Pramana – J. Phys. (2019) 92:92 © Indian Academy of Scienceshttps://doi.org/10.1007/s12043-019-1757-8

Design of polarisation-dependent multiband terahertzfrequency-selective surface using two resonators

S SASI PRINCY∗, B S SREEJA, E MANIKANDAN and S RADHA

Department of ECE, SSN College of Engineering, SSN Nagar, Kalavakkam, Chennai 603 110, India∗Corresponding author. E-mail: sasiprincys@ssn.edu.in

MS received 14 August 2018; revised 22 November 2018; accepted 29 November 2018;published online 4 April 2019

Abstract. The aim of this paper is to present the frequency-selective surface (FSS) filter operating in the terahertzregime, emphasising on its polarisation-dependent nature. The FSS filter consists of two-concentric hexagonal-shaped metal strips embossed on a gold layer over the teflon substrate, created in the form of a split ring resonator(SRR). The emphasised polarisation-dependent nature of the FSS structure has been proved by analysing thefrequency response. Numerical simulation has been done using the CST microwave studio software. Resonanceoccurs at five frequencies in the transverse electric (TE) mode and at four frequencies in the transverse magnetic(TM) mode, describing the polarisation-dependent nature of the proposed FSS filter structure.

Keywords. Frequency-selective surface filter; terahertz; polarisation.

PACS Nos 42.60.Da; 42.79.Ci

1. Introduction

The regime of the frequency spectrum which liesbetween the infrared region and the microwave regionis the terahertz region (sub-millimetre regime) whichpossesses certain characteristics of both infrared wavesand microwaves. This region of terahertz radiation isfrom 0.3 to 10 THz [1]. Recently, researchers haveincreasingly focussed on this electromagnetic spectrumbecause there are very few studies on this due to theunavailability of sources for generating terahertz radia-tion and the lack of mechanisms for its detection and,most importantly, the techniques for measuring suchhigh frequencies are not available. This has stirred activeresearch in this area, which is still in its infancy, such thatthe incredible merits of the terahertz frequency regionin the electromagnetic spectrum can be explored andused efficiently. Devices, such as frequency-selectivesurfaces, metamaterials, polarimetric devices, etc., haveplayed vital roles in the recent era of sub-millimetrerange. The novel characteristics of these devices enabletheir application in the terahertz band of frequency ofoperation because they satisfy the current needs of thecommunication, security, space science, biotechnologyand material characterisation domains [2]. The terahertzdevices provide key insights in the terahertz frequencydomain as they meet the requirements of being ableto operate in the sub-millimetre wavelengths [3]. The

frequency band of 0.1–10 THz is a very broad bandand its frequency spectrum has very much been thefocus of recent research trends because of its wide-ranging use in several fields. Devices such as filtersand absorbers with dual band, triple band, quad band,multiband and broadband responses are needed to oper-ate in the terahertz frequency band spectrum [4–7].The techniques of generating these types of frequencyresponses in the terahertz regime involve the use ofmultilayered frequency-selective surface (FSS) struc-tures and FSS with defected ground structures (DGS).Li and Ding [8] proposed a method of multilayeredmicrostructure where the rectangular ring resonatormetamaterial microstructure is stacked in a periodicmanner for achieving a broadband band-stop filter fre-quency response [8]. This response is achieved by meansof longitudinal coupling between the multilayered stackof rectangular ring resonators in the direction of trans-mission obtaining a bandwidth of 1.1 THz. Zhang etal [9] designed a double-layered metamaterial operat-ing in the terahertz regime using negative photoresistas an isolation layer. The substrate is a polyethy-lene terephthalate (PET) film and photolithography isthe fabrication process. Band-stop filter exhibiting afrequency response of 0.1 THz bandwidth has beenachieved due to the principle of coupling between thedouble-layered metamaterial microstructures. The mul-tilayered complementary metamaterial structure is also

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used for achieving second-order band-pass filter whichoperates in the terahertz frequency band [10]. The quartzcrystal is used for achieving the same along with thedesired properties of flat transmission in the pass-band,low insertion loss, steep skirts and high out-of-bandrejection. The filter design with DGS using microstripin terahertz frequency was proposed by Kumar andKartikeyan [11]. The proposal is for the band-stop fil-ter using the dumbbell-shaped DGS designed usingbenzocyclobutene as the substrate. The advantage ofemploying DGS is that compactness can be achievedby using a microstrip line of 50 �, λ/4 structure topol-ogy and wide-band frequency response. It achieves aninsertion loss of −25.8 dB at 1.4 THz. This finds appli-cations in the biomedical field for sensing, detectingand testing DNA [11]. Chaimool and Akkaraekthalin[12] proposed a wide-band band-pass filter with a widestop-band. This proposal uses a metamaterial-based res-onator and the DGS. The metamaterial-based resonatorhas a non-bi-anisotropic split-ring resonator (NB-SRR)structure and the DGS is a cross-shaped DGS cou-pled with the NB-SRR. The harmonics of first orderis eliminated using a technique called the zero-degreefeed structure and the second-order harmonics is treatedusing the embedded slot-loaded resonator at the inputand the output ports, thereby obtaining additional trans-mission zeros near the pass-band edges and increasedstop-band rejection. Thus, a wide-band band-pass filterwith a fractional bandwidth of 63% and low insertionloss in the stop-band has been achieved. Recently, afive-band band-stop filter using metamaterial has beenreported [13].

In the proposed design of the two-concentric hexa-gonal-shaped split-ring resonator structure, the aim ofachieving a multiband frequency response in the tera-hertz regime by considering the FSS structure solely isemphasised and accomplished.

2. Design

The design of the proposed two-concentric hexagonal-shaped FSS filter involves a dielectric substrate witha conductive metal coated over it. The dielectric sub-strate used for the simulation of the proposed structureis teflon, whose dielectric constant ε = 2.1 and the losstangent value tan δ = 0.0002. The conducting metalto be coated over the dielectric substrate is gold whosevalue of conductivity is σ = 4.56 × 107 S/cm. Thefollowing method can be adopted for achieving this two-layered structure: The metallic thin film can be depositedon the teflon substrate by the sputtering process. Thesputtering process allows smooth and uniform deposi-tion of the gold metal onto the teflon substrate, creatingthe required two-layered structure. Using fabrication

Figure 1. Schematic diagram of the hexagonal SRRstructure.

techniques, such as direct writing, this structure canbe fabricated easily [14]. The teflon substrate chosenis 250 μm thick and the conducting gold layer coatedover it is 2 μm thick. Then, laser micromachining canbe done to obtain the FSS filter.

The dimensional specification of the FSS filter is asfollows: the substrate is 500 μm long and 500 μm wide.The two-concentric hexagonal-shaped metallic stripsthat are embossed on the gold layer are 30 μm wideeach and 2 μm thick, which is the thickness of the goldlayer. The gap between the two-concentric hexagonalmetallic strips is 30 μm. The split ring resonator (SRR)has been obtained by removing gold metal of 40 μmwidth from the embossed hexagonal metallic strips. Theremoval of the metal is done at the right end of the outerhexagonal metallic strip and at the left end of the innerhexagonal metallic strip. The schematic diagram of thetwo-concentric hexagonal FSS filter is shown in figure 1.

Table 1 provides the numerical values of the param-eters of the designed two-concentric hexagonal-shapedSRR-FSS filter.

3. Results and discussion

The proposed FSS filter has been simulated usingthe CST microwave studio. The finite-element methodis adopted by CST while simulating the proposeddesign. The designed structure has been subjected to thex-polarised wave of terahertz frequency incident on itand it resonates at five frequencies in the transverse elec-tric (TE) mode and four frequencies in the transversemagnetic (TM) mode. The resonant frequencies in theTE mode are as follows: 0.078, 0.142, 0.3, 0.39 and 0.47

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Table 1. Design parameters of the proposed two-concentrichexagonal-shaped SRR-FSS filter structure.

Parameters Dimensions (μm)

l 500w 500r1 150r2 210g1 40g2 40w1 30w2 30

THz. The resonant frequencies in the TM mode are asfollows: 0.194, 0.308, 0.456 and 0.497 THz. The bound-ary conditions that are followed during the simulationare unit cell boundary conditions applied along the x-direction and y-direction for the purpose of obtainingan infinite array of the proposed structure by means ofreplication. The ports of the designed FSS filter are theFloquet ports implemented in the z-direction.

The Floquet ports are with two orthogonal modesand the transmission coefficient S21 is observed there.The transmission coefficient of the designed FSS filteris shown in figure 2. From figure 2, it is inferred that

the proposed FSS filter is polarisation-dependent. In theTE mode, the structure exhibits five resonant frequen-cies, whereas in the TM mode, the designed structureexhibits four resonant frequencies. The difference inthe frequency response is shown clearly in figures 2aand 2b, where both the TE and TM mode responsesof the designed FSS filter structure are plotted provingthe polarisation-dependent nature of the FSS filter. Thesame process is repeated for different incident angles,i.e. the angle between the wave vector and the z-axis inthe y–z plane and the corresponding frequency responseof the structure in the TE and TM modes has beenobtained and plotted and is shown in figures 3a and 3b,respectively.

The surface current of the five resonant frequen-cies is given as follows: as a matter of fact, thecurrent distribution is almost ideal due to the perfectconductance of the metals in the terahertz frequencyregime. This underlying mechanism in the terahertzband leads to optimal geometries, unit cell boundaryconditions and its excitation. The numerical simula-tion has also been done and therefore the results ofstrong and sharp resonances have been obtained. At0.078 THz, the first resonant frequency, the surfacecurrent is more concentrated on the outer hexagon in

Figure 2. (a) Five-band resonance in the TE mode and (b) four-band resonance in the TM mode.

Figure 3. Transmission coefficient for different angles of incidence in the (a) TE mode and (b) TM mode.

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which the current distribution is less at the split end andgradually increases along the hexagonal path, becom-ing more concentrated on the opposite side of the splitend. The inner hexagon is responsible for the secondresonant frequency of 0.142 THz. Similar to the firstresonant frequency, the concentration of the surfacecurrent at 0.142 THz is less at the split end side andmore on the opposite side as it gradually increases.At the third and fourth resonant frequencies, i.e. 0.3and 0.39 THz, the surface current distribution is moreconcentrated on the alternative corners with respect tothe split end of the outer and inner hexagons, respec-tively. At the split end, the current distribution is lesswhile it is concentrated on the next immediate hexagonal

corner and decreases in the consecutive hexagonalcorner and it increases in the next hexagonal cornerwhich is the opposite end of the split end of the hexago-nal metal strip. The fifth resonant frequency, i.e. at 0.47THz, the surface current diverges from the middle of theupper and lower arms of the outer hexagonal metal stripand it is more concentrated on the end corners of theupper and lower arms. The outer hexagon attributed tothe fifth resonant frequency. The depiction of the sur-face current distribution of the designed two-concentrichexagonal-shaped SRR-FSS filter in the TE mode isshown in figures 4a–4e representing the five resonantfrequencies of 0.078, 0.142, 0.3, 0.39 and 0.47 THz,respectively.

Figure 4. Surface current distribution in the TE mode at the five resonant frequencies: (a) 0.078 THz, (b) 0.142 THz,(c) 0.3 THz, (d) 0.39 THz and (e) 0.47 THz.

Figure 5. Electric field in the TE mode at the five resonant frequencies: (a) 0.078 THz, (b) 0.142 THz, (c) 0.3 THz,(d) 0.39 THz and (e) 0.47 THz.

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Figure 6. Magnetic field in the TE mode at the five resonant frequencies: (a) 0.078 THz, (b) 0.142 THz, (c) 0.3 THz,(d) 0.39 THz and (e) 0.47 THz.

The absolute electric field at the five resonantfrequencies of the designed two-concentric hexagonal-shaped split-ring FSS resonator in the TE mode is shownin figures 5a–5e. At the first resonant frequency of0.078 THz, the two-concentric hexagonal-shaped SRRshows more accumulation of the electric field in thesplit ends. At the second resonant frequency of 0.142THz, the electric field distribution in the two-concentrichexagonal-shaped SRR is highly accumulated only atthe split end and at the two successive arms from bothsides of the split end. At resonant frequencies 0.3 and0.39 THz, the absolute electric field is concentrated onthe outer and inner hexagon, respectively, and exhibitsmore concentration at the split end and its alternativecorners with respect to the split end. At the fifth res-onant frequency of 0.47 THz, the field distribution ismore concentrated in the outer hexagonal arms ratherthan at the corners as before.

The magnetic field distribution of the five resonantfrequencies of the TE mode is reciprocal to the electricfield distribution and is shown in figures 6a–6e.

4. Conclusion

We have designed a polarisation-sensitive multibandFSS filter using a flexible teflon substrate coated withgold as the conductive material. The TE and TM modesexhibiting five and four resonant frequencies, respec-tively, prove the polarisation dependency of the FSSfilter. The multiband characteristics of the FSS filterhave also been justified by the surface current and theelectric field distributions. The polarisation sensitiv-ity of the designed FSS filter is of great advantage indetecting and sensing applications using multibandreflector antennas, multiband radomes and spatial filters.

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