In this paper, we introduced a switchable dual-band and ultra-wideband terahertz wave absorber based on photoconductive silicon combining with vanadium dioxide (VO2). In the terahertz absorber, photoconductive silicon cross array, silicon dioxide layer, vanadium dioxide windmill type array, silicon dioxide dielectric layer, and gold ground plane are placed from the top layer to bottom layer in sequence. When VO2 is in a metallic state and the conductivity of photoconductive silicon is 2.5×10−4 S/m, the designed structure represents an ultra-wideband absorber with an absorption larger than 90% in the range of 3.14∼7.80 THz. As VO2 is in an insulation state and the conductivity of photoconductive silicon becomes 8.0×104 S/m, the designed device acts as two absorption bands, with a terahertz wave absorber with absorption more than 98% at 1.78∼2.90 THz and 7.35∼8.45 THz. The results show that the absorption band (dual-band or ultra-wideband) and absorption intensity (from 2% to 99%) can be switched by changing the phase transition of the VO2 and the conductivity of photoconductive silicon. Furthermore, the proposed device exhibits polarization insensitive and wide incident angles (lager than 70°) for TE- and TM- polarizations incidence.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Recent years, terahertz technology have been widely used in terahertz wireless communication [1–2], imaging , spectroscopy, and detection . As one of the core devices of these terahertz systems, terahertz absorber has become a research hot-spot. Developing a high efficiency and multifunctional terahertz absorber is one of the key issues. Most recently, various kinds of terahertz absorbers with single frequency, dual frequency band and multi-frequency have been proposed and studied [5–6]. However, most of the reported terahertz absorbers show that their performance including absorptivity and working bandwidth are fixed and can not be adjusted once these structures are designed without tunable materials. Until recently, several researchers investigated some controlled terahertz absorbers based on three-dimensional foam materials [7–8], all-silicon structures [9–10], metamaterials hybrid VO2 [11–12], liquid crystals [13–14], graphene [15–17], and Dirac semimetals [18–19]. Meanwhile, various multi-layer hybrid structures are exploited for broadband terahertz absorbers to overcome one absorption band defect [20–21]. But, most of these hybrid structures only shift the absorption peak of the center frequency [22–23] and have the drawback of low absorptivity or complicated structures. So far, it is still a huge challenge for researchers to design a switchable wide bandwidth and high absorption intensity simultaneously by using a simple structure. Thus, it is extremely necessary to develop an inexpensive way to generate a required dual-band and ultra-wideband terahertz absorbers having relatively stable operating frequency range and high absorbance with desired wide incident angle.
To solve these problems, we proposed a switchable dual absorption band and ultra-wideband terahertz absorber based on photoconductive silicon combining with vanadium dioxide matematerial. By using external ambient stimuli to change the phase transition of the VO2 and the conductivity of photoconductive silicon, ultra-broadband absorption and dual-band characteristics are realized and can be switched. The absorptivity in the operating frequency band can be adjusted from 2% to 99% by controlling the external ambient stimuli. The designed absorber shows polarization insensitive and high absorption performance within incident angle of 70°. It has a potential application prospect in the field of terahertz communication and sensing.
2. Terahertz absorber theory analysis
A three-dimensional (3D) schematic diagram of the proposed dual-band and ultra-wideband terahertz wave absorber based on photoconductive silicon combining with vanadium dioxide is depicted as Fig. 1. The proposed device is made of photoconductive silicon cross array pattern with a thickness of h4 = 1μm, silicon dioxide layer, VO2 windmill type array with a thickness of h3 = 0.8μm, silicon dioxide (SiO2) dielectric layer, and gold ground plane. The symmetrical dual radar structure array layer can be fabricated by large-scale synthesis, transfer and etching techniques. The chemical vapor deposition method is employed to fabricate this multilayer structure. The relative permittivity of SiO2 is 3.75 and the dielectric loss angle is 0.0004. The conductivity range of photoconductive silicon varies from 2.5×10−4 S/m to 5×105 S/m as the external laser pump intensity changes . Without laser-pumping, the default conductivity of the photoconductive silicon is 2.5×10−4 S/m, and the dielectric constant is 11.7. The absorptivity A(ω) of the terahertz wave absorber can be calculated as follows:
According to the equivalent impedance matching theory, the incident terahertz wave generates electromagnetic resonance in the designed structure and produces absorption. In order to ensure the terahertz wave into the designed structure, the surface impedance of the absorber needs perfectly match with the free space impedance. At this time, the terahertz wave absorptivity reaches the maximum value. The optimized geometric parameters of the proposed structure are set as h1=7.5μm, h2=6.5μm, P=15μm, R1=0.4μm, R2=2.4μm, R3=7.5μm, R4=6μm, R5=5μm, X1 = 9.6μm, Y1=9μm, and Y2=8μm. The optical properties of phase-change material vanadium dioxide (VO2) in terahertz region can be expressed by Drude model [25–26]
3. Results and discussions
To study the physical mechanism of the absorber, we analyzed the absorption spectra and electric field distribution of various number of VO2 windmill type structures including 4 vortex plates, 8 vortex plates, and 12 vortex plates, as shown in Fig. 2. As depicted in Fig 2(a), one can see that the maximum amplitude of the absorption peak is only 52% as the number of vortex sheets is 4. When the number of vortex sheets increase to 8, the maximum absorption of terahertz wave reaches 90% at a very narrow absorption frequency band, as illustrated in Fig 2(b). From Fig 2(c), it can be noted that the absorption is more than 90% in the terahertz frequency range of 3.14∼7.80 THz when the number of vortex sheets increase up to 12. Figures. 2(d)∼2(f) display the electric field diagrams of the three kind numbers of VO2 vortex structures at 6 THz (Here, we selected the electric field distribution at 6 THz, which is the center frequency of absorption band.). It proves that the terahertz absorption intensity and operating bandwidth of the absorber are closely related to the number of vortex sheets.
The influence of different layer structures on the absorptivity of the terahertz absorber is also discussed, as shown in Fig 3. As plotted in Fig 3(a), the absorptivity of the absorber based on photoconductive silicon cross array pattern structure can be negligible. If the absorber consists of only the VO2 windmill type array pattern, the absorption amplitude and operating bandwidth are obviously insufficient, as given in Figs. 3(b) and 3(c) shows the absorption intensity is over 90% in the terahertz band range from 3.14THz to 7.80 THz when the absorber is made of photoconductive silicon cross array pattern, silicon dioxide layer, VO2 windmill type array, silicon dioxide dielectric layer, and gold ground plane. Figure. 4 illustrates the absorption intensity and operating bandwidth variation of the absorber with different geometric parameters. As depicted in Fig 4(a), with the increase of the lower silicon dioxide dielectric layer thickness h1, the operating frequency band produces clearly blue-shift. Figure. 4(b) plots the variation of the terahertz wave absorption intensity with the increase of the upper silicon dioxide dielectric layer thickness h2. One sees that the absorption intensity and bandwidth of the absorber are weakened. In addition, it is worth noting that with the increase of the outer diameter of the vortex R3, the absorption band over than 90% is widened, as shown in Fig 4(c).
Figures. 5(a) and (b) show the absorption spectra of the designed absorber with different conductivities of photo-conductive silicon when VO2 is set in insulating state and in metallic state, respectively. From the Fig 5(a), it can be noted that the corresponding absorption changes from 60% to 99% in the wide absorption band of 3.14 THz to 7.80 THz as the conductivity of photo-conductive silicon increases from 2.5×10−4 S/m to 3.0 ×105 S/m. As for the Fig 5(b), it can be clearly found that the absorption varies from 4% to 99% for dual absorption band 1.78∼2.90 THz and 7.35∼8.45 THz. In addition, we simulated the absorption properties of the proposed absorber as a function of the conductivity of VO2 when the conductivities of photo-conductive silicon are set as σSi=2.5×10−4 S/m and σSi=8.0×104 S/m, as plotted in Figs. 5(c) and 5(d), respectively. It can be observed from the Fig 5(c) that the absorption of the proposed structure can be controlled from 2% to 99% in the broadband of 3.14∼7.80 THz as the conductivities of photo-conductive silicon equals 2.5×10−4 S/m. According to Figure.5(d), it can be also observed that the absorption of the proposed structure can be adjusted from 69% to 99% in the double absorption band when the conductivities of photo-conductive silicon σSi is set to 8.0 × 104 S/m. Both the conductivity of photo-conductive silicon and VO2 decides the absorption amplitude of the proposed structure.
Figure. 6(a) shows the absorption coefficient A, reflection coefficient R and transmission coefficient T of the absorber for TE and TM modes in the range of 0∼10 THz when σSi=2.5×10−4 S/m and VO2 is in metallic state (i.e. ambient temperature is 68 ℃). Obviously, the broadband absorbance is more than 90% in the range of 3.14∼7.80 THz and the bandwidth reach 4.66 THz. The corresponding real part and imaginary part curves of the equivalent impedance of the proposed absorber is illustrated in Fig 6(b). One can see that the real part of the surface impedance tends to 1 and the imaginary part is close to 0 in the range of 3.14∼7.80 THz. In this case, the equivalent impedance of the designed absorber matches the free space impedance in the required operating frequency band, and the absorber achieves high absorbance. When σSi=8.0×104 S/m and VO2 is in insulating state, the absorber exhibits dual-frequency absorption band, as shown in Fig 6 (d). The absorbance maintains more than 90% in the frequency range of 1.78∼2.90 THz and 7.35∼8.45 THz, respectively. Similarly, as given in Fig 6(d), the real part of the equivalent impedance equals to 1 and the imaginary part approximates to 0 in the two absorption bands, which leads to the maximum absorptance. It is consistent with the absorption spectrum. Figure 7 displays the electric field distribution of the proposed absorber in broadband operating frequency for TE (TM) modes at 3.14 THz, 6.0 THz, and 7.8 THz. As shown in Figs. 7(a) and 7(d), the electric field is mainly concentrated in the outer vortex sheets at 3.14 THz. As the frequency of the terahertz wave increases, the electric field is concentrated on the middle vortex sheets in Figs. 7(b) and 7(e) at 6.0 THz. At last, at the frequency of 7.80 THz, the electric field distributed on the middle vortex sheets is weaken, as depicted in Figs. 7(c) and 7(f). The phenomenon shows that the magnetic dipole resonance in different vortices regions will produce different absorption peaks, and the ultra-wideband of the proposed absorber comes from the superposition of different absorption peaks.
In addition, we investigated the absorption as a function of incidence angles and frequency under ultra-broadband operating absorber for TE- and TM- polarization incidence, as shown in Figure 8. From the figure, one can see that the absorption peaks are larger than 90% until the terahertz wave incident angle is close to 70°. For TE- (TM-) polarization incidence, the designed absorber exhibits a high-efficient absorption performance in a broad absorption band from 3.14 to 7.80 THz. As a whole, the absorptivity of the absorber decreases with the increase of incident angle. From the Figs. 8(a) and 8(b), one can deduce the terahertz wave propagation constant at the interface of the two media along z-direction kz = k1cosθi, where k1=2π/λ is the wave number in medium 1. As the incident angle increases, the impedance matching between the medium 1 and medium 2 is destroyed. It can be clearly seen that the absorption of the proposed absorber become weak. Similarly, for dual-band absorber, the absorption also decreases slightly with the increase of incident angle. The absorption in the two absorption bands for TE and TM modes remains more than 85% until the incident angle exceeds 70°, as shown in Fig 9. In order to show the novelty of the proposed absorber, the proposed structure is compared with the absorber described in different literatures. As shown in Table. 1, compared with the previously reported literature, the absorber described in this paper has obvious wide bandwidth.
To sum up, we designed a switchable dual-band and ultra-wideband terahertz absorber by introducing tunable medium (photoconductive silicon and VO2 metamaterial patterns). It is composed of a photoconductive silicon cross array, VO2 windmill type array, and a gold ground plane separated by two silicon dioxide dielectric layers. Via changing the phase transition of VO2 and the conductivity of photoconductive silicon, the device possesses a switchable dual absorption band (1.78∼2.90 THz and 7.35∼8.45 THz) and ultra-wideband absorption frequency range (3.14∼7.80 THz) performance. Furthermore, the absorptivity in the absorption band can be dynamically adjusted from 2% to 99% by changing the conductivity of the photoconductive silicon and VO2. The proposed absorber maintains high absorptivity in the terahertz regime over a wide range of incident angles up to 70°. Benefiting from the above excellent performances, the absorber has great potential in terahertz wave imaging, communication and stealth.
National Natural Science Foundation of China (61831012, 61871355); Zhejiang Key R & D Project of China (2021C03153); Fundamental Research Funds for the Provincial Universities of Zhejiang (2020YW20); Zhejiang Lab (2019LC0AB03).
The authors declare that there are no conflicts of interest related to this article.
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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