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Abstract
A tunable broadband terahertz (THz) absorber based on vanadium dioxide (VO2) is proposed, which is composed of a structured VO2 metamaterial layer, a dielectric layer, and a metal layer. The simulation results show that the peak absorption intensity of the absorber can be dynamically adjusted from 3.6% to 100% by changing the conductivity of VO2. When the conductivity of VO2 is 2×105 S/m, an excellent broadband absorption phenomenon appears, and the bandwidth of absorption rate greater than 90% is as wide as 4.10 THz from 3.25 THz to 7.35 THz. The impedance matching theory and the electric field distribution are used to explain the physical mechanism of the proposed absorber. In addition, the absorber shows the advantages of polarization-insensitiveness and wide-angle absorption. It is expected that this absorber can have potential applications such as modulators and photoelectric switches.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz (THz) wave has shown great application potential in the fields of wireless communication [1,2], non-destructive testing [3], and security detection [4]. To promote the development of THz technology, various functional devices based on metamaterials have been proposed, such as filters [5,6], polarization converters [7,8] and absorbers [9,10]. Among these devices, the perfect absorber has always been one of the central issues due to its extensive applications in stealth technology, imaging, solar cells, and electromagnetic pollution [11–14].
In 2008, Tao et al. designed and fabricated the first generation metamaterial absorber in the THz band [15]. In the following decades, many single-band absorbers, dual-band absorbers, and multi-band absorbers have been proposed and demonstrated to solve the problems of the narrow working bandwidth and fixed operating frequency range [16–18]. At present, the research on THz metamaterial absorbers has been extended to broadband absorption [19–29]. For example, Song et al. reported the THz absorption bandwidth of more than 90% absorptance can reach to 2.01 THz [27]. At the same time, the dynamic regulation of absorbers has also been achieved by combing metamaterials with semiconductors, graphene, liquid crystal, and phase transition materials [20,30–33], which provides more convenience to practical applications . As we known, the phase of vanadium dioxide (VO2) can be changed by the external temperature and its conductivity can vary several orders of magnitude. Thus, it is a feasible method to achieve tunable broadband absorption in the THz band by using VO2, the absorption bandwidth of more than 90% can arrive 2.45 THz and 3.30 THz, respectively [28,29]. However, the adjustable range that has been reported is still narrower than the actual application requirements, so the expansion of bandwidth is still needed to study continually.
In this paper, a tunable broadband THz absorber based on VO2, which is composed of a structured VO2 metamaterial layer, a dielectric spacer, and a metal layer, is proposed. The conductivity of the VO2 can be adjusted by changing the external temperature. When the temperature is 340 K, the conductivity of VO2 is 2×105 S/m, the bandwidth of absorption rate greater than 90% reaches 4.10 THz from 3.25 THz to 7.35 THz, the bandwidth has been enlarged 25% compared with previously reported. Through the analysis of the impedance matching theory and the electric field distribution, the physical mechanism of the absorber is explored. Furthermore, the effects of different polarization and incident angles on the absorption spectrum are also investigated. The designed absorber has potential applications such as modulators and photoelectric switches, which are quite important for the practical application of THz radiation.
2. Design and simulation
The structured unit cell of the designed absorber is shown in Fig. 1, which is composed of three layers, a VO2 combined pattern (cross-shaped and four L-shaped), a dielectric spacer (SiO2), and a gold (Au) ground plane. The period of the unit cell is P. The thickness of VO2, SiO2, and Au is defined as h1, h2, and h3. The length and width of L-shaped structures are L1 and W1. The length and width of the cross-shaped structure are L2 and W2. By analyzing the influence of geometric parameters on the absorption spectrum, the optimal geometric parameters are fixed and shown in Table 1.
Table 1. Parameters of the designed absorber.
The device can be fabricated as follows [34]. Firstly, a gold layer (the thickness is 0.2 μm) is coated on a substrate. Since the thickness of the gold layer is much larger than the skin depth of THz radiation in gold, the effect of substrate can be neglected. Secondly, the silicon dioxide layer (the thickness is 6.5 μm) needs to deposit on the gold layer by Plasma Enhanced Chemical Vapor Deposition process. Thirdly, the VOx film with a thickness of 0.2 μm is deposited onto silicon dioxide film by DC magnetron sputtering with a vanadium metal target and further annealed in low pressure O2 atmosphere to change VOx into VO2. Finally, the cross-shaped and four L-shaped structures are needed to etch on the surface of the VO2 layer by lithography technology. It should be pointed out that numbers of studies have shown that with carefully selected fabrication conditions the electronic and optical properties associated with the phase transition can be well reserved in the patterned VO2 structures, which shows the practicality of the proposed VO2 based metamaterial absorbers [35–37].
The electromagnetic response of the absorber is simulated by the commercial software FDTD Solutions. The periodic boundary conditions are employed in x- and y-directions and the perfect matching layer is set in z-direction. The Drude model is taken to describe the permittivity of VO2 in the THz range, which has a good agreement with the experimental data [38]. It is written as
3. Results and discussions
Figure 2 shows the reflection and absorption spectra of this proposed absorber with different conductivities of VO2 under normal incidence. The simulation results show that an excellent absorption bandwidth of more than 90% absorption rate is 4.10 THz in the frequency range from 3.25 THz to 7.35 THz with the central frequency around 5.20 THz when the conductivity of VO2 is 2000 S/cm. And there are two perfect absorption peaks located at f1=3.85 THz, f2=6.55 THz. Figure 2(b) shows that the peak absorption intensity can be adjusted from 3.6% to 100% when the conductivity of VO2 changes from 2×102 S/m to 2×105 S/m. This phenomenon mainly stems from the variations of permittivity of VO2. The real and imaginary parts of the permittivity under different conductivities of VO2 are displayed in Fig. 3. The results indicate that the imaginary part of the permittivity is much larger than that of the real part, and it increases rapidly with the conductivity. It leads to a remarkable change in spectral intensity. Figure 3 also shows that both the real and imaginary parts vary by 2–3 orders of magnitude, which means that VO2 can be used as a suitable metamaterial to design adjustable devices by controlling conductivity.
To explain the perfect absorption phenomenon of the designed absorber, the impedance matching theory is used . The effective impedance of the absorber can be expressed as [42]
Fig. 4. (a) Relative impedance in the conductivity of 2000 S/cm for VO2 and (b) Comparison of the absorption spectra between the solo units, and combined pattern.
To further explore the physical mechanism of two perfect absorption peaks, it is necessary to analyze the respective influence of the cross-shaped and four L-shaped parts that constitute the combination pattern of the first layer on the absorption spectrum. The absorption spectrum of the cross-shaped structure, four L-shaped structures, and combination patterns are shown in Fig. 4(b). Compared to the corresponding absorption spectra, it can be found that the absorber based on a single cross-shaped structure has only one single absorption band. However, when adding a set of surrounding L-shaped structures, a perfect broadband absorption is achieved. To further explain this phenomenon, Fig. 5 shows the electric field distribution for the two absorption peaks at 3.85 THz and 6.55 THz under x-polarized light incidence. When the frequency is 3.85 THz, the charges are concentrated on the two ends of the horizontal rectangular bar and ends of the four L-shaped structures. Combining with the analysis of the absorption spectrum in Fig. 4(b), it can be known that the first absorption peak is mainly caused by the electric dipole resonance excitation based on the cross-shaped structure. The purpose of adding four L-shaped structures is to increase the absorption rate. When the frequency is 6.55 THz, the electric field is distributed in the horizontal gap between cross-shaped and L-shaped. It means that the second absorption peak is caused by the coupling effect between components. In Fig. 5, it also can demonstrate that there is a coupling effect between adjacent unit cells.
Fig. 5. The electric field distribution of the top view at (a) 3.85 THz and (b) 6.55 THz of the combined pattern under x-polarized light incidence.
To explain the importance of the optimization structure parameters to the results, it is necessary to investigate the influences of the structure parameters on the absorber. For simplicity, one parameter is analyzed while other parameters keep unchanged in the following discussion. Figure 6(a) shows that the half bandwidth decreases as the period P increases because the coupling effect between adjacent unit cells is weakened. The maximum bandwidth of absorption rate greater than 90% is obtained at P=24 μm. The first peak has a blue shift with its absorption rate remaining almost unchanged, and the range between the two peaks decreases. Figure 6(b) presents the absorption spectrum of the device with different thicknesses of SiO2, it can be found that the bandwidth of absorption rate greater than 90% first increases and then decreases as the thickness of SiO2 (h2) increases. The maximum absorption bandwidth appears when h2 is equaled to 6.5 μm. The influences of the length L1 and width W1 of L-shaped structures on the absorption spectrum are shown in Fig. 7(a) and Fig. 7(b), respectively. In Fig. 7(a), the bandwidth increases as the length of L-shaped structures (L1) increases. The position of the first peak remains unchanged and the second peak has a blue shift. It is because that the first peak is mainly caused by the cross-shaped structure and the second peak is caused by the coupling effect between the cross-shaped and four L-shaped structures. For the same reason, in Fig. 7(b), as the width of L-shaped structures (W1) increases, the position of the first peak keeps unchanged, while the second peak has a blue shift with the decrease of absorption. Figure 8 describes the influences of the length L2 and width W2 of the cross-shaped structure on the absorption spectrum. In Fig. 8(a), the first peak has a red shift and the absorption bandwidth gradually increases as the length (L2) becomes larger. In Fig. 8(b), the two peaks move in the opposite direction and the absorption bandwidth is gradually increased. In summary, the best effect of the absorption bandwidth can be obtained by adjusting the geometric parameters of the structure.
Fig. 7. Influence of the (a) length L1 and (b) width W1 of L-shaped structures based on VO2 on the absorption spectrum when the cross-shaped structure is fixed.
Fig. 8. Influence of the (a) length L2 and (b) width W2 of the cross-shaped structue based on VO2 on the absorption spectrum when the L-shaped structures are fixed.
Finally, the absorption spectra for different polarization and incident angles are investigated. As shown in Fig. 9(a), the absorption spectra keep unchanged when the polarization angle increases from 0° to 90° under normal incidence. Therefore, this absorber has the advantage of polarization insensitivity. In addition, the oblique incidence of light should also be considered. For the TE polarization, the absorption rate remains larger than 90% until the incident angle reaches up to 60°, as shown in Fig. 9(b). For the TM polarization, the absorption rate maintains larger than 90% in a wide range of incident angle up to 40°, as shown in Fig. 9(c). When the incident angle further increases, the bandwidth becomes wider but the absorption rate drops gradually. The absorption performance of this designed absorber is better than those previously reported. The results of comparisons are shown in Table 2.
Fig. 9. The absorption spectrum (a) under different polarization angles (b) for the TE polarization and (c) for the TM polarization under different incidence angles.
Table 2. Comparison between references and our work.a
4. Conclusions
In conclusion, a tunable broadband THz metamaterial absorber based on VO2 is proposed. The device is composed of a set of a combined pattern (cross-shaped and four L-shaped) based on VO2 and a metal ground layer separated by a dielectric spacer. Simulation results indicate that the bandwidth of absorption rate more than 90% reaches 4.10 THz, which is covers from 3.25 THz to 7.35 THz when the conductivity of VO2 is 2000 S/cm. When the conductivity of VO2 changes from 2 S/cm to 2000 S/cm, the peak absorption intensity of the absorber can be dynamically adjusted from 3.6% to 100%. The electric field distributions for the two absorption peaks show that the first peak steams from the excitation of the electric dipole resonance on the cross-shaped structure and the second peak originates from the electric field coupling effect between the combined pattern. In addition, the absorber has the characteristics of polarization-insensitive and wide-angle absorption. For the TE polarization, the absorption intensity remains greater than 90% until the incidence angle reaches up to 60°. And for the TM polarization, the absorption intensity remains greater than 90% in a wide range of incident angle up to 40°. Therefore, the designed absorber may have good application prospects in the THz band, such as modulators and photoelectric switches.
Funding
National Natural Science Foundation of China (21976049); Natural Science Foundation of Hebei Province (B2021402006); Handan Science and Technology Research and Development Program (19422031008-5).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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