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Abstract
A dynamically adjustable ultra-wideband metamaterial perfect absorber (MPA) is proposed which consists of three resonance rings based on vanadium dioxide (VO2) and a metal ground layer separated by a dielectric spacer. The simulation results show that the terahertz (THz) absorption bandwidth of more than 90% absorptance reaches 3.30 THz, which covers from 2.34 to 5.64 THz, under different incident polarization angles. The range is better than that of previous VO2-based reports. Moreover, when the conductivity of VO2 changes from 200 S/m to 2×105 S/m, the absorption peak intensity can be adjusted continuously from 4% to 100%. The key is to optimize the geometric structure through interference cancellation and impedance matching theory, to achieve better absorption bandwidth and efficiency. Besides, the terahertz absorber has a wide-angle absorption effect both in TE and TM waves. Thus, the designed absorber may have many potential applications in modulating, sensing and imaging technology.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The wide application prospects of THz waves in wireless communications [1], sensors [2], and imaging [3] have attracted great interest in the last few decades. To promote the development of terahertz technology, a variety of functional devices based on different metamaterials have been proposed, such as filters [4,5], absorbers [6–8], polarization converters [9–11], and so on. Among these applications in the THz field, the MPA plays an important role, especially in imaging, stealth technology and sensing applications [12,13]. The MPA was firstly proposed by Landy et al. at microwave frequency band [14]. It consists of a top metal pattern and a bottom metal ground plane separated by a dielectric layer. After that, the MPA is widely developed, which is not only limited to the microwave frequency band but also developing towards full-spectrum applications such as terahertz, infrared, visible light and ultraviolet, and so on [15–18]. However, the aforementioned MPAs suffer from narrow absorption bandwidth and fixed electromagnetic absorption peaks. These imperfections restrict their practical application when considering a larger THz bandwidth and better flexibility.
Recently, a large number of researchers have devoted to realizing THz MPAs with broadband absorption capabilities and reconfigurable characteristics. To achieve broadband absorption, multiple resonant structures with different sizes and multi-layer structures with multi-thickness dielectric layers are proposed [19–22]. However, these structures are limited to complex fabrication process and difficulty controlling. Simultaneously, there are many tunable hybrid MPAs based on semiconductor, liquid crystal, graphene, and phase change material that have been reported to achieve reconfigurable characteristics [23–29]. Among these phase change materials, vanadium dioxide (VO2) shows an excellent transition behavior from an insulator phase to a metal phase at about 340 K [30], accompanied by a huge change in conductivity. Therefore, VO2 as an effective component of metamaterial is expected to achieve dynamically adjustable characteristics in the THz range. In recent years, some VO2-based MPAs have broadband and adjustable absorption characteristics [see [31–38] in Table 1]. However, the absorption bandwidth and the adjustable range have not yet reached the expectations of practical applications.
Table 1. Comparison of absorption performance between different VO2-based absorbers.
This paper presents a dynamically adjustable broadband terahertz absorber, which consists of three VO2 resonance rings and a metal ground layer separated by a dielectric spacer. Under different incident polarization angles, the THz absorption bandwidth with an absorption rate over 90% ranges from 2.34 to 5.64 THz (a bandwidth of 3.30 THz), which is better than that of previous VO2-based reports. Through changing the temperature, the conductivity of VO2 changes from 200 S/m to 2×105 S/m, that is, from an insulating phase to a metallic phase. It makes that the absorption peak intensity can be continuously adjusted from 4% to 100%. The physical mechanism of the perfect absorption can be explained by the interference cancellation theory and the impedance matching theory. Furthermore, the electric field distribution of two absorption peaks is also introduced to analyze the physical principle of the absorber. The absorber also has a wide-angle absorption effect both in TE and TM waves, which has a good application prospect in the terahertz range.
2. Design and simulation
Fig. 1 shows the unit cell of the designed ultra-wideband THz absorber, which contains three VO2 resonance rings and a metal ground layer separated by a dielectric spacer. The period of the unit cell is p = 30 μm. The thickness of the top layer VO2 is 0.2 μm. Figure 1(a) shows the top layer pattern with three equally spaced rings in different sizes, and the size of the circle radius from the outer to the inner is r1 = 12 μm, r2 = 8 μm, r3 = 4 μm, respectively. The width of the rings is all w = 3 μm. The Drude model is employed to describe the optical characteristics of VO2 in the THz range [39–41], which can be expressed as
3. Results
The reflection spectrum and absorption spectrum of the absorber are shown in Fig. 2(a) when the VO2 layer is in the metal phase with a conductivity of 2×105 S/m. It can be seen from the results that the absorption bandwidth greater than 90% absorptance is as high as 3.30 THz from 2.34 THz to 5.64 THz under normal incidence, which is wider than other MPAs in previous reports. The absorber has two absorption peaks that locate at 2.83 THz and 5.19 THz, respectively. The central frequency between the two peaks is 4.10 THz. Figure 2(b) shows the absorption intensity of the absorber at different polarization angles from 0° to 90°, indicating its polarization-insensitivity due to the symmetry of the design.
Fig. 2. (a) The reflection spectrum and absorption spectrum of the absorber. (b) The color diagram of the absorption spectrum with different polarization angles.
As Fig. 3 shown, when the conductivity changes from 200 S/m to 2×105 S/m, the reflectance spectrum and absorption spectrum of the absorber change accordingly. The absorptance of the absorber increases continuously from 4% to 100% with the increasing of conductivity, and the center frequency of the absorber stays nearly unchanged in the meantime. The reason for this phenomenon is the variation of the permittivity for VO2 with the change of conductivity. From the Fig. 3(c) and Fig. 3(d), it can be seen that the imaginary part of the permittivity for VO2 is much larger than the real part, and it increases rapidly as the conductivity adds up. The results indicate that the absorber has reconfigurable characteristics when actively controlling VO2. Furthermore, the properties of VO2 can be tuned by electrical [44] or thermal [45].
Fig. 3. (a) The reflection spectrum and (b) absorption spectrum with different conductivities of VO2. (c) The real parts and (d) imaginary parts of permittivity for VO2 in different conductivities.
4. Discussions
The absorption spectrum of the absorber varies with the thickness of the dielectric as shown in Fig. 4(a). For an absorber structure with a dielectric thickness of d = 9 μm, its center frequency is 4.10 THz corresponding to a wavelength of 73.17 μm in free space. The refractive index in the dielectric layer is $\sqrt \varepsilon $, thus the corresponding wavelength in the dielectric layer is 37.5 μm. The physical mechanism of the absorber can be explained by the interference cancellation principle that the thickness of the dielectric is 9 μm almost equaling to 1/4 of the center wavelength 37.5 μm in a dielectric layer, which meets the interference cancellation condition between the reflection and incidence. Therefore, the absorber exhibits a strong absorption effect when the thickness of the dielectric layer is 9 μm. Here, we can briefly describe the possible processing steps of the metasurface to show the experimental feasibility. First, a thickness of 9 μm silicon dioxide film can be deposited on gold substrate by Plasma Enhanced Chemical Vapor Deposition (PECVD) process. Then, the VOx film can be 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 [16]. Finally, based on the mask of the surface pattern, the surface of VO2 is etched through a photolithography process to form a multi-ring pattern.
Fig. 4. (a) The absorption spectrum of the absorber changes with the thickness of SiO2 and (b) the relative impedance in a conductivity of 2×105 S/m for VO2.
The impedance matching theory can also be applied to explain the perfect absorption phenomenon of the absorber. Here, the effective impedance of the MPA can be retrieved from [46]
Furthermore, the horizontal and vertical electric field distribution of the absorber at two absorption peaks which are located at 2.83 THz and 5.19 THz are shown in Fig. 5. As the results shown in Fig. 5(a), the horizontal electric field at the first peak is almost distributed in the gap between the outer ring and the middle ring, and a small amount is located between the two longitudinal units. Differently, in Fig. 5(b), the horizontal electric field at the second peak is nearly distributed in the gap between the middle ring and the inner ring, and a little distribution is located between the outer ring and the middle ring. The electric field distribution of the absorber in the vertical section is also added in Fig. 5(c) and (d). The electric field distribution at the two frequency peaks is similar to the electric field direction in the horizontal state, mainly concentrating between the rings, but it differs by 90 degrees. The above results indicate that the electric field distribution of the absorber mainly comes from the coupling effect between the rings and the two distant absorption peaks of the absorber give it a broadband absorption effect. Therefore, the multi-resonant ring structure is beneficial to the preparation of ultra-wideband absorbers.
Fig. 5. The electric field distribution of the top view at (a) 2.83 THz and (b) 5.19 THz in horizontal direction and (c) 2.83 THz and (d) 5.19 THz in vertical direction.
To explain the importance of the optimization structure parameters to the results, it is necessary to analyze the influence of geometric structure parameters on the absorber. Figure 6 shows the influence of r1, r2, r3, and the width w of the number of rings on the absorption spectrum. In Fig. 6(a), the absorption bandwidth of the absorber increases as the radius of the outer ring r1 becomes wider, but the total absorption efficiency decreases due to the weakening of the coupling effect between the outer ring and the middle ring. The first peak has a blue shift, while the second peak remains nearly unchanged. In Fig. 6(b), the overall bandwidth remains unchanged. But when the radius of middle ring r2 is 7 μm and 9 μm, it overlaps with the inner ring and the outer ring respectively, resulting that a part of the coupling effect is lost and the absorption efficiency decreases. In Fig. 6(c), the absorption efficiency increases as the size of the inner ring radius r3 increases, but the bandwidth changes a little. It is worth noting that when the r3 is 5μm, it coincides with the middle ring, resulting in a decrease in absorption efficiency due to the disappearance of the gap between two rings. In Fig. 6(d), the absorption bandwidth of the absorber increases as the width becomes wider, but the total absorption efficiency decreases. In the meantime, the absorption spectrum changes from single-peak absorption to double-peak absorption, which greatly increases the overall absorption bandwidth. What’s more, the effect of increasing or decreasing the number of VO2 resonance rings on the absorption performance is also considered in Fig. 6(e). The absorption efficiency of the absorber first increases and then decreases as the number of rings increases, and the absorption bandwidth increases as the number of rings increases. Among them, the absorption effect achieves the best when the number of rings is 3. It indicates that when the number of rings exceeds three, the overall absorption efficiency will be lowered due to the increasing in absorption peaks. These results above indicate the importance of geometric structure parameters adjustment to improve the absorption performance of the absorber. Finally, after a series of geometric structure parameters optimization, a perfect absorber with ultra-wideband absorption effect is obtained.
Fig. 6. The influence of the (a) outer ring radius r1, (b) middle ring radius r2, (c) inner ring radius r3, (d) the width w and (e) the number of the rings on the absorption spectrum.
Moreover, the large-angle absorption effect also plays an important role in describing the performance of the absorber for practical applications. The absorption spectrums with different incident angles in TE and TM polarization are demonstrated in Fig. 7. The unit structure of the absorber is isotropic due to the multi-resonant ring structure, resulting in the same angle absorption effect under two polarization conditions. The absorption rate remains greater than 75% until the incidence angle changes to 55°. When the incident angle further increases, the bandwidth becomes wider and the absorption rate drops dramatically. Thus, the above results show that the absorber has good large-angle absorptance and keeps the same absorption effect under the two polarization. Besides, the absorptance of the absorber under TM polarization is better than that of the previous reports whose absorptance are usually deteriorated by high-order diffraction [32,33,37,38].
Fig. 7. The absorption spectrums under different incident angles for (a) TE polarization and (b) TM polarization.
5. Conclusions
In this paper, a dynamically adjustable ultra-wideband THz absorber is proposed which is composed of three VO2 rings and a metal ground layer separated by a dielectric spacer. The results indicate that the THz absorption bandwidth of 90% absorptance reaches 3.30 THz (from 2.34 to 5.64 THz) under normal incidence. Through changing the temperature, the conductivity of VO2 changes from 200 S/m to 2×105 S/m, resulting that the absorption peak intensity continuously is adjusted from 4% to 100%. The physical mechanism of the perfect absorption can be verified through the wave-interference theory and the impedance matching theory respectively. The absorption bandwidth and efficiency have reached the best through optimizing the geometric structure. Furthermore, the electric field distribution of two absorption peaks shows that they come from the field coupling between the adjacent rings. Besides, the designed absorber has good polarization insensitivity and a wide-angle absorption effect. Thus, the dynamically adjustable ultra-wideband THz absorber may have many potential applications in the THz range containing modulators, sensors, optic-electro switches, and other optical devices.
Funding
National Key Research and Development Program of China (2018YFB2200504, 2019YFB2203700); National Natural Science Foundation of China (11674313, 61527820, 61674142, 62041502).
Acknowledgments
Thank you for State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences and College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences.
Disclosures
The authors declare no conflicts of interest.
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