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) transmission line based THz sensor using a perforated dumbbell shaped unit cell is presented. The proposed SSPP structure possesses better confinement ability as compared to the conventional microstrip line in the THz frequency regime which actually helps to realize the THz sensor with improved sensitivity. The designed SSPPs based structure is having under layer ground in order to obtain better conversion between conventional microstrip to SSPP structure due to efficient impedance matching between them in the microwave and THz frequency band. To design the THz dielectric sensor using the proposed highly confined SSPP transmission an appropriate capacitive slot on the dumbbell cells is created. Design and optimization of the proposed SSPP based resonant structure is performed using the CST-MWS in the THz and microwave frequency range. Thereafter, a prototype of the resonant SSPP (scaled for microwave region) sensor is fabricated, and accordingly the shift in the resonance frequency is measured after loading the sensor with the test specimen.

Index Terms — capacitive loading, surface plasmon polaritons, terahertz sensor

1. Introduction

The surface plasmons have been found to be very useful in several applications such as optic data storage, solar cell and special biological sensors [1]-[3]. Although the metals do not show surface plasmons behavior at microwave and terahertz (THz) frequencies, however, the propagation of surface plasmons at a lower frequency can be made possible by 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 employed in [3], where, the inductively loaded rectangular strip is employed to manipulate the propagation of surface plasmonic waves. Using microstrip to SSPPs mode converter, several slow wave structures in the THz and GHz frequency region have been realized by different research groups [6]-[7]. Most of the SSPP structures, described above have employed rectangular shaped unit cells in order to realize the equivalent surface plasmons in the microwave frequency region. Recently, some preliminary study was carried out to design SSPP based filter using dumbbell grooves without using a metallic ground [7]. However, the feeding mechanism of these types of structures is not properly

optimized in the microwave frequency region for obtaining the desired characteristics.

In this paper, the dumbbell shaped unit cell geometry is proposed to design the microstrip fed SSPP structure with the underlying ground in the microwave frequency band. The proposed dumbbell shaped SSPP structure with under layer ground is proven to be quite compact possessing improved confinement ability as compared to that of the conventional microstrip lines. This property of proposed SSPP structure is exploited here to design the THz/ microwave sensor with improved sensitivity by creating the capacitive slot on the dumbbell unit cell. The proposed SSPP sensor is quite novel because of the planar geometry as compared to earlier proposed non-planar SSPP dielectric sensors [8]-[9].

2. Design and analysis of unit cell

The schematic diagram of the proposed dumbbell shaped unit cell with its optimized physical parameters is shown in figure 1(a). To get more insight about its improved confinement ability, the dispersion diagram (k-β) analysis of the proposed dumbbell cell and the conventional microstrip line is carried out using eigen mode solver of the CST-MWS as shown in Fig. 1 (b). This diagram clearly shows that the dumbbell cell based transmission line with the standard feeding arrangement is well suited as the SSPP structure with having very high confinement ability as compared to that of the conventional microstrip line.

Fig. 1(a) Dumbbell unit cell with L1=0.75mm, W1=0.5mm and W2=2.55mm (b) Dispersion relation for proposed SSPP line and microstip line.

3. Simulation and optimization of SSPP sensor The numerical simulation model of the perforated dumbbell shaped SSPP sensor is shown in Figs. 2, where two different arrangements of capacitive slots along the line are considered. The position and interspacing of capacitive gaps along the dumbbell cells helps to control the resonance frequency of the SSPP sensor. This fact can easily be noticed from the reflection coefficient plots corresponding to two different arrangement of the capacitive slot, Figs.2 (a), (b), where the

[WeE2-5] 2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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unloaded resonance frequency is different for both the structures. Here it is to be noted that the induction of capacitive slot in the sensing region of Fig.4 helps to trap the maximum electric field at the resonance frequency which eventually facilitates better interaction between the electric field and test samples placed directly on the top of the 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. 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 of dumbbell shaped unit cell.

Fig. 2 Capacitive slot along the dumbbell cell at (a) alternate strips, (b) outer two strips.

Fig. 3 S-parameter plot of proposed THz sensor corresponding to capacitive slot position shown in Fig. 2 (a), (b).

4. Fabrication and measurement

The dimension of the designed sensor is scaled at the microwave frequency region in order to perform the experimental validation as currently, we don’t have THz measurement setup. A prototype of both the designed microwave SSPP sensors (Fig. 4) is fabricated on the 0.8 mm thick Roger RT-5880 substrate to measure the S-parameter using the measurement setup shown in Fig. 4. The plot of S-parameters under the unloaded and loaded condition is shown in Fig. 5 to demonstrate the sensitivity of the proposed sensor.

Fig. 4 Measurement setup along with the fabricated prototype of two sensors, capacitive slots in both the structures having at different positions. From this figure, it can be observed that both the fabricated sensors can clearly differentiate two materials having quite similar dielectric values. As a matter of fact, the unloaded resonance frequency of both the sensor is quite different

owing to the difference in the capacitive slot position and interspacing. This basically means that the proposed methodology can effectively be used to tune the resonance frequency of the sensor without any change in physical dimension in contrary to the conventional metamaterial unit cell loaded sensors.

Fig. 5 Measured S-parameters using fabricated sensors corresponding to 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 removes the need for integration of any other resonating element with the main transmission line, unlike the metamaterial unit cell based sensor. As in the proposed 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.

Acknowledgement

This work is partially supported by BIRAC/EE/2016205.

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