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Liquid Crystal Tunable Dielectric Metamaterial Absorber in the Terahertz Range

Liquid Crystal Tunable Dielectric Metamaterial Absorber in the Terahertz Range

applied sciences

Communication Tunable Absorber in the Terahertz Range

Shenghang Zhou 1, Zhixiong Shen 1,2, Ruiyun Kang 3, Shijun Ge 1,2,* and Wei Hu 1,2,*

1 Key Laboratory of Intelligent Optical Sensing and Manipulation, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China; [email protected] (S.Z.); [email protected] (Z.S.) 2 Institute for Smart Liquid , JITRI, Changshu 215500, China 3 Beijing New Oriental Foreign Language School at Yangzhou, Yangzhou 225006, China; [email protected] * Correspondence: [email protected] (S.G.); [email protected] (W.H.)

 Received: 30 October 2018; Accepted: 8 November 2018; Published: 10 November 2018 

Abstract: In this paper, we propose a tunable dielectric in the terahertz (THz) range. The absorber is composed of a silicon pillar array embedded in a (LC) layer, which is sandwiched by two electrodes. By way of varying the applied bias, the LC orientation can be continuously tuned. At a saturated bias, all LCs are vertically driven, and an absorption peak of 0.86 is achieved at 0.79 THz. When the bias is turned off, the same LCs are horizontally aligned, and the absorption peak degenerates into two smaller ones. A 47% modulation depth at 0.79 THz is obtained via numerical simulation with experimental feasibility considered. Such an active THz dielectric absorber may be utilized as part of various active THz apparatuses in THz imaging, sensing, switching, and filtering.

Keywords: dielectric absorber; liquid crystal; metamaterial; tunable; terahertz

1. Introduction Terahertz (THz) , the last exploited part in the electromagnetic spectrum, have great potential in security screening [1], biological detection [2], and communication [3]. Among various THz devices, an absorber that can efficiently capture energy is of vital importance. Metamaterial absorbers (MMAs) have drawn particular attention due to its capability of arbitrary and amplitude modulation via freely designing their [4]. After Tao et al. presented the first THz metallic MMA in 2008 [5], a tremendous amount of research on MMAs have been performed [6]. Integrated with functional materials including phase-change media [7], [8], graphene [9], and liquid crystals (LCs) [10,11], MMAs exhibit tunable electromagnetic responses. The used in traditional metallic MMAs are often the highest electrically conductive materials in order to achieve high-Q in MMAs. High electrical conductivity leads to high and subsequently high ohmic loss. For applications where thermal properties are vital—e.g., THz imaging, sensing, and filtering—-free devices are preferred. To address this issue, Liu et al. developed a dielectric THz absorber based on hybrid dielectric [12] and achieved an absorption of 97.5%. The functionality of this absorber is static. To further exploit dielectric MMAs with dynamic functions is meaningful. LC presents excellent electro-optic tunability thanks to its broadband large and external field responsibility. Through spatially controlling the tilt or azimuthal directors of LC, the dynamic phase or geometric phase of THz wave can be freely manipulated [13,14]. However, these components all require a large LC thickness to accumulate sufficient phase retardation, leading to a slow response and high operating . Recently, on the basis of the distinct influence on the resonance of of surrounding LC change, various active THz

Appl. Sci. 2018, 8, 2211; doi:10.3390/app8112211 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 2211 2 of 7 Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 7

metadevices have been demonstrated, such as active filters [15,16], converters [17], and metadevices have been demonstrated, such as active filters [15,16], polarization converters [17], phase shifters [18]. Here, both the cell gap and the operating voltage are drastically decreased. and phase shifters [18]. Here, both the cell gap and the operating voltage are drastically decreased. Through precisely designing the geometry of metadevices and optimizing the matching between the Through precisely designing the geometry of metadevices and optimizing the matching between the refractive index change of LC and electromagnetic resonant boundary condition, a dynamic tuning refractive index change of LC and electromagnetic resonant boundary condition, a dynamic tuning with large modulation depth can be reached. with large modulation depth can be reached. Here, we propose a tunable THz dielectric MMA, which is composed of a silicon pillar array embeddedHere, we in proposean LC layer a tunable driven THzby graphene dielectric electrodes. MMA, which The function is composed of this ofabsorber a silicon relies pillar on arraythe embeddedoverlapping in anbetween LC layer electric driven and by magnetic graphene electrodes. resonances The of function the silicon of pillar this absorber array. We relies precisely on the overlappingset the radius between of the electric pillar andto satisfy magnetic the boundarydipole resonances condition of at the the silicon bias pillarsaturated array. state, We precisely thus it setbehaves the radius as a ofperfect the pillar absorber. to satisfy While the turning boundary off the condition bias, the at overlapped the bias saturated resonances state, are thus broken it behaves due asto a the perfect refractive absorber. index While change turning of LC. As off a the result, bias, the the absorption overlapped peak resonances is split and are its amplitude broken due drops to the refractiveto half of index the original change one. of LC. As a result, the absorption peak is split and its amplitude drops to half of the original one. 2. Tunable Dielectric Metamaterial Absorber 2. Tunable Dielectric Metamaterial Absorber The configuration of the absorber is illustrated in Figure 1a. Both substrates were 500-μm-thick fusedThe silica configuration and covered of the with absorber graphene is illustrated electrodes. in Figure One of1a. them Both c substratesould be spincoated were 500- µ withm-thick a fused5-μm silica-thick and polydimethylsiloxane covered with graphene (PDMS) electrodes. film. The One silicon of them pillar could array be spincoated could be withfabricated a 5-µ m-thick on a polydimethylsiloxane60-μm-thick heavily-doped (PDMS) silicon film. on The silicon (SOI) pillar using array a Bosch could process. be fabricated Then it on wa as 60-bondedµm-thick to heavily-dopedthe substrate silicon with oxygen on insulator- (SOI)-treated using PDMS a Bosch [19 process.]. Subsequently, Then it was the bonded top SOI to handle the substrate was withstripped oxygen-plasma-treated off with a lateral force. PDMS The [ 19unit]. Subsequently,geometry of the the silicon top SOIpillar handle array wasis shown stripped in Figure off with 1b. a lateralThe PDMS force. layer The unitseparate geometryd the pillar of the array silicon from pillar the graphene array is shownelectrode in and Figure work1b.ed The as a PDMS refractive layer separatedindex matching the pillar layer array simultaneously. from the graphene Both electrodesubstrates and could worked be spincoated as a refractive with indexa sulfonic matching azo dye layer simultaneously.(SD1) alignment Both layer, substrates and then could separated be spincoated by a 110- withμm-thick a sulfonic Mylar azofilm dye to form (SD1) the alignment cell [20], layer, as andshown then in separated Figure 1a. by Finally, a 110-µ am-thick home-made Mylar high film-birefringence to form the cellLC [NJU20],- asLDn shown-4 with in n Figureo = 1.591 a.+ 0.006i Finally, and ne = 1.90 + 0.001i around 0.80 THz could be infiltrated [21] to obtain the tunable THz dielectric a home-made high-birefringence LC NJU-LDn-4 with no = 1.59 + 0.006i and ne = 1.90 + 0.001i around 0.80MMA. THz could be infiltrated [21] to obtain the tunable THz dielectric MMA.

Figure 1. (a) The decomposition diagram of the LC integrated dielectric MMA. The yellow arrows Figure 1. (a) The decomposition diagram of the LC integrated dielectric MMA. The yellow arrows indicate the alignment direction. (b) The unit of the pillar : lattice periodicity, indicate the alignment direction. (b) The unit dimension of the pillar resonator: lattice periodicity, p: p: 210 µm; radius, r: 64 µm; height, h: 60 µm. 210 μm; radius, r: 64 μm; height, h: 60 μm. We use hybrid dielectric waveguide resonances to explain the principle of this absorber. When the We use hybrid dielectric waveguide resonances to explain the principle of this absorber. When THz wave is incident along the z-axis (Figure1a), the dielectric pillar works like a common waveguide. the THz wave is incident along the z-axis (Figure 1a), the dielectric pillar works like a common The resonator supports two different groups of hybrid modes, HE and EH. For HE mode, H /E << 1; waveguide. The resonator supports two different groups of hybrid modes, HE and EH. z For z HE while for EH mode, Ez/Hz << 1. Three dimensions of the field variation in the silicon pillar are denoted mode, Hz/Ez << 1; while for EH mode, Ez/Hz << 1. Three dimensions of the field variation in the silicon by different indices, i.e., HE and EH . Here, only the lowest magnetic dipole mode (HE ) and pillar are denoted by differentnml indices,nml i.e., HEnml and EHnml. Here, only the lowest magnetic dipole111 electric dipole mode (EH111) are taken into account [22]. Each pillar is represented by an ideal Huygens’ Appl. Sci. 2018, 8, 2211 3 of 7

source, i.e., a pair of electric and magnetic oriented in the x- (EH111) and y- (HE111) direction respectively [23]. When both resonances occur at the same frequency, impedance matching is satisfied, which means the THz wave is totally absorbed. HE111 always exists for a given resonator, while EH111 has a cutoff condition. At a resonance of λ0, the cut off condition for a pillar is given by [24]: λ r = 0.61 0 (1) 2 2 1/2 nSi − nLC

Wherein, r is the radius of the pillar, and nSi and nLC are the refractive indices of silicon and surrounded LC respectively. For the lowest modes, the desired minimum height h of the resonator can be approximated as a half-wave dipole with

λ h = 0 (2) 2nSi

Therefore, the pillar parameters are set as shown in Figure1 to ensure both HE 111 and EH111 are supported at the same frequency when the surrounding refractive index was 1.59 (bias saturation state). After turning off the bias, the surrounding refractive index increased to 1.90, inducing the cutoff of EH111. The HE111 and EH111 no longer existed at the same frequency. Therefore, the absorption peak degenerated into two different parts, leading to the resonance vanishing at the same frequency. The simulations were carried out using the commercial Lumerical FDTD Solutions. The refractive index of the silicon was 3.41 + 0.12i. Since a thin layer high transparent PDMS was utilized, we only consider the real part of its refractive index, i.e., 1.60. The LC orientation FDTD module was applied to simulate the optical properties of the absorber along with the variation of LC directors. For the x-incident polarization, the refractive index of NJU-LDn-4 could be tuned from no to ne using an electric field. Compared to the visible and near-IR range, the feature size of metamaterials in the THz region is much larger and its influence on LC anchoring can be neglected, permitting a better tunability [25]. Schematics of LC orientations in a bias saturated state is illustrated in Figure2a. In this case, the environmental refractive index was not along x-axis. As shown in Figure2b, a strong absorption peak (calculated from corresponding transmittance and reflectance) was observed at 0.79 THz. The side view magnetic and electric field distributions of a single unit at 0.79 THz are illustrated in Figure2c,d. The images reveal that the magnetic dipoles lay along the y-axis while the electric dipoles lay along the x-axis. The distribution indicated that both dipoles were highly localized in the silicon pillar, further facilitating the overlap of HE111 and EH111. The impedance match was fulfilled as both the electric and magnetic resonances occurred at the same frequency [26]. After the bias was removed, the LC returned to the original orientation, as shown in Figure3a. For the same x-incident polarization, the refractive index changed to ne. As presented in Figure3b, the absorption peaks of EH111 and HE111 modes separated. Their magnetic and electric field distributions at corresponding resonant are presented. The resonant frequency of the magnetic dipole was 0.81 THz, and the corresponding side view distribution is shown in Figure3c. The resonant frequency of the electric dipole was 0.76 THz, and the distribution of the electric field in the x-z plane is shown in Figure3d. The opposite frequency shift caused the degeneration of the absorption peak, further inducing an obvious absorption reduction. It needs to be noticed that the reduction was caused by confinement but not the loss induced by the imaginary part variation of LC, which contributed less than 5% of total absorption change. Appl. Sci. 2018, 8, 2211 4 of 7 Appl.Appl. Sci.Sci. 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 77

FigureFigureFigure 2. 2. 2. (((aaa))) TheTheThe LC LC LC orientation orientation orientation ininin the the the biasbiasbias saturated saturated saturated state. state. ( ( (bbb))) Simulated Simulated Simulated spectra spectra spectra of of of reflectance, reflectance, reflectance, transmittance,transmitttransmittanceance,, and andand absorbance absorbanceabsorbance atatat biasbiasbias saturatedsaturated state.state. The ( (cc)) magneticmagnetic fieldfield field ofof of aa a singlesingle single unitunit unit inin in thethe the yyy--z-z z plane,planeplane, and, andand ( d (()dd electric)) electricelectric field fieldfield in inin the thethex- z xxplane--zz planeplane at 0.79atat 0.790.79 THz. THz.THz. The TheThe white whitewhite rectangles rectanglesrectangles in ( cinin,d )((cc represent,,dd)) representrepresent the edge thethe ofedgeedge the pillarofof thethe unit,pillarpillar same unit,unit, hereinafter.samesame hereinafter.hereinafter.

FigureFigureFigure 3. 3. 3.(a )(( Theaa)) The The LC orientation LC LC orientation orientation in the in biasin thethe OFF biasbias state. OFF OFF (b ) Simulatedstate. state. ( (bb)) spectraSimulated Simulated of reflectance, spectra spectra of of transmittance, reflectance, reflectance, andtransmittancetransmittance absorbance,, andand at bias absorbanceabsorbance OFF state. atat Thebiasbias ( OFFcOFF) magnetic state.state. TheThe field ((cc) of) magneticmagnetic a single unitfieldfield in ofof the aa singlesingley-z plane unitunit at inin 0.81 thethe THz. yy--zz andplaneplane the atat ( d 0.810.81) electric THz.THz. fieldandand thethe in the ((dd)) xelectricelectric-z plane fieldfield at 0.76 inin thethe THz. xx--zz planeplane atat 0.760.76 TTHzHz..

ToTo vividly vividly present present thethe evolutionevolution ofof thethe absorptionabsorption peak, we depict the dependency of of the the EH EH111111 To vividly present the evolution of the absorption peak, we depict the dependency of the EH111 andand HEHE111111 modes on on the the LC LC orientation orientation (φ (ϕ) )in in Figure Figure 4a.4a. The The LC LC wa wass set set as asa uniaxial a uniaxial medium medium in and HE111 modes on the LC orientation (φ) in Figure 4a. The LC was set as a uniaxial medium in insimulation simulation and and φ waϕ wass defined defined as the as angle the angle between between the optical the optical axis and axis z-axis and inz-axis the x in-z plane. the x-z Whenplane. simulation and◦ φ was defined as the angle between the optical axis and z-axis in the x-z plane. When Whenφ = 30°,ϕ =the 30 effective, the effective refractive refractive index (n indexeff) was ( n1.66) [2 was7], and 1.66 the [27 absorption], and the absorptionpeak began peakto degenerate began to. φ = 30°, the effective refractive index (neff) was 1.66eff [27], and the absorption peak began to degenerate. ◦ degenerate.As φ increase Asdϕ toincreased 40° (neff = to1.70), 40 (then degenerated= 1.70), the degenerated absorption peaks absorption already peaks exist alreadyed. In this existed. range, In the this As φ increased to 40° (neff = 1.70), theeff degenerated absorption peaks already existed. In this range, the resonanceresonance wawass veryvery sensitivesensitive toto thethe refractiverefractive indexindex change.change. WeWe plotplot thethe absorbanceabsorbance changechange alongalong φφ Appl. Sci. 2018, 8, 2211 5 of 7 range,Appl. Sci. the 2018 resonance, 8, x FOR wasPEER veryREVIEW sensitive to the refractive index change. We plot the absorbance change5 of 7 along ϕ at 0.79 THz in Figure4b. When the refractive index changed from no to ne, a 47% decrease at 0.79 THz in Figure 4b. When the refractive index changed from no to ne, a 47% decrease of of absorption was achieved. The absorption was stable in the range of 0–30◦. Over 30◦, it changed absorption was achieved. The absorption was stable in the range of 0–30°. Over 30°, it changed rapidly. This means that this absorber is suitable for measuring the refractive index in this range. rapidly.◦ This means that this absorber is suitable for measuring the refractive index in this range. AfterAfter 60 60°,, the the absorption absorption changed changed slowlyslowly again.again. Additionally, since since the the res resonanceonance is is only only sensitive sensitive to to thethe refractive refractive index index change change along along the the incident incident polarization, polarization, the the same same LC LCembedded embedded silicon silicon pillar pillar array isarray suitable is suitable for THz for wave THz wave polarization polarization detecting detecting as well as well through through rotating rotating the the sample. sample. Compared Compared to traditionalto traditional broadband broadband and frequency-tuning and frequency-tuning absorbers, absorbers, our device our presents device presents the switchable the switchable absorption, whichabsorption, was not which revealed was innot the revealed former in investigations. the former investigations. It may benefit It notmay only benefit the not THz only absorbers the THz but alsoabsorbers THz filters but also and THz sensors. filters Besides, and sensors. if we Besides, set the pillarif we set to the some pillar different to some radius, different our radius design, our may achievedesign a may broadband achieve absorptiona broadband in absorption a switched in OFF a switched state and OFF some statenear-unity and some near absorption-unity absorption at different frequenciesat different in frequencies a switched in ON a switched state. ON state.

◦ ◦ FigureFigure 4. 4.( a(a)) SimulatedSimulated absorption spectra spectra along along with with the the LC LC orientation orientation changes changes from from 0° to 090°.to (b 90) . ◦ ◦ (bThe) The absorption absorption at at0.79 0.79 THz THz when when the the LC LCorientation orientation change changedd from from 0° to 0 90°.to 90 .

3.3 Conclusions. Conclusions InIn summary, summary, we we introduced introduced an an active active THzTHz absorberabsorber composed of of a a silicon silicon pillar pillar array array integrated integrated withwith LC. LC. Through Through spatially spatially reorienting reorienting the the LC LC directors directors with with bias, bias, its it absorptions absorption peak peak degenerated degenerate intod twointo small two peaks, small peaks, and the and absorption the absorption at the resonance at the resonance frequency frequency half reduced. half reduce Thed proposed. The proposed design addsdesign a new adds candidate a new candidate beyond traditional beyond traditional broadband broadband and frequency and frequency tuning absorbers. tuning absorbers. The switchable The absorptionswitchable will absorption benefit various will benefit applications various applications such as energy such harvesting as energy and harvesting spatial light and spatial modulations. light Also,modulat the polarizationions. Also, sensitivitythe polarization of the proposedsensitivity device of themay proposed havepotential device may in polarization have potential detectors. in Thispolarization work may detectors. supply a This practical work approach may supply for active a practical THz metadevice approach for fabrication active THz and metadevice may further inspirefabricati variouson and advanced may further THz inspire apparatuses various advanced in sensing, THz switching, apparatuses imaging, in sensing, and filtering.switching, imaging, and filtering. Author Contributions: S.Z. conceived the idea of absorber and performed the numerical simulations;Author Contributions: Z.S. and R.K. S.Z analyzed. conceived the the simulations idea of tunable and metamaterial modified the absorber article; S.G.and perf andormed W.H. the supervised numerical the whole work. simulations; Z.S. and R.K. analyzed the simulations and modified the article; S.G. and W.H. supervised the Funding:whole work.This research was supported by the National Key Research and Development Program of China (2017YFA0303700), the National Natural Science Foundation of China (NSFC) (Nos. 61575093, 61490714, andFunding 61435008),: This the research Distinguished was supported Young by Scholars the National Fund of Key Jiangsu Research Province and Development (BK20180004), Program the Fundamental of China Research(2017YFA0303700), Funds for thethe CentralNational Universities, Natural Science and Foundation Jiangsu Donghai of China Silicon (NSFC) Industry (Nos. 61575093, Science and61490714, Technology and Innovation61435008), Center. the Distinguished Young Scholars Fund of Jiangsu Province (BK20180004), the Fundamental ConflictsResearch of Funds Interest: for The the authorsCentral declareUniversities, no conflict and Jiangsu of interest. Donghai Silicon Industry Science and Technology Innovation Center.

Conflicts of Interest: The authors declare no conflict of interest.

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