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.
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].
Index Terms — capacitive loading, surface plasmon
polaritons, terahertz sensor
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-) 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.
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
2.
Design and analysis of unit cell
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
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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.
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.
(a)
Fig. 5 Measured S-parameters using fabricated sensors corresponding to
capacitive slot at (a) alternate strips, (b) outer two strips.
5.
(a)
(b)
Fig. 2 Capacitive slot along the dumbbell cell at (a) alternate strips, (b) outer
two strips.
(a)
(b)
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 Sparameters under the unloaded and loaded condition is
shown in Fig. 5 to demonstrate the sensitivity of the
proposed sensor.
(b)
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.
References
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Fig. 4 Measurement setup along with the fabricated prototype of two sensors,
capacitive slots in both the structures having at different positions.
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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
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