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Article Reconfigurable Terahertz Using Split-Ring Meta- with Multifunctional Electromagnetic Characteristics

Yuhang Liao and Yu-Sheng Lin * State Key Laboratory of Optoelectronic Materials and Technologies, School of and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China; [email protected] * Correspondence: [email protected]; Tel.: +86-188-0205-4660

 Received: 1 July 2020; Accepted: 29 July 2020; Published: 30 July 2020 

Abstract: We propose a reconfigurable terahertz (THz) metamaterial (RTM) to investigate its multifunctional electromagnetic characteristics by moving the meta-atoms of split-ring (SRR) array. It shows the preferable and capable adjustability in the THz range. The electromagnetic characteristics of the proposed RTM device are compared and analyzed by moving the meta-atoms in different polarized transverse magnetic (TM) and transverse electric (TE) modes. The symmetrical meta-atoms of RTM device exhibit a resonant tuning range of several tens of GHz and the asymmetrical meta-atoms of RTM device exhibit the better tunability. Therefore, an RTM device with reconfigurable meta-atoms possesses the shifting, switching, electromagnetically induced transparency (EIT) switching and multiband to single-band switching characteristics. This proposed RTM device provides the potential possibilities for the use of THz-wave optoelectronics with tunable resonance, EIT analog and tunable multiresonance characteristics.

Keywords: metamaterial; terahertz; optical switching; electromagnetically induced transparency; resonator

1. Introduction In the last two decades, there has been increasing interest in terahertz (THz) metamaterial spanning the range of 0.1 THz to 10 THz [1,2]. are not natural materials. It means that they are artificially designed [3] and engineered to span the whole , including visible [4–6], [7–12], THz [13–23] and [24]. Metamaterials possess many extraordinary and useful optical properties such as negative [25], metalensing [26], cloaking [27] and perfect absorbance [28], etc. Many studies have reported about THz metamaterials being used in sensors, filters and absorbers [28–32]. Among these THz metamaterials, the split-ring resonator (SRR) is a simple plane structure that possesses unique electric and magnetic characteristics. Therefore, there have been many diversified metamaterials developed in the last few years [4–23], such as U-shaped SRR [33], three-dimensional (3D) SRRs [34], C-shaped SRRs [35], V-shaped SRRs [10], spiral-shaped SRRs [23] and H-shaped SRRs [36], etc. The capability of actively tunable metamaterials plays an important role in exploring flexible and applicable electromagnetic characteristics [35–37]. There have been several methods reported to achieve tunable metamaterials. These include—but are not limited to—electrical and magnetic driving, optical pumping, thermal tuning and mechanical control [21–23]. Through these controllable methods, the geometric morphology of the metamaterials can be reconfigured by curling, rotating and reshaping the metamaterial unit cell. Therefore, we can control numerous electromagnetic properties of metamaterials actively, such as resonant frequency, electromagnetically induced transparency (EIT) analog and multiband switching [14–19].

Appl. Sci. 2020, 10, 5267; doi:10.3390/app10155267 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 9

Appl. Sci. 2020, 10, 5267 2 of 9 properties of metamaterials actively, such as resonant frequency, electromagnetically induced transparency (EIT) analog and multiband switching [14–19]. InIn this this study, study, we we propose propose and and investigate investigate a a reconfigurable reconfigurable THz THz metamaterial metamaterial (RTM) (RTM) with with multifunctionalmultifunctional electromagnetic electromagnetic characteristics characteristics by moving by moving the meta-atoms the meta- ofatoms SRR array, of SRR which array, exhibits which theexhibits active tunabilitythe active intunability the THz in frequency the THz range.frequency By moving range. By the moving inner arrangement the inner arrangement of the SRR array, of the theSRR transmission array, the transmission spectra show spectra the tunable show the tunable resonances and the switchable and the switchable EIT analog. EIT analog. This RTM This designRTM design opens theopens door the todoor the to use the of use THz-wave of THz-wave optoelectronics optoelectronics in filters, in filter polarizers,s, polarizer switchess, and and sensorsensor applications. applications.

2.2 Design. Design and and Method Method FigureFigure1a,b 1a shows,b shows the the schematic schematic drawings drawings of the of proposed the proposed RTM RTM device device configured configure withd four with SRR four meta-atomsSRR meta-atoms and the and geometrical the geometrical denotations denotations of the of RTM the unitRTM cell, unit respectively. cell, respectively. The thickness The thickness of the of SRRthe meta-atomsSRR meta-atoms is 300 is nm300 onnm silicon on silicon on on insulator (SOI) (SOI) substrate. substrate. The The permittivities of of Au Au and and Si Si materialsmaterials are are assumed assumed as as constants constants in in this this study. study. They They are are 10 104 for4 for Au Au layer layer and and 10 10 for for Si Si substrate, substrate, respectivelyrespectively [37 [37].]. For For a convenienta convenient description, description, we we denote denote these these four four SRR SRR meta-atoms meta-atoms as SRR-1,as SRR SRR-2,-1, SRR- SRR-32, SRR and-3 and SRR-4. SRR The-4. The line line width width of eachof each SRR SRR meta- meta-atom is 6is µ6m μm and and the the period period of of SRR SRR array array is is 2 100100 ×100 100µ mμm(2P (xPx ×P Pyy).). TheThe lengthlength ((LL)) and and width width ( W(W)) of of each each SRR SRR meta-atom meta-atom are are 30 30µ m.μm. The The distance distance × × betweenbetween the the SRR SRR meta-atoms meta-atoms (G ()G is) is 2 µ2 m.μm. The The coordinates coordinates of of the the incident incident electromagnetic electromagnetic wave wave in in transversetransverse electric electric (TE) (TE) and and transverse transverse magnetic magnetic (TM) (TM) modes modes are are also also illustrated illustrated in Figure in Figure1a, where 1a, whereE, HEand, H andk are k theare the electric electric field, field, magnetic magnetic field field and and wave wave vector vector of of the the incident incident electromagnetic electromagnetic wave, wave, respectively.respectively. In In this this study, study, each each SRR SRR meta-atom meta-atom is is designed designed to to move move along along the thex -x or- ory-axis y-axis direction direction andand to to investigate investigate the the corresponding corresponding electromagnetic electromagnetic responses. responses. We We define define each each SRR SRR meta-atom meta-atom that that hashas its its own own reference reference position position denoted denoted as as ( x(xi,i,y yi),i), wherewherei i isis eacheach SRRSRR meta-atom,meta-atom, i.e.,i.e., ii = 1,1, 2, 2, 3 3 and and 4, 4,as as shown shown in in Figure Figure 1b1b.. The The center center points points of SRR of SRR meta meta-atoms-atoms are arethe theinitial initial reference reference positions positions whose whosevariations variations represent represent the movement the movement of each of SRR each meta SRR-atom. meta-atom. The initial The reference initial reference positions positions are (x1, y1) are= ( (−x16,1, y 16),1) = (x(2, 16,y2) = 16), (16, ( x16),2, y (2x)3,= y3(16,) = (− 16),16, − (x16)3, yand3) = (x(4, 16,y4) = (16,16) and−16), ( xrespectively.4, y4) = (16, The16), electromagnetic respectively. − − − − Thecharacteristics electromagnetic of SRR characteristics array will ofbe SRRinvestigated array will by be mov investigateding SRR meta by moving-atoms SRR to six meta-atoms different ( tox2 six, x4), di(ffyerent1, y2), (xy22,, yx4),), x (2y,1 (,xy2,2 ),x3 ()y and2, y4 (),y2x, 2y,(3)x positions.2, x3) and (y2, y3) positions.

y W (a) (b) THz （x , y ） （x , y ） 1 1 2 2 L x (E) ① ② y (H) G x z (k) Au O Py ③ ④ Si （x3, y3） （x4, y4） unit cell G Px

FigureFigure 1. 1.(a ()a Schematic) Schematic drawing drawing of of the the proposed proposed reconfigurable reconfigurable terahertz terahertz (THz) (THz) metamaterial metamaterial (RTM) (RTM) devicedevice with with four four split-ring split-ring resonator resonator (SRR) (SRR) meta-atoms; meta-atoms (;b ()b The) The geometric geometric parameters parameters of of four four SRR SRR meta-atoms.meta-atoms. The The geometric geometric dimensions dimensions are: areW: =W30 = 30µm, μm,L = L30 = 30µm, μm,G = G2 =µ 2m μm and andPx =PxP =y P=y 100= 100µm. μm. The The referencereference points points are are (x (,xy1, y)1=) =( (−16,16, 16), 16), ( x(x2,,y y2)) == (16, 16), ( x3,, yy3)) == (−(16,16, −16)16) and and (x (4x, y,4)y =) (16,= (16, −16).16). 1 1 − 2 2 3 3 − − 4 4 − InIn simulations, simulations, the the incident incident electromagnetic electromagnetic wave wave is a is plane a plane wave, wave, whose whose propagation propagation direction direction is perpendicularis perpendicular to the to thx-ye xplane.-y plane. The The boundary boundary conditions conditions are are set set periodic periodic for forx- yx-planey plane and and perfectly perfectly matchedmatched for for the thez plane.z plane. The The electromagnetic electromagnetic filed filed monitor monitor is is set set on on the the bottom bottom of of the the substrate substrate to to

Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 Appl. Sci. 2020, 10, 5267 3 of 9 calculate the transmission of the incident THz wave. The resonances of the engineered SRR meta- atoms can be expressed by [13] calculate the transmission of the incident THz wave. The resonances of the engineered SRR meta-atoms can be expressed by [13] 푐 퐺 0 r 푓퐿퐶 = ( )!√ (1) 2휋푃c0√휀 푊G fLC = 푠 (1) 2πP √εs W where 푐0 represents the velocity of in a vacuum, P is the period of SRR array (Px and Py in this where c0 represents the velocity of light in a vacuum, P is the period of SRR array (Px and Py in this study), εs is the relative of the materials in the SRR array, G is the gap width and W is the study), ε is the relative permittivity of the materials in the SRR array, G is the gap width and W is the width of sthe SRR meta-atom. width of the SRR meta-atom. 3. Results and Discussions 3. Results and Discussions Figure 2 presents the simulated transmission spectra of the RTM device obtained by moving the Figure2 presents the simulated transmission spectra of the RTM device obtained by moving the SRR-2 and SRR-4 meta-atoms along the positive x-axis direction in TE and TM modes. The values of SRR-2 and SRR-4 meta-atoms along the positive x-axis direction in TE and TM modes. The values y2 and y4 are kept at the initial positions. It has two resonances at 0.49 THz and 0.92 THz for the initial of y2 and y4 are kept at the initial positions. It has two resonances at 0.49 THz and 0.92 THz for the state (x2, x4) = (16, 16) in TE mode, which is shown as the black curve in Figure 2a. By moving the initial state (x2, x4) = (16, 16) in TE mode, which is shown as the black curve in Figure2a. By moving position of (x2, x4) from (16, 16) to (32, 32) along the positive x-axis direction in TE mode, the first the position of (x2, x4) from (16, 16) to (32, 32) along the positive x-axis direction in TE mode, the first resonance is blue-shifted from 0.49 THz to 0.54 THz. The tuning range is 50 GHz. The second resonance is blue-shifted from 0.49 THz to 0.54 THz. The tuning range is 50 GHz. The second resonance resonance is blue-shifted from 0.92 THz to 0.96 THz with a tuning range of 40 GHz. In TM mode, is blue-shifted from 0.92 THz to 0.96 THz with a tuning range of 40 GHz. In TM mode, there are three there are three initial resonances at 0.50 THz, 0.67 THz and 0.91 THz as shown by the black curve in initial resonances at 0.50 THz, 0.67 THz and 0.91 THz as shown by the black curve in Figure2b. The first Figure 2b. The first resonance is slightly red-shifted from 0.50 THz to 0.48 THz, the second resonance resonance is slightly red-shifted from 0.50 THz to 0.48 THz, the second resonance is red-shifted from is red-shifted from 0.67 THz to 0.65 THz, and the third resonance is red-shifted from 0.91 THz to 0.89 0.67 THz to 0.65 THz, and the third resonance is red-shifted from 0.91 THz to 0.89 THz. These results THz. These results indicate that the reconfigured SRR meta-atoms have minor THz tunability in TE indicate that the reconfigured SRR meta-atoms have minor THz tunability in TE and TM modes. and TM modes. In order to investigate the influence of moving the SRR-1 and SRR-2 meta-atoms, the values of x1 In order to investigate the influence of moving the SRR-1 and SRR-2 meta-atoms, the values of and x2 are kept at the initial positions while y1 and y2 are changed. The simulated transmission spectra x1 and x2 are kept at the initial positions while y1 and y2 are changed. The simulated transmission are obtained by moving the SRR-1 and SRR-2 meta-atoms along positive y-axis direction in TE and TM spectra are obtained by moving the SRR-1 and SRR-2 meta-atoms along positive y-axis direction in modes as shown in Figure3a,b, respectively. In TE mode, there are two initial resonances at 0.49 THz TE and TM modes as shown in Figure 3a,b, respectively. In TE mode, there are two initial resonances and 0.92 THz. When (y1, y2) moves to (32, 32), the resonances are almost identical. It is because the at 0.49 THz and 0.92 THz. When (y1, y2) moves to (32, 32), the resonances are almost identical. It is TE-field coupling effect within the inner SRR meta-atoms has no change along x-axis direction. In TM because the TE-field coupling effect within the inner SRR meta-atoms has no change along x-axis mode, there are three resonances at 0.50 THz, 0.67 THz and 0.91 THz for the initial state of (y1, y2) = (16, direction. In TM mode, there are three resonances at 0.50 THz, 0.67 THz and 0.91 THz for the initial 16). By moving the (y1, y2) position to above (20, 20), the first and third resonances vanish gradually, state of (y1, y2) = (16, 16). By moving the (y1, y2) position to above (20, 20), the first and third resonances and the second resonance shifts to 0.72 THz for (y1, y2) = (32, 32). These properties are suitable for the vanish gradually, and the second resonance shifts to 0.72 THz for (y1, y2) = (32, 32). These properties metamaterial to be used in the tunable polarized multiresonance application. are suitable for the metamaterial to be used in the tunable polarized multiresonance application.

FigureFigure 2. 2. SimulatedSimulated transmission transmission spectra spectra of of the the RTM device obtained by moving thethe SRR-2SRR-2 andand SRR-4SRR- 4meta-atoms meta-atoms along along the the positive positixve-axis x-axis direction direction in (a ) in transverse (a) transverse electric electric (TE) and (TE) (b) transverseand (b) transverse magnetic magnetic(TM) modes. (TM) modes.

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Figure 3. Simulated transmission spectra of the RTM device obtained by moving the SRR-1 and SRR- FigureFigure2 meta 3.-3.atoms Simulated along transmissiontransmissionpositive y-axis spectraspectra direction ofof thethe in RTMRTM(a) TE device device and ( obtainedb obtained) TM modes. by by moving moving the the SRR-1 SRR-1 and and SRR-2 SRR- meta-atoms2 meta-atoms along along positive positivey-axis y-axis direction direction in in (a )(a TE) TE and and (b )(b TM) TM modes. modes. Figure 4 shows the simulated transmission spectra of the RTM device obtained by moving the Figure4 shows the simulated transmission spectra of the RTM device obtained by moving the SRR-Figure2 and SRR4 shows-4 meta the-atoms simulated along transmission the positive spectray-axis direction of the RTM in TE device and TMobtained modes. by The moving x2 and the x4 SRR-2values and are SRR-4set to the meta-atoms initial positions, along the i.e. positive, (16, 16).y -axisThe y direction2 and y4 values in TE are and moved TM modes. from 16 The μmx2 toand 32 SRR-2 and SRR-4 meta-atoms along the positive y-axis direction in TE and TM modes. The x2 and x4 xμm4 values and from are set −16 to μm the to initial 0 μm, positions, respectively. i.e., (16,When 16). the The SRRy2-2and andy SRR4 values-4 meta are-atoms moved are from moved 16 µ mfrom to values are set to the initial positions, i.e., (16, 16). The y2 and y4 values are moved from 16 μm to 32 32theµ mposition and from (16, −1616)µ tom (20, to 0 −µ12),m, respectively.a sharp resonant When peak the at SRR-2 the first and resonance SRR-4 meta-atoms appears, which are moved is the μm and from −16− μm to 0 μm, respectively. When the SRR-2 and SRR-4 meta-atoms are moved from frombright the-mode position resonance (16, 16) in to the (20, EIT12), analog a sharp owing resonant to the peak stronger at the electromagnetic first resonance appears, coupling which is the position (16, −16) −to (20, −12),− a sharp resonant peak at the first resonance appears, which is the the bright-mode resonance in the EIT analog owing to the stronger electromagnetic coupling energy brightbetween-mode the asymmetrical resonance in SRRs. the EIT In TE analog mode, owing there toare the two stronger resonant electromagnetic modes of the RTM coupling device, energy which between the asymmetrical SRRs. In TE mode, there are two resonant modes of the RTM device, betweenare broadband the asymmetrical resonance generatedSRRs. In TE by mode, the localized there are surface two resonant plasmons modes within of the SRR RTM m etadevice,-atoms which and which are broadband resonance generated by the localized surface plasmons within SRR meta-atoms arethe broadbandnarrowband resonance resonance generated produced by by the the localized surface surfacediffraction plasmons wave fromwithin the SRR periodic meta -SRRatoms array. and andtheWhen narrowband the they narrowband are tuned resonance resonance to superimpose, produced produced bythese bythe thetwo surface surface resonances diffraction diffraction will couplewave wave from together from the the periodicand periodic then SRR SRRgenerate array. array. a When they are tuned to superimpose, these two resonances will couple together and then generate WhenFano- resonance.they are tuned Therefore, to superimpose, the RTM these device two exhibit resonancess the will EIT couple analog together characteristic. and then In generate order toa aFanounderstand Fano-resonance.-resonance. the interactions Therefore, Therefore, the between the RTM RTM the device device THz exhibit exhibitswave s and the the the EIT EIT SRR analog array, characteristic. characteristic. an electromagnetic In In order field to to understand the interactions between the THz wave and the SRR array, an electromagnetic field monitor understandmonitor is set the in interactionsthe SRR array. between The corresponding the THz wave electromagnetic and the SRR array,field distributions an electromagnetic are shown field in ismonitorFigure set in 5 the isto set SRRillust in array. therate SRR the The electricarray. corresponding The field corresponding (E-field) electromagnetic and electromagneticmagnetic field field distributions (H field-field), distributions are respectively. shown inare Figure Whenshown5 theto in illustrate the electric field (E-field) and magnetic field (H-field), respectively. When the position of Figureposition 5 ofto (illusty2, y4rate) changes the electric continuously, field (E- thefield) resonance and magnetic is slightly field blue (H--field),shifted respectively. with a tuning When range the of (20y2, GHz.y4) changes Figure continuously,4b shows the transmission the resonance spectra is slightly of the blue-shifted RTM device with in TM a tuning mode. rangeIt is clear of 20 that GHz. the position of (y2, y4) changes continuously, the resonance is slightly blue-shifted with a tuning range of Figure20change GHz.4b has Figure shows no remarkable the4b shows transmission the influence transmission spectra on the of the spectraresonances, RTM of device the which RTM in TM means devi mode.ce that in ItTM the is clearmode. RTM that device It theis clear changehas thata stable has the nochangeperformance remarkable has no by influenceremarkable moving on the influence the SRR resonances,-2 andon the SRR whichresonances,-4 meta means-atoms which that along the means RTM the that devicepositive the hasRTM y- aaxis stabledevice direction performance has a instable TM y byperformancemode. moving the by SRR-2 moving and the SRR-4 SRR meta-atoms-2 and SRR- along4 meta the-atoms positive along-axis the positive direction y in-axis TM direction mode. in TM mode. (a) (b) (a) (b)

TE mode TM mode TE mode TM mode

FigureFigure 4.4. SimulatedSimulated transmissiontransmission spectraspectra of of the the RTM RTM device device obtained obtained by by moving moving the the SRR-2 SRR-2 and and SRR-4 SRR- meta-atoms along positive y-axis direction in (a) TE and (b) TM modes. Figure4 meta -4.atoms Simulate alongd transmissionpositive y-axis spectra direction of the in RTM(a) TE device and (b obtained) TM modes. by moving the SRR-2 and SRR- 4 meta-atoms along positive y-axis direction in (a) TE and (b) TM modes.

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Figure 5. (a,b) E-field and (c,d) H-field distributions of the RTM device with (y2, y4) = (24, −8) μm in TE mode. FigureFigure 5.5. ((aa,,bb)) E-fieldE-field andand ((cc,d,d)) H-fieldH-field distributions distributionsof of the the RTM RTM device device with with ( y(y,2, y y)4)= = (24,(24, −8) µμmm inin 2 4 − TETE mode.mode. Figure 6a,b shows the simulated transmission spectra of RTM device by moving SRR-2 meta- atom along positive x-axis direction in TE and TM modes, respectively. In order to achieve a better FigureFigure6 a,b6a,b shows shows the the simulated simulated transmission transmission spectra spectra of RTM of RTM device device by moving by moving SRR-2 SRR meta-atom-2 meta- understanding of the physical mechanism related to the electromagnetic response, the corresponding alongatom along positive positivex-axis x- directionaxis direction in TE in andTE and TM TM modes, modes, respectively. respectively. In In order order to to achieve achieve aa betterbetter E-field and H-field distributions of SRR-2 meta-atom with x2 = 24 μm in TE mode are shown in Figure understandingunderstanding ofof thethe physical mechanism related to the electromagnetic response, the corresponding 7. In TE mode, there is a resonance at 0.49 THz for the initial stateµ of x2 = 16 μm as shown in Figure E-fieldE-field andand H-fieldH-field distributions distributions of of SRR-2 SRR-2 meta-atom meta-atom with withx2 x=2 =24 24 μmm in in TE TE mode mode are are shown shown in in Figure Figure7. 6a. When x2 value moves to 20 μm, there is an additional EIT response found at 0.45 THz. When SRR- In7. TEIn TE mode, mode, there there is ais resonance a resonance at 0.49at 0.49 THz THz for for the the initial initial state state of x of2 = x216 = µ16m μm as shownas shown in Figurein Figure6a. 2 meta-atom continuously movesµ to 24 μm, 28 μm and 32 μm, there are two EIT resonances at 0.45 When6a. Whenx2 value x2 value moves moves to 20to 20m, μm, there there is anis an additional additional EIT EIT response response found found at at 0.45 0.45 THz. THz. When When SRR-2 SRR- THz and 0.51 THz. There are nonuniform E-field distributions shown in Figure 7a–c. These meta-atom2 meta-atom continuously continuously moves moves to 24to µ24m, μm, 28 µ28m μm and and 32 µ 32m, μm, there there are twoare EITtwo resonancesEIT resonances at 0.45 at THz0.45 unbalanced electromagnetic result in the generations of Fano resonances. In TM mode, there andTHz 0.51 and THz. 0.51 There THz. are There nonuniform are nonuniform E-field distributions E-field distributions shown in Figure shown7 a–c. in Fi Thesegure unbalanced7a–c. These are three resonances at 0.50 THz, 0.67 THz and 0.91 THz as shown in Figure 6b. These three electromagneticunbalanced electromagnetic energies result energies in the result generations in the generations of Fano resonances. of Fano resonances. In TM mode, In TM there mode, are threethere resonances are insensitive to the movement of SRR-2 meta-atom along positive x-axis direction. It resonancesare three resonances at 0.50 THz, at 0.67 0.50 THz THz, and 0.67 0.91 THz THz and as shown0.91 THz in Figure as shown6b. These in Figure three 6 resonancesb. These three are indicates the proposed RTM device exhibits the tunable EIT resonance in TE mode and stable insensitiveresonances toare the insensitive movement to ofthe SRR-2 movement meta-atom of SRR along-2 meta positive-atom alongx-axis positive direction. x-axis It indicates direction. the It resonance in TM mode. proposedindicates RTM the proposed device exhibits RTM the device tunable exhibits EIT resonance the tunable in TE EIT mode resonance and stable in resonance TE mode in and TM mode.stable resonance in TM mode. (a) (b) (a) (b)

TE mode TM mode TE mode TM mode

FigureFigure 6. 6.SimulatedSimulated transmission transmission spectra spectra of the RTMRTM devicedevice obtained obtained by by moving moving the the SRR-2 SRR meta-atom-2 meta- atomalong along positive positivex-axis x-axis direction direction in (a in) TE (a) and TE and (b) TM(b) modes.TM modes. Figure 6. Simulated transmission spectra of the RTM device obtained by moving the SRR-2 meta- atom along positive x-axis direction in (a) TE and (b) TM modes.

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Figure 7. (a–c) E-field and (d–f) H-field distributions of the RTM device with x2 = 24 μm in TE mode.

To control the electromagnetic response of the proposed RTM design, SRR-2 and SRR-3 meta- atoms are moved symmetrically along positive and negative x-axis directions. x2 and x3 values are moved from the position of (16, −16) to (32, −32), while y2 and y3 values are kept at the initial positions. Figure 8 shows the simulated transmission spectra of the RTM device by moving SRR-2 and SRR-3 meta-atoms along positive and negative x-axis directions in TE and TM modes, respectively. In TE mode, when the position of (x2, x3) moves from (16, −16) to (32, −32), the resonances are almost kept constant as shown in Figure 8a. In TM mode, there are three resonances at 0.50 THz, 0.67 THz and 0.91 THz for the initial state of (x2, x3) = (16, −16) as the black curve shown in Figure 8b. By moving x2 and x3 values symmetrically along both x-axis directions from the position of (16, −16) to (32, −32), the first and third resonances will vanish gradually while the second resonance is stable at 0.67 THz. Such RTM design provides the possibility of optical application. FigureFigure 7. 7. ((aa––cc)) E E-field-field and and ( (dd––ff)) H H-field-field distributions distributions of of the the RTM RTM device device with with xx22 == 2424 μmµm in in TE TE mode mode.. Figure 9 shows the simulated transmission spectra of the RTM device by moving SRR-2 and SRR-To3To meta control -atoms thethe electromagneticalongelectromagnetic positive and response response negative of of the ythe-axis proposed proposed directions RTM RTM design,in TEdesign, and SRR-2 TMSRR and modes.-2 SRR-3and SRRIn meta-atoms TE-3 mometade,- 2 3 atomsthereare moved are are two moved symmetrically resonances symmetrically found along at positivealong 0.49 THzpositive and and negative and0.92 negative THzx-axis for directions.thex-axis initial directions. statex2 and of x(y23 andvalues, y ) x=3 (16,values are − moved16) are as 2 3 movedshownfrom the fromin positionFigure the position 9a of. By (16, movingof (16,16) − to 16)y (32, andto (32, 32),y −values32), while while symmetricallyy yand2 andy y3values va luesalongare are positive keptkept at andthe the initial initialnegative positions. positions. y-axis − − 2 3 Figuredirections,Figure 8 shows an additional the si simulatedmulated EIT resonance transmission appears spectra spectra at about of of the the 0.53 RTM RTM THz, device while by the moving one at SRR SRR-20.92 -THz2 and remains SRR SRR-3-3 metastable.meta-atoms-atoms In TM along alongmode, positive positive there are and and three negative negative resonances xx-axis-axis at directions directions0.50 THz, in 0.67 in TE TE THz and and andTM TM 0.91modes, modes, THz respectively. respectively.for the initial In Instate TE TE 2 3 2 3 mode,ofmode, (y , ywhen when) = (16, the the − 16)position position as the o ofblackf ( (xx2, ,curvexx3) )moves moves shown from from in (16,Figure (16, −16)16) 9b to.to By (32, (32, moving −32),32), the y the andresonances resonances y values are aresymmetrically almost almost kept kept 2 3 − − constantalongconstant positive as as shown shown and in innegative Figure Figure 8a8ya.-.axis In In TM TM directions, mode, mode, there there the firstare are three and theresonances third resonances at 0.50 THz, vanish 0.67 whileTHz and the 0.91second0.91 THz THz resonance for for the the initial initial remains state state of quite ( x2, , x stablex3)) == (16,(16, and −16) 16) constant. as as the the black black This curve curve means shown shown that in in these Figure Figure resonances 8b.b. By moving can bexx2 2 3 − 2 andswitchedand xx3 valuesvalues from symmetrically symmetricallytriple-band to along single along both-band both x- axisxres-axisonance directions directions in TM from frommode. the the position Therefore, position of (16,this of (16, −proposed16) to16) (32, to SRRs (32,−32), canthe32), 3 − − firstbethe used firstand as andthird an thirdEIT resonances switch resonances in will TE will modevanish vanish and gradually multi gradually-band while whileto singlethe the second- secondband resonanceswitch resonance in TM is isstable mode stable atby at 0.67 changing 0.67 THz. THz. SuchySuch2 and RTM RTMy3 values design design symmetrically. provides provides the the possibility possibility of of optical optical switch switch application. application. Figure 9 shows the simulated transmission spectra of the RTM device by moving SRR-2 and SRR-3 meta-atoms(a) along positive and negative y-axis(b) directions in TE and TM modes. In TE mode, there are two resonances found at 0.49 THz and 0.92 THz for the initial state of (y2, y3) = (16, −16) as shown in Figure 9a. By moving y2 and y3 values symmetrically along positive and negative y-axis directions, an additional EIT resonance appears at about 0.53 THz, while the one at 0.92 THz remains stable. In TM mode, there are three resonances at 0.50 THz, 0.67 THz and 0.91 THz for the initial state of (y2, y3) = (16, −16) as the black curve shown in Figure 9b. By moving y2 and y3 values symmetrically along positive and negative y-axis directions, the first and the third resonances vanish while the second resonance remains quite stable and constant. This means that these resonances can be switched from triple-band to singleTE-band mode resonance in TM mode. Therefore,TM mode this proposed SRRs can be used as an EIT switch in TE mode and multi-band to single-band switch in TM mode by changing y2 and y3 values symmetrically.

Figure 8.(a)Simulated transmission spectra of the RTM(b) device obtained by moving the SRR-2 and SRR-3 meta-atoms along positive and negative x-axis directions in (a) TE and (b) TM modes, respectively. Figure9 shows the simulated transmission spectra of the RTM device by moving SRR-2 and SRR-3 meta-atoms along positive and negative y-axis directions in TE and TM modes. In TE mode, there are two resonances found at 0.49 THz and 0.92 THz for the initial state of (y , y ) = (16, 16) as 2 3 − shown in Figure9a. By moving y2 and y3 values symmetrically along positive and negative y-axis directions, an additional EIT resonance appears at about 0.53 THz, while the one at 0.92 THz remains stable. In TM mode, there are three resonances at 0.50 THz, 0.67 THz and 0.91 THz for the initial state TE mode TM mode of (y , y ) = (16, 16) as the black curve shown in Figure9b. By moving y and y values symmetrically 2 3 − 2 3 along positive and negative y-axis directions, the first and the third resonances vanish while the second resonance remains quite stable and constant. This means that these resonances can be switched from

Appl. Sci. 2020, 10, 5267 7 of 9

Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 9 triple-band to single-band resonance in TM mode. Therefore, this proposed SRRs can be used as an EIT switchFigure 8. in Simulated TE mode transmission and multi-band spectra toof single-bandthe RTM device switch obtained in TMby mo modeving bythe changingSRR-2 and SRRy2 and- y3 values3 meta symmetrically.-atoms along positive and negative x-axis directions in (a) TE and (b) TM modes, respectively.

(a) (b)

TE mode TM mode

FigureFigure 9. SimulatedSimulated transmission transmission spectra spectra of of RTM RTM device device obtained obtained by by moving moving the the SRR SRR-2-2 and SRR SRR-3-3 metameta-atoms-atoms along along positive positive and and negati negativeve y--axisaxis directions in ( a) TE and (b) TM modes, respectively. 4. Conclusions 4. Conclusions In conclusion, we present a design of reconfiguring SRR meta-atoms, which exhibits the capability In conclusion, we present a design of reconfiguring SRR meta-atoms, which exhibits the to tune the electromagnetic responses of the incident THz wave by moving SRR meta-atom positions. capability to tune the electromagnetic responses of the incident THz wave by moving SRR meta-atom There are two tuning resonances with a tuning range of 50 GHz in TE mode when SRR-2 and SRR-4 positions. There are two tuning resonances with a tuning range of 50 GHz in TE mode when SRR-2 meta-atoms move along positive x-axis direction. On the other hand, there is one single-resonance and SRR-4 meta-atoms move along positive x-axis direction. On the other hand, there is one single- tuned from 0.67 THz to 0.72 THz in TM mode by moving SRR-1 and SRR-2 meta-atoms along positive resonance tuned from 0.67 THz to 0.72 THz in TM mode by moving SRR-1 and SRR-2 meta-atoms y-axis direction. This shows that the resonances can be tuned in a range of several tens of GHz in along positive y-axis direction. This shows that the resonances can be tuned in a range of several tens the situation where the SRR array is still axially symmetric even if the whole configurated pattern is of GHz in the situation where the SRR array is still axially symmetric even if the whole configurated changed. By moving the SRR-2 and SRR-4 meta-atoms along the positive y-axis direction, there is pattern is changed. By moving the SRR-2 and SRR-4 meta-atoms along the positive y-axis direction, an EIT resonance in TE mode. Furthermore, the resonances can be transformed from dual-band to there is an EIT resonance in TE mode. Furthermore, the resonances can be transformed from dual- multi-band in TE mode by moving the SRR-2 meta-atom along positive x-axis direction. When the band to multi-band in TE mode by moving the SRR-2 meta-atom along positive x-axis direction. position of SRR-2 and SRR-3 meta-atoms are moved along x- or y-axis direction, the switching function When the position of SRR-2 and SRR-3 meta-atoms are moved along x- or y-axis direction, the is transformed from triple-band to single-band in TM mode. This shows that the asymmetric SRR array switching function is transformed from triple-band to single-band in TM mode. This shows that the can change their EIT resonance from single-band, to dual-band and triple-band. This work provides asymmetric SRR array can change their EIT resonance from single-band, to dual-band and triple- a detailed investigation of the electromagnetic response of a planar SRR array and paves the way band. This work provides a detailed investigation of the electromagnetic response of a planar SRR for future studies on tunable metamaterials. It can be potentially used in many applications such as array and paves the way for future studies on tunable metamaterials. It can be potentially used in wearable electronic devices, tunable filters, active sensors, modulators and so on. many applications such as wearable electronic devices, tunable filters, active sensors, modulators and soAuthor on. Contributions: Conceptualization, Y.L.and Y.-S.L.; methodology, Y.L.and Y.-S.L.; software, Y.L.; validation, Y.L. and Y.-S.L.; formal analysis, Y.L. and Y.-S.L.; investigation, Y.L. and Y.-S.L.; resources, Y.L. and Y.-S.L.; data Authorcuration, Contributions: Y.L. and Y.-S.L.; Conceptualization, writing—original draftY.L. preparation, and Y.-S.L.; Y.L. methodology, and Y.-S.L.; Y writing—review.L. and Y.-S.L.; and software, editing, Y Y.L..L.; and Y.-S.L.; supervision, Y.-S.L.; project administration, Y.-S.L.; funding acquisition, Y.-S.L. All authors have read validation,and agreed Y to.L. the and published Y.-S.L.; formal version analysis, of the manuscript. Y.L. and Y.-S.L.; investigation, Y.L. and Y.-S.L.; resources, Y.L. and Y.-S.L.; data curation, Y.L. and Y.-S.L.; writing—original draft preparation, Y.L. and Y.-S.L.; writing—review Funding: This was funded by the financial support from the National Key Research and Development andProgram editing, of ChinaY.L. and (2019YFA0705000). Y.-S.L.; supervision, Y.-S.L.; project administration, Y.-S.L.; funding acquisition, Y.-S.L. All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflicts of interest. Funding: This research was funded by the financial support from the National Key Research and Development ProgramReferences of China (2019YFA0705000).

Conflicts1. Smith, of D.R.;Interest Padilla,: The W.J.;authors Vier, declare D.C.; Nemat-Nasser, no conflicts of S.C.;interest. Schultz, S. Composite Medium with Simultaneously Negative Permeability and Permittivity. Phys. Rev. Lett. 2000, 84, 4184–4187. [CrossRef][PubMed] Reference2. Smith, D.R.; Vier, D.C.; Kroll, N.; Schultz, S. Direct calculation of permeability and permittivity for 1. Smith,a left-handed D.R.; metamaterial. Padilla, W.J.; Appl. Vier, Phys. D.C.; Lett. Nemat2000-,Nasser,77, 2246–2248. S.C.; Schultz, [CrossRef S.] Composite Medium with Simultaneously Negative Permeability and Permittivity. Phys. Rev. Lett. 2000, 84, 4184–4187, doi:10.1103/physrevlett.84.4184.

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