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Spoof Surface Plasmon Based Planar Thz Sensor System Using Dumbbell Shaped Unit Cell

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Spoof Surface Plasmon Based Planar Thz Sensor System Using Dumbbell Shaped Unit Cell 2018 International Symposium on Antennas and Propagation (ISAP 2018) [WeE2-5] October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea Spoof Surface Plasmon based Planar THz Sensor System using Dumbbell Shaped Unit Cell M Jaleel Akhtar, Nilesh K Tiwari, Surya P Singh Department of Electrical Engineering, Indian Institute of Technology Kanpur, India Abstract - A spoof surface plasmon polariton (SSPP) optimized in the microwave frequency region for obtaining transmission line based THz sensor using a perforated the desired characteristics. dumbbell shaped unit cell is presented. The proposed SSPP structure possesses better confinement ability as compared to In this paper, the dumbbell shaped unit cell geometry is the conventional microstrip line in the THz frequency regime proposed to design the microstrip fed SSPP structure with which actually helps to realize the THz sensor with improved the underlying ground in the microwave frequency band. The sensitivity. The designed SSPPs based structure is having under proposed dumbbell shaped SSPP structure with under layer layer ground in order to obtain better conversion between ground is proven to be quite compact possessing improved conventional microstrip to SSPP structure due to efficient impedance matching between them in the microwave and THz confinement ability as compared to that of the conventional frequency band. To design the THz dielectric sensor using the microstrip lines. This property of proposed SSPP structure is proposed highly confined SSPP transmission an appropriate exploited here to design the THz/ microwave sensor with capacitive slot on the dumbbell cells is created. Design and improved sensitivity by creating the capacitive slot on the optimization of the proposed SSPP based resonant structure is dumbbell unit cell. The proposed SSPP sensor is quite novel performed using the CST-MWS in the THz and microwave frequency range. Thereafter, a prototype of the resonant SSPP because of the planar geometry as compared to earlier (scaled for microwave region) sensor is fabricated, and proposed non-planar SSPP dielectric sensors [8]-[9]. accordingly the shift in the resonance frequency is measured after loading the sensor with the test specimen. 2. Design and analysis of unit cell Index Terms — capacitive loading, surface plasmon The schematic diagram of the proposed dumbbell shaped polaritons, terahertz sensor unit cell with its optimized physical parameters is shown in figure 1(a). To get more insight about its improved 1. Introduction confinement ability, the dispersion diagram (k-β) analysis of the proposed dumbbell cell and the conventional microstrip The surface plasmons have been found to be very useful in line is carried out using eigen mode solver of the CST-MWS several applications such as optic data storage, solar cell and as shown in Fig. 1 (b). This diagram clearly shows that the special biological sensors [1]-[3]. Although the metals do not dumbbell cell based transmission line with the standard show surface plasmons behavior at microwave and terahertz feeding arrangement is well suited as the SSPP structure with (THz) frequencies, however, the propagation of surface having very high confinement ability as compared to that of plasmons at a lower frequency can be made possible by the conventional microstrip line. creating artificial corrugated metal structures popularly known as spoof surface plasmon polaritons (SSPP) [4]. These artificial structures can produce similar dispersion characteristic in the THz and microwave range, which can confine the electromagnetic waves at the interface between the conductor and the dielectric [4]. In the past, various SSPP devices have been designed and tested in the THz and GHz frequency band [3], [5]-[7]. For example, the microstrip based Quasi-TEM to SSP mode converter has been Fig. 1(a) Dumbbell unit cell with L1=0.75mm, W1=0.5mm and employed in [3], where, the inductively loaded rectangular W2=2.55mm (b) Dispersion relation for proposed SSPP line and microstip strip is employed to manipulate the propagation of surface line. plasmonic waves. Using microstrip to SSPPs mode converter, several slow wave structures in the THz and GHz frequency 3. Simulation and optimization of SSPP sensor region have been realized by different research groups [6]- The numerical simulation model of the perforated dumbbell [7]. Most of the SSPP structures, described above have shaped SSPP sensor is shown in Figs. 2, where two different employed rectangular shaped unit cells in order to realize the arrangements of capacitive slots along the line are considered. equivalent surface plasmons in the microwave frequency The position and interspacing of capacitive gaps along the region. Recently, some preliminary study was carried out to dumbbell cells helps to control the resonance frequency of design SSPP based filter using dumbbell grooves without the SSPP sensor. This fact can easily be noticed from the using a metallic ground [7]. However, the feeding reflection coefficient plots corresponding to two different mechanism of these types of structures is not properly arrangement of the capacitive slot, Figs.2 (a), (b), where the 129 2018 International Symposium on Antennas and Propagation (ISAP 2018) October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea unloaded resonance frequency is different for both the owing to the difference in the capacitive slot position and structures. Here it is to be noted that the induction of interspacing. This basically means that the proposed capacitive slot in the sensing region of Fig.4 helps to trap the methodology can effectively be used to tune the resonance maximum electric field at the resonance frequency which frequency of the sensor without any change in physical eventually facilitates better interaction between the electric dimension in contrary to the conventional metamaterial unit field and test samples placed directly on the top of the cell loaded sensors. sensing region. The numerical analysis of dielectric sensing capability corresponding to the proposed THz sensor is performed for both the structure as shown in Figs. 3, where the significant change in resonance frequency corresponds to a small change in dielectric constant can easily be observed. (a) (b) This basically means that proposed SSPPs based planar sensor is having the excellent sensitivity due to the localization of the electric field in the capacitive gap region Fig. 5 Measured S-parameters using fabricated sensors corresponding to of dumbbell shaped unit cell. capacitive slot at (a) alternate strips, (b) outer two strips. 5. Conclusion A novel design methodology to realize the planar spoof sensor at THz and microwave frequency region has been presented in this work. The proposed design technique (a) (b) removes the need for integration of any other resonating element with the main transmission line, unlike the Fig. 2 Capacitive slot along the dumbbell cell at (a) alternate strips, (b) outer metamaterial unit cell based sensor. As in the proposed two strips. structure, the capacitive slot is directly created on the dumbbell unit cell by its perforation which actually behaves as the sensing element. The improved confinement ability of the designed sensor helps to realize planar spoof sensor with improved sensitivity than that of a conventional metamaterial unit cell loaded microstrip based sensor. (a) (b) Acknowledgement Fig. 3 S-parameter plot of proposed THz sensor corresponding to capacitive slot position shown in Fig. 2 (a), (b). This work is partially supported by BIRAC/EE/2016205. 4. Fabrication and measurement References The dimension of the designed sensor is scaled at the [1] Barnes, W. L., Dereux, A. and Ebbesen, T. W, “Surface plasmon microwave frequency region in order to perform the subwavelength optics” Nature, vol. 424, pp. 824-830, 2003. experimental validation as currently, we don’t have THz [2] S. A. Maier, “Plasmonics: Fundamentals and applications,” New York, USA: Springer Verlag, 2007. measurement setup. A prototype of both the designed [3] Wenjuan Zhang, Guiqiang Zhu, Liguo Sun, and Fujianng Lin, microwave SSPP sensors (Fig. 4) is fabricated on the 0.8 mm “Trapping of surface plasmon wave through gradient corrugated strip thick Roger RT-5880 substrate to measure the S-parameter with underlayer ground and manipulating its propagation,” J. App. using the measurement setup shown in Fig. 4. The plot of S- Phys., vol. 106, no. 2, 2015. [4] J. B. Pendry, L. Martín-Moreno, and F. J. Garcia-Vidal, “Mimicking parameters under the unloaded and loaded condition is surface plasmons with structured surfaces,” Science, vol. 305, pp. shown in Fig. 5 to demonstrate the sensitivity of the 847–848, Aug. 2004. proposed sensor. [5] A. Kianinejad, Z. N. Chen and C. W. Qiu, “Design and Modeling of Spoof Surface Plasmon Modes-Based Microwave Slow-Wave Transmission Line,” IEEE Trans. Microw. Theory Tech, vol. 63, no. 6, pp. 1817-1825, June 2015. [6] P. Pal, S. P. Singh, N. K. Tiwari and M. J. Akhtar, “Novel spoof plasmonic based compact slow wave structure for terahertz and microwave applications,” 2016 16th Mediterranean Microwave Symposium (MMS), Abu Dhabi, , pp. 1-4, 2016. [7] Y. J. Zhou and B. J. Yang, “Planar spoof plasmonic ultra-wideband filter based on low-loss and compact terahertz waveguide corrugated with dumbbell grooves,” Appl. Opt., vol. 54, no. 14, pp. 4529–4533, 2015. Fig. 4 Measurement setup along with the fabricated prototype of two sensors, [8] P. Singh, “SPR Biosensors: Historical Perspectives and Current capacitive slots in both the structures having at different positions. Challenges”, Sens. Actuators B Chem., vol. 229, pp. 110-130, 2016. From this figure, it can be observed that both the fabricated [9] C. Lertvachirapaiboon et al., “Transmission surface plasmon sensors can clearly differentiate two materials having quite resonance techniques and their potential biosensor applications”, Biosens. Bioelectron, vol. 99, pp. 399–415, 2018. similar dielectric values. As a matter of fact, the unloaded resonance frequency of both the sensor is quite different 130.
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