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Enhanced Ultra-Sensitive Metamaterial Resonance Sensor www.nature.com/scientificreports OPEN Enhanced Ultra-Sensitive Metamaterial Resonance Sensor based on Double Corrugated Metal Received: 28 January 2019 Accepted: 7 May 2019 stripe for Terahertz Sensing Published: xx xx xxxx Sajad Niknam1, Mehran Yazdi 2 & Salman Behboudi Amlashi 3 In this paper, an ultra-sensitive metamaterial terahertz sensor is proposed. The resonance sensor is designed based on a novel double corrugation form to enhance the ability of the sensor in the terms of sensitivity, Q-factor and the maximum sensible thickness of an analyte. The introduced structure can support the spoof surface plasmon and can resonate strongly at the tuned frequencies. Moreover, the structure of the terahertz sensor is investigated thoroughly from diferent points of view including frequency shifts due to variations in the thickness or refractive index of the analyte. In addition, the sensitivity of the sensor is approximated with a biharmonic ftting function for diferent combinations of refractive index and analyte thickness as “sensitivity surface”. The sensor shows the maximum sensitivity of 1.75 THz/RIU for refractive index between 1–1.2 with a maximum thickness of 80 μm. Moreover, the simulation results approved that the double corrugation on the metal stripe improves the electromagnetic feld interaction in the metal part greatly in comparison with the previously reported works. According to this work, the proposed structure can be applied for terahertz sensing with more abilities to sense even thicker biologic tissues. In recent decades, many researches have been done in the feld of terahertz and many applications have been emerged through several investigation1,2. Accordingly, due to special interaction of the terahertz wave with bio- logic materials, biomedical applications of THz radiation are very much considered3–7. In fact, the low photon energy of THz spectra wave has made it very attractive in biological imaging and spectroscopy to the extent that many researchers have tried to provide new and more efective methods for synthesizing THz radiation based on these applications8–14. Recently, the sensing of the biologic material in the THz spectra has turned to a fascinating topic of research. Terefore, engineers and scientists examined novel methods for the identifcation of diferent analyte by THz spectroscopy including approaches based on transmission, absorption, shifs in fre- quency responses, refractive index sensing, or variations of phase15–22. Xie et al. applied an array of highly sensi- tive square-shaped slits on a silicon substrate for terahertz transmission spectroscopy of antibiotics at 0.3 THz23. In a similar approach, a Nano-slot-antenna array-based THz sensing method is used for the discrimination of carbohydrates in the frequency range between 0.5–2.5 THz based on both absorption and transmission spectra24. One of the most interesting techniques in terahertz sensing is the frequency shif sensing by which the shifs in the resonance modes of a transmission spectra of a structure is measured for the detection of the changes in the analyte characteristics25–31. Interestingly, the metallic nanotrenches coupled with water is reported as a high contrast terahertz sensor that can be applied to a various types of polar organic molecules based on resonant frequency shifs32. Although, in the mentioned report, resonant frequencies are appeared in sub THz frequencies that makes sensitivity parameter limited for the frequency shif detection. On the other hand, for such structures, there are signifcant losses due to attenuation in the terahertz regime as well as low confnement of electromag- netic feld with the structure33. Due to these losses, the Q-factor of the resonant modes has low values in several reported structures. Tus, the electromagnetic interaction with metals cannot be strong enough to cause sharp resonance dips in their structures. Contrary to what occurs in terahertz frequencies, surface plasmon polariton 1Department of Computer Science & Engineering and Information Technology, Shiraz University, Shiraz, Iran. 2Department of Communications & Electronics, School of Electrical and Computer Engineering, Shiraz University, Shiraz, Iran. 3Department of Electrical Engineering, Shahed University, Tehran, Iran. Correspondence and requests for materials should be addressed to M.Y. (email: [email protected]) SCIENTIFIC REPORTS | (2019) 9:7516 | https://doi.org/10.1038/s41598-019-44026-4 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Te structure of the proposed metamaterial sensor with the illumination of THz beams. Te analyte is superimposed on the sensor structure, which causes an environmental change in refractive index and consequently, shifs the resonance frequency. Te unit cell of metamaterial structure has the dimensions Py = 68 μm and PX = 24 μm. Te PI layer has a thickness equals to 15 μm. (SPP) sensing demonstrates tightly confned local feld that brings about higher Q-factors and sensitivity at very higher frequencies (visible wavelength)34–37 with noble metals such as silver and gold38. Conversely, these noble metals cannot bind the THz electromagnetic feld at its surface because of its dimensions relative to the terahertz wavelength scale. Although it is not possible to use such a feature in terahertz structures but many researches proved that corrugated metal structures (even with noble metals) could support surface waves greatly that can improve the confnement of local electromagnetic feld at the surface boundary of metals. Tese surface waves, named spoof surface plasmon (SSP) waves, can mimic the properties of SPPs in terahertz spectrum39–41. Because of SSP modes, tightly confned local felds can be observed and due to this, much diferent types of terahertz sen- sors are proposed based on metamaterial structures42–45. Te eye-catching characteristics of metamaterial struc- ture in controlling the electromagnetic feld make it a solution for improving the feld confnement in terahertz sensors. In this paper, a terahertz sensor is investigated based on metamaterial structure that its unit cell consists of a corrugated metal stripe. A novel corrugation form, as is called double corrugation, is perforated on the metal part and the obtained results show great improvement compared to similar reported works46. In addition, the sensitivity and the Q-factor of the SSP resonance modes are reported in this paper and are assessed statistically. Te proposed structure can be successfully applied for terahertz sensing for the variations in refractive index and thickness of an analyte. It is worth mentioning that the electric feld and surface current of the structure, which are depicted later, indicate a signifcant betterment than similar structures. As result, this improvement causes higher sensitivity as well as more ability for sensing thicker analyte. Structure Design A novel structure of metamaterial terahertz sensor is proposed in Fig. 1. As depicted, the metamaterial struc- ture consists of a unit cell with metal stripe, which is grooved in two sides and form symmetrical metal arms. Moreover, each arm is corrugated in two diferent orders but with the same intervals on the edges. Te metallic structure is placed on a layer of polyimide substrate. Te polyimide layer is transparent for these frequencies due to its low loss tangent of 0.005 with a dielectric constant 3 in these wavelengths47. With this corrugation form on the metal stripe, it is expected that the proposed structure can support spoof surface plasmon modes, which can be considered in dispersion diagram of the structure in the next section. As Fig. 1 Represents the structure, the dimensions Py and Px of the unit cell are 68 μm and 24 μm for a metal stripe with L = 26 μm. With a closer look at the dimensions of the metal structure, it can be noted that L is the length of the metal stripe as can be seen in Fig. 2. In fact, the metamaterial structure is formed from a periodic unit cell in directions x and y with periodicities Px and Py respectively. In each unit cell, the metal part is origi- nated from three repeating H-shaped metal stripes that stick together and one period is shown individually in Fig. 2. Te width of each repeating H-shaped metal stripe is constant through this paper and is W = 6 μm. Te length of metal stripe is L that can be tuned for diferent resonance frequencies. In addition to those, the gap sizes between metal arms, i.e., Lg and Wg, have fxed values of 2 μm and 3 μm respectively. Te other dimensions in the structure of the metal stripe such as C1 and C2, are the widths of toothed–shaped modifcations and have the sizes equal to C1 = 3C2 = 0.5 μm. It is necessary to state that the length of C2 is calculated for best possible results. In addition, H1 and H2 are the lengths of the tooth-shaped modifcations. As aforementioned, the inner edges in each metal arm are modifed in two ways, frst, they are corrugated with periodic grooves equidistantly with SCIENTIFIC REPORTS | (2019) 9:7516 | https://doi.org/10.1038/s41598-019-44026-4 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 2. Te unit cell of the metamaterial structure is formed from a periodic H-shaped metal which supports surface wave due to its corrugations. Te dimensions are included W = 6 μm, Lg = 2 μm, Wg = 3 μm, H1 = 2*H2 = 1 μm, C1 = 3*C2 = 0.5 μm and DC section of 1 μm that through the simulation are fxed for this period. Also, t is the thickness of metal stripe and is equal to 2 μm. Te size L can be tuned for the desired resonance frequency. Figure 3. Te dispersion diagram plotted for one period of corrugated metal stripe. For diferent sizes of the metal stripe (L), dispersion curve asymptote to a specifc frequency of SSP mode. Te dispersion diagram proves that the interaction of electromagnetic feld with metal has similar behavior to surface plasmon polariton in visible wavelength.
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