applied sciences Article Reconfigurable Terahertz Metamaterial Using Split-Ring Meta-Atoms with Multifunctional Electromagnetic Characteristics Yuhang Liao and Yu-Sheng Lin * State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics 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 resonator (SRR) array. It shows the preferable and capable adjustability in the THz frequency 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 resonance shifting, polarization 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 optics spanning the spectrum range of 0.1 THz to 10 THz [1,2]. Metamaterials are not natural materials. It means that they are artificially designed [3] and engineered to span the whole electromagnetic spectrum, including visible [4–6], infrared [7–12], THz [13–23] and microwaves [24]. Metamaterials possess many extraordinary and useful optical properties such as negative refractive index [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 resonances 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, switches 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 insulator on insulator (SOI) (SOI) substrate. substrate. The The permittivities 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-atom 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