Bifunctional terahertz absorber with a tunable and switchable property between broadband and dual-band

Hui Li; Hui Li; Jiang Yu; Jiang Yu

doi:10.1364/OE.401992

1. Introduction

Metamaterials, an artificially engineered periodic nanostructure with extraordinary electromagnetic properties, could have some special and broad applications in different regions [1,2]. In recent years, the metamaterial tunable THz device has become one of the most attractive aspects due to its promising applications in some fields, such as the modulator, biological imaging, and spectroscopy [3–6]. Among all the applications of metamaterials, the tunable metamaterial absorbers have aroused wide concern since Landy et al. reported a single-band perfect structure in 2008 [7]. Furthermore, various kind of absorbers, such as single-band [8], multi-band [9,10], and broadband [11,12] absorbers, are proposed by researchers to realize excellent absorption performance. However, the single-performance tunable absorber can no longer meet the requests in practical application, therefore, the flexible and intelligent tunable metamaterial device is more attractive for many systems.

Due to their unique electromagnetic, and optical properties of graphene and vanadium dioxide (VO₂) materials, making them considered to be a new pathway for designing tunable and switchable material absorber [11–24]. Compared to other new materials, graphene is a two-dimensional crystal composed of a monolayer of carbon atoms arranged in a honeycomb lattice, and the surface conductivity of graphene can be continuously adjusted by chemical doping or applying an external bias voltage [11–13]. Meanwhile, VO₂ is an ideal choice for controlling devices, which can realize the reversible phase transition from insulator to metal by thermal, electrical or optical methods and lead to a dramatic change in dielectric function and optical properties, so that the absorber combined with VO₂ has a wide dynamic range and high modulation depth for THz waves [14–16]. To achieve excellent absorption performance, Song et al. reported a multilayer absorber with switchable functionalities based on phase-transition property of VO₂, the result showed that structure be switched from a broadband absorber to a narrowband absorber [17]. Chen et al. by introducing VO₂ film into a multilayer structure, the designed system can be switched from an absorber to a transparent conducting metal [18]. Wang et al. proposed a dual-controlled switchable broadband terahertz absorber based on a hybrid of VO₂ and graphene, by changing the conductivity of VO₂ and the Fermi energy of graphene simultaneously, it found that the state of the proposed absorber can be switched from absorption to reflection [24]. Previous works indicate that the graphene and VO₂ are both hopeful choices for achieving dynamically tunable or switchable absorber in the THz range. In fact, a great challenge today is to design new structures where the change in optical properties can lead to more flexible and intelligent response.

In this paper, we propose an exquisite polarization-insensitive THz bifunctional device with excellent absorption performance over a wide range of incident angles based on hybrid graphene-VO₂ metamaterial. Moreover, by controlling these two independent parameters (the conductivity of VO₂ and the Fermi energy of graphene) reasonably, we can achieve the switchable and tunable property in both broadband and dual-band on the proposed multilayer structure. Besides, we can use the interference theory to explain the physical mechanism of the perfect absorption. The results indicate a new method for designing high-performance terahertz devices, such as modulator, thermal emitter, and photovoltaics device.

2. Design and methods

The schematic of the proposed absorber based on hybrid graphene-VO₂ metamaterials is given in Fig. 1. Apparently, the structure consists of six layers, from top to bottom, they are E-shaped VO₂ resonator array, ToPaS dielectric spacer, VO₂ film, monolayer graphene sheet, SiO₂ dielectric layer and fully metallic reflective layer. By controlling the property of insulator-to-metal transition (IMT) of VO₂, we are able to switch the structure between a unity-broadband absorber and a perfect dual-band absorber. When the VO₂ is in the fully metallic state, the thickness of the middle VO₂ reflective film is selected as $t2 = 1\; \mu m$, which is larger than skin depth in the THz region [16]. Besides, the complex electric permittivity of ToPaS dielectric spacer (thickness $t = 15\; \mu m$) is $\varepsilon = 2.35 + 0.01i$ [25]. Meanwhile, the lower dielectric layer is 38-µm-thick SiO₂ with the permittivity ${\varepsilon _s} = 3.8$ [26]. The structure dimensions of the proposed absorber by careful design are listed as follows: $P = 50\; \mu m,\; t1 = 2\; \mu m,\; R1 = 40\; \mu m,\; R2 = 38\; \mu m,\; tm = 0.5\; \mu m,\; G = 2\; \mu m$ and $W = 1\; \mu m$.

Fig. 1. The schematic of the unit cell of the proposed absorber consists of perspective, top, and side view.

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In the numerical simulation, VO₂ films undergo a IMT process at a critical temperature around 68°C [27]. Since metallic VO₂ has a high free carrier concentration, it can achieve a significant modulation depth in application [28]. Moreover, the insulator-state VO₂ is highly transparent to electromagnetic waves below 6.7 THz [29]. The optical permittivity of VO₂ in the THz range can be described by the Drude model [16–19]

(1)$$\varepsilon (\omega )= {\varepsilon _\infty } - \frac{{\omega _p^2(\sigma )}}{{{\omega ^2} + j\gamma \omega}}$$

where the ${\varepsilon _\infty } = 12\; $ is the dielectric permittivity at the infinite frequency, $\varepsilon (\omega )$ is defined as the permittivity at high frequency, $\gamma = 5.75 \times {10^{13}}\; rad/s\; $ is the collision frequency. And the plasma frequency ${\omega _p}$ can be approximately defined as $\omega _p^2 = \frac{{\sigma ({V{O_2}} )}}{{{\sigma _0}}}\omega _p^2({{\sigma_0}} )$, where ${\sigma _0} = 3 \times {10^5}\; S/m$ and ${\omega _p}({{\sigma_0}} )= 1.4 \times {10^{15}}\; S/m$ [28]. During the IMT process, the conductivity of VO₂ can be transformed via five orders of magnitude. In the calculation process, we can apply different permittivity to describe the different phase state of VO₂, such as the conductivity of VO₂ is $2 \times {10^5}\; S/m$ is taken to describe the metal phase and $10\; S/m$ to describe the insulator phase.

Meanwhile, in the modeled process, the thickness of the graphene film (${t_g}$) is assumed to be a typical value of 1 nm. The monolayer graphene sheet is molded as an equivalent 2D surface impedance layer and without thickness in this paper, and it was built from a closed planar square curve extruding to a surface. [30,31]. From the range of terahertz to optical frequency, the graphene’s surface conductivity ${\sigma _g}(\omega )$ can be described as ${\sigma _g}(\omega )= {\sigma _{inter}}(\omega )+ {\sigma _{intra}}(\omega )$, which consists of intra-band ${\sigma _{intra}}(\omega )$ and inter-band ${\sigma _{inter}}(\omega )$ contributions from the Kubo formula, respectively. However, according to the Pauli exclusion principle, the inter-band contribution is negligible compared to the intra-band part in the case of the photon energy $\hbar \omega \ll {E_f}$ and ${k_B}T \ll {E_f}$ in the low THz region. Therefore, the surface conductivity of graphene can be simplified to ${\sigma _g}(\omega )= {\sigma _{intra}}(\omega )$ and the intra-band ${\sigma _{intra}}(\omega )$ can be defined as [30,31]

(2)$${\sigma _{intra}}(\omega )= j\frac{{{e^2}}}{{\pi {\hbar ^2}({\omega - j2\mathrm{\Gamma}} )}}\mathop \smallint \nolimits_0^\infty \eta \left( {\frac{{\partial {f_d}({\eta ,{E_f},T} )}}{{\partial \eta }} - \frac{{\partial {f_d}({ - \eta ,{E_f},T} )}}{{\partial \eta }}} \right)d\eta $$

where ${f_d}({\eta ,{E_f},T} )= {({{e^{({\eta - {E_f}} )/{k_B}T}} + 1} )^{ - 1}}$, ${E_f}$ is the Fermi energy (chemical potential) of graphene, $\mathrm{\Gamma} = 1/({2\tau } )$ is the phenomenological scattering rate and $\tau = \mu {E_f}/({ev_F^2} )$ is the relaxation time, relating to the carrier mobility $\mu = {10^4}\; c{m^2}{V^{ - 1}}{s^{ - 1}}$ and Fermi velocity ${v_F} \approx 1.1 \times {10^6}\; m/s$, $\omega $ is the radian frequency of the incident wave, ${k_B}$ is the Boltzmann constant, $\hbar $ is the reduced Planck’s constant and $\hbar = h/({2\pi } )$, e is the charge of an electron, and $T = 300\; K$ is the Kelvin temperature. Besides, the complex surface impedance of monolayer graphene can be described by ${Z_G}(\omega )= 1/{\sigma _G}(\omega )$ [31]. In this work, the numerical simulation results can be obtained by the frequency domain finite element method (FEM) solver of the CST Microwave Studio, and the adaptive mesh refinement is used to improve the accuracy of simulation. Moreover, unit cell boundary conditions are assigned along both the x-direction and y-direction while open boundary condition is assigned along the z-direction of the proposed absorber. Meanwhile, using the S parameters in the calculation, the absorbance $A(\omega )$ can be obtained by $A(\omega )= 1 - R(\omega )- T(\omega )= 1 - {|{{S_{11}}} |^2} - {|{{S_{21}}} |^2}$, where $R(\omega )= {|{{S_{11}}} |^2}$ and $T(\omega )= {|{{S_{21}}} |^2}$ are defined as transmittance and reflectance, respectively.

3. Results and discussions

When the Fermi energy of graphene is set to 0.01 eV and the VO₂ was in a fully metallic state with conductivity of 200000 S/m, the designed system can realize broadband absorption. As shown in Fig. 2(a), there are most distinctive broad absorption band, and the bandwidth with absorptance over 90% are as wide as 2.92 THz. Besides, it is obvious that there are also two near-unity absorption peaks located at ${f_1} = 2.45$ THz and ${f_1} = 3.88$ THz, respectively. Additionally, the magnitude of the corresponding broad absorption spectrum can be continuous adjusted by a thermal control system to prompt the IMT property of VO₂. It is clear that with the conductivity of VO₂ changes from 10 S/m to 200000 S/m, the corresponding absorptance is adjusted from 17% to 90% for the absorption spectrum of 1.45 THz to 4.37 THz. Compared with the device when the VO₂ in the insulating state, the proposed device at 2.45 THz obtains almost 9 times absorptance enhancement. It means that we can regard the proposed structure as an amplitude modulator, and we can independently set the sensitivity of the absorber to the incident wave. On the other hand, When VO₂ is in the metallic state, only the upper three-layer structure realizes the absorbing properties, while the lower graphene sheet-dielectric-gold structure does not work. So, as shown in Fig. 2(c), when the Fermi energy of graphene raised from 0.2 eV to 0.8 eV, the absorption curves of the device are perfectly coincident.

Fig. 2. (a) Absorption spectrum of the proposed absorber with different phase of VO₂. (b) The real and imaginary parts of the relative impedance of the absorber. (c) Absorption curves of the device with different Fermi energies. When VO₂ is in the metal phase.

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To further understand the inherent mechanism of the proposed structure, the impedance matching theory is introduced. Therefore, the absorptance and the relative impedance can be defined as [32,33]:

(3)$$A(\omega )= 1 - R(\omega )= 1 - {\left|{\frac{{Z - {Z_0}}}{{Z + {Z_0}}}} \right|^2} = 1 - {\left|{\frac{{{Z_r} - 1}}{{{Z_r} + 1}}} \right|^2}$$

(4)$${Z_r} ={\pm} \sqrt {\frac{{{{({1 + {S_{11}}} )}^2} - S_{21}^2}}{{{{({1 - {S_{11}}} )}^2} - S_{21}^2}}}$$

where Z and ${Z_0}$ are the effective impedances of the proposed absorber and the free space, respectively. ${S_{11}}$ and ${S_{21}}$ are denoted as transmission coefficient and reflection coefficient. The real and imaginary parts of the relative impedance of the absorber are displayed in Fig. 2(b). One can clearly see that the real part is close to 1, and the imaginary part is close to 0 in the corresponding broad absorption spectrum, which means the impedance of the proposed absorber matches with that of the free space. So far, the structure can be simplified as a VO₂ resonator-dielectric-VO₂ reflective layer, as no incident light can transmit the proposed absorber due to the metal-phase VO₂ film is on the middle. It means that the transmittance of the structure is almost zero and the incident wave can be absorbed to the maximum extent.

To better grasp the absorption mechanism in the perfect absorption, we monitor the electric field distribution at 2.45 THz and 3.88 THz. As shown in Fig. 3(a) and Fig. 3(b), from the top view of the absorber, it is obvious that the electric field is mainly concentrated on both ends of the E-shaped VO₂ array at the resonance frequency of 2.45 THz. Since the frequency shifts to 3.33 THz, the electric field is not only concentrated on both ends of E-shaped VO₂ array but also strongly concentrated on its arm gaps. In addition, from the side view of the absorber, the electric field is mainly compressed into the upper of the structure when the middle VO₂ reflective layer was in a fully metallic state. As shown in Fig. 3(c), one can clearly see that a strong electric field is concentrated on the interface between the ToPaS spacer and the fully metal-phase VO₂ resonator, which means that the surface plasmon resonances (SPR) can also enhance the absorptance of the absorber [27,33]. Besides, it can be seen from Fig. 3(d) that there exists a classical magnetic resonance absorption mechanism between the top E-shaped VO₂ resonator and middle VO₂ reflective film [8,30].

Fig. 3. The electric field distribution of the proposed absorber at different perfect resonance frequencies (a-b) Top view; (c-d) Side view when the conductivity of VO₂ is set to 200000S/m.

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In this study, we introduced the interference theory to better understand the physical mechanism of the proposed absorber when the VO₂ material is in metal phase. As shown in Fig. 4(c), a modified asymmetric Fabry-Perot resonant model is displayed to indicates the incident wave multi-reflection process inside the ToPaS dielectric layer. In the calculation process, ${r_{23}} ={-} 1$, there was no incident wave transmission through the simplified structure due to the transmittance of the proposed absorber is almost 0 when the VO₂ is in the metal phase. Therefore, the amplitude of all reflected THz waves from the air-VO₂ metasurface interface can be modeled as [34,35]

(5)$$\tilde{r} = {\tilde{r}_{12}} - \frac{{{{\tilde{t}}_{12}}{{\tilde{t}}_{21}}{e^{i2\tilde{\beta }}}}}{{1 + {{\tilde{r}}_{21}}{e^{i2\tilde{\beta }}}}}$$

where the accumulated phase reflected by the middle VO₂ film can be described by the formula (6):

(6)$$\tilde{\beta } = {\beta _r} + i{\beta _i} = \sqrt {{{\tilde{\varepsilon }}_t}} {k_0}t$$

and ${\tilde{r}_{12}} = {r_{12}}{e^{\phi 12}}$ is the reflection coefficient of the incident terahertz wave propagating from the air to the E-shaped VO₂ film layer and then reflected back to the air. And ${\tilde{t}_{12}} = {t_{12}}{e^{\phi 12}}$ is the transmission coefficient of the incident THz wave transmitting from air to E-shaped VO₂ film and then transmitted back into air. Similarly, ${\tilde{r}_{21}} = {r_{21}}{e^{\varphi 21}}$ and ${\tilde{t}_{21}} = {t_{21}}{e^{\varphi 21}}$ are the reflection and transmission coefficients of the incident waves transmitting from air into the top E-shaped VO₂ film and ToPaS and reflected back into air again, respectively. Besides, ${\beta _r}$ is the propagation phase of the structure, and ${\beta _i}$ is related to the absorption of the dielectric layer, ${k_0}$ is the wave vector of the free space, and d is the thickness of the ToPaS spacer. Additionally, according to Eq. (6) as above, the absorptance of the absorber can be closed to 1 when $|{{{\tilde{r}}_{12}}} |= |{{{\tilde{t}}_{12}}{{\tilde{t}}_{21}} - {{\tilde{r}}_{12}}{{\tilde{r}}_{21}}} |$ and $\tilde{\beta } = 2n\pi ,\; ({n = 0,\; \pm 1,\; \pm 2 \ldots } )$ are synchronously realized in the same band.

Fig. 4. The (a) magnitudes and (b) phases of the transmission and reflection coefficients calculated by the interference theory. (c) Schematic of the multiple reflection and interference model. (d) Comparison of the absorption spectra calculated by interference theory and the absorption spectra simulated by CST.

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Figures 4(a) and 4(b) show the magnitude and phase of the transmission and reflection coefficients calculated by the Fabry-Perot resonant model. It can be seen that the incident THz waves have the same magnitude and phase, and almost existing a Pi phase shift before and after the resonant frequency in the reflectance spectrum [35]. Apparently, from the Fig. 4(d) we can see that the absorptance spectra simulated by CST is almost consistent with that calculated by interference theory.

When the conductivity of VO₂ was fixed at 200000 S/m and the period of the unit cell was unchanged, we performed simulations with different geometric parameters. As shown in Fig. 5(a), the absorption intensity of the absorber can be continuously adjusted with the increasing width of W from 1 µm to 7 µm. From the Fig. 5(b), one can clearly see that with the increasing radius of $R1$ from 38 µm to 42 µm, VO₂ patches are getting bigger, and spectra width becomes to broaden. In fact, the effective impedance Z of the absorber is determined by the effective permittivity of $\varepsilon (\omega )$ and permeability of $\mu (\omega )$. Different coupling mode effects between the E-shaped VO₂ resonator and middle metal-phase plane lead to the variation of the permeability $\mu (\omega )$. According to the equation $Z = \sqrt {\mu (\omega )/\varepsilon (\omega )} $ [32] and ${Z_r} = Z/{Z_0}$, the excellent impedance matching between the absorber and the free space will be destroyed, which causing the different spectrum. Therefore, we choose the radius of 38 µm for design. Meanwhile, from Fig. 5(c), it can be observed that the absorption curves of different gap width G have a high overlap ratio. As shown in Figs. 5(d)–5(f), although the gap width of G has changed, the incident electric energy is still excited steadily, which is the key to keep the absorption stable, and this may have great operability in actual production. Therefore, from the discussion above, we illustrate the optimization process of the structure.

Fig. 5. Effects of different parameters on broadband absorption performance. (a) different arm widths of W, (b) different outside diameter of the circle R1, and (c) different gap widths of G. (d) Electric field distributions resonance frequencies of 2.45 THz when VO₂ is in the metal phase.

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Moreover, by using thermal or electric stimulation to prompt the IMT property of VO₂ in both top and bottom layers simultaneously, the influences of different conductivity of VO₂ on the absorption performance are further investigated. As shown in Figs. 6(a)–6(h), when the Fermi energy of the graphene is set to 0.8 eV [36,37], the proposed hybrid metamaterial absorber can achieve dual-band absorption performance by decreasing the conductivity of VO₂ from 200000 S/m to 10 S/m in the THz range. From Fig. 7(a), we can find that there are two perfect absorption peaks (above 93%) at the frequencies of 0.81 THz and 3.17 THz when the VO₂ is in the insulating state. So far, the proposed bifunctional device can be considered as a four-layer structure of dielectric-graphene-dielectric-gold. In this state, the amplitude of the absorption spectrum can be continually tuned by changing the Fermi energy when the conductivity of VO₂ is fixed at 10 S/m. As shown in Fig. 7(a) and Fig. 7(b), due to the existence of the bottom gold reflective layer, we are able to switch the proposed structure between absorber and reflector in a wide frequency range, from 1 THz to 4 THz. On the other hand, we investigate the influence of the thickness of SiO₂ layer on the absorptance spectrum. From Fig. 7(c), it is obvious that when the thickness of SiO₂ increases from 36 µm to 40 µm, the dual absorption bands occur red-shift and the peak absorptance of the spectrum keeps stable since the changes of the thickness of SiO₂ influence the relative impedance of the device. And it indicates that the thickness of SiO₂ designed as 38um is better to achieve high absorption performance for the whole structure. So far, the detailed parameters settings are given in Table 1. It can be seen that we can realize different absorption performance by applying the parameters in both $\sigma ({V{O_2}} )$ and ${E_f}$ reasonably.

Fig. 6. (a-h) The calculated absorption spectrum of the absorber with different conductivity of VO₂.

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Fig. 7. The absorption and reflection spectrum (a) and contour map (b) of the proposed absorber with different Fermi energy, and the absorption spectrum with different thickness of SiO₂ spacer (c) when the conductivity of VO₂ is set to 10 S/m.

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Table 1. Comparison of properties using different parameters.

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To better investigate the influences of the proposed absorber on the incident energy loss, it is indispensable to explain the main contribution to THz absorption of each part in the designed structure. In the analysis above, we realized that the proposed device can be regarded as a dual-band absorber for THz waves when VO₂ is in the insulating state and the Fermi energy of graphene is fived at 0.8 eV. As shown in Fig. 8(a), the reflection and transmission spectrums of the SiO₂ spacer are simulated. And there are two distinctive reflection peaks located at 1 THz and 3 THz, and the frequency interval is $\Delta f = 2$ THz. Likewise, we can see from Fig. 8(c) that when a gold reflective layer is added to the bottom, it still has two absorption peaks with similar absorptance but lower amplitudes at 1.03 THz and 3.03 THz. The inherent reason for this phenomenon can be also explained by the Fabry-Perot resonance model. Indeed, the dielectric of SiO₂ is modeled to be the finite thickness in simulation to realize Fabry-Perot resonance [38–40]. Thus, the frequency interval between adjacent peaks can be defined as [38]

(7)$$\Delta f = \frac{c}{{2n{t_d}\cos \xi }}$$

where $\xi $ is the incident angle of the THz wave, c is the velocity of light in vacuum, n and ${t_d}$ are the refractive index and the thickness of the SiO₂ layer, respectively. According to the Eq. (7), the calculated frequency interval is $\Delta f = 2$ THz, which is in correspondence with the simulation results. Moreover, as shown in Fig. 8(b), the absorptance of the monolayer graphene to electromagnetic waves can only reach up to 50% in the free space, and the absorptance will gradually decrease as the frequency increases [41]. Hence, adhering to the principle of improving the absorptance, a monolayer graphene sheet without a pattern is placed on the top to achieve a sandwich metamaterial perfect absorber. From Fig. 8(d), we can clearly see that the absorptance has been enhanced. Compared with the previous structure, the absorption bandwidth and amplitude have been significantly improved since the impedance between the absorber and the free space has matched. However, that VO₂ is served as a dielectric and the upper ToPaS spacer exists leads to the different absorption and operating bandwidth according to the relative impedance theory. The calculation result obtained by the three-layer structure is only slightly different from the simulation result of the proposed device. Therefore, a dual-band perfect absorber is achieved.

Fig. 8. (a) The reflection and transmission spectra of the dielectric layer. (b) The absorption, transmission and reflection spectra of monolayer graphene sheet in the free space. (c) The absorption spectrum of dielectric-gold structure. (d) The absorption spectrum of the graphene sheet-dielectric-gold structure. When VO2 is in the insulator-phase.

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As the polarization independence and wide-angle tolerance of the proposed absorber plays an important role in practical applications so that it is necessary to investigated the oblique angle insensitive of the device. As shown in Fig. 9(a) and Fig. 10(a), for both broadband absorption and dual-band absorption, it is obvious that the absorption curves under different polarization angles have completely coincided under normal incidence, which indicates that the proposed absorber could work well in different polarization modes. In fact, the symmetric unit cell is the inherent reason for the perfect polarization independence.

Fig. 9. Absorption spectrum of the proposed absorber under different (a) polarization angle and incident angles of (b) TE polarization, (c) TM polarization when VO₂ is in the metal-phase.

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Fig. 10. Absorption spectrum of the proposed absorber under different (a) polarization angle and incident angles of (b) TE polarization, (c) TM polarization when VO₂ is in the insulator-phase.

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Next, we studied the effects of obliquely incident THz waves on the absorption spectrum in transverse electric (TE) and transverse magnetic (TM) polarization modes, respectively. For TE polarization, as can be seen from Fig. 9(b), the broad absorption band has excellent absorption performance in the range of incident angles of 0° to 85°. The absorption bandwidth remains almost unchanged until the incident angle exceeds 45°. For TM polarization, as shown in Fig. 9(c), the result is similar to TE polarization that the absorption peak splits into six for large incident angles. Additionally, when the incident angle continuous increases, both of the two broadbands decrease significantly and finally disappear. On the other hand, according to the transmission line theory [31,42], the propagation constant ${k_z}$ along z-direction is molded by ${k_z} = {k_p}({1 - si{n^2}\xi } )$, where ${k_p}$ is the wave number in dielectric layer. We can find that the phase-matching condition is not satisfied (${k_z} \ne {k_p}$) on the hybrid metasurface since ${k_z}$ increases with the increasing incident angel $\theta $. The excellent impedance matching between the absorber and the free space will be destroyed, which will weaken the absorptance. When the VO₂ of the structure changes from metal to insulator phase, for TE polarization, as shown in Fig. 10(b), the two absorption bands remain perfect performance until the incident angle raises up to 65° for the first band and to 85° for the second band. Meanwhile, the second band has a blue-shift and the bandwidth turns narrower gradually with the incident angle increases. For TM polarization, we can see from Fig. 10(c) that the first absorption band becomes wider for large incidence angle and there is a slight blue-shift for the second absorption spectrum. And the absorption performance for both bands remain stable until the incident angle raises up to over 85°. So, the function of the absorber is almost insensitive to incidence angle.

4. Conclusion

In conclusion, we proposed and demonstrated a terahertz absorber with broadband absorption and dual-band absorption properties on a simple structure. Switchable functionalities properties are achieved based on the phase transition of VO₂. When VO₂ is in the metallic state, it is an ultra-broadband absorber. Numerical simulation results indicate that the magnitude of the spectrum can be dynamically adjusted when the Fermi energy of graphene is fixed at 0.01 eV. Polarization-insensitive properties are achieved and the device maintains excellent absorption performance over a wide range of incident angles up to 50°. In contrast, when the VO₂ changes from metal to insulator phase, the designed device can be regarded as a tunable dual-band perfect absorber at the resonance frequency of 0.81 THz and 3.17 THz. As the Fermi energy of graphene increases from 0.01 eV to 0.8 eV, the amplitude of the two absorptance peaks can be continually adjusted. And this system also shows the properties of ultra-wide incident angle and polarization independence. The Fabry-Perot resonance model can explain the inherent mechanism reasonably. In a word, we present a new method for designing high-performance terahertz devices, such as modulator, thermal emitter, and photovoltaics device.

Funding

National Natural Science Foundation of China (61162004).

Disclosures

The authors declare no conflicts of interest.

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	VO₂		Graphene
Function	$σ (V O_{2})$	$E_{f}$	$σ (V O_{2})$	$E_{f}$
Broadband absorption	200000 S/m	/	/	0.01 eV-0.8 eV
Broadband reflection	10 S/m	/	/	0.01eV
Dual-band absorption	10 S/m	/	/	0.8 eV

Bifunctional terahertz absorber with a tunable and switchable property between broadband and dual-band

Abstract

1. Introduction

2. Design and methods

3. Results and discussions

4. Conclusion

Funding

Disclosures

References

Cited By

Figures (10)

Tables (1)

Equations (7)

Optics Express