Metamaterial absorbers have attracted considerable attention for applications in the terahertz range. In this Letter, we report the design, fabrication, and characterization of a terahertz dual band metamaterial absorber that shows two distinct absorption peaks with high absorption. By manipulating the periodic patterned structures as well as the dielectric layer thickness of the metal–dielectric–metal structure, significantly high absorption can be obtained at specific resonance frequencies. Finite-difference time-domain modeling is used to design the structure of the absorber. The fabricated devices have been characterized using a Fourier transform IR spectrometer. The experimental results show two distinct absorption peaks at 2.7 and 5.2 THz, which are in good agreement with the simulation. The absorption magnitudes at 2.7 and 5.2 THz are 0.68 and 0.74, respectively

A. Khalid

Avitzour

David R. S. Cumming

James Grant

Landy

Qin Chen

Shalaev

Shimul C. Saha

Smith

Yong Ma

English

Crossref

Optics Letters

A terahertz polarization insensitive dual band metamaterial absorber

Ma, Y.

Chen, Q.

Grant, J.

Saha, S.C.

Khalid, A.

Cumming, D.R.S.

Enlighten: Publications

Enlighten

       Ma, Y., Chen, Q., Grant, J., Saha, S.C., Khalid, A. and Cumming, D.R.S. (2011) A terahertz polarization insensitive dual band metamaterial absorber. Optics Letters, 36 (6). pp. 945-947. ISSN 0146-9592  http://eprints.gla.ac.uk/50483  Deposited on: 15 April 2011   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk A terahertz polarization insensitive dual bandmetamaterial absorberYong Ma, Qin Chen, James Grant, Shimul C. Saha, A. Khalid, and David R. S. Cumming*Department of Electrical and Electronic Engineering, University of Glasgow, Rankine Building,Oakfield Avenue, Glasgow, G12 8LT, UK*Corresponding author: David.Cumming.2@glasgow.ac.ukReceived December 7, 2010; revised February 4, 2011; accepted February 14, 2011;posted February 15, 2011 (Doc. ID 139140); published March 10, 2011Metamaterial absorbers have attracted considerable attention for applications in the terahertz range. In this Letter,we report the design, fabrication, and characterization of a terahertz dual band metamaterial absorber that showstwo distinct absorption peaks with high absorption. Bymanipulating the periodic patterned structures as well as thedielectric layer thickness of the metal–dielectric–metal structure, significantly high absorption can be obtained atspecific resonance frequencies. Finite-difference time-domain modeling is used to design the structure of the ab-sorber. The fabricated devices have been characterized using a Fourier transform IR spectrometer. The experimentalresults show two distinct absorption peaks at 2.7 and 5:2THz, which are in good agreement with the simulation. Theabsorption magnitudes at 2.7 and 5:2THz are 0.68 and 0.74, respectively. © 2011 Optical Society of AmericaOCIS codes: 230.5750, 230.7408, 220.0220.Recently, electromagnetic (EM) metamaterials havebeen extensively studied at wavelengths from the visibleto the microwave band [1–3]. One potential applicationfor metamaterials is that they can be designed to makean EM absorber with a high absorption coefficient.The permittivity and permeability of a metamaterialabsorber can be engineered to maximize the absorptionof both electric and magnetic radiation and minimize theunwanted reflection due to impedance mismatch at theair–metamaterial interface. Metamaterial absorbers havebeen reported from the microwave to the visible band[4,5]. Most investigators have focused on the designand fabrication of metamaterial absorbers with a singleresonant frequency. In the terahertz frequency band,these metamaterial absorbers can be used to make a ter-ahertz detector with a specifically selective frequencyband. However, for spectroscopic applications, terahertzmultiband absorbers are required. Multiple band meta-material absorbers have been reported [6,7]. These de-vices are based on using electric split-ring resonatorsin the top metal layer. Multiple band absorbers may alsobe made using multiple vertically stacked metallic layers,each absorption band corresponding to a specific layer[8]. However, both of these structures are very compli-cated, placing considerable demands on both designand fabrication.In this Letter, a novel (to our knowledge) single pat-terned layer (planar) terahertz dual band metamaterialabsorber with a simple unit cell structure has been de-signed and fabricated. The top layer pattern consistsof two concentric “square rings” that exhibit two distinctabsorption bands due to strong magnetic resonances. Inaddition, the absorption resonances are insensitive to thepolarization of the incident beam due to the symmetricstructure in the unit cell, which provides more efficientabsorption for the nonpolarized incident beam.The dual band terahertz metamaterial absorber con-sists of three layers: a metallic plane layer on the bottom,a dielectric layer, and finally a second metallic layer withsquare ring unit cells on the top, as shown in Fig. 1. Theabsorber structure is fabricated on a 400-μm-thick siliconwafer used as the supporting substrate. Polyimide (PI-2545) is used as the dielectric layer to separate the twometallic layers. The thickness of the polyimide layer is4 μm. On the top layer, there are two metallic square ringsin each unit cell. The width of the metal strip is fixed at0:5 μm. The length of the side of the outer square ring is19 μm, and the length of the side of the inner square ringis 11 μm. Both of the two metallic layers are made of goldwith a conductivity of 4 × 107 S=m and a thickness of220 nm. The repeat period is 28 μm. We used the finite-difference time-domain method (Lumerical 6.5) to modelthe terahertz metamaterial absorber structure. The di-electric constant of the polyimide was assumed to be2:4þ 0:005i. The incident beam is assumed to be normalto the absorber surface. The thickness of the polyimidelayer is adjusted to be 4 μm in order to optimize the im-pedance matching of the air–sample interface.Figure 2 shows the calculated absorbance spectrum ofthe dual band absorber (sample A). The calculated ab-sorption coefficient, A, is obtained by A ¼ 1 − T − R,where T and R are transmission and reflection coeffi-cients, respectively. Since the bottom layer is a continu-ous metallic layer that has a thickness much greater thanFig. 1. (Color online) Terahertz dual band metamaterial absor-ber design and implementation: (a) sketch of the of top metallayer structure, (b) cross section of the absorber layers, (c) and(d) scanning electron micrographs of the dual band absorber.March 15, 2011 / Vol. 36, No. 6 / OPTICS LETTERS 9450146-9592/11/060945-03$15.00/0 © 2011 Optical Society of Americathe skin depth of the incident beam, the transmittanceshould be zero. Consequently, the calculation of the ab-sorbance can be simplified to A ¼ 1 − R. As shown inFig. 2(a), there are two distinct absorption peaks, whichare located at 2.7 and 5:0THz. In order to verify the originof these two absorption peaks, we also simulated anothertwo absorber structures with single square rings only forcomparison: sample B with the outer square ring patternand sample C with the inner square ring pattern. The per-iod is the same in all cases. As shown in Fig. 2(a), samplesB and C have a single absorption peak located at 2.8 and5:1THz, respectively. The simulation results reveal thatthe two absorption peaks in the dual band absorberare contributed by the outer square ring and inner squarering, respectively. The slight difference in the resonantfrequencies is due to the loading effect when both ofthe square ring patterns are combined into the dual bandabsorber.Figure 2(c) shows the calculated effective permittivity(ε0 þ iε00) and permeability (μ0 þ μ00) for the dual bandabsorber. They have similar behavior to earlier work[9]. As shown in Fig. 2(c), both ε0 and μ0 cross zero at2.7 and 5:0THz, which results in the impedance matchingat these frequencies. Meanwhile μ00 has two positive re-sonances at the corresponding frequencies. Thereforethe absorption resonances are magnetic resonances.As has been shown, a metamaterial absorber with across-shape metallic pattern on the top layer may havestrong magnetic resonance results from excitation of adipole resonance along the metal strip [8]. We suggestthat the strong magnetic resonances in our dual band ab-sorber are also induced by the magnetic polariton due tothe excitation of a dipole resonance along the side of thesquare. The resonant frequencies are determined by thelength of the sides of the metal square. The resonantstrength is tuned and optimized by adjusting the thick-ness of the dielectric layer, as shown in Fig. 2(b). Bothof the two frequency peaks are optimized when the thick-ness is in the range between 2.9 and 5:0 μm. Figure 3shows the EM absorption distribution of the dual bandabsorber at two absorption peak frequencies. As shownin Fig. 3, at 2:7THz the incident terahertz radiation ismainly trapped in the outer metallic square ring andthe adjacent dielectric layer. Similarly, at 5:0THz the ab-sorption is mainly trapped in the inner metallic squarering and its adjacent dielectric layer. The above resultsconfirm that the two absorption peaks are contributedby the inner and outer square ring patterns respectively.Experimental devices were fabricated as follows:20 nm of Ti, followed by 200 nm of Au, was depositedonto the sample as a reflection layer. A polyimide layer(PI-2545 from HD MicroSystems) was then spin-cast as adielectric layer for the absorber. Electron beam lithogra-phy was used to define the metallic pattern layer. Finally,20 nm of Ti, followed by 200 nm of Au, was evaporated,and the pattern transfer was completed by metal liftoff.Scanning electron micrograph images of the sample areshown in Figs. 1(c) and 1(d). The unit cell structure isrepeated in a 28 μm period in both x and y directions.Very good uniformity was achieved across the 1:5 cm ×1:5 cm device area.We used a Bruker IFS 66v/S Fourier-transform IR spec-trometer to characterize the fabricated devices by mea-suring their reflectance spectra. A wire grid polarizermade on high-density polyethylene was used to ensurethe incident beam is linearly polarized [10]. The incidentangle of the illumination was at 30° to the normal of theabsorber array for the reflectance measurements. Fivesamples were characterized: two of them (sample Band sample C) are single band absorbers with innerand outer square ring patterns only, respectively. Thepolyimide thickness of both samples is 4 μm. The otherthree samples were dual band absorber samples with dif-ferent polyimide layer thickness. The thickness of thepolyimide layer for each sample was measured by aFig. 2. (Color online) (a) Simulated absorption spectra forthree different terahertz absorber geometries, (b) simulatedeffect on absorption spectra for single band absorbers withdifferent thicknesses of polyimide, (c) calculated effectivepermittivity and permeability. The permeability is scaled by afactor of 10.Fig. 3. (Color online) Magnitude of the Poynting vector in thedual band absorbers: (a) plan view of top metal layer at 2:7THz,(b) cross section through center at 2:7THz, (c) plan view of topmetal layer at 5:0THz, (d) cross section through center at5:0THz.946 OPTICS LETTERS / Vol. 36, No. 6 / March 15, 2011surface profilometer. The measured thicknesses were3.4, 4.8 and 6:5 μm, respectively. Figure 4(a) shows themeasured spectra of the dual band absorber (sampleA) compared with two single band absorbers (sampleB and sample C). As shown in Fig. 4(a), there are twodistinct absorption peaks at 2.7 and 5:2THz. This resultreveals that two peaks originate from the resonancescaused by the inner and outer square ring, respectively.Figure 4(b) shows the measured absorption spectra ofthe three dual band absorber samples with various thick-nesses. The absorption increases when the polyimidelayer thickness increases from 3.4 to 4:8 μm. Both ofthe two absorption peaks are at a maximum for the sam-ple with a dielectric thickness of 4:8 μm. The absorptiondecreases when the thickness is above 6:5 μm, which isalso accompanied by a slight redshift of the absorptionpeak frequency. These trends are in good agreement withthe simulation results, as shown in Fig. 2(b). The experi-mentally obtained absorption is 68% at 2:7THz and 74% at5:2THz, which is lower than in the simulation. The mea-sured absorption peak frequencies are also slightly differ-ent from the simulation. We attribute these discrepanciesto possible variation in the thickness of the polyimidelayer and deviation of the simulation data for the dielec-tric constant of the polyimide from the actual materialparameters. We also have found through simulation thatthere is a dependence on the angle of incidence of theradiation that gives rise to a small (<5%) shift in the re-sonant frequencies, which is within the margin of errorwe observe. The off-resonance absorption is lower thanin previous reports for other devices, where 30% wasachieved [6,7]. We have also studied the polarization de-pendence of the dual band absorber devices, as shown inFig. 4(c). The absorption response is not sensitive to theincident polarization due to the 90° rotational symmetryof the top layer pattern. The Q factors are 6.75 and 4.4 forthe two resonant peaks at 2.7 and 5:2THz, respectively.In conclusion, we have designed and fabricated a po-larization insensitive terahertz dual band metamaterialabsorber constructed with simple periodically patternedstructures. The fabricated samples were characterized bymeasuring the reflectance spectra. Two distinct absorp-tion peaks were found at 2:7THz and 5:2THz, which is ingood agreement with the simulation. The absorptionmagnitude at two distinct peaks is lower than the simula-tion but nonetheless is superior to other devices thathave been published to date. While we presently envisagenormal incidence usage only for the absorber, we notethat it would be of interest to further study the effectof angle of incidence on the characteristics of the device.However, the simple structure and design of the dualband absorber will make it easier to fabricate multipleband absorbers as well as broad band absorbers infuture.References1. D. R. Smith, J. B. Pendry, and M. C. K. Wiltshire, Science305, 788 (2004).2. V. M. Shalaev, W. Cai, U. K. Chettiar, H.-K. Yuan, A. K.Sarychev, V. P. Drachev, and A. V. Kildishev, Opt. Lett.30, 3356 (2005).3. W. Cai and V. Shalaev, Optical Metamaterials (Springer,2010).4. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J.Padilla, Phys. Rev. Lett. 100, 207402 (2008).5. Y. Avitzour, Y. A. Urzhumov, and G. Shvets, Phys. Rev. B 79,045131 (2009).6. Q.-Y. Wen, H.-W. Zhang, Y.-S. Xie, Q.-H. Yang, and Y.-L. Liu,Appl. Phys. Lett. 95, 241111 (2009).7. H. Tao, C. M. Bingham, D. Pilon, K. Fan, A. C. Strikwerda, D.Shrekenhamer, W. J. Padilla, X. Zhang, and R. D. Averitt, J.Phys. D 43, 225102 (2010).8. Y. Q. Ye, Y. Jin, and S. He, J. Opt. Soc. Am. B 27, 498 (2010).9. X. Liu, T. Starr, A. F. Starr, andW. J. Padilla, Phys. Rev. Lett.104, 207403 (2010).10. Y. Ma, A. Khalid, T. D. Drysdale, and D. R. S. Cumming, Opt.Lett. 34, 1555 (2009).Fig. 4. (Color online) (a) Experimental spectra of three typesof absorbers; (b) experimental spectra of terahertz dual bandabsorber with different polyimide layer thicknesses 3.4, 4.8,and 6:5 μm; (c) polarization dependence of terahertz dual bandabsorber.March 15, 2011 / Vol. 36, No. 6 / OPTICS LETTERS 947

Optical Metamaterials (Springer,

http://eprints.gla.ac.uk/50483/1/50483.pdf

A terahertz polarization insensitive dual band metamaterial absorber

Abstract

Similar works

Full text

Available Versions

Crossref

Enlighten: Publications

Enlighten