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Novel terahertz metasurfaces based on complementary coupled split ring resonators
Optical Materials 99 (2020) 109596
Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Novel terahertz metasurfaces based on complementary coupled split ring resonators Ibraheem Al-Naib Biomedical Engineering Department, College of Engineering, Imam Abdurahman Bin Faisal University, Dammam 31441, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Terahertz time-domain spectroscopy Metasurfaces Babinet’s principle Biomedical sensing
In this paper, we propose a terahertz metasurface unit cell based on an efficient coupling between two completely symmetric complementary split-ring resonators via a coupling slot. This coupling allows the excitation of a dark eigenmode resonance that is otherwise forbidden. In turn, the radiation losses are highly suppressed and the excited eigenmode features a quite large quality factor. Interestingly, this kind of coupling leads to the resto ration of the symmetry of the unit cell of the coupled resonators with respect to the terahertz field illumination. More importantly, we show that the quality factor of this dark mode resonance can be tuned by modifying the coupling slot length and reach a high value of 67.4. Furthermore, a laser beam machining technique can be easily exploited during the fabrication process of this design. Besides using this design as a sharp bandpass filter, it can be utilized as an efficient biosensor. We show that a sensitivity level of 6.3 � 104 nm/RIU is indeed doable using the proposed structure. This design may emerge as a potential candidate for future biosensors and hence utilized during the development of new cancer biomarkers.
1. Introduction In the last twenty years, a novel type of materials has been explored, namely metamaterials for 3D structures and metasurfaces for 2D planar structures. The design flexibility of this type of materials offered intriguing optical properties that attracted a lot of attention and proved to be potential candidates for a wide spectrum of applications. In turn, various structures have been developed for diverse applications across the electromagnetic spectrum [1–4]. A plethora of terahertz (THz) de vices has been designed and developed such as high-performance filters, perfect absorbers, modulators, and biomedical sensors [5–7]. More specifically, utilizing THz technology for different biomedical applica tions has motivated many scientists to come up with novel designs and devices [7–9]. More importantly, the identification of chemical and bimolecular analytes without labeling the samples using terahertz technology has been of particular interest [10–16]. For instance, biosensors are crucial devices that can be utilized in cancer diagnosis and in drug discovery. Interestingly, they can be designed to determine drug effectiveness at various target sites and to detect emerging cancer biomarkers. Unfor tunately, there is a number of challenges facing different techniques: (i) Various techniques rely on labeling the targets by adding some dyes to the targets that in turn may change the original properties of the targets.
(ii) Some other techniques are invasive. Luckily, there is no need to stain the samples when various optical techniques are used. Moreover, they are non-invasive, but the photon energy is high enough to change the target properties. Hence, terahertz frequencies with meV photon energy represent a good candidate to solve this problem. However, many bio sensors suffer from low field confinement and hence low light-matter interaction that leads a very low sensitivity. Therefore, there is a crucial need to confine the field in a small area, i.e. using high quality-factor resonators. Conventional terahertz metasurfaces are typically composed of metallic symmetric structures on top of a dielectric substrate. Hence, a dipole resonance with in-phase current distribution can be easily excited in such structures and feature a quite broadband response that leads to a low quality-factor (Q-factor). The main reason behind this low Q-factor is the high level of ohmic and radiation losses. The ohmic losses due to the dielectric substrates at THz frequencies can be minimized by using substrates with very low absorption coefficient. Likewise, using high conductivity metals such as gold or aluminum leads to diminished ohmic losses. Therefore, we are left with the radiation loss channel as the main reason behind the broadband response. Hence, scientists have proposed novel designs with out-of-phase current distribution in order to limit the radiation loss [17,18]. Fano-type resonators are considered one of the well-known techniques
E-mail address: [email protected]. https://doi.org/10.1016/j.optmat.2019.109596 Received 3 November 2019; Received in revised form 21 November 2019; Accepted 1 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
I. Al-Naib
Optical Materials 99 (2020) 109596
Fig. 1. Schematic of the metasurface unit cells consists of: (a) two comple mentary symmetric split-ring resonators with the detailed geometric di mensions and (b) two complementary coupled split-ring resonators explaining the coupling slot length (2s). The inset of the figure shows the polarization of the electric field illumination.
Fig. 2. Transmission (a) and reflection (b) amplitude spectra of the comple mentary split-ring resonators (red dashed line) and complementary coupled split-ring resonators (green solid line) unit cells. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
by exciting an out-of-phase eigenmode resonance. It can be easily designed by breaking the symmetry of symmetric split-ring resonators, with respect to the illumination electric field, in order to achieve a sharp response and hence high Q-factors [19–21]. This kind of resonator consists of two metal arms with different lengths. In order to enhance the metasurface perfor mance, various designs have been proposed based on this scheme [22,23]. Moreover, other strategies have been developed with supercells made of multiple resonators that sustain a sharp resonance [24–28]. Another approach to achieve sharp resonances was developed by utilizing electromagnetic-induced-transparency (EIT) via coupling a radiative element with a subradiant element [29–31]. Quite complicated and expensive photolithographic processes have to be followed in order to fabricate such designs as those metallic resonators have to be patterned on top of the dielectric substrates. Moreover, the THz signal that is measured at the other side of the substrate suffers a quite large reduction due to the high refractive index of the used substrates. Furthermore, small changes in the fabricated dimensions lead to a quite large degradation in the perfor mance of the device. Therefore, in this article, a terahertz metasurface unit cell based on efficient complementary coupled split-ring resonators (CC-SRRs) is proposed. This design is derived by applying Babinet’s principle [32] to the design in Ref. [25]. The structures can be easily fabricated using a laser beam machining technique that has been proposed recently [33]. The transmission and reflection responses of the uncoupled and coupled C-SRRs are discussed in detail. Moreover, the sharpness of the spectral response will be investigated by sweeping the length of the coupling slot. This investigation will reveal the viability of this device as a potential candidate for biomedical sensing applications. The advantage of such an approach is that there will be neither a need for photolithographic techniques nor a necessity for a supportive substrate. More importantly, the design is not prone to small deviations in the dimensions compared to the breaking symmetry of resonators.
2. Sensor design Fig. 1 depicts the schematic of both unit cells consist of two com plementary split-ring resonators (C-SRRs) in part A and two comple mentary coupled split-ring resonators (CC-SRRs) in part B of the figure, respectively. The geometrical dimensions are shown in Fig. 1(a) with dimensions of the side length of l ¼ 60 μm, a gap of g ¼ 5 μm, a width of w ¼ 20 μm for each resonator and a lattice constant of p ¼ 130 μm. The two split-ring resonators are coupled through a coupling slot with a length of (2s) ¼ 70 μm. The thickness of the metal aluminum sheet is 200 nm. We utilized Computer Simulation Technology (CST) Microwave Studio simulation package to perform the simulations. More precisely, we used the frequency-domain solver of CST package, which is based on the finite integration technique. It is a very robust technique in simu lating responses with spectral responses that feature high Q-factor. In order to mimic the actual configuration, periodic boundary conditions have been adopted with normal incidence plane-wave excitation. Babinet’s principle has been applied to get this design and hence the yellow color represents metal and the white color represents air. The inset of Fig. 1 depicts the terahertz field polarization of the electric and magnetic field illumination, where the E-field is parallel to the gaps of the resonators. Hence, only the dipole resonance can be excited in CSRRs. In this paper, we are going to answer the following question: What will happen when these two complementary resonators that are completely symmetric are coupled through the coupling slot as shown in Fig. 1(b)? 3. Results and discussion Fig. 2 presents the simulated transmission and reflection amplitude spectra for both designs depicted in Fig. 1. The red dashed-curves are for the complementary split-ring resonators unit cell and the green solidcurves are for the complementary coupled split-ring resonators unit
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Optical Materials 99 (2020) 109596
Fig. 5. Resonance frequency shift versus analyte thickness with a refractive index of n ¼ 1.6 of the CC-SRRs unit cell.
which is the ratio between the resonance frequency (fr) of 0.943 THz and full-width half-maximum (FWHM) bandwidth of 15.4 GHz. Hence, a remarkable Q-factor of 61.2 is achieved as a result of out-of-phase dipole moments in the two resonators of the CC-SRRs. In turn, leading to a strong reduction of radiation losses as a result of destructive interference. In order to better understand the excitation of this resonance, we carried out near-field simulations for the spatial electric and magnetic field normal to the surface of the structure at 0.943 THz. The results are shown in Fig. 3(a) and (b) for the electric and magnetic fields, respec tively. As one may expect for a complementary structure after applying Babinet’s principle, the electric field is focused away from the gaps and the magnetic field is confined at the areas very close to the gap edges. This suggests that areas with high field confinement can be coated with unidentified substances for biosensing [34]. In turn, this leads to mini mizing the required amount for sensing and increase volumetric sensitivity. It is evident that the coupling slot led to this strong resonance and hence it is quite important to study its length effect on the resonance. Therefore, we carried out a parametric study of the slot length (2s) with values of d ¼ 0, 10, 20, 30, and 35 μm as shown in Fig. 4. The slope of the transmission response between the peak and the dip is ranging between 0.94%/GHz and 3.5%/GHz for the values of d ¼ 10 and 35 μm, respectively. Moreover, the Q-factor (and the corresponding amplitude depth) is decreasing (increasing) and ranging from 67.4 (4.9%) to 61.1 (27.5%), respectively. These results show that there is rather a trade-off between the possible spectral sharpness and the amplitude depth of the resonances [35]. Nevertheless, we observe that there is a quite small reduction in the Q-factor due to a large change of the coupling slot length. Hence, this proves that the performance of this design is robust against small deviations in the geometrical. One of the key issues in sensing is studying the light-matter inter action. Hence, we study next the effect of increasing the analyte thick ness on the performance of the CC-SRRs proposed design. The refractive index (n) of different biomolecules varies between 1.4 and 2.0 [36]. Therefore, an average value of 1.6 was chosen to perform this study. Analyte thickness ranging between 0.1 and 16 μm has been chosen to cover the possible thickness of potential analytes. Fig. 5 shows the re sults where the resonance frequency features a redshift of 34, 55, 77.2, 84, 117.7, 159.5, and 197.5 GHz for analyte thickness of 0.1, 0.5, 1, 2, 4, 8, and 16 μm, respectively. A significant shift is achieved even with a quite small thickness of 0.1 μm. An exponential relation between the resonance frequency shift and the analyte thickness is observed as shown in Fig. 5. It saturates beyond the thickness of 8 μm. This result suggests that the THz light analyte interaction decreases exponentially as the analyte thickness is increased and starts saturating beyond the
Fig. 3. Simulated spatial electric (a) and magnetic (b) field distribution at 0.943 THz of the complementary coupled split-ring resonators unit cell.
Fig. 4. Transmission (a) and reflection (b) amplitude spectra of CC-SRRs unit cell for a sweep of half slot length (s) of 0, 10, 20, 30, and 35 μm.
cell. For the C-SRRs design shown in Fig. 1(a) and for the given electric field polarization, only the dipole eigenmode can be excited (not shown here as it takes place at higher frequencies than 1.1 THz). Indeed, no resonance response between 0 and 1.1 THz is observed in this case as expected. Conversely, for the CC-SRRs design shown in Fig. 1(b), both transmission and reflection amplitude results feature a very sharp resonance at 0.943 THz. The slop between the peak and the dip in the transmission response is 3.4%/GHz. The sharpness of the spectral response can be calculated quantitatively by calculating the Q-factor, 3
I. Al-Naib
Optical Materials 99 (2020) 109596 [3] M. Choi, S.H. Lee, Y. Kim, S.B. Kang, J. Shin, M.H. Kwak, K.Y. Kang, Y.H. Lee, N. Park, B. Min, A terahertz metamaterial with unnaturally high refractive index, Nature 470 (2011) 369–373. [4] H.-T. Chen, A.J. Taylor, N. Yu, A review of metasurfaces: physics and applications, Rep. Prog. Phys. 79 (2016) 76401. [5] J.F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A.J. Taylor, W. Zhang, Thinfilm sensing with planar terahertz metamaterials: sensitivity and limitations, Opt. Express 16 (2008) 1786–1795. [6] H. Tao, W.J. Padilla, X. Zhang, R.D. Averitt, Recent progress in electromagnetic metamaterial devices for Terahertz applications, IEEE J. Sel. Top. Quantum Electron. 17 (2011) 92–101. [7] J.F. O’Hara, W. Withayachumnankul, I. Al-Naib, A review on thin-film sensing with terahertz waves, J. Infrared, Millim. Terahertz Waves 33 (2012) 245–291. [8] I. Al-Naib, R. Singh, M. Shalaby, T. Ozaki, R. Morandotti, Enhanced Q-factor in optimally coupled macrocell THz metamaterials: effect of spatial arrangement, IEEE J. Sel. Top. Quantum Electron. 19 (2013) 8400807. [9] I. Al-Naib, W. Withayachumnankul, Recent progress in terahertz metasurfaces, J. Infrared, Millim. Terahertz Waves 38 (2017) 1067–1084. [10] M. Brucherseifer, M. Nagel, P. Haring Bolivar, H. Kurz, A. Bosserhoff, R. Buettner, Label-free probing of the binding state of DNA by time-domain terahertz sensing, Appl. Phys. Lett. 77 (2000) 4049–4051. [11] I. Al-Naib, Thin-Film sensing via fano resonance excitation in symmetric terahertz metamaterials, J. Infrared, Millim. Terahertz Waves 39 (2018) 1–5. [12] L. Cong, S. Tan, R. Yahiaoui, F. Yan, W. Zhang, R. Singh, Experimental demonstration of ultrasensitive sensing with terahertz metamaterial absorbers: a comparison with the metasurfaces, Appl. Phys. Lett. 106 (2015) 31107. [13] R. Singh, W. Cao, I. Al-Naib, L. Cong, W. Withayachumnankul, W. Zhang, Ultrasensitive terahertz sensing with high-Q Fano resonances in metasurfaces, Appl. Phys. Lett. 105 (2014) 171101. [14] M. Gupta, Y.K. Srivastava, M. Manjappa, R. Singh, Sensing with toroidal metamaterial, Appl. Phys. Lett. 110 (2017) 121108. [15] Y.K. Srivastava, L. Cong, R. Singh, Dual-surface flexible THz Fano metasensor, Appl. Phys. Lett. 111 (2017) 201101. [16] Y.K. Srivastava, R.T. Ako, M. Gupta, M. Bhaskaran, S. Sriram, R. Singh, Terahertz Sensing of 7nm Dielectric Film with Bound States in the Continuum Metasurfaces, 2019, p. 151105. [17] C. Jansen, I.a.I. Al-Naib, N. Born, M. Koch, Terahertz metasurfaces with high Qfactors, Appl. Phys. Lett. 98 (2011) 51109. [18] M.F. Limonov, M.V. Rybin, A.N. Poddubny, Y.S. Kivshar, Fano resonances in photonics, Nat. Photonics 11 (2017) 543–554. [19] V.A. Fedotov, M. Rose, S.L. Prosvirnin, N. Papasimakis, N.I. Zheludev, Sharp trapped-mode resonances in planar metamaterials with a broken structural symmetry, Phys. Rev. Lett. 99 (2007) 147401. [20] R. Singh, I.A.I. Al-Naib, M. Koch, W. Zhang, Sharp Fano resonances in THz metamaterials, Opt. Express 19 (2011) 6312–6319. [21] B. Luk’yanchuk, N.I. Zheludev, S.A. Maier, N.J. Halas, P. Nordlander, H. Giessen, C.T. Chong, The Fano resonance in plasmonic nanostructures and metamaterials, Nat. Mater. 9 (2010) 707–715. [22] R. Singh, I.A.I. Al-Naib, M. Koch, W. Zhang, Asymmetric planar terahertz metamaterials, Opt. Express 18 (2010) 13044–13050. [23] R. Singh, I. Al-Naib, W. Cao, C. Rockstuhl, M. Koch, W. Zhang, The fano resonance in symmetry broken terahertz metamaterials, IEEE Trans. Terahertz Sci. Technol. 3 (2013) 820–826. [24] I. Al-Naib, R. Singh, C. Rockstuhl, F. Lederer, S. Delprat, D. Rocheleau, M. Chaker, T. Ozaki, R. Morandotti, Excitation of a high-Q subradiant resonance mode in mirrored single-gap asymmetric split ring resonator terahertz metamaterials, Appl. Phys. Lett. 101 (2012) 71108. [25] I. Al-Naib, E. Hebestreit, C. Rockstuhl, F. Lederer, D. Christodoulides, T. Ozaki, R. Morandotti, Conductive coupling of split ring resonators: a path to THz metamaterials with ultrasharp resonances, Phys. Rev. Lett. 112 (2014) 183903. [26] I. Al-Naib, Y. Yang, M.M. Dignam, W. Zhang, R. Singh, Ultra-high Q even eigenmode resonance in terahertz metamaterials, Appl. Phys. Lett. 106 (2015) 11102. [27] Y.K. Srivastava, M. Manjappa, L. Cong, W. Cao, I. Al-Naib, W. Zhang, R. Singh, Ultrahigh-Q Fano resonances in terahertz metasurfaces: strong influence of metallic conductivity at extremely low asymmetry, Adv. Opt. Mater. 4 (2016) 457–463. [28] M. Manjappa, Y.K. Srivastava, L. Cong, I. Al-Naib, R. Singh, Active photoswitching of sharp fano resonances in THz metadevices, Adv. Mater. 29 (2017) 1603355. [29] N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. S€ onnichsen, H. Giessen, Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing, Nano Lett. 10 (2010) 1103–1107. [30] P. Tassin, L. Zhang, T. Koschny, E.N. Economou, C.M. Soukoulis, Low-loss metamaterials based on classical electromagnetically induced transparency, Phys. Rev. Lett. 102 (2009) 6–9. [31] R. Singh, I.A.I. Al-Naib, Y. Yang, D. Roy Chowdhury, W. Cao, C. Rockstuhl, T. Ozaki, R. Morandotti, W. Zhang, Observing metamaterial induced transparency in individual Fano resonators with broken symmetry, Appl. Phys. Lett. 99 (2011) 201107. [32] I.A.I. Al-Naib, C. Jansen, M. Koch, Applying the Babinet principle to asymmetric resonators, Electron. Lett. 44 (2008) 1228–1229. [33] N. Born, R. Gente, I. Al-Naib, M. Koch, Laser beam machined free-standing terahertz metamaterials, Electron. Lett. 51 (2015) 1012–1014.
Fig. 6. Resonance frequency shift versus analyte refractive index with a thickness of 4 μm of the CC-SRRs.
analyte thickness of 8 μm. Even with the same or similar thickness of the unidentified analyte, various analytes might have different refractive indices. Hence, we performed another set of simulations and for a sweep of the refractive index of the analyte with a certain thickness and investigate the effect of that onto the sensor performance. Fig. 6 shows the resonance frequency shift by sweeping the refractive index with values of 1.2, 1.4, 1.6, 1.8 and 2.0 for an analyte thickness of 4 μm. The results reveal a linear relationship between the resonance frequency shift and the refractive index. The resulting resonance frequency shift (df) is 41, 79, 117, 150, and 186 GHz as the refractive index is increased from 1.2 up to 2.0. Finally, we calculate the sensitivity as � � �dλ� co df � �¼ � �dn� f 2 dn r where “co” is the speed of light in free space. Calculating that for the case when the refractive index equals 2.0 results in a very large sensitivity of 6.3 � 104 nm/RIU. 4. Conclusions In summary, a novel terahertz metasurface design based on com plementary coupled two split-ring resonators is proposed. The perfor mance of the design has been carefully evaluated and found to support out-of-phase currents in the CC-SRRs compared to C-SRRs. Moreover, we demonstrated that the coupled design frequency response features Qfactor as high as of 67.4. More interestingly, we studied the performance of the proposed structure for biosensing applications and a quite high sensitivity of 6.3 � 104 nm/RIU has been attained. More interestingly, laser beam machining technique can be utilized to fabricate the design proposed in this paper. Future biomedical sensing systems can utilize such designs to achieve a high level of sensitivity during the develop ment process of new cancer biomarkers. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] C.M. Soukoulis, S. Linden, M. Wegener, Negative refractive index at optical wavelengths, Science 315 (2007) 47–50. [2] C. Wu, A.B. Khanikaev, R. Adato, N. Arju, A.A. Yanik, H. Altug, G. Shvets, Fanoresonant asymmetric metamaterials for ultrasensitive spectroscopy and identification of molecular monolayers, Nat. Mater. 11 (2011) 69–75.
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Optical Materials 99 (2020) 109596
[34] I. Al-Naib, Biomedical sensing with conductively coupled terahertz metamaterial resonators, IEEE J. Sel. Top. Quantum Electron. 23 (2017) 4700405. [35] N. Vieweg, F. Rettich, A. Deninger, H. Roehle, R. Dietz, T. G€ obel, M. Schell, Terahertz-time domain spectrometer with 90 dB peak dynamic range, J. Infrared, Millim. Terahertz Waves 35 (2014) 823–832.
[36] R. Yahiaoui, A.C. Strikwerda, P.U. Jepsen, Terahertz plasmonic structure with enhanced sensing capabilities, IEEE Sens. J. 16 (2016) 2484–2488.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Novel terahertz metasurfaces based on complementary coupled split ring resonators Ibraheem Al-Naib Biomedical Engineering Department, College of Engineering, Imam Abdurahman Bin Faisal University, Dammam 31441, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Terahertz time-domain spectroscopy Metasurfaces Babinet’s principle Biomedical sensing
In this paper, we propose a terahertz metasurface unit cell based on an efficient coupling between two completely symmetric complementary split-ring resonators via a coupling slot. This coupling allows the excitation of a dark eigenmode resonance that is otherwise forbidden. In turn, the radiation losses are highly suppressed and the excited eigenmode features a quite large quality factor. Interestingly, this kind of coupling leads to the resto ration of the symmetry of the unit cell of the coupled resonators with respect to the terahertz field illumination. More importantly, we show that the quality factor of this dark mode resonance can be tuned by modifying the coupling slot length and reach a high value of 67.4. Furthermore, a laser beam machining technique can be easily exploited during the fabrication process of this design. Besides using this design as a sharp bandpass filter, it can be utilized as an efficient biosensor. We show that a sensitivity level of 6.3 � 104 nm/RIU is indeed doable using the proposed structure. This design may emerge as a potential candidate for future biosensors and hence utilized during the development of new cancer biomarkers.
1. Introduction In the last twenty years, a novel type of materials has been explored, namely metamaterials for 3D structures and metasurfaces for 2D planar structures. The design flexibility of this type of materials offered intriguing optical properties that attracted a lot of attention and proved to be potential candidates for a wide spectrum of applications. In turn, various structures have been developed for diverse applications across the electromagnetic spectrum [1–4]. A plethora of terahertz (THz) de vices has been designed and developed such as high-performance filters, perfect absorbers, modulators, and biomedical sensors [5–7]. More specifically, utilizing THz technology for different biomedical applica tions has motivated many scientists to come up with novel designs and devices [7–9]. More importantly, the identification of chemical and bimolecular analytes without labeling the samples using terahertz technology has been of particular interest [10–16]. For instance, biosensors are crucial devices that can be utilized in cancer diagnosis and in drug discovery. Interestingly, they can be designed to determine drug effectiveness at various target sites and to detect emerging cancer biomarkers. Unfor tunately, there is a number of challenges facing different techniques: (i) Various techniques rely on labeling the targets by adding some dyes to the targets that in turn may change the original properties of the targets.
(ii) Some other techniques are invasive. Luckily, there is no need to stain the samples when various optical techniques are used. Moreover, they are non-invasive, but the photon energy is high enough to change the target properties. Hence, terahertz frequencies with meV photon energy represent a good candidate to solve this problem. However, many bio sensors suffer from low field confinement and hence low light-matter interaction that leads a very low sensitivity. Therefore, there is a crucial need to confine the field in a small area, i.e. using high quality-factor resonators. Conventional terahertz metasurfaces are typically composed of metallic symmetric structures on top of a dielectric substrate. Hence, a dipole resonance with in-phase current distribution can be easily excited in such structures and feature a quite broadband response that leads to a low quality-factor (Q-factor). The main reason behind this low Q-factor is the high level of ohmic and radiation losses. The ohmic losses due to the dielectric substrates at THz frequencies can be minimized by using substrates with very low absorption coefficient. Likewise, using high conductivity metals such as gold or aluminum leads to diminished ohmic losses. Therefore, we are left with the radiation loss channel as the main reason behind the broadband response. Hence, scientists have proposed novel designs with out-of-phase current distribution in order to limit the radiation loss [17,18]. Fano-type resonators are considered one of the well-known techniques
E-mail address: [email protected]. https://doi.org/10.1016/j.optmat.2019.109596 Received 3 November 2019; Received in revised form 21 November 2019; Accepted 1 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
I. Al-Naib
Optical Materials 99 (2020) 109596
Fig. 1. Schematic of the metasurface unit cells consists of: (a) two comple mentary symmetric split-ring resonators with the detailed geometric di mensions and (b) two complementary coupled split-ring resonators explaining the coupling slot length (2s). The inset of the figure shows the polarization of the electric field illumination.
Fig. 2. Transmission (a) and reflection (b) amplitude spectra of the comple mentary split-ring resonators (red dashed line) and complementary coupled split-ring resonators (green solid line) unit cells. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
by exciting an out-of-phase eigenmode resonance. It can be easily designed by breaking the symmetry of symmetric split-ring resonators, with respect to the illumination electric field, in order to achieve a sharp response and hence high Q-factors [19–21]. This kind of resonator consists of two metal arms with different lengths. In order to enhance the metasurface perfor mance, various designs have been proposed based on this scheme [22,23]. Moreover, other strategies have been developed with supercells made of multiple resonators that sustain a sharp resonance [24–28]. Another approach to achieve sharp resonances was developed by utilizing electromagnetic-induced-transparency (EIT) via coupling a radiative element with a subradiant element [29–31]. Quite complicated and expensive photolithographic processes have to be followed in order to fabricate such designs as those metallic resonators have to be patterned on top of the dielectric substrates. Moreover, the THz signal that is measured at the other side of the substrate suffers a quite large reduction due to the high refractive index of the used substrates. Furthermore, small changes in the fabricated dimensions lead to a quite large degradation in the perfor mance of the device. Therefore, in this article, a terahertz metasurface unit cell based on efficient complementary coupled split-ring resonators (CC-SRRs) is proposed. This design is derived by applying Babinet’s principle [32] to the design in Ref. [25]. The structures can be easily fabricated using a laser beam machining technique that has been proposed recently [33]. The transmission and reflection responses of the uncoupled and coupled C-SRRs are discussed in detail. Moreover, the sharpness of the spectral response will be investigated by sweeping the length of the coupling slot. This investigation will reveal the viability of this device as a potential candidate for biomedical sensing applications. The advantage of such an approach is that there will be neither a need for photolithographic techniques nor a necessity for a supportive substrate. More importantly, the design is not prone to small deviations in the dimensions compared to the breaking symmetry of resonators.
2. Sensor design Fig. 1 depicts the schematic of both unit cells consist of two com plementary split-ring resonators (C-SRRs) in part A and two comple mentary coupled split-ring resonators (CC-SRRs) in part B of the figure, respectively. The geometrical dimensions are shown in Fig. 1(a) with dimensions of the side length of l ¼ 60 μm, a gap of g ¼ 5 μm, a width of w ¼ 20 μm for each resonator and a lattice constant of p ¼ 130 μm. The two split-ring resonators are coupled through a coupling slot with a length of (2s) ¼ 70 μm. The thickness of the metal aluminum sheet is 200 nm. We utilized Computer Simulation Technology (CST) Microwave Studio simulation package to perform the simulations. More precisely, we used the frequency-domain solver of CST package, which is based on the finite integration technique. It is a very robust technique in simu lating responses with spectral responses that feature high Q-factor. In order to mimic the actual configuration, periodic boundary conditions have been adopted with normal incidence plane-wave excitation. Babinet’s principle has been applied to get this design and hence the yellow color represents metal and the white color represents air. The inset of Fig. 1 depicts the terahertz field polarization of the electric and magnetic field illumination, where the E-field is parallel to the gaps of the resonators. Hence, only the dipole resonance can be excited in CSRRs. In this paper, we are going to answer the following question: What will happen when these two complementary resonators that are completely symmetric are coupled through the coupling slot as shown in Fig. 1(b)? 3. Results and discussion Fig. 2 presents the simulated transmission and reflection amplitude spectra for both designs depicted in Fig. 1. The red dashed-curves are for the complementary split-ring resonators unit cell and the green solidcurves are for the complementary coupled split-ring resonators unit
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I. Al-Naib
Optical Materials 99 (2020) 109596
Fig. 5. Resonance frequency shift versus analyte thickness with a refractive index of n ¼ 1.6 of the CC-SRRs unit cell.
which is the ratio between the resonance frequency (fr) of 0.943 THz and full-width half-maximum (FWHM) bandwidth of 15.4 GHz. Hence, a remarkable Q-factor of 61.2 is achieved as a result of out-of-phase dipole moments in the two resonators of the CC-SRRs. In turn, leading to a strong reduction of radiation losses as a result of destructive interference. In order to better understand the excitation of this resonance, we carried out near-field simulations for the spatial electric and magnetic field normal to the surface of the structure at 0.943 THz. The results are shown in Fig. 3(a) and (b) for the electric and magnetic fields, respec tively. As one may expect for a complementary structure after applying Babinet’s principle, the electric field is focused away from the gaps and the magnetic field is confined at the areas very close to the gap edges. This suggests that areas with high field confinement can be coated with unidentified substances for biosensing [34]. In turn, this leads to mini mizing the required amount for sensing and increase volumetric sensitivity. It is evident that the coupling slot led to this strong resonance and hence it is quite important to study its length effect on the resonance. Therefore, we carried out a parametric study of the slot length (2s) with values of d ¼ 0, 10, 20, 30, and 35 μm as shown in Fig. 4. The slope of the transmission response between the peak and the dip is ranging between 0.94%/GHz and 3.5%/GHz for the values of d ¼ 10 and 35 μm, respectively. Moreover, the Q-factor (and the corresponding amplitude depth) is decreasing (increasing) and ranging from 67.4 (4.9%) to 61.1 (27.5%), respectively. These results show that there is rather a trade-off between the possible spectral sharpness and the amplitude depth of the resonances [35]. Nevertheless, we observe that there is a quite small reduction in the Q-factor due to a large change of the coupling slot length. Hence, this proves that the performance of this design is robust against small deviations in the geometrical. One of the key issues in sensing is studying the light-matter inter action. Hence, we study next the effect of increasing the analyte thick ness on the performance of the CC-SRRs proposed design. The refractive index (n) of different biomolecules varies between 1.4 and 2.0 [36]. Therefore, an average value of 1.6 was chosen to perform this study. Analyte thickness ranging between 0.1 and 16 μm has been chosen to cover the possible thickness of potential analytes. Fig. 5 shows the re sults where the resonance frequency features a redshift of 34, 55, 77.2, 84, 117.7, 159.5, and 197.5 GHz for analyte thickness of 0.1, 0.5, 1, 2, 4, 8, and 16 μm, respectively. A significant shift is achieved even with a quite small thickness of 0.1 μm. An exponential relation between the resonance frequency shift and the analyte thickness is observed as shown in Fig. 5. It saturates beyond the thickness of 8 μm. This result suggests that the THz light analyte interaction decreases exponentially as the analyte thickness is increased and starts saturating beyond the
Fig. 3. Simulated spatial electric (a) and magnetic (b) field distribution at 0.943 THz of the complementary coupled split-ring resonators unit cell.
Fig. 4. Transmission (a) and reflection (b) amplitude spectra of CC-SRRs unit cell for a sweep of half slot length (s) of 0, 10, 20, 30, and 35 μm.
cell. For the C-SRRs design shown in Fig. 1(a) and for the given electric field polarization, only the dipole eigenmode can be excited (not shown here as it takes place at higher frequencies than 1.1 THz). Indeed, no resonance response between 0 and 1.1 THz is observed in this case as expected. Conversely, for the CC-SRRs design shown in Fig. 1(b), both transmission and reflection amplitude results feature a very sharp resonance at 0.943 THz. The slop between the peak and the dip in the transmission response is 3.4%/GHz. The sharpness of the spectral response can be calculated quantitatively by calculating the Q-factor, 3
I. Al-Naib
Optical Materials 99 (2020) 109596 [3] M. Choi, S.H. Lee, Y. Kim, S.B. Kang, J. Shin, M.H. Kwak, K.Y. Kang, Y.H. Lee, N. Park, B. Min, A terahertz metamaterial with unnaturally high refractive index, Nature 470 (2011) 369–373. [4] H.-T. Chen, A.J. Taylor, N. Yu, A review of metasurfaces: physics and applications, Rep. Prog. Phys. 79 (2016) 76401. [5] J.F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A.J. Taylor, W. Zhang, Thinfilm sensing with planar terahertz metamaterials: sensitivity and limitations, Opt. Express 16 (2008) 1786–1795. [6] H. Tao, W.J. Padilla, X. Zhang, R.D. Averitt, Recent progress in electromagnetic metamaterial devices for Terahertz applications, IEEE J. Sel. Top. Quantum Electron. 17 (2011) 92–101. [7] J.F. O’Hara, W. Withayachumnankul, I. Al-Naib, A review on thin-film sensing with terahertz waves, J. Infrared, Millim. Terahertz Waves 33 (2012) 245–291. [8] I. Al-Naib, R. Singh, M. Shalaby, T. Ozaki, R. 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Fig. 6. Resonance frequency shift versus analyte refractive index with a thickness of 4 μm of the CC-SRRs.
analyte thickness of 8 μm. Even with the same or similar thickness of the unidentified analyte, various analytes might have different refractive indices. Hence, we performed another set of simulations and for a sweep of the refractive index of the analyte with a certain thickness and investigate the effect of that onto the sensor performance. Fig. 6 shows the resonance frequency shift by sweeping the refractive index with values of 1.2, 1.4, 1.6, 1.8 and 2.0 for an analyte thickness of 4 μm. The results reveal a linear relationship between the resonance frequency shift and the refractive index. The resulting resonance frequency shift (df) is 41, 79, 117, 150, and 186 GHz as the refractive index is increased from 1.2 up to 2.0. Finally, we calculate the sensitivity as � � �dλ� co df � �¼ � �dn� f 2 dn r where “co” is the speed of light in free space. Calculating that for the case when the refractive index equals 2.0 results in a very large sensitivity of 6.3 � 104 nm/RIU. 4. Conclusions In summary, a novel terahertz metasurface design based on com plementary coupled two split-ring resonators is proposed. The perfor mance of the design has been carefully evaluated and found to support out-of-phase currents in the CC-SRRs compared to C-SRRs. Moreover, we demonstrated that the coupled design frequency response features Qfactor as high as of 67.4. More interestingly, we studied the performance of the proposed structure for biosensing applications and a quite high sensitivity of 6.3 � 104 nm/RIU has been attained. More interestingly, laser beam machining technique can be utilized to fabricate the design proposed in this paper. Future biomedical sensing systems can utilize such designs to achieve a high level of sensitivity during the develop ment process of new cancer biomarkers. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] C.M. Soukoulis, S. Linden, M. Wegener, Negative refractive index at optical wavelengths, Science 315 (2007) 47–50. [2] C. Wu, A.B. Khanikaev, R. Adato, N. Arju, A.A. Yanik, H. Altug, G. Shvets, Fanoresonant asymmetric metamaterials for ultrasensitive spectroscopy and identification of molecular monolayers, Nat. Mater. 11 (2011) 69–75.
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