Paper SPIE (9883-15) Submission EPE16-EPE101-31

8/17/2019 Paper SPIE (9883-15) Submission EPE16-EPE101-31

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Design of an optical sensor based on plasmonic nanostructures

Marwa M. Tharwat,* Haya AlSharif,† Haifaa Alshabani,† Eilaf Qadi,† and Maha Sultan.†

Department of Electrical Engineering, Faculty of Engineering,

King Abdulaziz University (KAU), Jeddah 21432, Saudi Arabia*[email protected], † [email protected]

ABSTRACT

Plasmonic nano-structured array sensors have been highlighted by their tremendously promising applications, such as the

surface plasmon resonance (SPR) optical biosensors. In this paper, within the visible spectrum region, the opticaltransmission properties of a metallic thin film deposited over dielectric films of various refraction indices are

investigated. With finite difference time domain (FDTD) method, we investigate the optical transmission spectra of such

plasmonic structures based on both nano-holes and nano-disc arrays. This investigation includes monitoring the

modification in both the transmission resonance wavelengths and peak transmittance. The results of this study provide a

better understanding of the interaction between light and plasmonic nano-hole and nano-disc arrays. It shows that thechanging the shapes of the nano-holes can affect the resonance wavelengths and the intensity of transmitted spectra and

alter its resonance peak transmittance values. We found that the interaction coupling between the localized plasmons

(LSP) and the propagating surface plasmons (PSP) can be tuned to boost the performance of the optical sensor.

Keywords: FDTD; Plasmonics; SPR; and Sensitivity.

1. INTRODUCTION

The fabrication of low-cost optical biosensors, for fast, real-time identification of diseases, is required in the low-

income countries to monitor the health, warn of diseases, and provide the early detection before the spread of a

disease in case of infectious diseases [1], [2]. Among all optical biosensors construction, the best method of detection

known is based on an observation that was done for the first time at 1912 by Wood [3-5], and explained at 1968 when

Otto [6], and Kretschmann and Raether [7], at the same year, reported the excitation of surface plasmons (SP).

Plasmonic metamaterials has been recently recognized as a new border of engineering, optics and nanoscience since the

discovery of their extraordinary optical transmission (EOT) and confinement of optical field [8]. EOT is identified by

multiple peaks and dips in the transmission spectrum. The motivating optical properties are determined by the localizedand propagating surface plasmon resonances. These plasmonic resonances of the metasurfaces can be adapted by proper

tuning of the physical and geometrical parameters of such structures [9-15] rather than their synthesis. By extension,

these resonances are very sensitive to their surrounding dielectric and hence provide a pathway for refractive index

sensing [12], [17].

The performance of biochemical sensor has been widely improved by the development of surface plasmon resonance

(SPR) based sensors. Since EOT was reported, several plasmonic configurations, with different material, shape, and size,

have been studied. Some of these configurations were gold mushroom arrays with super sensitivity [9], circular

shaped [12], elliptical shaped [13], X-shaped [15] and H-shaped [16]. Many detection schemes have been developedand the field is rapidly growing to incorporate new methodologies and applications. SPR-based biosensors achieve

higher sensitivities in different types of analyses relative to other label-free sensors, such as in electrochemical [18],

interferometric [19], and other systems [20, 21]. Among all the ongoing research efforts, one common factor remains a

key driving force: continued improvement of biosensor’s performance.


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In this paper, we simulate an optical sensor based on a plasmonic configuration using a gold disk-in-hole nano-array.

Conceptually, we can consider that the annular aperture arrays are formed by stacking nano-disk arrays to the top ofcorresponding nano-hole arrays. In the disk-in-hole nano-arrays, two new coupled plasmon resonance modes are

simultaneously obtained under normal incidence. The two resonant modes originate from the robust interplay and

coupling interaction between the localized surface plasmons (LSP) comes from the nano-disk arrays and propagating

surface plasmons (PSP) caused by the nano-hole arrays. Strong electric field is confined in the gap between the nano-

disk and nano-hole arrays. Using the finite difference time domain (FDTD) method, we theoretically investigate the

influence of changing the outer and inner diameters on the coupling effect and consequently on the sensor performance.

Structural parameters can be changed by the software precisely and facilely. The results can be very useful for practical

experiments.

SPR is induced by the interactions between the free electrons of the metal and photons of the polarized light. Any disease

in the human body is accompanied by a disorder in any function in a certain organ in the body that is responsible of a

certain enzyme or hormone. Consequently, we hypothesize that any slight change of the refractive index of the

surrounding serum, due to the presence of small fluctuation of the sensed enzyme or hormone, will be indirectly detected

by observing the shift of either the SPR wavelength or intensity of the used gold nano-arrays. Optical simulation and

modeling tools will help in the selection and combination between the sensor and the detected biological molecules. The

performance of the designed sensor is determined through calculating its sensitivity, selectivity, full width half

maximum, and the figure of merit.

The main advantage of the reported sensor over those found in the literature is the ability to change its structural

parameters independently. Therefore, based on the application, we can tune the line width at a resonance wavelengthvalue. Additionally, the spatial distribution of magnetic and electric fields was presented to provide a better

understanding of the interaction between light and plasmonic nano-hole arrays.

This paper is organized as follows: the reported structure, FDTD simulation parameters, and evaluation techniques are

described in Section 2. Section 3 represents the performance characteristics of the proposed plasmonic sensor based on

different structures at different dimensions and discussions for its performance and the physical interpretation. Finally,Section 4 provides conclusion of the obtained results.

2. STRUCTURE DESCRIPTION AND METHODOLOGY

In this study, finite-difference time-domain (FDTD) method was used to measure and analyze the optical transmissionspectra of the reported plasmonic optical sensor. The FDTD algorithm is a numerical method of full-wave techniques

used to model some electromagnetic problems and to solve Maxwell’s equations for different mate rials. To apply and

simulate FDTD method, OptiFDTD simulation tool from Optiwave Inc. was used in this paper.

A schematic of the designed plasmonic sensor based on gold film perforated with disk-in-hole nano-array is illustrated inFig. 1(a). The substrate and air were used as a refractive index sensing medium and a cladding, respectively. Figure 1(b)

shows an enlarged unit cell, of period ( P ) and film thickness (h), having a nano- annular of inner diameter ( Di) and outer

diameter ( Do). The relative permittivity εr (ω) of the gold film can be defined using Lorentz-Drude model as follows:

,1

22

2

N

m mom

mmr

i

f

(1)

Where ε∞ denotes the permittivity at infinite frequency, f m is a function of position specifying the oscillator strengths, and Γ m is the damping coefficient. The incident wave frequency and the resonant frequencies are respectively represented by

ω and ωom. For the substrate layer, ε (ω) was assumed as n2 (n is the refractive index).

The simulation cell is 425 nm × 425 nm × 1000 nm in the Cartesian coordinates x, y, and z . An absorbing boundary

condition was rendered in the z -direction using anisotropic perfect matching layer (PML), while, periodic boundary

conditions (PBC) were used in the x and y directions. In our simulation, the period, film thickness, and substrate

refractive index are fixed at 425 nm, 200 nm and 1.5, respectively. In order to realize a broadband simulation on the

dispersive gold film, Gaussian modulated electromagnetic plane wave source was used. The continuous waves are

centered at 680 nm, linearly polarized in y-direction, and convoluted with a Gaussian envelope function.


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Figure 1. (a) Original layout and (b) enlarged single cell layout.

In the time domain, the light pulse has a half width of 0.8 × 10−14

s and an offset time of 0.1 × 10−14

s. The simulation

was performed at normal incidence ( z -direction) of the plane wave through the nano-hole arrays. The mesh size should

be small enough to catch the wave attenuation within the skin depth. As a result, so the calculation mesh resolution was

set as 5 nm (< 0.1 λ) to get accurate results. The simulation runs for 12,000 time step for a calculation time of 100 fs. An x- y observation area will perform and calculate the transmission spectral analysis at 400 nm away from the air/Au filminterface. Furthermore, to enhance the sensitivity of such optical sensor and get most accurate results, some sensing

performance characteristics would be calculated and analyzed.

Any rhymed modification in the transmission spectra, such as spectral shift; intensity change or both, is beneficial for

refractive index sensing. Although the spectral shift is simple and direct method in determining refractive index of the

medium, the intensity change can also be an efficient way. It is preferable if one uses both. The efficiency of a designedsensor is determined through calculating its resonance wavelength sensitivity (S λ ), resonance peak sensitivity (SI), line

width (Γ), and figure of merit (FOM). In the case of spectral shift investigation, the spectral sensitivity (S λ ) is the

measure of resonance wavelength per unit change of the refractive index. FOM is defined as the ratio of full width at half

maximum to Sλ at the resonance wavelength. In the case of resonance peak realization, the intensity sensitivity (S I) is the

change in the intensity per unit change in refractive index. In that case, a modified FOM is used to add the effect of peak

intensity enhancement or quenching. The line width is defined as the full width at half maximum. In general, high FOMand small line width are associated with highly selective sensor.

3. RESULTS AND DISCUSSIONS

First, the impact of changing the diameter of nano-array on the transmission spectra of the proposed plasmonic sensors,

based on both nano-disc and nano-hole arrays, has been investigated. We stare on line width at the resonance

wavelength. The modification of the line width due to the change of nano-array diameter is summarized for the case

nano-disc and nano-hole arrays in Table 1 and 2, respectively.

Based on the fact that small line width is associated with highly selective sensor, we can select the proper value of the

nano-array diameter. For example, For example, the plasmonic sensor based on the nano-disc array, array with Di = 150

nm is the best choice to get minimum Γ . The optical sensor founded on nano-hole array with Do = 200 nm shows a

minimum linewidth.

Second, we investigate the effect of changing the refractive index of the substrate on the transmission spectra of the

plasmonic sensors based on nano-disc and nano-hole arrays. During this study, n changes within a range of 1.5 ‒ 1.6 with

0.2 step for nano-disc array with Di = 150 nm and nano-hole array with Do = 200 nm.

The inset of Fig. 2(a) illustrates the schematic of the gold nano-disc array of diameter ( Di = 150 nm). Figure 2(a)

demonstrates the modification of the transmission spectra due to the change of the refractive index of the substrate.

Figure 2(a) shows that transmission spectra exhibit a dip in the visible region (≈ 650 nm). The change in the refractive

index is observed as shifting the transmitted wavelengths of that dip to longer wavelength as n increases. The change in

the resonance wavelength with different refractive index is illustrated in Fig. 2(c).


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Table 1. Line width at the resonance wavelength of the optical sensor based on nano-disc array

at different values of nano-disc diameters.

Enlarged single cell layout. Disc diameter (nm) Line width (nm)

100 151.21

150 33.59

200 77.29

250 252.40

300 >307.73

350 141.28

400 83.20

Table 2. Line width at the resonance wavelength of the optical sensor based on nano-hole arrayat different values of nano-hole diameters.

Enlarged single cell layout. Hole diameter (nm) Line width (nm)

200 46.8

250 53.8

300 59.9

350 71.4

400 182.1

Using linear fitting, the refractive index sensitivity was found to be equal to 387.13 nm/RIU, and maximum FOM equal

18.12.

The schematic of the gold nano-hole array of diameter ( Do = 200 nm) is illustrated in the inset of Fig. 2(b). The

modification of the transmission spectra due to the change of the refractive index of the substrate is shown in Fig. 2(b).

In comparison to the nano-disc arrays, the results of Fig. 2(b) exhibit the presence of an enhanced transmission peak inthe visible regime (≈ 625 nm). One can notice minimizing and red shifting of the wavelengths of that peaks as the

refractive index increases. The change in the resonance wavelength with different refractive index is illustrated in Fig.

2(d). Using linear fitting, the refractive index sensitivity was found to be equal to 220.91 nm/RIU, and maximum FOM

equal 4.87.

As can be seen from the obtained results, the plasmonic sensor based on nano-disc array exhibits higher sensitivity and

FOM. Unfortunately, it is sophisticated to fabricate such nano-disc structures in a uniform lattice arrangement. On the

other hand, it is a matter of interest to point out that there are several successful trials, for fabrication of nano-holestructures, have been achieved in the literature. However, the plasmonic sensor based on nano-hole array offers lower

sensitivity and FOM. By extension, it shows low transmission peaks. It worth noting that increasing the nano-hole

diameter may enhance the transmission peaks. Yet, increasing the nano-hole diameter results in thicker linewidth as

presented in Table 2.

Finally, we benefit from both higher sensitivity of the nano-disc array and soft fabrication process of the nano-hole

arrays to achieve a highly selective plasmonic sensor. A schematic of the designed plasmonic sensor based on gold film

perforated with disk-in-hole nano-array is illustrated in the inset Fig. 3(a). We here keep P , h, Di, and Do constants at 425

nm, 200 nm, 200 nm, and 150 nm respectively. The variation of the transmission spectra due to the change of the

refractive index of the substrate is shown in Fig. 3(a). Figure 3(a) shows that the transmission spectra exhibit an

enhanced peak in the infrared region (≈ 1050 nm). The resonance wavelength red shifts with as the refractive index

increases. The change in the resonance wavelength with different refractive index is illustrated in Fig. 2(b). Using linear

fitting, the refractive index sensitivity was found to be equal to 110.67 nm/RIU, and maximum FOM equal 1.71.


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Figure 2. Impact of changing the substrate refractive index on (a) and (b) the transmission spectra and on (c) and (d) theresonance wavelength. The plasmonic sensor is based on (a) and (c) nano-disc array and on (c) and (d) nano-hole array.

CONCULSION

In conclusion, we comprehensively investigate the performance of the optical sensor based on plasmonic nanostructures.

Evaluation techniques for plasmonic sensors based on both nano-disc and nano-hole arrays are studied. It was found that,

squeezing the best performance of the optical sensor can be done through disk-in-hole configuration. In such structure,

there are two types of plasmonic resonances are excited. The localized surface plasmons (LSP) are stimulated on the

middle sub-wavelength gold discs and the propagating surface plasmons (PSP) are induced at the surrounding nano-hole

interfaces. A trade-off between fabrication process complexity and sensitivity is achieved.


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Figure 3. Impact of changing the substrate refractive index on (a) the transmission spectra and on (b) the resonancewavelength of the plasmonic sensor based on disk-in-hole nano-array.

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Paper SPIE (9883-15) Submission EPE16-EPE101-31