Passively Tunable Terahertz Filters Using Liquid Crystal Cells Coated with Metamaterials

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Article Passively Tunable Terahertz Filters Using Liquid Crystal Cells Coated with Metamaterials

Wei-Fan Chiang 1, Yu-Yun Lu 2, Yin-Pei Chen 2, Xin-Yu Lin 2, Tsong-Shin Lim 2, Jih-Hsin Liu 3, Chia-Rong Lee 1,* and Chia-Yi Huang 2,*

1 Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan; [email protected] 2 Department of Applied Physics, Tunghai University, Taichung 40704, Taiwan; [email protected] (Y.-Y.L.); [email protected] (Y.-P.C.); [email protected] (X.-Y.L.); [email protected] (T.-S.L.) 3 Department of Electrical Engineering, Tunghai University, Taichung 40704, Taiwan; [email protected] * Correspondence: [email protected] (C.-R.L.); [email protected] (C.-Y.H.)

Abstract: Liquid crystal (LC) cells that are coated with metamaterials are fabricated in this work. The LC directors in the cells are aligned by rubbed polyimide layers, and make angles θ of 0◦, 45◦, and 90◦ with respect to the gaps of the split-ring resonators (SRRs) of the metamaterials. Experimental results display that the resonance frequencies of the metamaterials in these cells increase with an increase in θ, and the cells have a maximum frequency shifting region of 18 GHz. Simulated results reveal that the increase in the resonance frequencies arises from the birefringence of the LC, and the LC has a birefringence of 0.15 in the terahertz region. The resonance frequencies of the metamaterials are shifted by the rubbing directions of the polyimide layers, so the LC cells coated

with the metamaterials are passively tunable terahertz ﬁlters. The passively tunable terahertz ﬁlters exhibit promising applications on terahertz communication, terahertz sensing, and terahertz imaging.

Citation: Chiang, W.-F.; Lu, Y.-Y.; Chen, Y.-P.; Lin, X.-Y.; Lim, T.-S.; Liu, Keywords: liquid crystals; metamaterials; terahertz filters J.-H.; Lee, C.-R.; Huang, C.-Y. Passively Tunable Terahertz Filters Using Liquid Crystal Cells Coated with Metamaterials. Coatings 2021, 11, 1. Introduction 381. https://doi.org/10.3390/ Metamaterials have attracted much attention due to their unusual electromagnetic res- coatings11040381 onance [1–7]. The resonance frequencies of metamaterials are sensitive to their geometrical structures [1–3] and the refractive indices of the media that surround the metamateri- Academic Editor: You Seung Rim als [4–7]. Therefore, metamaterials can be used to develop frequency filters, intensity modulators, and sensors. Liquid crystals (LCs) have been used to tune the resonance Received: 2 March 2021 frequencies of terahertz metamaterials due to their large birefringences [6,7]. Chen et al. Accepted: 25 March 2021 deposited a dual frequency LC layer on a terahertz metamaterial [6]. The resonance spec- Published: 26 March 2021 trum of the terahertz metamaterial can be red-shifted and blue-shifted by switching the frequency of a given voltage because the dual frequency LC exhibits positive and neg- Publisher’s Note: MDPI stays neutral ative dielectric anisotropies at low and high frequencies. Therefore, the dual frequency with regard to jurisdictional claims in LC layer that is deposited on the terahertz metamaterial can be used to develop electri- published maps and institutional affil- cally controllable terahertz filters. Deng et al. deposited an LC layer between a terahertz iations. metamaterial and a metal layer [7]. The resonance frequency of the terahertz metamaterial is red-shifted by 11.4 (12.6) GHz at the irradiation of terahertz waves with TE (TM) polarization as a voltage that is applied to the LC layer increases from zero to 10 V. There- fore, the metamaterial-imbedded LC cell can be used to develop electrically controllable Copyright: © 2021 by the authors. terahertz absorbers. Licensee MDPI, Basel, Switzerland. The application of the large voltages to the LC layers causes the shift of the resonance This article is an open access article spectra of the metamaterials in the previous works [6,7]. The electrically controllable tera- distributed under the terms and hertz devices in these works consume a lot of power to filter incident terahertz waves. This conditions of the Creative Commons Attribution (CC BY) license (https:// drawback hinders the usage of the terahertz devices in practical applications. It is of great creativecommons.org/licenses/by/ interest to researchers to develop LC-based metamaterials with zero power consumption. 4.0/).

Coatings 2021, 11, 381. https://doi.org/10.3390/coatings11040381 https://www.mdpi.com/journal/coatings Coatings 2021, 11, x FOR PEER REVIEW 2 of 8

great interest to researchers to develop LC-based metamaterials with zero power con- Coatings 2021, 11, 381 2 of 8 sumption. This work fabricates LC cells coated with terahertz metamaterials, and studies the effect of the rubbing directions of the alignment layers of the cells on the electromagnetic resonanceThiswork of the fabricates metamaterials. LC cells The coated resonanc withe frequencies terahertz metamaterials, of the terahertz and metamaterials studies the effectincrease of thewith rubbing an increase directions in the ofangles the alignment between the layers directors of the of cells the onLC the and electromagnetic the gaps of the resonanceSRRs. A finite-difference of the metamaterials. time-domain The resonance simulation frequencies depicts ofthat the the terahertz increase metamaterials in the reso- increasenance frequencies with an increase arises from in the the angles birefringenc betweene theof the directors LC, and of the LC andhas a the birefringence gaps of the SRRs.of 0.15 A in finite-difference the terahertz region. time-domain The LC simulation cells coated depicts with thatthe theterahertz increase metamaterials in the resonance are frequenciespassively tunable arises filters from with the birefringence zero power consumption, of the LC, and and the can LC be has used a birefringencein terahertz im- of 0.15aging, in terahertz the terahertz sensing region. and terahertz The LC cellscommunication. coated with the terahertz metamaterials are passively tunable filters with zero power consumption, and can be used in terahertz imaging,2. Materials terahertz and Methods sensing and terahertz communication. 2. MaterialsFigure 1a and presents Methods the schematic configuration of LC cells that are coated with terahertz metamaterials. Each of the LC cells are fabricated using two 188-μm-thick polyes- Figure1a presents the schematic configuration of LC cells that are coated with terahertz ter (PET) substrates, which are separated by two plastic spacers with a thickness of 500 metamaterials. Each of the LC cells are fabricated using two 188-µm-thick polyester (PET) μm. A 200-nm-thick silver SRR array is coated on one of the PET substrates by photoli- substrates, which are separated by two plastic spacers with a thickness of 500 µm. A thography, metal evaporation, and lift-off process. Figure 1b presents the dimensions of 200-nm-thick silver SRR array is coated on one of the PET substrates by photolithography, one of the SRRs in the array. The SRR has a linewidth (w), split gap (g), short side (s), long metal evaporation, and lift-off process. Figure1b presents the dimensions of one of the side (l), period in the x direction, and period in the y direction of 6 μm, 20 μm, 50 μm, 60 SRRs in the array. The SRR has a linewidth (w), split gap (g), short side (s), long side (l), periodμm, 70 inμm, the andx direction, 80 μm, respectively. and period in Six-hundred-nanometer-thick the y direction of 6 µm, 20 µm, polyimide 50 µm, 60 µ (PI)m, 70layersµm, and(AL-1426CA, 80 µm, respectively. Daily Polymer) Six-hundred-nanometer-thick are coated on both PET substrates polyimide as homogenously (PI) layers (AL-1426CA, aligning Dailylayers Polymer)and are rubbed are coated using on a both rubbing PET substratesmachine. The as homogenously directors in the aligning LC cells layers thatand are ° ° ° arecoated rubbed with using the terahertz a rubbing metamaterials machine. The make directors angles in theθ of LC 0 , cells 45 , thatand are90 coatedwith respect with the to terahertzthe gaps of metamaterials the SRRs of the make metamaterials. angles θ of A 0◦ nematic, 45◦, and LC 90 (HTW114200-100,◦ with respect to Fusol the gaps Material) of the SRRsis used of in the the metamaterials. cells. The nematic A nematic LC has LC been (HTW114200-100, used to develop Fusol high-performance Material) is used optical in the cells.devices The [8,9]. nematic However, LC has the beenLC has used not to been develop used high-performanceto develop terahertz optical devices. devices Therefore, [8,9]. However,the LC is used the LC in this has notwork. been The used LC layers to develop with terahertza thickness devices. of 500 Therefore,μm are so thick the LC that is usedterahertz in this waves work. experience The LC layerstheir refractive with a thickness indices significantly of 500 µm are [10,11]. so thick The that LC terahertzhas ordi- wavesnary and experience extraordinary their refractiverefractive indicesindices significantlyof 1.513 and [101.779,,11]. respectively, The LC has ordinaryin the visible and extraordinarylight regime. refractive indices of 1.513 and 1.779, respectively, in the visible light regime.

Figure 1. (a) Schematic conﬁguration of liquid crystal (LC) cells that are coated with terahertz metamaterials. θ is an angle Figure 1. (a) Schematic configuration of liquid crystal (LC) cells that are coated with terahertz metamaterials. θ is an angle betweenbetween thethe directordirector ofof thethe LCLC andand thethe gapsgaps ofof thethe split-ringsplit-ring resonatorsresonators (SRRs).(SRRs). ((bb)) DimensionsDimensions ofof SRR.SRR. 3. Results and Discussion 3. Results and Discussion Figure2 displays the experimental terahertz spectra of three metamaterials that are Figure 2 displays the experimental terahertz spectra of three metamaterials that are coated with the PI layers. These spectra are obtained using a commercial terahertz spec- coated with the PI layers. These spectra are obtained using a commercial terahertz spectrometer (TPS 3000, TeraView, Cambridge, UK) in transmission mode, and the chamber in trometer (TPS 3000, TeraView, Cambridge, UK) in transmission mode, and the chamber the spectrometer is ﬁlled with nitrogen gas to prevent terahertz waves from absorbing mois- in the spectrometer is filled with nitrogen gas to prevent terahertz waves from absorbing ture. The polarization of incident terahertz waves is set parallel to the x axis of Figure1a. moisture. The polarization of incident terahertz waves is set parallel to the x axis of Figure The PI-coated metamaterials have transmission peaks in their terahertz spectra due to 1a. The PI-coated metamaterials have transmission peaks in their terahertz spectra due to the absorption of the electromagnetic resonance of the metamaterials. The transmission the absorption of the electromagnetic resonance of the metamaterials. The transmission peaks are at an identical frequency of 0.588 THz, and the two PI-coated metamaterials have peaks are at an identical frequency of 0.588 THz, and the two PI-coated metamaterials similar resonance spectra. Therefore, the three PI-coated metamaterials can be used to study the effect of the rubbing directions of the PI layers on the electromagnetic resonance of the metamaterials.

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have similar resonance spectra. Therefore, the three PI-coated metamaterials can be used Coatings 2021, 11, 381 have similar resonance spectra. Therefore, the three PI-coated metamaterials can be used3 of 8 to study the effect of the rubbing directions of the PI layers on the electromagnetic resonance of the metamaterials.

0 PI-coated metamaterial 1 PI-coated metamaterial 2 -5 PI-coated metamaterial 3

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-25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Frequency (THz) Frequency (THz) Figure 2. Experimental spectra of three metamaterials thatthat are coatedcoated withwith polyimidepolyimide (PI)(PI) layers.layers.

A simulation is performed using a softwaresoftware basedbased onon finite-differencefinite-difference time-domaintime-domain method to verify thethe experimentalexperimental spectraspectra ofof thethe PI-coatedPI-coated metamaterials.metamaterials. AA simulatedsimulated SRR has the same same geometrical geometrical dimensions dimensions as as the the SRR SRR of of Figure Figure 1b,1b, and and is is deposited deposited on on a 188-a 188-μm-thickµm-thick PET PET substrate substrate with with an an area area of of 70 70 μµmm × 8080 μµm.m. A A 600-nm-thick 600-nm-thick PI film film isis coated onon thethe simulatedsimulated SRR.SRR. The The permittivity permittivity of of the the PET PET substrate substrate (PI (PI film) film) is 3.0is 3.0 (1.21) (1.21) in inthe the simulation, simulation, and and was was obtained obtained from from its its time-domain time-domain spectrum. spectrum. A periodicalA periodical boundary bound- arycondition condition is set is inset the in the simulation, simulation, and and the the conductivity conductivity of silverof silver in thein the simulated simulated SRR SRR is 7 is6.30 6.30× ×10 107S/m. S/m. Figure3 3 displays displays thethe simulatedsimulated spectrumspectrum ofof thethe PI-coatedPI-coated metamaterials.metamaterials. These metamaterials have a simulated peak at 0.5880.588 THz.THz. Therefore, the peakpeak frequencyfrequency of the simulated spectrum spectrum of of Figure Figure 3 verifiesverifies that that of of the the experimental experimental spectra spectra of of Figure Figure 2. The2. The inset inset in inFigure Figure 33 presents presents the the near-field near-field distribution distribution of of thethe simulatedsimulated SRRSRR thatthat areare coated with the PI filmfilm atat 0.5880.588 THz.THz. The near fieldfield ofof thisthis SRRSRR exhibitsexhibits thethe maximummaximum strength at its gap. This result depicts that the electromagnetic resonance of the PI-coated metamaterials is an inductive-capacitive mode atat 0.5880.588 THz.THz.

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-15 V/m Transmittance (dB) Transmittance Transmittance (dB) Transmittance -20 Min -25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Frequency (THz) Frequency (THz) Figure 3. Simulated spectrum ofof PI-coatedPI-coated metamaterials.metamaterials. TheTheinsert insertpresents presents the the near-ﬁeld near-field distribu- distri- butiontion of theof the simulated simulated SRR SRR with with the the PI ﬁlmPI film at 0.588at 0.588 THz. THz.

An SRRSRR cancan be be considered considered as as an inductor-capacitoran inductor-capacitor circuit, circuit, and itsand resonance its resonance frequency fre- quencyis given is by given [12] by [12] 1 f = √ , (1) 2π LC

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1 = , (1) 2√ where L is an inductance of the inductor, and C is a capacitance of the capacitor. L is de-

Coatings 2021, 11, 381 termined by the enclosed area of the SRR, and C is proportional to the permittivities4 of 8 of the media that surround the SRR. Equation (1) depicts that the resonance frequency of the inductive-capacitive mode of a metamaterial is inversely proportional to the refractive whereindex Lofis a an dielectric inductance layer of the that inductor, is deposited and C is on a capacitancethe metamaterial of the capacitor. [12]. Therefore,L is the PI- determinedcoated metamaterials by the enclosed will area be ofsensitive the SRR, to and changeC is proportional in the refractive to the permittivitiesindices of the dielectric oflayers the media that are that deposited surround the on SRR. them. Equation (1) depicts that the resonance frequency of the inductive-capacitiveFigure 4 displays mode the ofexperimental a metamaterial spectra is inversely of the proportional three LC tocells the that refractive are coated with indexthe terahertz of a dielectric metamaterials layer that is at deposited θ = 0°, 45 on°, theand metamaterial 90°. The metamaterial-coated [12]. Therefore, the PI- LC cells at θ coated= 0°, 45 metamaterials°, and θ = 90 will° have be sensitive resonance to change peaks in at the 0.533 refractive THz, indices 0.542 ofTHZ, the dielectric and 0.551 THz, re- layers that are deposited on them. spectively.Figure4 displaysThe resonance the experimental frequencies spectra of of the the threeLC cells LC cells increase that are with coated an with increase the in θ, and terahertzthe cells metamaterialshave a maximum at θ = 0 freque◦, 45◦, andncy-shifting 90◦. The metamaterial-coated region of 18 GHz. LC The cells increase at θ = 0◦, in the reso- 45nance◦, and frequenciesθ = 90◦ have resonancearises from peaks the at birefringence 0.533 THz, 0.542 of THZ, the andLC. 0.551As the THz, cell respectively. is at θ = 0° (90°), the TheLC resonancedirector is frequencies parallel (perpendicular) of the LC cells increase to the with gaps an increaseof the SRRs in θ, andof the the metamaterial. cells have In ad- adition, maximum the frequency-shiftingpolarized direction region of ofincident 18 GHz. tera Thehertz increase waves in the is resonance parallel frequencies to these gaps. There- arises from the birefringence of the LC. As the cell is at θ = 0◦ (90◦), the LC director is fore, the incident terahertz waves experience an extraordinary (ordinary) refractive index parallel (perpendicular) to the gaps° of° the SRRs of the metamaterial. In addition, the polarizedof the LC direction in the cell of incident at θ = terahertz0 (90 ). wavesThis result is parallel depicts to these that gaps. the LC Therefore, has an the extraordinary incident(ordinary) terahertz refractive waves index experience as the an extraordinarymetamaterial (ordinary) has a resonance refractive index frequency of the LC at 0.533 THz in(0.551 the cell THz). at θ =Therefore, 0◦ (90◦). This the resultLC birefringence depicts that the causes LC has the an increase extraordinary in the (ordinary) resonance frequen- refractivecies as the index angles as the between metamaterial the LC has director a resonances and frequency the SRR at 0.533gaps THz are (0.551increased. THz). Resonance Therefore,frequencies the of LC plasmonic birefringence materials causes thecan increase be tune ind theby resonanceactive methods frequencies such as as the external volt- angles between the LC directors and the SRR gaps are increased. Resonance frequencies of age, pump light, and heat or by passive methods such as the change in their geometrical plasmonic materials can be tuned by active methods such as external voltage, pump light, anddimensions heat or by [13,14]. passive methodsThe results such asin theFigure change 4 re inveal their that geometrical the resonance dimensions frequency [13,14]. of a met- Theamaterial-coated results in Figure 4LC reveal cell thatcan the be resonancepassively frequency tuned by of athe metamaterial-coated rubbing direction LC of cell its alignment canlayers. be passively The metamaterial-coated tuned by the rubbing directionLC cells ofare its alignmentpassively layers.tunable The terahertz metamaterial- filters, and ex- coatedhibit promising LC cells are applications passively tunable on terahertzterahertz ﬁlters, communication, and exhibit promising terahertz applications sensing, on and terahertz terahertzimaging. communication, A situation in terahertz which sensing,photoalignment and terahertz layers imaging. replace A situation the PI inlayers which in this work photoalignment layers replace the PI layers in this work may fabricate light-controllable terahertzmay fabricate metamaterials. light-controllable Efforts are terahertz being made metamaterials. at the authors’ Efforts laboratory are tobeing fabricate made at the au- light-controllablethors’ laboratory terahertz to fabricate metamaterials, light-controll the resultsable of terahertz which will metamaterials, be published in thethe results of nearwhich future. will be published in the near future.

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θ = 0°

Transmittance Transmittance (dB) -30 θ = 45° θ = 90° -40 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Frequency (THz) Figure 4. 4.Experimental Experimental spectra spectra of three of three metamaterial-coated metamaterial-coated LC cells at LCθ = cells 0◦, 45 at◦, θ and = 0 90°,◦ 45. °, and 90°.

A simulation is performed using software to verify the experimental spectra of the A simulation is performed using software to verify the experimental spectra of the three metamaterial-coated LC cells. A dielectric layer with a refractive index of n is depositedthree metamaterial-coated on the simulated PI-coated LC cells. SRR A dielectric (the inset of layer Figure with3). Figurea refractive5 displays index the of n is depos- terahertzited on the spectra simulated of the simulated PI-coated PI-coated SRR (the SRR inset with of the Figure dielectric 3). layerFigure at n5 =displays 1.55, 1.62, the terahertz andspectra 1.70. of This the SRR simulated has simulated PI-coated resonance SRR frequencies with the dielectric of 0.551, 0.542, layer and at 0.533n = 1.55, THz 1.62, at and 1.70. nThis= 1.55, SRR 1.62, has and simulated 1.70, respectively. resonance The frequencies resonance frequency of 0.551, of the0.542, simulated and 0.533 PI-coated THz at n = 1.55, SRR1.62, with and the 1.70, dielectric respectively. layer at nThe= 1.70 resonance (1.55) equals frequency that of theof the metamaterial-coated simulated PI-coated LC SRR with

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Coatings 2021, 11, 381 5 of 8 the dielectric layer at n = 1.70 (1.55) equals that of the metamaterial-coated LC cell at θ = 0° (90°). The polarized direction of the incident terahertz waves is parallel (perpendicular) ◦ ◦ cellto the at θ LC= 0 director(90 ). The of polarizedthe metamaterial-coated direction of the incident LC cell terahertz at θ = waves0° (90 is°), parallel so the LC has an (perpendicular) to the LC director of the metamaterial-coated LC cell at θ = 0◦ (90◦), so the extraordinary (ordinary) refractive index ne (no) of 1.70 (1.55) in the terahertz region. This LC has an extraordinary (ordinary) refractive index n (n ) of 1.70 (1.55) in the terahertz result reveals that the LC exhibits a birefringencee oof 0.15 in the terahertz region, and the region. This result reveals that the LC exhibits a birefringence of 0.15 in the terahertz region, andLC birefringence the LC birefringence increases increases the theresonance resonance freq frequenciesuencies ofof the the metamaterials metamaterials as the as the angles anglesbetween between the directors the directors and and the the gaps gaps of of their their SRRs areare increased. increased.

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-20 n = 1.55 Transmittance (dB) n = 1.62 n = 1.70 -30 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Frequency (THz) Figure 5. 5.Simulated Simulated spectra spectra of three of three metamaterial-coated metamaterial-coated LC cells atLCn =cells 1.55, at 1.62, n =and 1.55, 1.70. 1.62,n is and the 1.70. n is the refractive index index of aof dielectric a dielectric layer layer that is that deposited is deposited on the simulated on the simulated PI-coated SRR. PI-coated SRR.

The effective refractive index of the LC in the metamaterial-coated LC cell that involves The effective refractive index of the LC in the metamaterial-coated LC cell that in- θ = 45◦ can be determined by an equation of the index ellipse of the LC, and it is given by volves θ = 45° can be determined by an equation of the index ellipse of the LC, and it is s given by 1 = ne f f 2 2 2 2 , (2) cos θ/ne + sinθ /no 1 = , (2) where θ is an angle between the LC director between the⁄ polarized+ direction⁄ of the incident ◦ terahertz waves. Substituting ne = 1.70, no = 1.55, and θ = 45 into Equation (2) yields nwhereeff = 1.62 θ. isn effanequals angle the between refractive the index LC director of the LC be intween the metamaterial-coated the polarized direction LC cell of the inci- at θ = 45◦, as presented in Figure5. This result verifies that the rubbing directions of dent terahertz waves. Substituting ne = 1.70, no = 1.55, and θ = 45° into Equation (2) yields the PI layers of the cells can be used to passively tune the resonance frequencies of the metamaterialsneff = 1.62. neff inequals the metamaterial-coated the refractive index LC of cells the at LCθ = in 0◦, the 45◦ ,metamaterial-coated and 90◦. The thickness LC cell at θ (600= 45 nm)°, as of presented the PI layers in exceedsFigure that5. This (200 result nm) of verifies the metamaterials. that the rubbing Therefore, directions the LC of the PI layers are of anisotropicthe cells can on be the used PI layers. to passively This fact tune reveals the that resonance the effective frequencies refractive indexof the metamate- ◦ ofrials the in LC the in themetamaterial-coated metamaterial-coated LC cellcells that at involvesθ = 0°, 45θ =°, 45andcan 90 be°. The calculated thickness from (600 nm) of Equationthe PI layers (2). exceeds that (200 nm) of the metamaterials. Therefore, the LC layers are ani- Figure4 displays that the transmittances of the metamaterials at their experimental sotropic on the PI layers. This fact reveals that the effective refractive index of the LC in resonance frequencies decrease with an increase in θ. The decrease° in the transmittances arisesthe metamaterial-coated from the anisotropic absorptionLC cell that of involves the LC in θ the = terahertz45 can be region. calculated The terahertz from Equation (2). wavesFigure are extraordinary 4 displays (ordinary) that the waves transmittances in the metamaterial-coated of the metamaterials LC cell at θat= their 0◦ (90 ◦experimental). Inresonance addition, frequencies most LCs in thedecrease terahertz with region an haveincrease a small in θ (large). The absorptiondecrease in coefficient the transmittances forarises extraordinary from the (ordinary)anisotropic waves absorption [15,16]. Therefore, of the LC the in anisotropic the terahertz absorption region. of theThe terahertz LCwaves in the are terahertz extraordinary region decreases (ordinary) the experimentalwaves in the resonance metamaterial-coated transmittances ofLC the cell at θ = 0° metamaterials° with the increase in θ. Figure5 presents that the metamaterial-coated LC cells have(90 ). similar In addition, transmittances most LCs at the in simulated the terahertz resonance region frequencies have a small of the (large) metamaterials. absorption coeffi- Thecient similar for extraordinary resonance transmittances (ordinary) arise waves from [15,16]. the fact thatTherefore, the anisotropic the anisotropic absorption ofabsorption of the LCLC is in not the considered terahertz in region the simulation. decreases the experimental resonance transmittances of the metamaterialsAnother LC with (E7, Merck,the increase Kenilworth, in θ. NJ,Figure USA) 5 presents is used to that study the the metamaterial-coated effect of the LC birefringencecells have similar of the LCtransmittances on the electromagnetic at the simulated resonance resonance of metamaterials. frequencies Metamaterial- of the metamate- coatedrials. The LC cells similar have resonance the same geometric transmittances configuration arise as from the cells the of fact Figure that1a, the but areanisotropic filled absorption of the LC is not considered in the simulation. Another LC (E7, Merck, Kenilworth, NJ, USA) is used to study the effect of the birefringence of the LC on the electromagnetic resonance of metamaterials. Metamaterial- coated LC cells have the same geometric configuration as the cells of Figure 1a, but are filled with the E7 LC. Figure 6 presents the experimental spectra of the metamaterial-

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with the E7 LC. Figure6 presents the experimental spectra of the metamaterial-coated cells coated cells filled with the E7◦ LC at θ◦ = 0° and 90°. These cells have resonance frequencies ﬁlledcoated with cells the filled E7 LC with at θthe= 0E7and LC at 90 θ. = These 0° and cells 90° have. These resonance cells have frequencies resonance of frequencies 0.533 THz of 0.533 THz and 0.542 ◦THz at θ = 0◦° and θ = 90°, respectively. Therefore, the metamaterial- andof 0.533 0.542 THz THz and at θ0.542= 0 THzand atθ = θ 90= 0°, respectively.and θ = 90°, respectively. Therefore, the Therefore, metamaterial-coated the metamaterial- cells coated cells filled with the E7 LC have a frequency shifting region of 9 GHz. ﬁlledcoated with cells the filled E7 LCwith have the aE7 frequency LC have shiftinga frequency region shifting of 9 GHz. region of 9 GHz. 0 0

-10 -10

-20 -20

Transmittance (dB) -30 Transmittance (dB) -30 θ = 0° θ = 0° θ = 90° θ = 90° -40 -400.3 0.4 0.5 0.6 0.7 0.8 0.3 0.4 0.5 0.6 0.7 0.8 Frequency (THz) Frequency (THz) Figure 6. Experimental spectra of metamaterial-coated LC cells filled with E7 LC at θ = 0° and 90°. FigureFigure 6.6. ExperimentalExperimental spectraspectra ofof metamaterial-coatedmetamaterial-coated LCLC cellscells filledfilled withwithE7 E7LC LCat atθ θ= =0 0◦°and and 9090◦°.. A simulation is performed using the software to verify the experimental spectra of AA simulationsimulation is is performed performed using using the the software software to verifyto verify the the experimental experimental spectra spectra of the of the metamaterial-coated cells filled with the E7 LC. A dielectric layer with a refractive metamaterial-coatedthe metamaterial-coated cells filledcells filled with thewith E7 the LC. E7 A dielectricLC. A dielectric layer with layer a refractive with a refractive index of index of n is deposited on the simulated PI-coated SRR (the inset of Figure 3). Figure 7 nindexis deposited of n is deposited on the simulated on the PI-coatedsimulated SRR PI-coated (the inset SRR of (the Figure inset3). Figureof Figure7 displays 3). Figure the 7 displays the terahertz spectra of the simulated PI-coated SRR with the dielectric layer at n terahertzdisplays the spectra terahertz of the spectra simulated of the PI-coated simulated SRR PI-coated with the SRR dielectric with the layer dielectric at n = layer 1.62 and at n = 1.62 and 1.70. This SRR has simulated resonance frequencies of 0.542 THz and 0.533 THz 1.70.= 1.62 This and SRR 1.70. has This simulated SRR has resonancesimulated frequenciesresonance frequencies of 0.542 THz of and 0.542 0.533 THz THz and at 0.533n = 1.62THz at n = 1.62 and 1.70, respectively. The resonance frequency of the simulated PI-coated SRR andat n 1.70,= 1.62 respectively. and 1.70, respectively. The resonance The resonance frequency frequency of the simulated of the simulated PI-coated PI-coated SRR with SRR the with the dielectric layer at n = 1.70 (1.62) equals that of the metamaterial-coated LC cell dielectricwith the dielectric layer at n =layer 1.70 at (1.62) n = 1.70 equals (1.62) that equals of the metamaterial-coatedthat of the metamaterial-coated LC cell filled LC with cell filled with the E7 LC◦ at◦ θ = 0° (90°). In addition, the polarized direction of the incident thefilled E7 with LC attheθ =E7 0 LC(90 at). θ In= 0 addition,° (90°). In the addition, polarized the directionpolarized of direction the incident of the terahertz incident terahertz waves is parallel (perpendicular) to the LC director in the metamaterial-coated wavesterahertz is parallelwaves is (perpendicular) parallel (perpendicular) to the LC to director the LC indirector the metamaterial-coated in the metamaterial-coated LC cell LC cell filled with the E7 LC◦ at θ◦ = 0° (90°). Therefore, the E7 LC has an extraordinary filledLC cell with filled the with E7 LC the at E7θ = LC 0 (90at θ). = Therefore, 0° (90°). Therefore, the E7 LC the has E7 an extraordinaryLC has an extraordinary (ordinary) (ordinary) refractive index ne (no) of 1.70 (1.62) in the terahertz region. This result reveals refractive(ordinary) index refractivene (no index) of 1.70 ne ( (1.62)no) of in1.70 the (1.62) terahertz in the region. terahe Thisrtz region. result revealsThis result that reveals the E7 that the E7 LC exhibits a birefringence of 0.08 in the terahertz region. Because the birefrin- LCthat exhibits the E7 LC a birefringence exhibits a birefringence of 0.08 in the of terahertz 0.08 in the region. terahertz Because region. the Because birefringence the birefrin- (0.08) gence (0.08) of the E7 LC is smaller than that (0.15) of the HTW114200-100 LC, the fre- ofgence the E7(0.08) LC isof smallerthe E7 thanLC is that smaller (0.15) than of the that HTW114200-100 (0.15) of the HTW114200-100 LC, the frequency LC, shifting the frequency shifting region (9 GHz) of the metamaterial-coated LC cell filled with the former regionquency (9 shifting GHz) of region the metamaterial-coated (9 GHz) of the meta LCmaterial-coated cell filled with LC the cell former filled with is smaller the former than is smaller than that (18 GHz) of the metamaterial-coated LC cell filled with the latter. thatis smaller (18 GHz) than of that the (18 metamaterial-coated GHz) of the metamaterial-coated LC cell filled with LC the cell latter. filled with the latter.

0 0

-10 -10

-20 -20 n = 1.62

Transmittance (dB) Transmittance n = 1.62

Transmittance (dB) Transmittance n = 1.70 n = 1.70

-30 -300.3 0.4 0.5 0.6 0.7 0.8 0.3 0.4 0.5 0.6 0.7 0.8 Frequency (THz) Frequency (THz) FigureFigure 7.7.Simulated Simulated terahertz terahertz spectra spectra ofmetamaterial-coated of metamaterial-coated LC cells LC ﬁlled cells withfilled E7 with LCat E7n LC= 1.62 at Figure 7. Simulated terahertz spectra of metamaterial-coated LC cells filled with E7 LC at nand = 1.62 1.70. and 1.70. n = 1.62 and 1.70.

Coatings 2021, 11, 381 7 of 8

4. Conclusions This work fabricated the metamaterial-coated LC cells with the LC directors that make the angles of 0◦, 45◦, and 90◦ with respect to the gaps of the SRRs of the metamaterials. They are used to study the effect of the rubbing directions of the PI layers of the cells on the electromagnetic resonance of the metamaterials. The experimental results reveal that the resonance frequencies of the metamaterial-coated LC cells increase with the increase in the angles, and the cells have a maximum frequency shifting region of 18 GHz. The simulated results depict that the shift of the resonance spectra of the cells arises from the LC birefringence, and the LC has a birefringence of 0.15 in the terahertz region. The metamaterial-coated LC cells are passively tunable terahertz ﬁlters due to the rubbing directions of the PI layers. The passively tunable terahertz ﬁlters can be used in terahertz sensing, terahertz communication, and terahertz imaging.

Author Contributions: Conceptualization, C.-Y.H.; methodology, W.-F.C.; software, W.-F.C.; vali- dation, W.-F.C.; formal analysis, W.-F.C., Y.-Y.L., Y.-P.C., X.-Y.L., and C.-Y.H.; investigation, W.-F.C., C.-R.L., and C.-Y.H.; resources, T.-S.L., J.-H.L., and C.-Y.H.; data curation, W.-F.C. and C.-Y.H.; writing—original draft preparation, W.-F.C. and C.-Y.H.; writing—review and editing, W.-F.C. and C.-Y.H.; visualization, W.-F.C. and C.-Y.H.; supervision, C.-R.L. and C.-Y.H.; project administration, C.-Y.H.; funding acquisition, C.-Y.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Technology (MOST) of Taiwan under Contract No. MOST 107-2112-M-029-005-MY3. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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