investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal

Investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal Mohammad P Hokmabadi, Abubaker Tareki, Elmer Rivera, Patrick Kung, Robert G Lindquist, and Seongsin M Kim Citation: AIP Advances 7, 015102 (2017); doi: 10.1063/1.4973638 View online: http://dx.doi.org/10.1063/1.4973638 View Table of Contents: http://aip.scitation.org/toc/adv/7/1 Published by the American Institute of Physics Articles you may be interested in Tunable quasi-monochromatic near-field radiative heat transfer in s and p polarizations by a hyperbolic metamaterial layer AIP Advances 121, 013106013106 (2017); 10.1063/1.4973530 AIP ADVANCES 7, 015102 (2017) Investigation of tunable terahertz metamaterial perfect absorber with anisotropic dielectric liquid crystal Mohammad P Hokmabadi,1 Abubaker Tareki,2 Elmer Rivera,1 Patrick Kung,1 Robert G Lindquist,2,a and Seongsin M Kim1,a Electrical and Computer Engineering Department, University of Alabama, Tuscaloosa, Alabama 35487, USA Electrical and Computer Engineering Department, University of Alabama in Huntsville, Huntsville, Alabama 35816, USA (Received September 2016; accepted 22 December 2016; published online January 2017) In this letter, we report the unique design, simulation and experimental verification of an electrically tunable THz metamaterial perfect absorber consisting of complementary split ring resonator (CSRR) arrays integrated with liquid crystal as the subwavelength spacer in between We observe a shift in resonance frequency of about 5.0 GHz at 0.567 THz with a V bias voltage at 1KHz between the CSRR and the metal backplane, while the absorbance and full width at half maximum bandwidth are maintained at 90% and 0.025 THz, respectively Simulated absorption spectrum by using a uniaxial model of LC matches perfectly the experiment data and demonstrates that the effective refractive index of LC changes between 1.5 and 1.7 by sweeping a kHz bias voltage from V to V By matching simulation and experiment for different bias voltages, we also estimate the angle of LC molecules versus the bias voltage Additionally, we study the created THz fields inside the spacer to gain a better insight of the characteristics of tunable response of this device This structure and associated study can support the design of liquid crystal based tunable terahertz detectors and sensors for various applications © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4973638] Metamaterials are well-known for their exotic electromagnetic characteristics such as negative refractive index, subwavelength imaging, and invisibility cloak that are impossible to elicit from natural material.1–5 Perfect metamaterial absorber is one of the metamaterial structures that has been recently the subject of growing interest.6–11 It completely absorbs electromagnetic radiation in a very narrow spectrum which is adjustable by fashioning its configuration and dimensions This exclusive feature of metamaterial absorber is very promising in designing low cost, ultrasensitive, and easy-to-use in room-temperature sensors and detectors particularly for THz band to fully exploit wide-range potential applications of this scientifically rich spectrum.12–16 Although the response of metamaterials is customized by tailoring their dimensions and configurations, in many applications it is highly desired to dynamically tune the frequency response of narrowband sensors and detectors Controlling the frequency response of metamaterials have been performed by several approaches e.g MEMS, photo-doping, and temperature.17–22 Very recently, liquid crystals (LCs) with voltage dependent birefringent properties have demonstrated great potential for modulating THz response of metamaterials.23–26 Several LC tunable THz metamaterial filters were realized by embedding metamaterials inside LC cells where either two Indium Tin Oxide layers or the metamaterial structure made of noble metal along with a conductive wire were used as two electrodes to apply a bias voltage on LC cell and thereby to align LC molecules in the direction of created bias electric field.25–29 However, these approaches usually necessitate applying a large bias voltage to fully utilize birefringent characteristic of LCs due to the creation of non-uniform and omnidirectional bias electric filed inside a Corresponding authors: lindqur@uah.edu; seongsin@eng.ua.edu 2158-3226/2017/7(1)/015102/8 7, 015102-1 © Author(s) 2017 015102-2 Hokmabadi et al AIP Advances 7, 015102 (2017) LC To better employ LC birefringent properties, two conductive but THz-transparent electrodes are required such that a uniform and unidirectional bias electric field is created between electrodes Conventional THz metamaterials made of noble metals not only create a non-uniform bias electric field but also are highly reflective at their resonance frequencies where the most interesting properties of metamaterials come from In contrast, complementary metamaterials made of noble metals, e.g CSRR, are highly transparent at their resonance frequencies and thus are practical as low-cost and easy-to-fabricate THz electrodes In this work, integrating a CSRR into a metal backplane via a subwavelength liquid crystal cell as the spacer, we numerically design and experimentally realize an electronically tunable terahertz perfect metamaterial absorber Unlike previously reported THz LC metamaterials absorber, we fabricate and assemble the structure without needing for etching and structural supporting layer, therefore we fill the whole space between CSRR and backplane with LC cell.23,26 We show that while the resonance frequency is tuned by about GHz at 0.567 THz through varying a kHz applied voltage from V to V, the measured absorbance and full width at half maximum (FWHM) bandwidth are preserved at 90% and 0.025 THz, respectively Furthermore, we apply a uniaxial model for LC to simulate absorption spectrum in different bias voltages and we obtain a perfect matching between simulation and experiment thereby we estimate the angle of LC molecules versus the applied bias voltage The schematic illustration of the designed device along with its front projection (i.e CSRR) are depicted in Figs 1(a) and 1(b), respectively where all dimensions are in micrometer The structure is basically consisted of three layers; CSRR made of 400 nm Cu on 127 µm polyimide substrate, followed by LC film as the spacer, and 400 nm Cu backplane which is made on a 400 µm Si substrate Additionally, two 200 nm polyvinyl alcohol (PVA) layers were spun on Cu backplane and CSRR as the rubbing layers for initial alignment of LC molecules in y direction (incoming polarization direction) To design the structure, finite element method was utilized with periodic conditions for all side boundaries and two ports were applied for the front and back boundaries Total number of mesh elements that were used in simulation is equal to 776971 The structure is illuminated by a continuous THz wave at a normal incident angle with polarization along the gap of SRR In simulation, we used 6×107 S/m for Cu conductivity, 1.77 for refractive index of polyimide, and 1.70 for refractive index of PVA A uniaxial anisotropic model was utilized for LC where an ordinary refractive index (in x direction) of no = 1.5 and extraordinary refractive index (in y direction) of ne = 1.7 with attenuation coefficient of αe = [cm☞1 ] and αo = 10 [cm☞1 ] were considered for LC to initially design the structure without applying a bias voltage.31,32 To design the structure two parameters were considered to be optimized: first the structure of CSRR to both maintain a resonance frequency at a target THz frequency and also support a uniform bias electric field between CSRR and backplane which is crucial for orienting LC molecules and second the thickness of LC film between CSRR and backplane to obtain perfect absorption at the target THz frequency To experimentally realize this device, two separate layers are initially fabricated First a 10 nm/400 nm Ti/Cu was deposited on 400 µm Si wafer by e-beam evaporation to make backplane layer Then by applying a standard UV-photolithography followed by e-beam evaporation and lift-off, a 10 nm/400 nm Ti/Cu was deposited on 127 µm Kapton polyimide substrate to form CSRR layer For each layer, a 200 nm PVA was spin coated on Cu and rubbed to provide a homogenous alignment of the LC film with a director axis initially parallel to the polarization of the incoming THz beam To incorporate LC film between layers, a 25 µm mylar sheet was used as a spacer on top of backplane layer The mylar was cut at a given active area of the device and the gap was filled with LC film Then, CSRR layer which was additionally supported by a thick Teflon film was positioned on top of the mylar and eventually the whole layers were assembled together by using four clamps LC material in this device is E7 and the film thickness is 25 µm Figures 1(c) and 1(d) are a picture of the assembled sample and an optical microscopic image of CSRR, respectively To characterize the fabricated sample, we used THz time domain spectroscope in reflection mode such that the absorption was obtained by A = – R, with R as the reflection A schematic demonstration of initial alignments of LC molecules between two electrodes is illustrated in Fig 1(e) where the applied voltage is equal to zero By increasing the applied voltage, LC molecules will be eventually oriented in a direction normal to the surface of the electrodes (Fig 1(f)) As a result, 015102-3 Hokmabadi et al AIP Advances 7, 015102 (2017) the incident THz field inside the spacer will experience different refractive indices of LC dependent upon the applied bias voltage and hence a change in absorber response is expected to be observed Figure 2(a) compares simulated absorption spectrum (red) with the measured spectrum (black) where the applied voltage between two electrodes is zero in experiment and a uniaxial model is used for refractive index of LC with nxx = nzz = no = 1.5 and nyy = ne = 1.7 in simulation where αe = [cm☞1 ] and αo = 10 [cm☞1 ] were considered for the loss of LC in THz spectrum.31,32 As observed in the figure, there is a good matching between simulation and experiment The measured absorption spectra versus kHz applied voltage with a square waveform is depicted in Fig 2(b) where the amplitude of applied bias voltage varies from V to 10 V and results in a redshift of resonance frequency We apply kHz for modulating the bias voltage since this modulation frequency offers large change in refractive index of E7 LC cell in our target THz spectrum without generating impurities or triggering FIG (a) Schematic demonstration of LC metamaterial absorber where the incident polarization is normal to absorber and polarization is along the gap of CSRR, (b) CSRR with all dimensions in micrometer, (c) picture of assembled sample, (d) optical microscopic image of CSRR, (e) artistic side view of the structure where the applied bias voltage is zero and LC molecules are initially aligned in the direction on incoming THz polarization, and (f) artistic side view of structure when the applied bias voltage is much larger than zero resulting in orientation of LC molecules normal to the surface of CSRR and backplane 015102-4 Hokmabadi et al AIP Advances 7, 015102 (2017) FIG (a) Comparison between simulated (red) and measured (black) absorption spectrum of the LC metamaterial absorber and (b) measured absorption spectrum versus applied bias voltage from V to 10 V the formation of charges in the LC that may screen the external filed.26,30 The created kHz electric field between electrodes tends to rotate the LC molecules from a parallel/vertical orientation (0 V in Fig 1(e)) to a perpendicular orientation with respect to the electrodes (10 V in Fig.1(f)) This results in a change of the effective refractive index for the extraordinary wave In other words, the refractive index of THz electric field components will decrease from ∼1.7 to ∼1.5 along the vertical (y) direction, however, will remain constant along the horizontal (x) direction, and will increase from ∼1.5 to ∼1.7 along the longitudinal (z) direction.31,32 Figure 3(a) demonstrates a scatter graph of the measured resonance frequency versus bias voltage where the x axis is in logarithmic scale As observed from Figs 2(b) and 3(a), while FWHM bandwidth and peak of absorption preserve at 0.025 THz and 90%,the resonance frequency is redshifted from ∼0.567 THz to ∼0.53 THz by increasing the bias voltage The curve in Fig 3(a) is indeed representative of a typical LC modulation where the modulation is saturated around V To gain a better insight of the function of device at different bias voltages, we simulated the structure by using a uniaxial model of LC where LC’s refractive index is modeled by  nxx = no  0   nyy = neffy nLC =    0 nzz = neffz  (1) n2 n2 n2 n2 = = e o e o and neffz where θ is the angle between LC’s director in which neffy n2 cos2 θ+n2 sin2 θ n2 sin2 θ+n2 cos2 θ e o e o axis and y axis (inset of Fig 3(b)).33 It is worth mentioning that this angle is indeed an average 015102-5 Hokmabadi et al AIP Advances 7, 015102 (2017) FIG (a) Measured resonance frequencies versus applied voltage with estimated angle of LC’s director axis (θ) obtained by simulation (horizontal axis is in logarithmic scale), and (b) simulated resonance frequency versus angle of LC’s director axis (θ) angle of LC molecules over the whole thickness of LC film In this equation, θ governs the effect of bias voltage on the direction of LC molecules Figure 3(b) shows simulated resonance frequency of LC metamaterial absorber versus θ in which a redshift in resonance frequency from ∼ 0.567 THz to ∼0.562 THz is observed which is in a good agreement with experiment Comparing Figs 3(a) and 3(b) additionally shows that the simulated resonance frequency versus θ and the measured resonance frequency versus bias voltage also follow a very similar trend We compared and matched simulated resonance frequency versus θ with experimental resonance frequency versus bias voltage to estimate the angle of LC’s director (θ) in different bias voltages The result are marked in Fig 3(a) and are summarized in Table I As seen from Fig 3(a), by changing the bias voltage from V to 0.3 V the resonance frequency remains unchanged although simulation predicts change in θ from 0◦ to 30◦ From 0.5 V up to V, the resonance frequency drops and θ monotonically increases For bias voltages equal or larger than V the resonance frequency stops shifting and θ gets equal to 90◦ indicating that all LC molecules are perpendicularly oriented relative to CSRR and backplane Despite a good agreement between simulation and experiment and estimation of the associated angle of molecules versus bias voltage, the shift in resonance frequency is smaller than an expected TABLE I Estimated angle of LC’s director axis versus bias voltage obtained by simulation Voltage (V) 0.1 0.2 0.3 0.5 0.8 10 Θ◦ 10 20 30 40 50 60 70 80 90 90 90 015102-6 Hokmabadi et al AIP Advances 7, 015102 (2017) TABLE II Comparison of the functionality of reported LC tunable metamaterial structures Metamaterial Structure Fish-scale28 Cross29 SRR-CW27 SRR25 ERR Absorber 26 CSRR Absorber (our study) Amplitude Modulation (%) Frequency Modulation (%) 20 22% 19% 4.3 30 2.5 1.1 1.1 4.6 0.8 LC Type Bias Voltage (V) LC mixture 1825 2002 B NLC 2002C NLC LC 1859 5CB E7 20 200 300 80 shift if one could fully utilize the ∼0.15 birefringence shift of E7 LC film.26,30,31 It is also recognized that the trend shown in Table I is not the typical behavior of a nematic liquid crystal, E7 with homogeneous alignment, which generally shows a threshold behavior with negligible tilt angle below FIG THz electric filed (V/m) and current density distribution (A/m2 ) on CSRR (a) and backplane (b) at resonance frequency in V case, (c) applied bias electric field oriented in longitudinal direction demonstrating a uniform bias electric field between CSRR and backplane to tune LC molecules in z direction, (d) and (e) side views of THz electric filed vectors inside LC on x-y plane which is located µm away from CSRR (d) and backplane (e) at 0V 015102-7 Hokmabadi et al AIP Advances 7, 015102 (2017) the critical voltage, Vc.34 The functionality of our LC tunable metamaterial absorber is compared with similar metamaterial structures in Table II To gain a better understanding of the absorption mechanism of this device as well as explanation of the unexpected small frequency shift we analyzed the simulated THz electric field distribution in this structure Figures 4(a) and 4(b) show the THz electric field and current density distribution created on CSRR and Cu backplane at resonance frequency in V case As the incident THz wave shines the structure, it excites a dipole moment and an oscillating current in CSRR as seen in Fig 4(a) This created dipole moment induces another dipole moment on backplane with an opposite direction of that in CSRR such that the induced current on Cu backplane is in reverse direction of that in CSRR as well In other words, the dipole moment in CSRR scatters THz radiation in backward and forward directions thereby the incident THz beam is partially reflected from CSRR and partially transmitted into the LC film The transmitted wave undergoes a multiple reflection between Cu backplane and CSRR and the compounded wave of multiple reflections is eventually reflected back from CSRR At resonance frequency this compounded reflected radiation gets out of phase with the initially reflected beam, therefore they cancel out each other and a complete absorption is obtained The simulated bias electric field between CSRR and backplane is observed in Fig 4(c) where it shows the existence of a uniform electric field inside spacer which guaranties longitudinal (z direction) alignment of LC molecules at voltages greater than V However, the explanation of unexpected small frequency shift is found in the complex electric field distribution of the THz wave in the LC film Figures 4(d) and 4(e) are side view illustrations of the structure in which the THz electric field vectors inside LC at µm away from CSRR (Fig 4(d)) and µm away from Cu backplane (Fig 4(e)) are demonstrated As observed in these figures, THz electric field possesses complex polarization inside LC Although in the vicinity of backplane (Fig 4(e)) the polarization tends to be oriented in longitudinal (z) direction, the electric field is more likely polarized in all directions in a close vicinity of CSRR (Fig 4(d)) The optimum THz polarization inside LC cell to fully utilize ∼0.15 birefringence of E7 LC is indeed in either y or z directions However as observed in Figs 4(d) and 4(e), THz wave inside LC is not fully polarized in y or z directions As a result, THz radiation does not experience the maximum change in refractive index of LC when LC molecules rotate from y to z direction by applying a bias voltage from V to V In summary, we numerically designed and experimentally fabricated a LC tunable THz perfect metamaterial absorber We used CSRR as the frequency selective layer which is both conductive and also transparent for narrowband THz radiation The space between CSRR and Cu backplane was filled with E7 LC without needing for etching and supportive dielectric layer Although the resonance frequency at 0.567 THz shifts by ∼5 GHz via applying a bias voltage at 1KHz from V to V, the FWHM bandwidth and absorbance maintains at 0.025 THz and 90%, respectively Simulation by using a uniaxial model of LC matched very well with experiment through which we estimated LC’s director angle for different bias voltages Analyzing the electric field of THz radiation showed 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