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Tunable Terahertz Metamaterial Using an Electric Split-Ring Resonator with Polarization-Sensitive Characteristic

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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 p"ε 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,"c 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
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