prevent the transmission of THz waves when the conductivity is equal to $200000\; S/m$. Therefore, the designed THz bi-functional device can theoretically reach nearly perfect absorptance when the reflectance is as low as possible.
3. Results and discussions
The absorption spectra of the TE (the direction of the electric field is parallel to the x-axis) and TM (the direction of the electric field is parallel to the y-axis) polarization are presented in Fig. 2(a) and Fig. 2(b), respectively. Meanwhile, the initial conductivity of VO2 is 200000 S/m, and the Fermi energy of graphene is 0.1 eV. The proposed device exhibits an excellent absorption performance under normal incidence because VO2 acts as a metallic plate. In the TM polarization mode [Fig. 2(a)], the most distinctive broad absorption band is observed that 90% absorptance reaches 0.6 THz from 0.4 to 1 THz. And two perfect absorption peaks are located at ${f_1} = 0.474$ THz and ${f_2} = 0.831$ THz. On the other hand, as shown in Fig. 2(b), it can be seen that broadband absorptance higher than 62% is obtained in the same frequency range. The full width at half maximum (FWHM) bandwidth of the absorptance spectrum exceeds 0.7 THz. Clearly, two perfect absorption peaks are located at ${f_3} = 0.468$ THz and ${f_4} = 0.891$ THz, respectively. In this mode, the proposed system can be regarded as a dual-band absorber. Also, the manipulation of the polarization angle can regulate the absorption performance of the proposed multi-functional THz device after fabrication.
Fig. 2. The absorption performance of the designed system; (a) TM polarization mode, (b) TE polarization mode. Real and imaginary parts of the relative impedance with different (c) TM polarization modes and (d) TE polarization modes. The absorption spectra of the meta-device when the Fermi energy of graphene is changed; (e) TM polarization mode, (f) TE polarization mode.
Impedance matching theory is usually used to investigate the physical mechanism of the proposed THz functional device. When VO2 is in the metallic state and the thickness of the film is large enough to block the transmitted THz wave, the transmittance reaches the minimum. If the effective impedance of the device matches that of the free space, relatively high-absorptance can be achieved. The relative impedance (${Z_r}$) of the device can be expressed as [43,44]:
And the corresponding broadband absorptance is defined as:
As we can be observed in Fig. 3(b) and Fig. 3(c), when VO2 is in the metallic state, the THz functional device achieves broadband absorption, with only two resonators in one structure. In other words, the hybrid gold-VO2 nano-film and the middle VO2 reflective layer naturally form a metal-dielectric-metal cavity that provides different resonant modes. The amplitudes in the y-direction of the maximum electric field distributions $|{{E_y}} |$ at the two absorption peaks are displayed in Fig. 3(f) and Fig. 3(g), respectively, to illustrate the absorption mechanism. Interestingly, for y-polarized incidence, classical dipole resonances occur on the hybrid metasurface at 0.474 THz as well as 0.831 THz. On the other hand, the solid white line in Figs. 3(d)–3(e) and Figs. 3(h)–3(i), we can confirm that typical dipole resonance behavior is induced by the y-polarized incidence in the target THz range [45]. Nevertheless, at the perfect absorption peak of 0.831 THz, dipole resonance is observed. However, as shown in Fig. 3(e) and Fig. 3(i) (white line), the broadband absorption spectrum is produced by the superposition of the dipole resonance effect of the outer square ring and the inner ring, resulting in fundamental dipole resonance behavior. More importantly, we also investigate the role of multipoles in forming the broadband absorbing. Figure 3(a) shows the normalized power radiated by electric P, magnetic M, toroidal T, electric quadrupole EQ, and magnetic quadrupole MQ [46]. Actually, the contributions of all multipoles are the same for varied frequencies, however, as shown in Fig. 3(a), the electric dipole resonance P moment dominates. Simply put, the electric dipole intensities are very high in comparison with other multipoles at the frequency range of broadband absorption. In the low THz region, the conductivity of gold ($4.561 \times {10^7}\; S/m$) is sufficiently high to regard the device as an ideal conductor [47] that reflects THz waves in the free space. Moreover, when the conductivity of VO2 is fixed at $2 \times {10^5}\; S/m$, the device acts as a dissipative medium with high absorption characteristics [48]. Thus, the VO2 in the hybrid meta-device can also broaden the absorption spectrum, and it is different from another absorbers only with purely gold resonators [4].
Fig. 3. (a) Contributions of the five strongest multipolar excitations of the meta-device. Top view of the proposed unit cell structure; (b) original state and (c) metallic state of VO2. The surface current distributions in the TM polarization mode at the perfect resonance peaks of (d), (h) 0.474 THz, and (e), (i) 0.831 THz. The magnitudes of the maximum electric field distributions in the TM polarization mode at the perfect resonance peaks of (f) 0.474 THz, and (g) 0.831 THz.
The subsequent discussion is based on the normal incidence to improve our understanding of the broadband modulation performance of the proposed THz multi-functional device in the absorption mode. In fact, the internal physical mechanism of continuous modulation is primarily caused by the change in permittivity of VO2 with a change in the room temperature. During the insulator-to-metal transition process, the conductivity of VO2 changes by five orders of magnitude. According to the Fig. 4(a) and Fig. 4(b), one can clearly see that the imaginary parts are much larger than the real parts at different VO2 conductivities. Moreover, compared with the imaginary parts, the real parts of the permittivity remain almost unchanged. Therefore, as depicted Fig. 4(c) and Fig. 4(d), it is obvious that the real parts mainly affect the target resonance frequency range since the position of the broadband spectrum is unchanged. In contrast, because the imaginary parts mainly control the intensity, the magnitude of the broad absorption spectrum indicates tunable properties.
Fig. 4. (a) and (b) are the real and imaginary pars of the relative permittivity of VO2, (c) and (d) show the absorption intensities in the TM and TE polarization modes, and (e) and (f) show the modulation depths in the TM and TE polarization modes at different conductivities, respectively.
As discussed above, the proposed THz functional device can be regarded as a tunable absorber when VO2 reaches the fully metallic state. The modulation depth of the THz absorber is calculated as ${\rho _A} = ({{A_{on}} - {A_{off}}} )/{A_{on}}$ [49]. Figure 4(c) and Fig. 4(d) show that the magnitude of the absorption spectra for both TE and TM polarization modes increase as the VO2 conductivity increases. Especially, at 0.474 THz, the magnitude can be continually adjusted from 8.5% to nearly 100% as$\; \sigma ({V{O_2}} )$ increases from 10 S/m to 200000 S/m in the TM polarization mode. As shown in Fig. 4(e), (a) modulation depth of more than 90% is observed in the corresponding broadband spectrum from 0.4 to 1 THz. Moreover, the maximum modulation depth (91.5%) is realized at the peak absorption frequency. In the TE polarization mode, one conclusion can be obtained by the same calculation process, the maximum absorptance modulation depth of 92.1% is achieved at 0.856 THz. An advantage of the hybrid gold-VO2 metasurface design in this unit cell structure is that the device will be tunable once it is fabricated. In addition, the modulation performance of the designed THz functional device in the absorption spectrum in different polarization modes is listed in Table 1.
Table 1. Modulation performance of the designed THz functional device in the absorption process.
Thermal stimulation is used to induce the IMT of VO2 in the top and middle films simultaneously. The calculated transmission spectra of the proposed device are displayed in Figs. 5(a)–5(f) under normal TE polarization incidence. In the simulation process, different conductivity values describe the different phase states of VO2; such as, 200000 S/m is used to define the metal phase, and 10 S/m defines the insulator phase. One can clearly see that a broadband transparent window with relatively high transmission intensity is obtained when VO2 is in the fully insulating state at room temperature, and the Fermi energy of graphene is fixed at 0.1 eV. Meanwhile, the transparent window is located between the two resonance dips at 0.579 THz and 1.043 THz, respectively.
Fig. 5. (a-f) The calculated absorptance spectrum of the designed device at different VO2 conductivities.
Then, the electric field distributions of the two resonance dips are investigated to determine the potential physical mechanism of the proposed hybrid unit cell structure. Surprisingly, electromagnetically-induced transparency (EIT) behavior [50] is observed as the conductivity of VO2 decreases from 200000 S/m to 10 S/m without changing the other parameters. As shown in Fig. 6(b) and Fig. 6(c), the hybrid gold-VO2 nanorod metasurface is decomposed into an I-shaped resonator (ISR) and a U-shaped resonator (USR) when VO2 reaches the fully insulating state. Figure 6(d) depicts the calculated transmission spectra of the ISR, the USR and their combined hybrid-shaped resonator (HSR), respectively. It can be found that the two transmission dips in Fig. 6(d) emanate from the two sub-resonances of ISR and USR. Simply put, the broadband EIT window is due to the coupling between these two bright modes. Especially, the magnitude of the maximum electric field distributions $|{{E_x}} |$ in Fig. 6(c) indicates that the resonance position at 0.579 THz corresponds to the occurrence of the bright dipole mode in the USR. Similarly, Fig. 6(d) shows that the resonance dip position at 1.043 THz corresponds to the excitation of another bright dipole mode in the ISR. More importantly, within the EIT window at 0.871 THz, the electric field distribution indicates that the meta-device weakly interacts with the THz waves, as shown in Fig. 6(g). Besides, we also investigate the role of multipoles in forming the broadband EIT window. Figure 6(a) shows the normalized power radiated by electric P, magnetic M, toroidal T, electric quadrupole EQ, and magnetic quadrupole MQ [46]. Clearly, as shown in Fig. 6(a), the electric dipole resonance P also moment dominates. Simply put, the electric dipole intensities are extremely high in comparison with other multipoles at the target frequency range. Therefore, the strong broadband transparent window induced by EIT is caused by the coupling resonators between the two bright modes [51].
Fig. 6. (a) Contributions of the five strongest multipolar excitations of the meta-device. Top view of the proposed unit cell structure; (b) original state and (c) insulating state of VO2. (d) Transmission intensity of the sole ISR, the sole USR and their combined HSR, respectively. The magnitudes of the maximum electric field distributions in the TE polarization mode at resonance dips of (e) 0.579 THz, (f) 1.043 THz, and (g) 0.871 THz.
So far, the designed bi-functional device can be described as a four-layer perfectly tunable THz modulator with a gold-spacer-graphene-spacer structure. Likewise, the modulation depth in this transmission mode is calculated as ${\rho _T} = ({{T_{on}} - {T_{off}}} )/{T_{on}}$ [49]. As we can see from Fig. 7(c), in the TE polarization mode, the transmission intensity of the broadband transparent window in the target THz range decreases from 91% to 49% as the Fermi energy of graphene increases from 0 to 0.9 eV. Apparently, in the frequency range of 0.1-1.2 THz, the maximum transmission modulation depth of 70.5% is obtained. On the other hand, as shown in Fig. 7(d), the broadband transmission spectrums of this proposed structure can be continually adjusted in the TM polarization mode, and the peak transmittance at 0.4 THz declines gradually from 96.5% to 25.4% as ${E_f}$ increases. Meanwhile, the maximum transmittance modulation intensity is 71.1%.
Fig. 7. (a) and (b) are the real and imaginary pars of the surface impedance of graphene, (c) and (d) is the transmission intensity in the TE and TM polarization modes, (e) and (f) is the modulation depth in the TE and TM polarization modes for different Fermi energies, respectively.
The underlying mechanism of the tunable properties is related to the frequency-dependent surface impedance of graphene at different Fermi energies. Figure 7(a) and Fig. 7(b) show the real and imaginary parts of the surface impedance as a function of the Fermi energy, respectively. In the target THz region, the real part remains almost constant, but the imaginary part increases linearly as the frequency increases. It means that the real parts of the surface impedance mainly decide the resonance frequency; however, the imaginary parts mainly affect the loss of magnitude. Besides, according to the equation of ${E_f} = \hbar {v_F}{({\pi n} )^{1/2}}$ [49], when Fermi energy of graphene increases, the number of charge carriers n raises dramatically which enhances the conductivity. Therefore, it can be concluded that the surface impedance of graphene can be effectively adjusted by changing the Fermi energy. Since the imaginary part increases linearly with the frequency, the transmission spectra will be tunable in the target range.
Next, we demonstrate the switching property of the proposed unit cell structure. We define two states: “ON” describes the state of high transmittance when the Fermi energy is 0.1 eV, and “OFF” describes the state of low transmittance when the Fermi energy is 0.9 eV. As we can see from Fig. 8(a) and Fig. 8(b), fundamental dipole resonance modes were caused by the y-polarized or x-polarized transmittance spectrums in the “ON” state, it means that a electric dipole resonance is excited by a TE or TM polarized incidence in the frequency range of interest, as shown in Fig. 6(a) [45,52]. In the TE polarization mode, the FWHM bandwidth of the strong transparent window exceeds 0.42 THz [in Fig. 8(a)]. Simply put, the transmittance of the proposed functional THz device gradually decreased as the Fermi energy of graphene increased. The EIT-like window was not observed. The results in the TM polarization mode are similar to those of the TE polarization mode, but the corresponding transmittance is smaller when the Fermi energy of graphene increases to 0.9 eV, indicating better modulation performance. In addition, the modulation performance of the designed THz functional device in the transmission spectrum in different polarization modes is presented in Table 2.
Fig. 8. Theoretical demonstration of the optical active control of mode switching in the (a) TE-polarized transmission spectra, and (b) TM-polarized transmission spectra.
Table 2. Modulation performance of the designed THz functional device in the transmission process.
In practice, it is also in great demand for THz functional devices have to operate accurately under oblique incidence. As shown in Fig. 9(a) and Fig. 9(b), the transmission spectra exhibit changes as the incident angle increases in the TE and TM polarization modes. More precisely, the performance at the first resonance dip position at 0.4 THz is relatively stable for TE polarization when the incident angle is more than 75°. At the second resonance dip of 0.831 THz, splitting occurs when the incident angle is larger than 45° and gradually disappears. In contrast, as shown in Fig. 9(b), the transmittance performance is almost stable when the incident angle is smaller than 70° in the TM polarization mode. In this work, the performance of the designed THz bi-functional device at different incident angles is a crucial characteristic in imaging, thermophotovoltaics, and wireless communication.
Fig. 9. Calculated transmission spectra of the proposed metamaterial device at normal incidence with different incident angles for (a) TE polarization and (b) TM polarization. Calculated absorption spectra of the proposed metamaterial device at normal incidence with different incident angles for (c) TE polarization and (d) TM polarization.
Besides, we also simulated the absorption intensity at different incident angle to demonstrate the relationship between the absorptance spectrum and the incident angle, as shown in Fig. 9(c) and Fig. 9(d). One can clearly see that the target bandwidth position is almost unchanged as the incident angle increases from 0° to 85°. As the above discussion, in the TE polarization mode, the proposed system exhibits a dual-band absorption performance. Clearly, the absorption intensity remains above 90% in the first band until the incident angle reaches 85°. However, the absorption bandwidth becomes narrower and splits into two in the second band when the incident angle exceeds 30°. On the other hand, as displayed in Fig. 9(d), when the incident angle is raised from 0° to 60°, the designed unit cell structure exhibits perfect absorption property in the broadband spectrum. As the incident angle further increases, the absorption spectrum gradually disappears. Simply put, the remarkable absorption/transmission performance of the designed THz device in different polarization modes will make it highly suited for smart applications. Such as a two-dimensional apertureless near-field THz imaging using a quantum cascade laser (QCL) source and a scattering probe. Different polarization modes may allowing image resolutions to be enhanced on the sample surface [53].
4. Conclusion
A polarization-sensitive switchable metamaterial THz device consisting of hybrid non-patterned graphene and VO2 was proposed. It has a bi-functional design and exhibits broadband absorption as well as modulation behavior. When VO2 is in the insulating state at room temperature, the interaction between the top I-shaped and U-shaped gold nanorod causes the formation of an EIT window phenomenon in the TE polarization mode. The electric field distribution indicates that the EIT window is accomplished by two coupled bright modes. By increasing the Fermi energy of graphene from 0 to 0.9 eV, the device can act as a transmittance modulator, achieving a modulation depth of 70.5%. In the TM polarization mode, the device can be operated as a broadband transmittance modulator when the Fermi energy is changed. And the maximum modulation intensity is enhanced to 71.1%. When VO2 reaches the fully metallic state in the phase change process and does not have any geometrical changes, the device can be flexibly switched from a transmittance modulator to a broadband absorber in the TM polarization mode. Furthermore, a distinctive absorption spectrum with an absolute bandwidth of 0.55 THz is obtained. In the TE polarization mode, the designed meta-device operates as a dual-band near-perfect absorber with an absorptance of more than 97%. In this state, the maximum absorptance modulation depth of 91.5% and 92.1% are obtained in the TM and TE polarization modes, respectively. Besides, the impedance matching theory and electric field distribution are used explain the inherent mechanism. The proposed functional device provides good performance at different incident angles. In summary, the designed reconfigurable device shows promise for use in tunable THz optoelectronic systems.
Funding
National Natural Science Foundation of China (61162004).
Disclosures
The authors declare no conflicts of interest.
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