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Article Tunable Dual Broadband Terahertz Absorber Based on Vanadium Dioxide

Xiao-Fei Jiao 1,2,3, Zi-Heng Zhang 1,2,3, Tong Li 4, Yun Xu 1,2,3 and Guo-Feng Song 1,2,3,*

1 Institute of , Chinese Academy of Sciences, Beijing 100083, China; [email protected] (X.-F.J.); [email protected] (Z.-H.Z.); [email protected] (Y.X.) 2 College of Materials and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China 3 Beijing Key Laboratory of Inorganic Stretchable and Flexible Information Technology, Beijing 100083, China 4 School of Physics and Electronic Sciences, Changsha University of Science and Technology, Changsha 410004, China; [email protected] * Correspondence: [email protected]

 Received: 15 August 2020; Accepted: 13 October 2020; Published: 17 October 2020 

Abstract: With the rapid development of terahertz technology, tunable high-efficiency broadband functional devices have become a trend. In this research, a dynamically tunable dual broadband terahertz absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The metamaterial is composed of patterned VO2 on the top layer, gold on the bottom layer and silicon dioxide (SiO2) as the middle layer. Simulation results show that two bandwidths of 90% absorption reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz under normal incidence. By changing the conductivity of VO2, the absorptance dynamically tuned from 2% to 94%. Moreover, it is verified that absorptance is insensitive to the angle. The physical origin of this absorber is revealed through interference theory and matching impedance theory. We further investigate the physical mechanism of dual broadband absorption through analysis. This design has potential applications in imaging, and stealth technology.

Keywords: terahertz; metamaterial; vanadium dioxide; perfect absorber

1. Introduction Terahertz (THz) generally refer to electromagnetic waves with a ranging from 0.1 to 10.0 THz. They have a wide range of applications in the fields of , sensors and imaging [1–3]. Because of their bright future, they have aroused great interest in the past few decades. With the rapid development of THz technology, the performance requirements of related functional devices are gradually increasing. Traditional natural materials have inherent shortcomings such as performance limitations, bulky and difficulty of integration, which can no longer meet the current needs for optic applications. (MM) composed of periodic subwavelength arrays can solve the above problems. Because MM have the advantages of high efficiency, tunability and easy integration, various functional devices based on MM have been proposed, such as optical filters, absorbers and polarization converters [4–9]. Among various devices, the metamaterial perfect absorber (MPA) has received widespread attention due to its significant role in imaging, stealth technology and sensing applications [10–12]. The first MPA working in the band was proposed by Landy et al. in 2008 [13]. A ––metal (MIM) configuration is used to demonstrate MPAs. The basic unit structure of MPAs usually consists of three functional layers: a patterned metal layer on the top layer, a metal layer on the bottom layer and a dielectric layer in the middle. Under the guidance of this design, many types of MPAs have been reported, and the research

Appl. Sci. 2020, 10, 7259; doi:10.3390/app10207259 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7259 2 of 9 on them is not limited to the microwave frequency range but gradually extends to all spectra related to technology, including THz, , visible and ranges [14–20]. So far, various MM have realized absorbers that work at THz . However, once the structure of the traditional MM absorber is determined, the electromagnetic response cannot be adjusted, and it has a fixed peak frequency and absorptance, which limits their practical application. MPAs based on semiconductors, graphene and liquid crystals have been reported to realize the dynamic tunable characteristics [21–23]. Vanadium dioxide (VO2), as a phase change material, varies greatly during the phase transition from insulator to metal by changing the temperature [24]. This is caused by the transformation of the structure from the insulating monoclinic phase to the metallic tetragonal phase around 68 ◦C. Therefore, combining MM with VO2 is a promising method to achieve dynamic tunable characteristics in the THz range. In 2018, Song et al. presented an active absorption device based on VO2 metamaterials, 90% absorptance bandwidth reaches 0.33 THz with a central frequency of 0.55 THz, and absorptance at peak frequencies can be continuously tuned from 30% to 100% [25]. In order to improve operation bandwidth, in 2019, Bai et al. designed a broadband THz absorber that the bandwidth of absorption rate above 90% is 1.25 THz, and the absorption rate can be continuously tuned from 15% to 96% [26]. To achieve multi-band absorption, in 2020, Huang et al. presented a dual broadband absorber, the bandwidths with the absorption above 80% were 0.88 THz and 0.77 THz in the frequency ranges of 0.56–1.44 THz and 2.88–3.65 THz, and the absorptance can be continuously adjusted from 20% to 90% [27]. Although the performance of absorbers based on vanadium dioxide metamaterials has been improved, the characteristics of the broad bandwidth, multi-band and active control still require more research to meet the expectations of practical applications. For the classic three-layer structure absorber, the material and the structure of the top layer design are the most critical parts. In this work, we propose a tunable dual broadband THz absorber, which consists of a VO2 structure with two perpendicularly intersecting ellipses and a circular hole in the center, and a metal separated by a dielectric spacer. The conductivity of VO2 can be controlled by changing the temperature, thereby inducing the transition of VO2 from an insulator to a metal. Under thermal control, the absorptance of the two bands of the absorber can be continuously tuned. The physical mechanism of MPA is investigated by the impedance matching and interference theory. The electric field analysis is discussed to a deeper understanding of the absorption mechanism. Moreover, it is verified that the absorptance is independent of polarization through the incidence of different polarization angles. The influence of the geometrical parameters of the structure on the absorber is discussed. This kind of tunable dual broadband perfect absorber has considerable application prospects in the field of THz technology.

2. Materials and Methods The design structure of the absorber is shown in Figure1. It consists of a three-layer structure: VO2 on the top layer, a dielectric layer in the middle layer and a metal substrate on the bottom layer. The period of the structural unit is p = 30 µm. The top layer is composed of VO2 with a thickness of 0.08 µm. As is shown in Figure1b, the pattern is composed of two perpendicularly intersecting ellipses and a circular hole in the center. The lengths of the semi-major axis and semi-minor axis of the ellipse and the radius of the inner circle are a = 14 µm, b = 9 µm and c = 6 µm, respectively. The dielectric constant of VO2 can be expressed by the Drude model [28], which is:

2 ωp(σ) ε(ω) = ε (1) ∞ − (ω2 + iγω) where ε and γ are 12 and 5.75 1013 rad/s, respectively. The relationship between plasma frequency ∞ × and conductivity is: 2 σ 2 ωp(σ) = ωp(σ0) (2) σ0 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Appl. Sci. 2020, 10, 7259 3 of 9

() = () (2) 5 15 with σ0 = 3 10 S/m and ωp(σ0) = 1.4 10 rad/s. The middle layer is silicon dioxide (SiO2) with × 5 × 15 awith thickness = 3 of× 10 12 µS/mm and and a () = 1.4 × ( ε10) of rad/s. 3.8 [29 The]. The middle bottom layer layer is silicon is composed dioxideof (SiO gold2) with with a athickness thickness of of 12 0.2 μmµm and and a permittivity a conductivity (ε) ofof 4.563.8 [29].10 The7 S/ m.bottom As shown layer is in composed Figure1a, of the gold incident with a × wavethickness propagates of 0.2 μ alongm and the a conductivityZ-axis from topof 4.56 to bottom, × 107 S/m. and As the shown electric in field Figure polarization 1a, the incident direction is alongpropagates the X-axis. along Thethe spectralZ-axis from response top to ofbottom, the structure and the is electric calculated field bypolarization the finite elementdirection method. is along Periodicthe X-axis. boundary The spectral conditions response are of applied the structure in the X is and calculated Y directions, by theand finite a perfectelement matching method. Periodic layer is appliedboundary in theconditions Z direction. are applied The absorptance in the X and can Y be dire calculatedctions, and by a calculating perfect matching the reflection layer is coe appliedfficient in (Sthe11) Z and direction. the transmission The absorptance coefficient can (S be21 ).calculat The expressioned by calculating of absorptance the reflection is: coefficient (S11) and the transmission coefficient (21). The expression of absorptance is: 2 2 A(ω) = 1 R(ω) T(ω) = 1 S11(ω) S21(ω) (3) () =1−− (−) −() =1−− |()−| − |()| (3) duedue toto thethe thickness thickness ofof metalmetal isis significantlysignificantly largerlargerthan than thetheskin skindepth depth toto ensureensure nono transmission,transmission, T((ω))= = 0.0. TheThe absorptanceabsorptance herehere cancan bebe expressedexpressed as:as:

() =1−() =1− | () 2 | (ω) = 1 R(ω) = 1 S11(ω) (4)(4) − −

FigureFigure 1. 1.( (aa)) Three-dimensional Three-dimensional schematic schematic of of the the absorber. absorber. ( b(b)) Top Top view view of of the the unit unit cell. cell. 3. Results 3. Results The absorption and reflection spectrum of the absorber when VO2 patterns are in the The absorption spectrum and reflection spectrum of the absorber when VO2 patterns are in the metal phase are shown in Figure2a. The results show that there are two absorption bandwidths of 90% metal phase are shown in Figure 2a. The results show that there are two absorption bandwidths of absorptance reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz 90% absorptance reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 under normal incidence. Moreover, there are also four perfect absorption peaks located at 2.3 THz, THz under normal incidence. Moreover, there are also four perfect absorption peaks located at 2.3 3.6 THz, 8.8 THz and 10.4 THz, respectively. Due to the influence of the polarization angle is one of THz, 3.6 THz, 8.8 THz and 10.4 THz, respectively. Due to the influence of the polarization angle is the most important properties of the absorber, we simulated the absorption spectrum under different one of the most important properties of the absorber, we simulated the absorption spectrum under polarization angles. Figure2b is the color map under di fferent polarization angles. Obviously, due to different polarization angles. Figure 2b is the color map under different polarization angles. the symmetry of the structure, polarization is insensitive. Obviously, due to the symmetry of the structure, polarization is insensitive. Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2020, 10, 7259 4 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 9

Figure 2. When the conductivity of VO2 is 2 × 105 S/m. (a) The reflection and absorption spectra of the

dual broadband absorber. (b) Color map of absorption5 spectra with different polarization angles. Figure 2. When the conductivity of VO2 is 2 10 S/m. (a) The reflection and absorption spectra of the Figure 2. When the conductivity of VO2 is 2× × 105 S/m. (a) The reflection and absorption spectra of the dual broadband absorber. (b) Color map of absorption spectra with different polarization angles. Thedual conductivitybroadband absorber. of VO (b2 ) canColor be map controlled of absorption by spectraadjusting with the different temperature. polarization The angles. change of

2 conductivityThe conductivity can change of VOthe2 dielectriccan be controlled constant by of adjusting VO . Therefore, the temperature. the absorptance The change of the of absorber conductivity can 2 The conductivity of VO can be controlled by adjusting the temperature.2 The change of becan continuously change the dielectric tuned by constant changing of VOthe2 te. Therefore,mperature. the When absorptance the conductivity of the absorber of VO can changes be continuously from 200 conductivity5 can change the dielectric constant of VO2. Therefore, the absorptance of the absorber5 can S/mtuned to by2 × changing 10 S/m, we the can temperature. tune the absorption When the conductivityfrom 2% to 94% of VO at 2thechanges corresponding from 200 Sfrequency/m to 2 10 range,S/m, be continuously tuned by changing the temperature. When the conductivity of VO2 changes× from 200 andwe canthe tunecentral the position absorption of the from peak 2% keeps to 94% unchange at the correspondingd, as shown in frequencyFigure 3. When range, the and conductivity the central 5 S/m to 52 × 10 S/m, we can tune the absorption from 2% to 94% at the corresponding frequency range, isposition 2 × 10 S/m, of the VO peak2 appears keeps as unchanged, a metallic asphase, shown and in the Figure best 3performance. When the conductivityof the absorber is 2is achieved.105 S/m, and the central position of the peak keeps unchanged, as shown in Figure2 3. When the conductivity× TheVO2 absorptionappears as apeaks metallic that phase, appear and in thethe best two performance absorption ofbands the absorberof VO at is low achieved. conductivity The absorption can be is 2 × 105 S/m, VO2 appears as a metallic phase, and the best performance of the absorber is achieved. explainedpeaks that by appear wave in interference the two absorption theory. The bands two of absorption VO2 at low peaks conductivity in Figure can 3b be are explained at 3.21 THz by wave and The absorption peaks that appear in the two absorption bands of VO2 at low conductivity can be 9.63interference THz, while theory. the corresponding The two absorption peaks in Figure 3areb are93 atμm 3.21 and THz 31 μ andm, respectively. 9.63 THz, while The explained by theory. The two absorption peaks in Figure 3b are at 3.21 THz and refractivethe corresponding index in the vacuum dielectric wavelengths layer is √ are, and 93 theµm corresponding and 31 µm, respectively. wavelengths The in the refractive dielectric index layer in 9.63 THz, while the corresponding vacuum wavelengths are 93 μm and 31 μm, respectively. The arethe 48 dielectric μm and layer16 μm. is The√ε, thickness and the corresponding of the dielectric wavelengths layer is d = 12 in μ them, dielectricand the length layer is are precisely 48 µm andthe in the dielectric layer is √, and the corresponding wavelengths in the dielectric layer corresponding16 µm. The thickness 1/4 and of 3/4 the wavelengths, dielectric layer which is d =meet12 µs m,the and destructive the length interference is precisely condition the corresponding between are 48 μm and 16 μm. The thickness of the dielectric layer is d = 12 μm, and the length is precisely the reflection1/4 and 3/ 4and wavelengths, incidence. When which the meets structural the destructive phase of VO interference2 is not a metallic condition phase, between the position reflection of andthe corresponding 1/4 and 3/4 wavelengths, which meets the destructive interference condition2 between absorptionincidence. Whenpeak is the determined structural phaseby the of thicknes VO2 iss not of athe metallic dielectric phase, layer. the The position presence of the of absorption VO is to tune peak 2 reflection and incidence. When the structural phase of VO is not a metallic phase, the position of the2 theis determined amplitude byof theabsorption thickness peaks. of the As dielectric the conductivity layer. The increases, presence ofthe VO dielectric2 is to tune constant the amplitude of VO absorption peak is determined by the thickness of the dielectric layer. The presence of VO2 is to tune changes,of absorption and peaks.the insulating As the conductivityphase gradually increases, approaches the dielectric the metal constant phase, of VOand2 changes,the absorptance and the the amplitude of absorption peaks. As the conductivity increases, the dielectric constant of VO2 graduallyinsulating increases. phase gradually approaches the metal phase, and the absorptance gradually increases. changes, and the insulating phase gradually approaches the metal phase, and the absorptance gradually increases.

FigureFigure 3. ( 3.a)( Reflectiona) Reflection and and (b) ( babsorption) absorption spect spectrara with with different different conductivity conductivity of of VO VO2. 2.

4. Discussion 4. DiscussionFigure 3. (a) Reflection and (b) absorption spectra with different conductivity of VO2. Impedance matching theory can be used to explain the phenomenon of perfect absorption. Impedance matching theory can be used to explain the phenomenon of perfect absorption. The The4. Discussion effective impedance of the MPA can be expressed as [30]: effective impedance of the MPA can be expressed as [30]: Impedance matching theory can be used to explain the phenomenon of perfect absorption. The s uv t 2 2 effective impedance of the MPA can be expressedµe f f as(1( [30]:1 + S 11)) −S Z = = == − 21 (5)(5) e f f 2 εe f f (1 − ) −2 (1 S11) S (1− + ) −−21 = = (5) (1 − ) − Appl. Sci. 2020, 10, 7259 5 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 9 wherewhere µe f f andand εe f f are are the the e effectiveffective permeability permeability andand eeffectffectiveive permittivity,permittivity, respectively.respectively. It is well well knownknown that that impedance impedance matching matching can can be be achieved as both of them reach the same. The The real real and and imaginaryimaginary parts parts of of the the impedance impedance are are derived derived from from the the SS parameter.parameter. As As is is shown shown in in Figure Figure 44,, whenwhen 5 thethe VO VO22 conductivityconductivity is is 2 2 × 10105 S/m,S/m, the the real real parts parts of of the the impedance impedance are are close close to to 1, 1, and and the the imaginary imaginary × partsparts of of the the impedance impedance are are close close to to 0 0 in in the the frequency frequency range range from from 1.87 1.87 to to 4.19 4.19 THz THz and and from from 8.70 8.70 to to 10.7310.73 THz. This means that the impedanceimpedance ofof thethe absorberabsorber gradually gradually matches matches with with free free space, space, which which is isthe the typical typical perfect perfect absorber absorber feature feature [27 [27].].

5 Figure 4. The relative impedance with the conductivities of VO2 isis 2 2 × 10105 S/m.S/m. 2 × ToTo further understand the the physical physical mechanism mechanism of of the the absorber, absorber, we we conducted an an electric electric field field analysis.analysis. Figure Figure 5 shows the electric fieldfield distributiondistribution at the four peaks.peaks. The firstfirst resonance peakpeak is is located located at at 2.3 2.3 THz, THz, as as shown shown in in Figure Figure 55a;a; thethe electricelectric fieldfield isis mainlymainly concentratedconcentrated in the gap betweenbetween the long axis ofof thethe ellipseellipse ofof thethe twotwo structural structural units. units. The The second second resonance resonance peak peak is is located located at at3.6 3.6 THz, THz, as shownas shown in Figurein Figure5b; the5b; electricthe electric field isfiel mainlyd is mainly concentrated concentrated at the at long the axis long of axis the ellipseof the ellipseand the and edge the of edge the innerof the ringinner of ring the of unit the structure. unit structure. The combination The combination of the of overlapping the overlapping regions regions of the oftwo the resonance two resonance spectra spectra provides provides a guarantee a guarantee for high-e for high-efficiencyfficiency broadband broadband absorption absorption [31]. The[31]. third The thirdresonance resonance peak ispeak located is located at 8.8 THz, at as8.8 shown THz, inas Figureshown5c; in the Figure electric 5c; field the is electric mainly concentratedfield is mainly in concentratedthe inner ring in and the the inner junction ring of and the twothe perpendicularjunction of the ellipses. two perpendicular The fourth resonance ellipses. peak The is locatedfourth resonanceat 10.4 THz, peak as is shown located in at Figure 10.4 THz,5d; the as electricshown in field Figure is mainly 5d; the concentrated electric field is at mainly the junction concentrated of two atellipses the junction and the of gap two area ellipses of the and corresponding the gap area unit. of the Both corresponding resonance spectra unit. provideBoth resonance the basis spectra for the providesecond broadbandthe basis for absorption. the second Duebroadband to the symmetry absorption. of Due the structure,to the symmetry similar of phenomena the structure, also similar apply phenomenawhen the structure also apply is under when the the Y-polarized structure is incident under the wave. Y-polarized incident wave. Appl. Sci. 2020, 10, 7259 6 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 9

Figure 5. TheThe electric electric field field distribution distribution in in the the top top view view at 2.3 at2.3 THz THz (a),( 3.6a), 3.6THz THz (b), (8.8b), THz 8.8 THz (c) and (c) 10.4 and 10.4THz THz(d). (d).

To illustrateillustrate the the importance importance of geometricof geometric structure structure parameters, parameters, it is necessary it is necessary to verify to the verify influence the ofinfluence geometric of geometric structureparameters structure parameters on the absorber. on the Figure absorber.6 shows Figure the influence6 shows the of the influence major axis of the of themajor ellipse axis aof, thethe minorellipse axis a, the of theminor ellipse axisb ,of the the radius ellipse of b the, the inner radius circle of ctheand inner the thicknesscircle c and of the mediumthickness dof on the the medium absorptance d on the of absorptance the absorber. of th Ase shownabsorber. in As Figure shown6a, in when Figure the 6a, major when axis the of major the ellipseaxis of the becomes ellipse longer, becomes the longer, absorption the absorption bandwidth bandwidth becomes wider; becomes however, wider; thehowever, absorption the absorption efficiency decreases.efficiency decreases. When the When major the axis major of the axis ellipse of the becomes ellipse shorter,becomes the shorter, absorption the absorption bandwidth bandwidth becomes narrower,becomes narrower, but the absorption but the absorption becomes higher. becomes In Figurehigher.6b, In it Figure is obvious 6b, it that is obvious the minor that axis the of minor the ellipse axis hasof the the ellipse same changinghas the same law. changing In Figure 6law.c, conversely, In Figure when6c, conversely, the radius wh of theen the inner radius circle of becomes the inner shorter, circle thebecomes absorption shorter, bandwidth the absorption becomes bandwidth wider, and becomes the absorption wider, and effi theciency absorption decreases. efficiency When decreases. the radius ofWhen the innerthe radius circle of becomes the inner longer, circle the becomes absorption longer, bandwidth the absorption becomes bandwidth narrower, becomes and the absorptionnarrower, becomesand the absorption higher. The becomes position higher. of the resonance The position peak of varies the resonance with the changepeak varies of the with major the axis change and minorof the axismajor of theaxis ellipse and minor or the radiusaxis of of the the ellipse inner circle. or the The radius absorptance of the andinner bandwidth circle. The will absorptance change with and the changebandwidth of the will resonance change with peak the position. change Asof the shown resonance in Figure peak6d, position. when the As thickness shown in of Figure SiO 2 increases,6d, when the absorptionthickness of peak SiO2gradually increases, shiftsthe absorption to low frequencies. peak gradually At the shifts same to time, low the frequencies. number of At absorption the same peakstime, the increases number and of absorption the bandwidth peaks narrows. increases When and the bandwidth thickness of narrows. SiO2 decreases, When the the thickness absorption of peakSiO2 decreases, gradually shiftsthe absorption to high frequency, peak gradually and the shifts bandwidth to high becomes frequency, larger. and However,the bandwidth the number becomes of absorptionlarger. However, peaks andthe number the absorptance of absorption decrease. peaks The and position the absorptance of the resonance decrease. peak The is apositionffected byof the thicknessresonance of peak the medium.is affected The by position the thickness of the resonanceof the medium. peak and The the position absorptance of the ofresonance the entire peak absorber and arethe aabsorptanceffected by the of the SiO entire2 and VOabsorber2. VO 2areand affected SiO2 determine by the SiO the2 and impedance VO2. VO2 ofand the SiO proposed2 determine system the andimpedance have a significantof the proposed influence system on the absorptanceand have a and significant bandwidth. influence The results on the further absorptance confirmed and the importantbandwidth. role The of results geometrical further structure confirmed parameters the import inant absorber role of geometrical performance. structure After optimizing parameters the in structuralabsorber performance. parameters, aAfter high-performance optimizing the tunable structural dual parameters, broadband a terahertz high-performance absorber is tunable obtained. dual broadband terahertz absorber is obtained. Appl. Sci. 2020, 10, 7259 7 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 9

5 Figure 6. When σ is 2 10 5 S/m, the absorption spectrum changes with the major axis of the ellipse (a), Figure 6. When is 2 ×× 10 S/m, the absorption spectrum changes with the major axis of the ellipse (thea), the minor minor axis axis of the of the ellipse ellipse (b), ( theb), the radius radius of the of the inner inner circle circle (c) and(c) and the thicknessthe thickness of SiO of 2SiO(d).2 (d). 5. Conclusions 5. Conclusions In conclusion, we propose a tunable dual broadband THz . Two VO2 ellipses In conclusion, we propose a tunable dual broadband THz metamaterial absorber. Two VO2 that intersect perpendicularly with a circular hole dug out of the center are used as the top structure, ellipses that intersect perpendicularly with a circular hole dug out of the center are used as the top the middle layer is SiO2, and the top layer is gold. Based on interference theory and impedance structure, the middle layer is SiO2, and the top layer is gold. Based on interference theory and matching theory, we obtained a tunable absorber with which two bandwidths of 90% absorption reach impedance matching theory, we obtained a tunable absorber with which two bandwidths of 90% as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz under normal incidence. absorption reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz By changing the conductivity of the VO2, dynamic absorption control from 2% to 94% can be achieved. under normal incidence. By changing the conductivity of the VO2, dynamic absorption control from Moreover, due to the symmetry of the structure, the absorptance is insensitive to the polarization angle. 2% to 94% can be achieved. Moreover, due to the symmetry of the structure, the absorptance is We envision that our high-performance, tunable, dual broadband absorber may have potential in a insensitive to the polarization angle. We envision that our high-performance, tunable, dual vast range of applications in imaging, modulation and stealth technology. broadband absorber may have potential in a vast range of applications in imaging, modulation and stealthAuthor technology. Contributions: Conceptualization, X.-F.J.; formal analysis, X.-F.J.; visualization, X.-F.J.; writing, original draft preparation, X.-F.J.; writing, review and editing, X.-F.J., T.L. and Z.-H.Z.; supervision, G.-F.S. and Y.X. All of Authorthe authors Contributions: discussedthe Conceptualization, results and commented X.-F.J.; form on theal analysis, manuscript. X.-F.J.; All visualizatio authors haven, X.-F.J.; read and writing, agreed original to the published version of the manuscript. draft preparation, X.-F.J.; writing, review and editing, X.-F.J., T.L. and Z.-H.Z.; supervision, G.-F.S. and Y.X. All ofFunding: the authorsThis discussed work was the supported results an byd commented National Key on the Research manuscr andipt. Development All authors have Plan re (No.2016YFB0402402).ad and agreed to the The Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB43010000. National Key published version of the manuscript. Research and Development Plan (No.2016YFB0400601). National Basic Research Program of China (973 Program) Funding:(No. 2015CB351902). This work Nationalwas supported Science andby National Technology Key Major Research Project and (2018ZX01005101-010). Development Plan National(No.2016YFB0402402) Natural Science. Foundation of China (Grant No. 61835011). National Natural Science Foundation of China (Grant No. U1431231). theKey Strategic Research Priority Projects Research of the FrontierProgram Science of the Chin of theese Chinese Academy Academy of Sciences, of Sciences Grant No. (No. XDB43010000. QYZDY-SSW-JSC004). National KeyBeijing Research Science and and Development Technology Projects Plan (No.2016YFB0400601). (Grant No. Z151100001615042). National Basic Research Program of China (973 Program)Conflicts of(No. Interest: 2015CB351902).The authors National declare Science no conflict and Technology of interest. Major Project (2018ZX01005101-010). National Natural Science Foundation of China (Grant No. 61835011). National Natural Science Foundation of China (Grant No. U1431231). Key Research Projects of the Frontier Science of the Chinese Academy of Sciences (No. QYZDY-SSW-JSC004). Beijing Science and Technology Projects (Grant No. Z151100001615042).

Conflicts of Interest: The authors declare no conflict of interest.

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