applied sciences
Article Tunable Terahertz Metamaterial Using an Electric Split-Ring Resonator with Polarization-Sensitive Characteristic
Tao Xu 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]
Received: 14 June 2020; Accepted: 1 July 2020; Published: 6 July 2020
Abstract: We present a tunable terahertz (THz) metamaterial using an electric split-ring resonator (eSRR), which exhibits polarization-sensitive characteristics. The proposed eSRR is composed of double symmetrical semicircles and two central metal bars. By changing the lengths of two metal bars, the electromagnetic responses can be tuned and switched between dual-band and triple-band resonances in transverse magnetic (TM) mode. Furthermore, by moving the bottom metal bar to change the gap between the two metal bars, the first resonance is stable at 0.39 THz, and the second resonance is gradually blue-shifted from 0.83 to 1.33 THz. The tuning range is 0.50 THz. This means that the free spectrum ranges (FSR) could be broadened by 0.50 THz. This proposed device exhibits a dual-/triple-band switch, tunable filter, tunable FSR and polarization-dependent characteristics. It provides an effective approach to perform tunable polarizer, sensor, switch, filter and other optoelectronics in THz-wave applications.
Keywords: metamaterial; terahertz optics; optical switch; optical filter; tunable resonator
1. Introduction It is generally accepted that the terahertz (THz) range is the frequency range of 0.1–10 THz. In recent years, THz technology has attracted growing attention with the progress of THz sources and instrumentation [1,2]. THz technology has been widely used in many fields, including but not limited to filtration [3], molecule identification [4], imaging [5,6] and environmental monitoring systems [7] because it has many fascinating electromagnetic characteristics, such as being harmless to the human body and having artificial magnetism and high chemical sensitivity [8–11]. However, the electromagnetic responses of general natural materials to THz waves are always very weak and cannot meet the requirements for real-world applications. Currently, people put much effort into incremental THz-wave efficiency by using metamaterials [12–14] to enhance the coupling effect between the THz wave and the metamaterial. Metamaterials are special materials with artificial micro/nanostructures. By adjusting their geometric dimensions, they have many unique electromagnetic responses, including asymmetric transmission, negative refraction index, perfect absorption, and superlens [15–19], and they can work in visible, infrared (IR), THz and microwave spectra ranges [20–23]. Such a unique electromagnetic response can be used in THz optics applications. To date, there has been a significant amount of literature reporting on tunable metamaterials [24,25] by using a split-ring resonator (SRR) [26,27], complementary SRR (CSRR) [28], electric SRR (eSRR) [29], F-shaped SRR [30], V-shaped SRR [31], I-shaped SRR [32], U-shaped SRR [33], C-shaped SRR [34], and three-dimensional SRR [35], etc. However, the tuning ranges of these proposed THz metamaterials are not large enough for the THz filter application. Therefore, the requirement of realizing a tunable THz filter with a large tuning range has become a hot topic of scientific research.
Appl. Sci. 2020, 10, 4660; doi:10.3390/app10134660 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 9
filter application. Therefore, the requirement of realizing a tunable THz filter with a large tuning Appl.range Sci. has2020 become, 10, 4660 a hot topic of scientific research. 2 of 9 In this study, we propose a tunable THz metamaterial using an eSRR to exhibit polarization- sensitiveIn this characteristics. study, we propose An aeSRR tunable is composed THz metamaterial of double using symmetrical an eSRR to exhibitsemicircles polarization-sensitive and two central characteristics.metal bars on Ansilicon eSRR on is composedan insulator of double (SOI) symmetricalsubstrate. By semicircles changing and the two relevant central geometrical metal bars ondimensions silicon on of an the insulator semicircles (SOI) and substrate. two central By changing metal bars, the relevantthe resonances geometrical of the dimensions eSRR device of theare semicirclesalways stable and in two transverse central metal electric bars, (TE) the mode, resonances and the of resonances the eSRR device of TM are polarized always stablemode incan transverse be tuned electricwith a tuning (TE) mode, range and of 0.50 the resonancesTHz by moving of TM the polarized bottom metal mode bar. can beMoreover, tuned with the aresonances tuning range can ofbe 0.50switched THz by between moving dual-band the bottom and metal triple-band bar. Moreover, resonanc the resonanceses by changing can be the switched lengths between of two metal dual-band bars. and triple-band resonances by changing the lengths of two metal bars. 2. Materials and Methods 2. Materials and Methods Figure 1 shows schematic drawings of the 3D view of the eSRR device and the corresponding geometricalFigure1 showsdenotations schematic of the drawings unit cell. of theThe 3D eSRR view device of the is eSRR composed device andof double the corresponding symmetrical geometricalsemicircles denotationsand two central of the metal unit cell. bars The on eSRRthe SOI device substrate. is composed The thickness of double symmetricalof the Au layer semicircles is kept andconstant two centralat 300 nm. metal The bars corresponding on the SOI substrate.geometrical The denotations thickness of of the the eSRR Au layer unit cell is kept are represented constant at 300in Figure nm. The 1b. corresponding They are lengths geometrical (L1, L2) of denotations the metal bars, of the the eSRR gap unit(g) between cell are representedthe two metal in bars, Figure gaps1b. They(g1) between are lengths semicircles (L1, L2) ofand the bars, metal and bars, outer the and gap inner (g) between radiuses the (R two, r) metalof semicircles. bars, gaps The (g1 )metal between line semicircleswidth is kept and as bars, a constant and outer at 4 and μm. inner The radiuses period of (R the, r) ofeSRR semicircles. is 80 μm The × 80 metal μm. lineThe widthproposed is kept eSRR as adevice constant is designed at 4 µm. Thebased period on finite of the difference eSRR is 80timeµm domain80 µm. (FDTD) The proposed solutions. eSRR The deviceperiodic is boundary designed × basedconditions on finite are diappliedfference along time domainthe x- and (FDTD) y-axis solutions. directions, The while periodic the boundary boundary conditionscondition of are the applied z-axis alongdirection the isx- a and perfectlyy-axis matched directions, layer while (PML). the boundaryThe monitor condition of resonant of the frequencyz-axis direction is set under is a perfectlythe eSRR matcheddevice to layer calculate (PML). the The transmission monitor of resonantof the incident frequency THz is wave. set under The theresonant eSRR devicefrequency to calculate (fLC) can the be transmissionexpressed by of[36]: the incident THz wave. The resonant frequency (fLC) can be expressed by [36]: r ! gg = 11 = c0c0 eqeq fLCLC = p = (1)(1) π l √εε w 2π LLeqeqCCeqeq l c c w wherewherec 0c0is is the the light light velocity velocity in in a a vacuum, vacuum,Leq Leqand andC eqCeqare are the the equivalent equivalent inductance inductance and and capacitance capacitance of eSRR,of eSRR,εc isεc theis the relative relative permittivity permittivity of of the the materials materials within within the the eSRR, eSRR,l isl is the the size size of of models, models,w wis is thethe metallicmetallic lineline width,width, andand ggeqeq isis thethe equivalentequivalent gapgap widthwidth ofof thethe eSRR.eSRR. TheThe resonantresonant frequencyfrequency ofof thethe eSRReSRR cancan be be tuned tuned by by changing changing diff erentdifferent parameters. parameters. According According to Equation to Equation (1), the (1), impact the ofimpact metallic of thicknessmetallic thickness is minor [is37 minor–39]. Here,[37–39]. the Here, Au layer the Au is designed layer is designed to be 300 to nm be in 300 thickness. nm in thickness.
FigureFigure 1.1. SchematicSchematic drawings drawings of of (a (a) )3D 3D view view of of an an electric electric split-ring split-ring resonator resonator (eSRR) (eSRR) array array and and (b) (geometricalb) geometrical denotations denotations of an of aneSRR eSRR unit unit cell. cell.
3. Results and Discussion 3. Results and Discussion In our design, the TE electric field is perpendicular to the g1 parameter, and the transverse magnetic In our design, the TE electric field is perpendicular to the g1 parameter, and the transverse (TM) electric field is perpendicular to the g parameter. To clarify the electromagnetic responses of magnetic (TM) electric field is perpendicular to the g parameter. To clarify the electromagnetic the eSRR device, we first simulate the transmission spectra of only two metal bars or two semicircles responses of the eSRR device, we first simulate the transmission spectra of only two metal bars or to figure out the source of the individual resonance. Figure2a,b show the transmission spectra of Appl.Appl. Sci. Sci. 20202020, ,1010, ,x 4660 FOR PEER REVIEW 33 of of 9 9 two semicircles to figure out the source of the individual resonance. Figures 2a and 2b show the transmissionmetal bars with spectra different of metalL1 values bars with in TE different and TM modes,L1 values respectively. in TE and ByTM increasing modes, respectively. the L1 value By of increasingthe bottom the metal L1 value bar from of the 12 bottomµm to 24metalµm underbar from the 12 condition μm to 24 of μLm2 =under12 µ m,the there condition is no resonanceof L2 = 12 μinm, TE there mode is andno resonance the electromagnetic in TE mode responses and the el canectromagnetic be switched responses between dual-band can be switched and triple-band between dual-bandresonances and in TM triple-band mode, as shownresonances in Figure in TM2a,b, mode respectively., as shown By in increasing Figure 2a,b, thegap respectively. between two By increasingsemicircle-shaped the gap metamaterialsbetween two semicircle-shaped (g0) from 6 to 10 µ metamaterialsm under the condition (g0) from of6 Rto =1020 μµmm, under there the are conditiontwo stable of resonances R = 20 μm, inthere TE modeare two and stable one resonances stable resonance in TE mode in TM and mode, one as stable shown resonance in Figure in2 TMc,d, mode,respectively. as shown Therefore, in Figure we 2c,d, investigate respectively. the electromagnetic Therefore, we responsesinvestigate of the the electromagnetic eSRR device with responses different ofL1 thevalues eSRR by device keeping with the different parameters L1 values of g1 by= 6 keepingµm, L2 =the12 parametersµm, and R of= 20g1 =µ m,6 μ asm, shownL2 = 12 inμm, Figure and R3. =In 20 TE μm, mode, as shown there in are Figure two stable3. In TE resonances mode, there ( ωare1, twoω2) stable at 0.69 resonances and 1.21 THz,(ω1, ω2 as) at shown 0.69 and in 1.21 Figure THz,3a. asMeanwhile, shown in Figure by increasing 3a. Meanwhile,L1 values, by the increasing electromagnetic L1 values, responses the electromagnetic can be tuned responses and switched can be between tuned anddual-band switched (L1 between= L2) and dual-band triple-band (L1 ( L=1 L,2) Land2) resonances triple-band in (L TM1 ≠ L mode,2) resonances as shown in inTM Figure mode,3b. as The shown first in(ω Figure1) and second3b. The ( firstω2) resonances(ω1) and second are both (ω2 stable) resonances at 0.39 are and both 1.34 stable THz, whileat 0.39 the and third 1.34 (ω THz,3) resonance while the is thirdgradually (ω3) resonance red-shifted is fromgradually 1.71 tored-shifted 0.83 THz from by increasing 1.71 to 0.83 theTHzL 1byvalue increasing from the 8 to L 241 valueµm. from The tendencies8 to 24 μm. Theof resonances tendencies of and resonancesL1 values and of L1 thevalues eSRR of the device eSRR device are summarized are summarized in Figure in Figure3c. 3c. The The corresponding corresponding electricelectric (E) (E) and and magnetic magnetic (H) (H)field field distributions distributions with L1 with = 24 μL1m= are24 illustratedµm are illustrated in Figure 4. inIn FigureTE mode,4. Inthe TE E- andmode, H-field the energies E- and H-fieldare distributed energies around are distributedthe semicircles around at 0.69 the (ω semicircles1) and 1.21 THz at 0.69 (ω2). (ω In1) the and TM 1.21 mode, THz the(ω2 E-). Inand the H-field TM mode, energies the are E- distributed and H-field around energies the aresemicircles distributed at 0.39 around THz the(ω1 semicircles). Such a result at 0.39 shows THz that(ω1). the Such first a resultresonance shows of that the theeSRR first comes resonance from ofth thee semicircle eSRR comes structures. from the The semicircle E-field structures.energy is distributedThe E-field on energy the upper is distributed and lower on parts the upper of the and uppe lowerr metal parts bar, of and the the upper H-fiel metald energy bar, and is distributed the H-field inenergy the center is distributed of the upper in the metal center bar of monitored the upper at metal 1.34 barTHz monitored (ω2). This means at 1.34 that THz the (ω2 second). This meansresonance that ofthe the second eSRR resonanceis caused by of the upper eSRR isbar. caused When by f = the 0.83 upper THz bar.(ω3), Whenthe E-fieldf = 0.83 energy THz is ( ωdistributed3), the E-field on theenergy upper is distributedand lower parts on the of upperthe lower and metal lower bar, parts an ofd the H-field lower metal energy bar, is anddistributed the H-field in the energy center is ofdistributed the lower inmetal the centerbar, which of the indicates lower metal that bar,the th whichird resonance indicates of that the the eSRR third is resonancegeneratedof by the the eSRR lower is bar.generated Therefore, by the when lower the bar. length Therefore, of the whenlowerthe bar length changes of the(different lowerbar L1 values), changes the (diff electromagneticerent L1 values), responsesthe electromagnetic of the eSRR responses can be tuned of the and eSRR switched can be between tuned andthe dual-band switched betweenand triple-band the dual-band resonances and intriple-band TM mode. resonances in TM mode.
Figure 2. Transmission spectra of metal bars with different L1 values in (a) TE mode and (b) transverse Figure 2. Transmission spectra of metal bars with different L1 values in (a) TE mode and (b) transverse magnetic ™ mode and those of semicircle-shaped metamaterial with different g0 values in (c) TE mode magnetic TM mode and those of semicircle-shaped metamaterial with different g0 values in (c) TE andmode (d) and TM ( dmode,) TM mode,respectively. respectively. Appl. Sci. 2020, 10, 4660 4 of 9 Appl.Appl. Sci.Sci. 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 99
FigureFigure 3. 3. TransmissionTransmission spectra spectra of of eSRR eSRR with with different didifferentfferent LL11 valuesvalues in in ( (aa)) TE TE and and ((bb)) TMTM modes.modes. ( (cc)) The The
relationshipsrelationships between between resonances resonances andand LL11 valuesvalues are are shown shown in in ( (aa,,bb).).
FigureFigure 4.4. E-E- andand H-fieldH-fieldH-field distributionsdistributions ofof eSRReSRR withwith LL11 == 2424 μμµmm inin TETE andand TMTM modes. modes.
WhenWhen two two metal metal barsbars areare are equalequal equal atat at aa a certaincertain certain lengthlength length (i.e.,(i.e., (i.e., LL L == = LL11L ==1 LL=22 byLby2 keepingkeepingby keeping RR == 2020R μμ=mm20 andandµm gg1 and1 == 66 μgμ1m),m),= 6 thetheµm correspondingcorresponding), the corresponding electromagneticelectromagnetic electromagnetic responsesresponses responses ofof thethe eSRReSRR of the devicedevice eSRR deviceareare shownshown are shown inin FigureFigure in Figure5.5. ThereThere5. areThereare twotwo are stablestable two resonances stableresonances resonances ((ωω11,, ωω22) () ω atat1 , 0.690.69ω2) atandand 0.69 1.211.21 and THzTHz 1.21 inin THzTETE mode,mode, in TE asas mode, shownshown as in shownin FigureFigure in 5a. Figure5a. InIn TMTM5a. mode,Inmode, TM the mode,the firstfirst the resonanceresonance first resonance ((ωω11)) isis veryvery (ω1) stablestable is very atat stable 0.390.39 THz,THz, at 0.39 whilewhile THz, thethe while secondsecond the resonanceresonance second resonance ((ωω22)) isis graduallygradually (ω2) is red-shiftedgraduallyred-shifted from red-shiftedfrom 1.711.71 toto 0.820.82 from THzTHz 1.71 byby to increasingincreasing 0.82 THz thethe by LL increasingvaluevalue fromfrom 8 the8 toto 2424L value μμm,m, asas from shownshown 8 toinin Figure 24Figureµm, 5b.5b. as TheThe shown tuningtuning in rangeFigurerange ofof5 b. thethe The secondsecond tuning resonanceresonance range of andand the thethe second freefree spectrumspectrum resonance rara andngenge the (FSR)(FSR) free isis spectrum 0.890.89 THz.THz. range ThisThis characteristiccharacteristic (FSR) is 0.89 isis THz. veryvery suitableThissuitable characteristic forfor THzTHz filterfilter is andand very sensorsensor suitable applicapplic forations.ations. THz The filterThe tendencytendency and sensor ofof resonancesresonances applications. andand LL The valuesvalues tendency ofof thethe eSRReSRR of resonances devicedevice areare summarizedandsummarizedL values inin of FigureFigure the eSRR 5c.5c. FigureFigure device 66 represents arerepresents summarized thethe correspondingcorresponding in Figure5c. E-E- Figure andand H-fieldH-field6 represents distributionsdistributions the corresponding withwith LL == 1212 μμm.m. E- ItIt isandis worthworth H-field mentioningmentioning distributions thatthat thethe with E-fieldE-fieldL = energyenergy12 µm. isis It distribudistribu is worthtedted mentioning onon thethe upperupper that andand the lowerlower E-field partsparts energy ofof thethe twotwo is distributed metalmetal barsbars andonand the thethe upper H-fieldH-field and energyenergy lower isis distributeddistributed parts of the inin two thethe center metalcenter of barsof thethe and twotwo the metalmetal H-field barsbars monitoredmonitored energy is distributedatat 1.331.33 THzTHz in in TM theTM mode,centermode, whichwhich isis obviouslyobviously differentdifferent fromfrom thethe casecase wherwheree thethe lengthslengths ofof twotwo metalmetal barsbars areare notnot equalequal ((LL11 ≠≠ LL22).). Appl. Sci. 2020, 10, 4660 5 of 9 of the two metal bars monitored at 1.33 THz in TM mode, which is obviously different from the case where the lengths of two metal bars are not equal (L1 , L2). Appl.Appl. Sci.Sci. 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 99
Figure 5. Transmission spectra of eSRR with different L (L = L1 = L2) values in (a) TE and (b) TM modes. Figure 5. Transmission spectra ofof eSRReSRR withwith didifferentfferent L L( L(L= = LL11 = LL22)) values values in in ( (aa)) TE TE and and ( (bb)) TM TM modes. modes. ((cc)) The The relationshipsrelationships relationships ofof of resonancesresonances resonances andand and LL valuesvaluesL values inin ((aa,,b inb).). ( a,b).
FigureFigure 6. 6. E-E- and and H-field H-fieldH-field distributions distributions of of eSRR eSRR with with LL == 1212 μμµmm inin TETE andand TMTM modes.modes.
TheThe lower lower metal metal bar bar is is connectedconnected to to thethe suspendingsuspensuspendingding microelectromechanical microelectromechanical systems systems (MEMS) (MEMS) combcomb driver driver moving moving along along the the y-axis y-axis direction direction to to change change the the gapgap betweenbetween twotwo metalmetal barsbars (i.e.,(i.e., gg value)value)value) andand determinedeterminedetermine tunability tunabilitytunability and andand applicability. applicability.applicability. The TheThe transmission trtransmissionansmission spectra spectraspectra of the ofof eSRR thethe eSRReSRR with diwithwithfferent differentdifferentg values gg valuesarevalues shown areare shownshown in Figure inin Figure7Figure. There 7.7. areThereThere two areare stable twotwo stable resonancesstable resonancesresonances in TE in mode,in TETE mode,mode, as shown asas shownshown in Figure inin FigureFigure7a. In 7a.7a. TM InIn TMmode,TM mode,mode, the the firstthe firstfirst resonance resonanceresonance (ω1 (()ωω is11)) very isis veryvery stable stablestable at atat 0.39 0.390.39 THz THzTHz and andand the thethe second secondsecond resonance resonanceresonance ( (ω(ωω222))) isis gradually gradually blue-shiftedblue-shifted fromfrom 0.830.83 toto 1.331.33 THzTHz byby increasingincreasing thethe gg valuevalue fromfrom from 00 0 toto to 1212 12 μμµm,m, asas shownshown inin FigureFigure 77b.7b.b. TheThe tuningtuningtuning range rangerange is isis 0.50 0.500.50 THz. THz.THz. That ThatThat means meansmeans that thatthat FSR FSRFSR can cancan be broadened bebe broadenedbroadened by 0.50 byby THz.0.500.50 THz.THz. Therefore, Therefore,Therefore, the eSRR thethe eSRRcouldeSRR could becould controlled bebe controlledcontrolled to broaden toto broadenbroaden the tuning thethe of tuningtuning FSR by ofof increasing FSRFSR byby increasingincreasing the g value thethe and gg valuevalue narrowed andand narrowed bynarrowed increasing byby increasingtheincreasingL value, thethe which LL value,value, can whichwhich be used cancanin bebe a usedused tunable inin aa bandwidthtunabletunable bandwidthbandwidth filter. The filter.filter. tendencies ThThee tendenciestendencies of resonances ofof resonancesresonances and g andand gg valuesvalues ofof thethe eSRReSRR devicedevice areare summarizedsummarized inin FigureFigure 7c.7c. InIn orderorder toto furtherfurther understandunderstand thethe physicalphysical mechanismmechanism ofof electromagneticelectromagnetic responsesresponses ofof thethe eSRR,eSRR, thethe correspondingcorresponding E-E- andand H-fieldH-field distributionsdistributions ofof thethe eSRReSRR withwith gg == 66 μμmm areare plottedplotted inin FigureFigure 8.8. TheThe E-fieldE-field energyenergy isis distributeddistributed onon thethe upperupper andand lowerlower partsparts ofof thethe twotwo metalmetal bars,bars, andand thethe H-fielH-fieldd energyenergy isis distributeddistributed inin thethe centercenter ofof thethe twotwo metalmetal barsbars monitoredmonitored atat 1.301.30 THzTHz inin TMTM mode.mode. Therefore,Therefore, byby movingmoving thethe lowerlower metalmetal barbar toto aa differentdifferent position,position, thethe Appl. Sci. 2020, 10, 4660 6 of 9 values of the eSRR device are summarized in Figure7c. In order to further understand the physical mechanism of electromagnetic responses of the eSRR, the corresponding E- and H-field distributions of the eSRR with g = 6 µm are plotted in Figure8. The E-field energy is distributed on the upper and lower parts of the two metal bars, and the H-field energy is distributed in the center of the two metal barsAppl.Appl. Sci.Sci. monitored 20202020,, 1010,, xx FORatFOR 1.30 PEERPEER THz REVIEWREVIEW in TM mode. Therefore, by moving the lower metal bar to a different66 ofof 99 position, the eSRR device exhibits different optical characteristics. This design provides an effective eSRReSRR devicedevice exhibitsexhibits differentdifferent opticaloptical characteristics.characteristics. ThisThis designdesign providesprovides anan effectiveeffective approachapproach toto realizerealize thethe approach to realize the tunable THz filter, polarizer and sensor applications. tunabletunable THzTHz filter,filter, polarizerpolarizer andand sensorsensor applications.applications.
FigureFigure 7.7. TransmissionTransmission spectraspectra ofof anan eSRReSRR withwith didifferentdifferentfferent g gg values valuesvalues in inin (((aaa))) TETETE andandand (((bb))) TMTMTM modes.modes.modes. ( (cc)) The The relationshipsrelationships ofof resonancesresonances andandand gg valuesvalues inin (((aaa,,b,bb).).).
FigureFigure 8.8. E-E- and and H-field H-fieldH-field distributionsdistributions ofof eSRReSRR withwith gg == 66 μµμmm in in TE TE and and TM TM modes. modes.
4.4. ConclusionsConclusions InIn conclusion,conclusion, aa tunabletunable THzTHz metamaterialmetamaterial usingusing eSRReSRR isis presentedpresented toto demonstratedemonstrate thethe electromagneticelectromagnetic responsesresponses couldcould bebe tunedtuned byby chanchangingging geometricalgeometrical parameters.parameters. ByBy changingchanging thethe lengthlength ofof thethe lowerlower metalmetal bar,bar, thethe electromagneelectromagnetictic responsesresponses cancan bebe tunedtuned andand switchedswitched betweenbetween dual-banddual-band andand triple-bandtriple-band resonances.resonances. TheThe tuningtuning rangesranges ofof thethe FSRFSR cancan bebe broadenedbroadened byby 0.500.50 THzTHz andand narrowednarrowed byby 0.890.89 THzTHz byby changingchanging gg andand LL values,values, respectively.respectively. TheThe eSRReSRR devicedevice exhibitsexhibits tunabletunable andand switchableswitchable polarization-sensitivepolarization-sensitive characteristcharacteristics.ics. TheThe gapgap betweenbetween semicirclessemicircles andand centralcentral metalmetal Appl. Sci. 2020, 10, 4660 7 of 9
4. Conclusions In conclusion, a tunable THz metamaterial using eSRR is presented to demonstrate the electromagnetic responses could be tuned by changing geometrical parameters. By changing the length of the lower metal bar, the electromagnetic responses can be tuned and switched between dual-band and triple-band resonances. The tuning ranges of the FSR can be broadened by 0.50 THz and narrowed by 0.89 THz by changing g and L values, respectively. The eSRR device exhibits tunable and switchable polarization-sensitive characteristics. The gap between semicircles and central metal bars would not influence the electromagnetic response of the eSRR, which indicates that the eSRR device has high stability to allow manufacturing tolerance. These results pave the way for the use of eSRR devices in tunable filters, switches, detectors, sensors and other THz optoelectronics applications.
Author Contributions: Conceptualization, T.X. and Y.-S.L.; methodology, T.X. and Y.-S.L.; software, T.X.; validation, T.X. and Y.-S.L.; formal analysis, T.X. and Y.-S.L.; investigation, T.X. and Y.-S.L.; resources, T.X. and Y.-S.L.; data curation, T.X. and Y.-S.L.; writing—original draft preparation, T.X. and Y.-S.L.; writing—review and editing, T.X. and Y.-S.L.; visualization, T.X. and 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. Funding: This research was funded by the financial support from the National Key Research and Development Program of China (2019YFA0705000). Conflicts of Interest: The authors declare no conflict of interest.
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