Optically Transparent Metamaterial Absorber Using Inkjet Printing Technology

materials

Article Optically Transparent Metamaterial Absorber Using Inkjet Printing Technology

Heijun Jeong 1, Manos M. Tentzeris 2,* and Sungjoon Lim 1,*

1 School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, Seoul 06974, Korea; [email protected] 2 School of Electrical and Computer Engineering, College of Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA * Correspondence: [email protected] (M.M.T.); [email protected] (S.L.); Tel.: +82-2-820-5827 (S.L.)

Received: 30 August 2019; Accepted: 14 October 2019; Published: 17 October 2019

Abstract: An optically transparent metamaterial absorber that can be obtained using inkjet printing technology is proposed. In order to make the metamaterial absorber optically transparent, an inkjet printer was used to fabricate a thin conductive loop pattern. The loop pattern had a width of 0.2 mm and was located on the top surface of the metamaterial absorber, and polyethylene terephthalate films were used for fabricating the substrate. An optically transparent conductive indium tin oxide film was introduced in the bottom ground plane. Therefore, the proposed metamaterial absorber was optically transparent. The metamaterial absorber was demonstrated by performing a full-wave electromagnetic simulation and measured in free space. In the simulation, the 90% absorption bandwidth ranged from 26.6 to 28.8 GHz, while the measured 90% absorption bandwidth was 26.8–28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

Keywords: electromagnetic absorber; metamaterials; transparent absorber; inkjet printing

1. Introduction Metamaterialsareartificialstructureswithinfinitelyarrangedunitcells[1]. Sincethesetypesofstructures can be used to control the permittivity and permeability of materials, they have many electromagnetic applications; for example, they are used in frequency-selective surfaces [2,3],super lenses [4,5],terahertz applications [6,7], artificial magnetic conductors [8,9], and high-impedance surfaces [10,11], physical sensors [12,13], chemical sensors [14,15], biosensors [16,17], invisible cloaking [18,19], imaging [20,21], antennas [22–24], and circuits [25–27]. Conventionally, high-loss materials have been used to fabricate electromagnetic wave absorbers. For example, a wedge-tapered absorber, which is based on ferrite or a composite material, can excellently absorb electromagnetic waves [28–30]. However, this absorber is bulky and costly. The Jaumann absorber was proposed in 1994 to overcome these drawbacks of the wedge-tapered absorber [31]. The Jaumann absorber is based on a resistive sheet and has a resonance structure; moreover, it has a small size and high absorptivity. However, the material size should be a quarter wavelength (λ/4), and, at low frequencies, it has a bulky size. A metamaterial absorber [32–34] with a small thickness can have high absorptivity since wave absorption occurs through electromagnetic resonance. Furthermore, its fabrication is easy and involves low cost. Therefore, metamaterial absorbers have been researched in various electromagnetic ﬁelds such as stealth technology [35,36], electromagnetic interference/electromagnetic compatibility [37,38], or radio frequency (RF) sensor applications [39,40]. Recently, the development of transparent conductive materials has led to research on optically transparent electromagnetic devices [41–43]. This futuristic topic elicits interest in metamaterial

Materials 2019, 12, 3406; doi:10.3390/ma12203406 www.mdpi.com/journal/materials Materials 2019, 12, 3406 2 of 13 absorber research because of its high performance in achieving transparency. In general, optically transparent metamaterial absorbers can be used for smart window applications. Because the main function of the window is optical transparency, the optically transparent metamaterial can absorb the electromagnetic wave at the specific frequency. Therefore, the proposed absorber can be used for smart windows of buildings or aircraft [44–46]. According to these demands, metamaterial absorbers have been researched actively for realizing optically transparent absorbers [46–48], and typically, indium tin oxide (ITO) films have been used for realizing such absorbers [44,49,50]. By using ITO films for preparing conductive patterns and the ground layer, a metamaterial absorber that is optically transparent and that has high absorptivity can be realized. However, the ITO film is costly and its fabrication process is complex. In particular, since the structure of a metamaterial absorber should have infinitely arranged unit cells, the practical use of ITO-film-based optically transparent metamaterial absorbers has limitations. This paper proposes an optically transparent metamaterial absorber that can be fabricated using inkjet printing technology. In order to make the absorber optically transparent, thin conductive ring patterns were fabricated through inkjet printing technology, and the substrate used was a polyethylene terephthalate (PET) film having transparent properties. Inkjet printing [51–53] is an additive fabrication process that is cost effective and simple. Moreover, it can be used for printing on the any type of substrate. Thus, the proposed metamaterial absorber has the advantages of cost effectiveness and simplicity, unlike the fabrication process for ITO-film-based transparent metamaterial absorbers. The proposed optically transparent metamaterial absorber was successfully tested by performing a full-wave electromagnetic simulation and experimental measurements.

2. Numerical Simulations The numerical simulation involving full-wave analysis by using the ANSYS High-Frequency Structure Simulator (HFSS, ANSYS, Washington, PA, USA) was used to design the proposed metamaterial absorber. Figure1 shows the geometry of the unit cell of the proposed metamaterial absorber. To achieve the feature of optical transparency, apart from using two PET films (dielectric constant εr = 3 and loss tangent = 0.12) for the substrate and an ITO film for the bottom layer, as shown in Figure1a, a thin square conductive loop was introduced with a width of 0.2 mm on the top layer of the PET substrate. The dimensions of the substrate and conductive pattern were a = 3 mm, w = 0.2 mm, and l = 2 mm, where a is the length of the substrate and w and l are the width and length of the conductive pattern, respectively. Adhesive tape of length 0.05 mm (t2)(εr = 3 and loss tangent = 0.05) was used to bind both PET substrates, as shown in Figure1c. The thicknesses of the upper and lower PET substrates were t1 = 0.25 mm and t3 = 0.2 mm, respectively. The bottom layer was fully covered with a 5 Ω (Rs) ITO conductive sheet to prevent wave transmission. In order to achieve the best performance and quantify the sharpness, the parameter values were determined by conducting a parametric simulation study. The sharpness in the resonance can be defined by the following equation:

Resonance f requency fr Sharpness (Quality-factor) = = , (1) 3dB bandwidtdh f f 2 − 1 where, fr is resonance frequency, f 1 is lower frequency of 3 dB bandwidth, and f 2 is higher frequency of 3 dB bandwidth, respectively. Figure2 shows the simulated S-parameters for di ﬀerent values of the parameters. When the conductive loop width was varied from 0.1 to 0.2, and 0.3 mm, the resonance frequency varied from 26.5 to 29 GHz and the sharpness varied from 35 to 413, and 48, respectively as shown in Figure2a. the width was set at 0.2 mm on the basis of the proposed concept and the fabrication process capability. Next, when the length was varied from 1.8 to 2.0, and 2.2 mm, the resonance frequency varied from 31.5 to 24 GHz and the sharpness varied from 64 to 413, and 95, respectively as shown Figure2b. The length was determined to be 2.0 mm to match the resonance frequency of Materials 2018, 11, x FOR PEER REVIEW 2 of 13

Recently, the development of transparent conductive materials has led to research on optically transparent electromagnetic devices [41–43]. This futuristic topic elicits interest in metamaterial absorber research because of its high performance in achieving transparency. In general, optically transparent metamaterial absorbers can be used for smart window applications. Because the main function of the window is optical transparency, the optically transparent metamaterial can absorb the electromagnetic wave at the specific frequency. Therefore, the proposed absorber can be used for smart windows of buildings or aircraft [44–46]. According to these demands, metamaterial absorbers have been researched actively for realizing optically transparent absorbers [46–48], and typically, indium tin oxide (ITO) films have been used for realizing such absorbers [44,49,50]. By using ITO films for preparing conductive patterns and the ground layer, a metamaterial absorber that is optically transparent and that has high absorptivity can be realized. However, the ITO film is costly and its fabrication process is complex. In particular, since the structure of a metamaterial absorber should have infinitely arranged unit cells, the practical use of ITO-film-based optically transparent metamaterial absorbers has limitations. This paper proposes an optically transparent metamaterial absorber that can be fabricated using inkjet printing technology. In order to make the absorber optically transparent, thin conductive ring patterns were fabricated through inkjet printing technology, and the substrate used was a polyethylene terephthalate (PET) film having transparent properties. Inkjet printing [51–53] is an additive fabrication process that is cost effective and simple. Moreover, it can be used for printing on the any type of substrate. Thus, the proposed metamaterial absorber has the advantages of cost effectiveness and simplicity, unlike the fabrication process for ITO-film-based transparent metamaterial absorbers. The proposed optically transparent metamaterial absorber was successfully tested by performing a full-wave electromagnetic simulation and experimental measurements.

2. Numerical Simulations The numerical simulation involving full-wave analysis by using the ANSYS High-Frequency Structure Simulator (HFSS, ANSYS, Washington, PA, USA) was used to design the proposed metamaterial absorber. Figure 1 shows the geometry of the unit cell of the proposed metamaterial absorber. To achieve the feature of optical transparency, apart from using two PET films (dielectric Materials 2019, 12, 3406 3 of 13 constant εr = 3 and loss tangent = 0.12) for the substrate and an ITO film for the bottom layer, as shown in Figure 1a, a thin square conductive loop was introduced with a width of 0.2 mm on the top layer of28 the GHz. PET The substrate. thickness The of thedimensions unit cell wasof the varied substrate to analyze and conductive the correlation pattern between were the a = thickness3 mm, w (t)= 0.2 of mm,the substrate and l = 2 andmm, the where simulation a is the result.length Whenof the thesubstrate thickness and wasw and varied l are fromthe width 0.4 to and 0.5, length and 0.6 of mm, the conductivethe resonance pattern, frequency respectively. slightly changedAdhesive from tape 28 of to length 27 GHz 0.05 and mm the (t2 sharpness) (εr = 3 and varied loss tangent from 27 = to 0.05) 413, wasand 95,used respectively to bind both as shownPET substrates, in Figure 2asc. shown Lastly, in when Figure the 1c. resistivity The thicknesses of the bottom of the ground upper planeand lower was variedPET substrates from 3 to were 7 Ω, wet1 = observed0.25 mm and that transmissiont3 = 0.2 mm, respectively. coeﬃcient varied The bottom from 31layer to was27 dB fully and covered then to − − with24 dB, a 5 asΩ shown(Rs) ITO in conductive Figure2d. sheet to prevent wave transmission. −

Materials 2018, 11, x FOR PEER(a REVIEW) (b) 3 of 13

(c)

FigureFigure 1.1. GeometryGeometry of of the the unit unit cell cell of theof the proposed proposed absorber: absorber: (a) perspective, (a) perspective, (b) top, (b) and top, ( cand) side (c views) side ofviews the unitof the cell. unit cell.

TheIn order absorptivity to achieveA( ωthe) can best be performance defined as and quantify the sharpness, the parameter values were determined by conducting a parametric simulation study. The sharpness in the resonance can be A(ω) = 1 Γ(ω) T(ω), (2) defined by the following equation: − − Resonance frequency f where Γ(ω) and T(Sharpnessω) are the (Quality-factor) reflection coefficient== and transmission coefficient,r , respectively. − (1) Since the proposed metamaterial absorber is fully covered3dB bandwidtdh with a conductive f21 groundf plane under the bottom layer, the transmission coefficient is zero. Therefore, the highest absorptivity performance of thewhere, metamaterial fr is resonance absorber frequency, can be achieved f1 is lower by frequency minimizing of the 3 dB reflection bandwidth, coefficient, and f which2 is higher can befrequency defined asof follows:3 dB bandwidth, respectively. Figure 2 shows the simulated S-parameters forZ differentZM values of the parameters. When the Γ(ω) = 0 − , (3) conductive loop width was varied from 0.1 to 0.2, Zand0 + 0.3ZM mm, the resonance frequency varied from 26.5 to 29 GHz and the sharpness varied from 35 to 413, and 48, respectively as shown in Figure 2a. where Z0 is the free-space impedance (377 Ω) and ZM is the metamaterial absorber impedance. the width was set at 0.2 mm on the basis of the proposed concept and the fabrication process Thus, the highest absorptivity can be achieved when Z0 and ZM are equal, since the reflection coefficient iscapability. minimized. Next, when the length was varied from 1.8 to 2.0, and 2.2 mm, the resonance frequency variedIn orderfrom 31.5 to achieve to 24 GHz the highest and the absorptivity sharpness performance,varied from 64 the to normalized 413, and 95, impedance respectively and as reflection shown coeFigurefficient 2b. The was length simulated; was determined these parameters to be 2.0 are mm shown to match in Figure the resonance3. Figure3 afrequency shows the of normalized28 GHz. The impedance.thickness of Thethe unit proposed cell was metamaterial varied to absorberanalyze the has correlation 365.2 Ω of between real impedance the thickness and 3.99(t) ofΩ theof − imaginarysubstrate and impedance the simulation at 27.7 GHz.result. Therefore, When the itthickn can beess observed was varied that from the normalized 0.4 to 0.5, and real 0.6 impedance mm, the resonance frequency slightly changed from 28 to 27 GHz and the sharpness varied from 27 to 413, and 95, respectively as shown in Figure 2c. Lastly, when the resistivity of the bottom ground plane was varied from 3 to 7 Ω, we observed that transmission coefficient varied from −31 to −27 dB and then to −24 dB, as shown in Figure 2d.

(a) (b)

Materials 2018, 11, x FOR PEER REVIEW 3 of 13

(c)

Figure 1. Geometry of the unit cell of the proposed absorber: (a) perspective, (b) top, and (c) side views of the unit cell.

In order to achieve the best performance and quantify the sharpness, the parameter values were determined by conducting a parametric simulation study. The sharpness in the resonance can be defined by the following equation: Resonance frequency f Sharpness (Quality-factor) ==r , − (1) 3dB bandwidtdh f21 f where, fr is resonance frequency, f1 is lower frequency of 3 dB bandwidth, and f2 is higher frequency of 3 dB bandwidth, respectively. Figure 2 shows the simulated S-parameters for different values of the parameters. When the conductive loop width was varied from 0.1 to 0.2, and 0.3 mm, the resonance frequency varied from 26.5 to 29 GHz and the sharpness varied from 35 to 413, and 48, respectively as shown in Figure 2a. the width was set at 0.2 mm on the basis of the proposed concept and the fabrication process capability. Next, when the length was varied from 1.8 to 2.0, and 2.2 mm, the resonance frequency varied from 31.5 to 24 GHz and the sharpness varied from 64 to 413, and 95, respectively as shown Figure 2b. The length was determined to be 2.0 mm to match the resonance frequency of 28 GHz. The Materials 2019, 12, 3406 4 of 13 thickness of the unit cell was varied to analyze the correlation between the thickness (t) of the substrate and the simulation result. When the thickness was varied from 0.4 to 0.5, and 0.6 mm, the andresonance normalized frequency imaginary slightly impedancechanged from are 28 0.95 to and27 GHz 0, respectively.and the sharpness The optimizedvaried from impedance 27 to 413, correspondsand 95, respectively to the minimized as shown reflection in Figure coe 2c.ffi Lastly,cient, as when shown the in resistivity Figure3b. of The the proposed bottom ground metamaterial plane absorberwas varied has from a reflection 3 to 7 Ω coe, weffi observedcient of 35.1that dBtransmission at 27.7 GHz. coe Thus,fficient we varied achieved from 99.9%−31 to absorptivity −27 dB and − atthen this to frequency. −24 dB, as shown in Figure 2d.

Materials 2018, 11, x FOR PEER REVIEW 4 of 13 (a) (b)

FigureFigure 2.2.Plot Plot of of simulated simulated S-parameters S-parameters versus versus frequency frequency fordi forﬀerent different values values of parameters: of parameters: (a) width (a) ofwidth the squareof the square conductive conductive loop, ( bloop,) length (b) length of the of square the square conductive conductive loop, loop, (c) thickness (c) thickness of the of unit the cell,unit andcell, (andd) resistivity (d) resistivity of the of bottom the bottom ground ground plane. plane.

FigureThe absorptivity4 shows a simulatedA(ω) can be electric defined field as distribution and vector current density at 27.7 GHz. In Figure4a, the electric field is strongly distributed at both edges of the square ring, implying ()ω = −Γ ()ω − ()ω that the width and length are the mainA factors1 determiningT , the resonance frequency. This can(2) be verified from Figure2a,b. Similarly, the vector current shows strong flows at the top and bottom where Γ(ω) and T(ω) are the reflection coefficient and transmission coefficient, respectively. Since the sides of the square ring (Figure4b). Additionally, antiparallel flows were observed, which are part of proposed metamaterial absorber is fully covered with a conductive ground plane under the bottom a circulating loop as shown in Figure4c. layer, the transmission coefficient is zero. Therefore, the highest absorptivity performance of the metamaterial absorber can be achieved by minimizing the reflection coefficient, which can be defined as follows: ZZ− Γ(ω) = 0 M , + (3) ZZ0 M where Z0 is the free-space impedance (377 Ω) and ZM is the metamaterial absorber impedance. Thus, the highest absorptivity can be achieved when Z0 and ZM are equal, since the reflection coefficient is minimized. In order to achieve the highest absorptivity performance, the normalized impedance and reflection coefficient was simulated; these parameters are shown in Figure 3. Figure 3a shows the normalized impedance. The proposed metamaterial absorber has 365.2 Ω of real impedance and −3.99 Ω of imaginary impedance at 27.7 GHz. Therefore, it can be observed that the normalized real impedance and normalized imaginary impedance are 0.95 and 0, respectively. The optimized impedance corresponds to the minimized reflection coefficient, as shown in Figure 3b. The proposed metamaterial absorber has a reflection coefficient of −35.1 dB at 27.7 GHz. Thus, we achieved 99.9% absorptivity at this frequency.

Materials 2018, 11, x FOR PEER REVIEW 5 of 13

Materials 2019, 12, 3406 5 of 13 Materials 2018, 11, x FOR PEER REVIEW 5 of 13

(a) (b)

Figure 3. (a) Normalized impedance and (b) simulated S-parameters and absorptivity plotted against frequency.

Figure 4 shows a simulated electric field distribution and vector current density at 27.7 GHz. In Figure 4a, the electric field is strongly distributed at both edges of the square ring, implying that the width and length are the main factors determining the resonance frequency. This can be verified from Figure 2a,b. Similarly, the(a) vector current shows strong flows at the top and( bbottom) sides of the square ring Figure(Figure 3. 3. 4b).(a)( aNormalized )Additionally, Normalized impedance impedanceantiparallel and and(b flows) simulated (b) were simulated S-parametersobserved, S-parameters which and absorptivity are and part absorptivity of plotted a circulating plottedagainst loop as shownfrequency.against in frequency. Figure 4c.

Figure 4 shows a simulated electric field distribution and vector current density at 27.7 GHz. In Figure 4a, the electric field is strongly distributed at both edges of the square ring, implying that the width and length are the main factors determining the resonance frequency. This can be verified from Figure 2a,b. Similarly, the vector current shows strong flows at the top and bottom sides of the square ring (Figure 4b). Additionally, antiparallel flows were observed, which are part of a circulating loop as shown in Figure 4c.

(a) (b)

(a) (b) (c)

Figure 4.4. (a) SimulatedSimulated electricelectric ﬁeldfield distribution:distribution: (b) top view and (c) side view ofof thethe vectorvector currentcurrent density atat 27.727.7 GHzGHz onon thethe proposedproposed metamaterialmetamaterial absorber.absorber.

Figure5 5 shows shows thethe simulated simulated reflectionreflection coecoefficientfficient ofof thethe proposedproposed metamaterialmetamaterial absorberabsorber for didifferentfferent incidentincident angles.angles. The proposed metamaterial absorber has a 10 dB bandwidthbandwidth from 26.626.6 toto 28.8 GHzGHz underunder normalnormal incidence. incidence. Under Under oblique oblique incidence, incidence, the the 10 10 dB dB bandwidth bandwidth is keptis kept from from 26.6 26.6 to 28.8to 28.8 GHz GHz at at 10 ◦10°in in the the transverse transverse electric electric (TE) (TE) mode mode(c) and and 20 20°◦ in in thethe transversetransverse magneticmagnetic (TM) mode. However,Figure whenwhen 4. (a) Simulated thethe incidentincident electric angleangle field isis varieddistribution:varied fromfrom ( 0b0°◦) totopto 9090°, view◦, thethe and 1010 (c dBdB) side bandwidthbandwidth view of the is vector shiftedshifted current fromfrom 26.426.4 toto 28.5density GHz at at at 27.7 20° 20◦ GHz inin the the on TE the TE mode proposed mode and and metamaterial from from 26.8 26.8 to toabsorber. 29 29 GHz GHz at at30° 30 in◦ inthe the TM TM mode. mode. Nevertheless, Nevertheless, the theangular angular stability stability of the of the proposed proposed absorber absorber is isnot not competitive competitive to to other other metamaterial metamaterial absorbers with wider Figure incidence 5 shows angles the [54 simulated–56] because reflection we focused coefficient on the opticalof the proposed transparency metamaterial in this work. absorber An optical for differenttransparent incident metamaterial angles. absorberThe proposed with angularmetamaterial stability absorber will be has the a next 10 dB work. bandwidth from 26.6 to 28.8 GHz under normal incidence. Under oblique incidence, the 10 dB bandwidth is kept from 26.6 to 28.8 GHz at 10° in the transverse electric (TE) mode and 20° in the transverse magnetic (TM) mode. However, when the incident angle is varied from 0° to 90°, the 10 dB bandwidth is shifted from 26.4 to 28.5 GHz at 20° in the TE mode and from 26.8 to 29 GHz at 30° in the TM mode. Nevertheless, the angular stability of the proposed absorber is not competitive to other metamaterial absorbers with

Materials 2018, 11, x FOR PEER REVIEW 6 of 13

Materialswider incidence2019, 12, 3406 angles [54–56] because we focused on the optical transparency in this work.6 ofAn 13 optical transparent metamaterial absorber with angular stability will be the next work.

(a) (b)

FigureFigure 5.5. Simulated reflectionreflection coecoefficientfficient of the proposed metamaterial absorber for didifferentfferent incidentincident anglesangles underunder ((aa)) TETE polarizationpolarization andand ((bb)) TMTM polarization.polarization.

3.3. Experimental Measurements ToTo experimentallyexperimentally verifyverify thethe simulationsimulation results,results, wewe fabricatedfabricated aa prototypeprototype ofof thethe metamaterialmetamaterial absorber.absorber. The The conductive conductive square square ring ring was was printed printed using using a FUJIFILM a FUJIFILM Dimatix Dimatix materials materials printer printer(DMP- (DMP-2831,2831, FUJIFILM, FUJIFILM, Minato, Minato, Tokyo, Tokyo,Japan) Japan)with a with1 pl cartridge a 1 pl cartridge (DMC-11601 (DMC-11601,, FUJIFILM, FUJIFILM, Minato, Minato, Tokyo, Tokyo,Japan) Japan)and ANP and silver ANP silvernanoparticle nanoparticle ink (DGP ink (DGP40LT-15C, 40LT-15C, ANP, ANP, Bugang-myeon, Bugang-myeon, Sejong, Sejong, Korea). Korea). To µ µ Tofabricate fabricate the the 200 200 μm mof ofline line width, width, the the fabricated fabricated line line width width was was set set as as 150 150 μmm because the printedprinted lineline waswas a little bit bit spread. spread. The The vertical vertical and and horizont horizontalal lines lines are are observed observed at atdifferent diﬀerent drop drop spacing spacing as µ asshown shown in inTable Table 1.1 .When When the the drop drop spacing spacing is is 15 15 μm,m, the the vertical vertical and horizontal lines are unstableunstable µ becausebecause thethe ink ink spills spills out out of theof the line. line. When When the dropthe drop spacing spacing is increased is increased from 15 from to 25 15m, to it 25 is observedμm, it is fromobserved Table from1 that Table the vertical 1 that linethe isvertical stable, line but theis stab horizontalle, but the line horizontal is not clear. line When is not the clear. drop spacingWhen the is µ increaseddrop spacing from is 25increased to 35 m, from both 25 vertical to 35 μ andm, both horizontal vertical lines and arehorizontal clearly printed.lines are However, clearly printed. when µ theHowever, drop spacing when the is increased drop spacing from is 35 increased to 45, and from 55 35m, to the 45, vertical and 55 andμm, horizontalthe vertical inks and are horizontal leaking µ becauseinks are theleaking drop because spacing the is toodrop far. spacing Therefore, is too the far. drop Therefore, spacing the is setdrop as spacing 35 m tois realizeset as 35 the μm best to µ linerealize shape. the Inbest addition, line shape. when In the addition, drop spacing when isthe set drop as 35 spacingm, the is vertical set as and35 μ horizontalm, the vertical linewidth and hashorizontal only 2 andlinewidth 4.5 percentages has only 2 of and tolerance, 4.5 percentages respectively, of tolerance, as shown respectively, in Figure6. Finally,as shown the in cartridge Figure 6. Materials 2018, 11, x FOR PEER REVIEW µ 7 of 13 headFinally, is setthe at cartridge 0◦ and the head drop is set spacing at 0° and is determined the drop spacing at 35 m. is determined In addition, at three 35 μ nozzlesm. In addition, with 100 three dpi resolution were used to print the designed pattern. nozzlesTable with 1. Comparison 100 dpi resolution of the fabricated were used vertical to print and thehorizontal designed line pattern. according to the different drop spacings. Table 1. Comparison of the fabricated vertical and horizontal line according to the diﬀerent Table 1. Comparison of the fabricated vertical and horizontal line according to the different drop drop spacings. Dropspacings. Spacing Vertical Line Horizontal Line Drop Spacing Vertical Line Horizontal Line Drop Spacing Vertical Line Horizontal Line

15 μm 1515µ mμm

(unstable) (unstable) (unstable)

25 μm

35 μm

Materials 2018, 11, x FOR PEER REVIEW 7 of 13

Table 1. Comparison of the fabricated vertical and horizontal line according to the different drop Materialsspacings. 2018, 11 , x FOR PEER REVIEW 7 of 13 Drop Spacing Vertical Line Horizontal Line Table 1. Comparison of the fabricated vertical and horizontal line according to the different drop

spacings.

Drop Spacing Vertical Line Horizontal Line

15 μm

Materials 2019, 12, 3406 7 of 13 (unstable)

Table 1. Cont. (unstable) Drop Spacing Vertical Line Horizontal Line

25 μm

25 µm 25 μm

35 µμmm

Materials35 20182018 μm,, 1111 ,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1313 Materials 2018, 11, x FOR PEER REVIEW 8 of 13

45 μm 45 µμmm

5555 µμmm 55 μm

Figure 6. Comparison graph of vertical and horizontal line tolerance varying to the drop spacing. Figure 6. Comparison graph of vertical and horizontal lineline tolerancetolerance varyingvarying toto thethe dropdrop spacing.spacing. Figure 7 shows the fabricated prototype and a magnified view of the printed surface. The prototypeFigure had 7 shows dimensions the fabricated of 198 mm prototype × 198 mm and and a 67magnified × 67 unit view cells. of The the horizontal printed surface. and vertical The prototypewidths of thehad printed dimensions surface of were198 mm 0.196 × 198and mm0.193 and mm, 67 respectively, × 67 unit cells. as shownThe horizontal in Figure and 7, and vertical they widthswere very of the close printed to the surface value of were 0.2 mm0.196 used and in 0.193 the simulation.mm, respectively, as shown in Figure 7, and they were Figurevery close 8 shows to the avalue schematic of 0.2 mmand used a photograph in the simulation. of the measurement setup. For performing measurements of the prototype, the distance was set between the prototype sample and a horn

Materials 2019, 12, 3406 8 of 13 Materials 2018, 11, x FOR PEER REVIEW 8 of 13

FigureFigure 6.6. Comparison graphgraph ofof verticalvertical andand horizontalhorizontal lineline tolerancetolerance varyingvarying toto thethe dropdrop spacing.spacing.

FigureFigure7 shows7 shows the the fabricated fabricated prototype prototype and a and magniﬁed a magnified view of theview printed of the surface. printed The surface. prototype The had dimensions of 198 mm 198 mm and 67 67 unit cells. The horizontal and vertical widths of prototype had dimensions of× 198 mm × 198 mm× and 67 × 67 unit cells. The horizontal and vertical thewidths printed of the surface printed were surface 0.196 were and 0.1930.196 mm,and 0.193 respectively, mm, respectively, as shown in as Figure shown7, in and Figure they 7, were and very they closeMaterialswere tovery 2018 the close, value11, x FORto of the 0.2PEER value mm REVIEW usedof 0.2 in mm the used simulation. in the simulation. 9 of 13 Figure 8 shows a schematic and a photograph of the measurement setup. For performing measurements of the prototype, the distance was set between the prototype sample and a horn antenna as 0.5 m for far-field conditions. To avoid unexpected reflected signals, the measurement was performed in an anechoic chamber and a Salisbury screen absorber was placed behind the sample. An Agilent E8361A programmable network analyzer (AGILENT, Santa Clara, CA, USA) and two horn antennas (frequency range: about 26.5–33 GHz) were used for the measurement, as shown in Figure 8a. The reflected signals were measured from the metamaterial plane and the reverse side of the ground plane. Next, the metamaterial plane and ground plane were compared to obtain the reflection coefficient, which was referenced to the reverse side of the ground plane.

Figure 7. Fabricated prototype and magniﬁedmagnified view of the printed surface.surface.

Figure8 shows a schematic and a photograph of the measurement setup. For performing measurements of the prototype, the distance was set between the prototype sample and a horn

(a) (b)

Figure 8. (a) Schematic and (b) a photograph of the measurement setup.

Finally, the measured reflection coefficients were obtained as shown in Figure 9a. The measured reflection coefficients had a bandwidth of −10 dB from 26.8 to 28.2 GHz, and the measured reflection coeﬃcient at the resonance frequency of 27.5 GHz was −28.5 dB. Equation (2) was used for calculating the 90% absorption bandwidth, which ranged from 26.8 to 28.2 GHz as shown in Figure 9b. Since, the metamaterial absorbers have an infinite periodic structure, their absorptivity depends on the polarization angle of the incident electromagnetic wave. However, practical applications require an absorber whose performance can be kept constant even with varying polarization incidence. Therefore, metamaterial absorbers are required to have a polarization insensitive characteristic for the practical applications. Figure 10b shows the measurement of the prototype at various polarization angles to demonstrate its polarization insensitivity. As shown in Figure 10b, the prototype results hardly changed when the polarization angle was changed because the proposed metamaterial absorber was designed by symmetric structure. To verify the excellence of the proposed work, the proposed optically transparent metamaterial absorber is compared with metamaterial absorbers proposed by other studies, and the comparison is shown in Table 2. As you can see in Table 2, the proposed metamaterial absorber has the advantage not only of being cost effective compared with using ITO sheet for conductor but also of advanced transparency compared with electro-textile or other metal mesh fabric methods. Hence, from the entire numerical simulation and experimental measurements, one can simply infer that the proposed metamaterial absorber has high absorptivity having polarization insensitivity.

Materials 2018, 11, x FOR PEER REVIEW 9 of 13

Materials 2019, 12, 3406 9 of 13 antenna as 0.5 m for far-field conditions. To avoid unexpected reflected signals, the measurement was performed in an anechoic chamber and a Salisbury screen absorber was placed behind the sample. An Agilent E8361A programmable network analyzer (AGILENT, Santa Clara, CA, USA) and two horn antennas (frequency range: about 26.5–33 GHz) were used for the measurement, as shown in Figure8a. The reflected signals were measured from the metamaterial plane and the reverse side of the ground plane. Next, the metamaterial plane and ground plane were compared to obtain the reflection coeFigurefficient, 7. whichFabricated was prototype referenced and to magnified the reverse view side of the of theprinted ground surface. plane.

(a) (b)

Figure 8. ((aa)) Schematic Schematic and and ( (bb)) a a photograph photograph of of the the measurement measurement setup.

Finally, the measured reflectionreflection coecoefficientsfficients werewere obtainedobtained asas shown in Figure9 9a.a. TheThe measuredmeasured reflection coefficients had a bandwidth of 10 dB from 26.8 to 28.2 GHz, and the measured reflection reflection coefficients had a bandwidth of −10 dB from 26.8 to 28.2 GHz, and the measured reflection coefficient at the resonance frequency of 27.5 GHz was 28.5 dB. Equation (2) was used for calculating coefficient at the resonance frequency of 27.5 GHz was −28.5 dB. Equation (2) was used for calculating the 90% 90% absorption absorption bandwidth, bandwidth, which which ranged ranged from from 26.8 to 26.8 28.2 to GHz 28.2 as GHz shown as in shown Figure in9b. Figure Since, 9theb. metamaterialSince, the metamaterial absorbers absorbershave an infinite have an periodic infinite periodicstructure, structure, their absorptivity their absorptivity depends depends on the polarizationon the polarization angle of anglethe incident of the incidentelectromagnetic electromagnetic wave. However, wave. However,practical applications practical applications require an absorberrequire an whose absorber performance whose performance can be cankept be consta kept constantnt even even with with varying varying polarization polarization incidence. incidence. Therefore, metamaterial absorbers are required to have a polarization insensitive characteristic for the practical practical applications. Figure Figure 10b10b shows shows the the meas measurementurement of the the prototype prototype at at various various polarization polarization angles to demonstrate its polarization insensitivit insensitivity.y. As As shown shown in in Figure 10b,10b, the prototype results hardly changedchanged when when the the polarization polarization angle angle was changedwas changed because because the proposed the proposed metamaterial metamaterial absorber absorberwasMaterials designed 2018 was, 11 designed by, x FOR symmetric PEER by REVIEW symmetric structure. structure. 10 of 13 To verify the excellence of the proposed work, the proposed optically transparent metamaterial absorber is compared with metamaterial absorbers proposed by other studies, and the comparison is shown in Table 2. As you can see in Table 2, the proposed metamaterial absorber has the advantage not only of being cost effective compared with using ITO sheet for conductor but also of advanced transparency compared with electro-textile or other metal mesh fabric methods. Hence, from the entire numerical simulation and experimental measurements, one can simply infer that the proposed metamaterial absorber has high absorptivity having polarization insensitivity.

(a) (b)

FigureFigure 9.9. ComparisonComparison graphgraph betweenbetween simulationsimulation andand measurementmeasurement results:results: plotsplots ofof ((aa)) reﬂectionreflection coecoefficientﬃcient and and ( b(b)) absorptivity absorptivity versus versus the the frequency. frequency.

To verify the excellence of the proposed work, the proposed optically transparent metamaterial absorber is compared with metamaterial absorbers proposed by other studies, and the comparison is shown in Table2. As you can see in Table2, the proposed metamaterial absorber has the advantage

(a) (b)

Figure 10. (a) Simulated and (b) measured reflection coefficients plotted against the frequency for different polarizations.

Table 2. Comparison of the proposed optically transparent metamaterial absorber with previously reported metamaterial absorbers.

Transparency Method Frequency Transparency 1 Reference Cost Range [GHz] (>90%) [55] Electro-textile on acryl substrate 27–35 Low No [56] Metal mesh fabrics using screen printing 1.5–2.5 Low No [57] Metal mesh fabrics on ceramic substrate 2.35–2.75 High No [54] Metal mesh fabrics on glass sheet 2.4, 3.7, 5.7 Low No substrate [46] ITO sheet 6.06–14.66 High Yes Present Metal mesh introduced using inkjet 26.8–28.2 Low Yes study printing 1 Transparency (%) = (1 − opaque area/total area) × 100; ITO: indium tin oxide.

4. Conclusions This paper proposes an optically transparent metamaterial absorber fabricated using inkjet printing technology. To make the metamaterial absorber optically transparent, a thin conductive loop pattern was introduced on the top surface and PET films were used to fabricate the substrate. The thin conductive pattern was prepared using an inkjet printer, and its width was 0.2 mm. Thus, an

Materials 2018, 11, x FOR PEER REVIEW 10 of 13

Materials 2019, 12, 3406 10 of 13 not only of being cost eﬀective compared with using ITO sheet for conductor but also of advanced transparency compared( witha) electro-textile or other metal mesh fabric methods.(b) Hence, from the entire numericalFigure simulation9. Comparison and graph experimental between simulation measurements, and measurement one can simplyresults: plots infer of that (a) thereflection proposed metamaterialcoefficient absorberand (b) absorptivity has high absorptivity versus the frequency. having polarization insensitivity.

(a) (b)

Figure 10. ((aa)) Simulated Simulated and and ( (bb)) measured measured reflection reflection coefficients coefficients plotted plotted against against the the frequency for for differentdifferent polarizations.

Table 2. Comparison of the proposed optically transparent metamaterial absorber with previously Table 2. Comparison of the proposed optically transparent metamaterial absorber with previously reported metamaterial absorbers. reported metamaterial absorbers. Reference Transparency Method Frequency Range [GHz] Cost Transparency 1 (>90%) Transparency Method Frequency Transparency 1 Reference[55] Electro-textile on acryl substrate 27–35 LowCost No Range [GHz] (>90%) [56] Metal mesh fabrics using screen printing 1.5–2.5 Low No [55] Electro-textile on acryl substrate [57] Metal mesh fabrics on ceramic substrate 2.35–2.7527–35 HighLow NoNo [56][54] Metal Metal mesh mesh fabrics fabrics on glassusing sheet screen substrate printing 2.4, 3.7, 1.5–2.5 5.7 Low Low No No [57][46] Metal mesh fabrics ITO sheet on ceramic substrate 6.06–14.66 2.35–2.75 High High Yes No Present[54] study Metal Metal mesh mesh introduced fabrics using on inkjet glass printing sheet 26.8–28.2 Low Yes 2.4, 3.7, 5.7 Low No 1 Transparencysubstrate (%) = (1 opaque area/total area) 100; ITO: indium tin oxide. − × [46] ITO sheet 6.06–14.66 High Yes 4. ConclusionsPresent Metal mesh introduced using inkjet 26.8–28.2 Low Yes studyThis paper proposes anprinting optically transparent metamaterial absorber fabricated using inkjet 1 printing technology. Transparency To make (%) the =metamaterial (1 − opaque area/total absorber area) optically × 100; ITO: transparent, indium tin a oxide. thin conductive loop pattern was introduced on the top surface and PET ﬁlms were used to fabricate the substrate. 4. Conclusions The thin conductive pattern was prepared using an inkjet printer, and its width was 0.2 mm. Thus,This an opticallypaper proposes transparent an optically metamaterial transparent absorber metamaterial was realized absorber with small fabricated width andusing optically inkjet printingtransparent technology. substrate. To The make metamaterial the metamaterial absorber absorber was simulated optically transparent, using a full-wave a thin conductive electromagnetic loop patternsimulator was and introduced measured on with the a top free-space surface measurementand PET films setup. were Theused numerical to fabricate simulation the substrate. indicated The thinthat conductive the 90% absorption pattern was bandwidth prepared of using the metamaterialan inkjet printer, absorber and its ranged width fromwas 0.2 26.6 mm. to 28.8 Thus, GHz, an while experimental measurements yielded a range from 26.8 to 28.2 GHz. Furthermore, the proposed metamaterial absorber has a polarization insensitive characteristic. In conclusion, it is successfully demonstrated by the numerical simulation and measurement results.

Author Contributions: Conceptualization, S.L.; methodology, H.J.; software, H.J.; validation, H.J; formal analysis, H.J.; investigation, M.M.T.; resources, M.M.T.; data curation, H.J.; writing—original draft preparation, H.J.; writing—review and editing, S.L.; visualization, H.J.; supervision, S.L., M.M.T.; project administration, S.L.; funding acquisition, S.L. Funding: This research received no external funding. Materials 2019, 12, 3406 11 of 13

Acknowledgments: This research was supported by the Chung-Ang University Research Grants in 2019, National Research Foundation of Korea (NRF)-2018-Fostering Core Leaders of the Future Basic Science Program/Global Fellowship Program, and NSF-EFRI. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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