Optik 127 (2016) 7514–7519

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Optik

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High-performance bimetallic film surface plasmon resonancesensor based on film thickness optimization

Shujing Chena,∗, Chengyou Linb

a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,School of Materials Science and Technology, China University of Geosciences, Beijing 100083, Chinab College of Science, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e i n f o

Article history:Received 28 March 2016Accepted 24 May 2016

Keywords:Surface plasmon resonanceBimetallic filmSensitivityResolution

a b s t r a c t

The performance of a surface plasmon resonance (SPR) sensor with silver-gold bimetal-lic film based on angular interrogation was presented. To achieve minimal reflectance atresonance angle, the thicknesses of silver and gold layers of the bimetallic sensor wereoptimized at first. Then, the sensitivity and full-width-half-maximum (FWHM) of SPR sen-sors with optimum silver-gold thicknesses were investigated. With the increasing of thethickness of silver layer in bimetallic film, both the sensitivity and FWHM of the SPR sensordecreases, which indicates that the bimetallic sensor can obtain higher sensitivity than theSPR sensor with single silver layer, and smaller FWHM than the one with single gold layer.In addition, by analyzing the electric field inside the bimetallic SPR sensor, we studied theunderlying physics of the improvement of the bimetallic sensor performance.

© 2016 Elsevier GmbH. All rights reserved.

1. Introduction

Surface plasmon resonance (SPR) sensors have been widely used in the area of biological [1,2] and chemical [3,4], becauseof their advantages of simple, direct, high-sensitivity and real-time detection. One of the most common configurations is theKretschmann configuration based on the angular interrogation, in which the resonance is realized by a dip in the reflectivityversus incidence angle. Usually, the performance of a SPR sensor is evaluated by its three features. The first is the sensorsensitivity, which is defined as the resonance angle or wavelength shift per analyte refractive index unit [5]. The second isthe sensor resolution [5], described by the smallest change in the bulk refractive index that produces a detectable changein the sensor output. Generally, the formula of the resolution in an angular-interrogation SPR sensor can be expressed asthe ratio of the sensitivity and the noise level. Additionally, the noise level of the sensor is proportional to the spectrumwidth (full-width-half-maximum, FWHM) [5]. So the resolution of the SPR sensor can be improved by decreasing FWHM ofthe SPR reflectivity dip curve [6,7]. The last one is the chemical stability of the metal film under extreme environmental. Inthe last two decades, many approaches were proposed to enhance the sensitivity, resolution and chemical stability of the

SPR sensor. For example, by adding a top layer of dielectric thin film, the sensitivity of the guided-wave SPR sensor can beimproved [8–10]. By using long-range surface plasmon, the high-resolution SPR sensors were realized [11–14]. Particularly,double-metal-layer (such as silver-gold bi-layer) can be used in a SPR sensor to enhance the sensitivity and stability [15,16].

∗ Corresponding author.E-mail address: chenshujing@cugb.edu.cn (S. Chen).

http://dx.doi.org/10.1016/j.ijleo.2016.05.0850030-4026/© 2016 Elsevier GmbH. All rights reserved.

S. Chen, C. Lin / Optik 127 (2016) 7514–7519 7515

HbbStraF

wpt

2

[fima

wp

w

a

o

Fig. 1. Schematic diagrams of (a) a bimetallic film SPR sensor and (b) its bi-layer mode.

In 2002, sensitivity improvement of the SPR sensor with silver-gold bimetallic film were studied theoretically by Wu ando [17]. The sensitivity and signal to noise ratio of a step-index fiber optic SPR sensor with bimetallic layers were analyzedy Sharma and Gupta in 2005 [18]. The sensitivity and stability of the SPR sensor with double metal layer were optimizedy Ong et al. [15]. Also, they optimized film thickness for maximum evanescent field enhancement of the bimetallic filmPR sensor [19] in 2006. The bimetallic film configuration also was utilized to improve both the sensitivity and stability ofhe commercial SPR instrument by Chen et al. in 2011 [20], and the SPR optical fiber sensor by Huang et al. in 2014 [21],espectively. Besides, the resolution enhancement in SPR sensor based on waveguide coupled by combining a bimetallicpproach were presented by Lee et al. in 2010 [12]. However, a bimetallic film SPR sensor with high sensitivity and smallWHM simultaneously has not been proposed until now.

In this paper, the sensitivity and FWHM of SPR sensors with bimetallic film based on angular interrogation (Fig. 1(a))ere studied under the optimized thicknesses of silver and gold layers for achieving minimal reflectivity, and the underlyinghysics of the improvement of the bimetallic sensor performance were investigated by analyzing the electric field insidehe bimetallic SPR sensor.

. Calculation methods

Theoretical analysis of the plane-wave radiation interacting with stratified layers was presented by several investigators22,23]. Fig. 1(b) is the schematic diagram of the bi-layer mode of the SPR sensor shown in Fig. 1(a), where the bimetalliclm can be considered as two independent layers, and the prism and the analyte are seen as the incident and emergentedium respectively. The electric and magnetic field amplitudes at the first boundary (E0 and H0) of this bi-layer structure

re connected to those at the last boundary (E3 and H3) by the total characteristic matrix[E0

H0

]=

⎡⎣ cos ı1

i

�1sin ı1

i�1 sin ı1 cos ı1

⎤⎦

⎡⎣ cos ı2

i

�1sin ı2

i�2 sin ı2 cos ı2

⎤⎦

[E3

H3

], (1)

here ıi (i = 1, 2) is the phase factor of the silver or the gold layer, and ıi = 2�� nidi cos �i (i = 1, 2) for both TM and TE

olarization. ni, di and �i denote the refractive index, layer thickness and incident angle for each layer in the bi-layer structure.The optical admittance �i is given by

�i ={

ni/ cos �i p-wave(TM)

ni cos �i s-wave(TE), (2)

here the subscripts 0,1,2,3 represent the optical admittance in prism, silver layer, gold layer and analyte respectively.The equivalent optical admittance at the first boundary of the bi-layer structure is Y = H0/E0, and the optical admittance

t the last boundary (E3 and H3) is �3 = H3/E3, so the relationship of E and H in matrix form can also be written as

E0

[1

Y

]=

⎡⎣ cos ı1

i

�1sin ı1

i�1 sin ı1 cos ı1

⎤⎦

⎡⎣ cos ı2

i

�2sin ı2

i�2 sin ı2 cos ı2

⎤⎦[

1

�3

]E3. (3)

The characteristic matrix of the bi-layer structure can be written as[B]

=

⎡⎣ cos ı1

i

�1sin ı1

⎤⎦

⎡⎣ cos ı2

i

�2sin ı2

⎤⎦[

1]

. (4)

C

i�1 sin ı1 cos ı1 i�2 sin ı2 cos ı2�3

In this case, the reflectivity R of the bi-layer structure can be derived by R = | �0B−C�0B+C |2, where �0 is the optical admittance

f incident medium.

7516 S. Chen, C. Lin / Optik 127 (2016) 7514–7519

Fig. 2. Reflectance at resonance angle (Rc) with varying silver-gold thickness. The analyte is (a) air and (b) water respectively.

Fig. 3. (a) The optimized thicknesses of silver-gold bi-layer in the bimetallic film SPR sensor to achieve minimal reflectance at resonance angle (Rc) for airand water analyzing; and (b) the total thicknesses of the optimized bimetallic film SPR sensors.

In a Kretschmann prism-coupling device, the resonance angle �c, where the minimum reflectivity Rc is realized, changeswith the varied refractive index of the analyte. The angular sensitivity of a SPR sensor is defined as the ratio of resonanceangle variation ��c and the variation of refractive index of the sample �n,

S� = (��c

�n). (5)

3. Numerical calculation results and analysis

In this paper, reflectivity at resonance angle (Rc), FWHM and sensitivity of bimetallic film surface plasmon resonancesensors with different layer thicknesses were discussed. All simulations were performed at incident light wavelength � =633nm. Also, a BK7 glass prism with refractive index np = 1.51509 was used in our prism-coupling device. The permittivityof silver and gold used for simulation were εAg = −17.81 + 0.676i and εAu = −10.98 + 1.464i, respectively.

Fig. 2 shows the reflectance at resonance angle of the bimetallic film SPR sensor under different thicknesses of gold andsilver layers. In the simulation, the calculating thickness range of silver and gold are 0–52 nm and 0–46 nm respectively, toinvolve the sensor structure with single metal layer. According to the principle of conservation of energy, if the reflectivityat resonance angle Rc reaches its lowest value, the maximum amount of energy from the incident light could be coupledto surface plasmons (i.e., the best coupling coefficient), which can help the SPR sensor to achieve high sensitivity [7] andresolution [6,24]. From Fig. 2, we can see that near-zero Rc can be realized when the gold and silver layers take the appropriatethicknesses, called the optimized thicknesses of silver-gold bi-layer. The values of the optimized thicknesses of silver-goldbi-layer in the bimetallic film SPR sensor for air and water analyzing are presented in Fig. 3.

As shown in Fig. 3(a), although the optimized thicknesses of silver-gold bi-layer are slightly different for two analytes(air and water), the thickness of one metal layer decreases with the increasing of the thickness of the other metal layer in

both cases. The results show that the total thickness of bimetallic film is limited when achieving the minimal Rc. In Fig. 3(b),the total thicknesses of bimetallic film in the optimized SPR sensors are shown. As the silver-layer thickness in optimizedthicknesses of silver-gold bi-layer increases, the total thickness of bimetallic film decreases at first, and then increases forboth air and water analytes. The minimum total thickness is 42.6 nm and 43.7 nm for the analyte of air and water respectively.

S. Chen, C. Lin / Optik 127 (2016) 7514–7519 7517

Fig. 4. Reflectivity curves of three different configurations versus angle of incidence for the analyte of (a) air and (b) water.

Table 1The angle shift and FWHM of the three SPR sensor configurations.

Configuration Resonance angle shiftfor air analyte (degree)

Resonance angle shift forwater analyte (degree)

FWHM for airanalyte (degree)

FWHM for wateranalyte (degree)

Single layer (silver) 0.5666 1.1514 0.2497 1.1950Bimetallic layer (silver-gold) 0.5930 1.3460 0.9266 4.5648Single layer (gold) 0.6132 1.4644 2.3070 7.9093

Hr

bSataogsSfils

tS6obf

Fig. 5. Sensitivity of optimized bimetallic film SPR sensors with varying the optimum thickness of silver-gold layer.

owever, for the SPR sensor with single gold layer, 46 nm and 45.7 nm are the optimized thickness for air and water analytesespectively, while for the one with single silver layer, 50 nm and 52 nm are the optimized thicknesses.

In order to analyze the sensitivity and FWHM of bimetallic film SPR sensors with the optimized thicknesses of silver-goldi-layer, the shift of SPR reflectance curves for three configurations (the single silver layer SPR sensor, the single gold layerPR sensor and the bimetallic silver-gold layers SPR sensor) with the changing refractive index of analyte (from 1.0 to 1.01nd 1.325 to 1.335 respectively) were studied firstly, as shown in Fig. 4. The optimized thickness of silver-gold used inhe simulation is 30–13.1 nm for the analyte of air, and 30–14.2 nm for the analyte of water. The shift of resonance anglend FWHM of the three configurations are summarized in Table 1. It can be seen that the shift of the resonance angle of theptimized bimetallic film SPR sensor is larger than the SPR sensor with single silver layer, but smaller than the one with singleold layer. The results indicate that the sensitivity of the optimized bimetallic film SPR sensor is better than the sensor withingle silver layer, but poor than the one with single gold layer. On the other hand, FWHM of the optimized bimetallic filmPR sensor shows totally contrary rules. Comparing with the SPR sensors with single metal layer, the optimized bimetalliclm SPR sensor can achieve a higher sensitivity than the single silver layer sensor and smaller FWHM than the single gold

ayer sensor. So, the bimetallic film SPR sensor integrates the small FWHM of the sensor with single silver layer and highensitivity of the sensor with single gold layer.

To investigate the sensitivity of bimetallic film SPR sensors under different optimized thicknesses of silver-gold layer,he sensitivity of bimetallic film SPR sensor for the analyte of air and water are presented in Fig. 5. The sensitivity of thePR sensor with single gold layer (the thickness of silver is 0 nm) and single silver layer (the thickness of gold is 0 nm) are1.32 and 56.66/RIU for air analyte, and 146.44 and 115.14/RIU for water analyte. For bimetallic film SPR sensors with the

ptimized thicknesses of silver-gold layer, the sensitivity decreases with the increasing of the thickness of silver layer in theimetallic film for both air and water analytes. So, the gold layer on the top of the silver layer can not only protect the silverrom oxidation, but also improve the sensitivity of the sensor. Furthermore, we notice that the sensitivity of the sensor with

7518 S. Chen, C. Lin / Optik 127 (2016) 7514–7519

Fig. 6. The maximal electric field inside the three SPR sensors for the analyte of water versus incident angle.

Fig. 7. FWHM of bimetallic film SPR sensor with varying the optimum thickness of silver-gold layer.

water analyte is much higher than the one with air analyte. The reason for that is far from the cut-off wavelength of totalreflection, the sensitivity of a prism-coupling SPR sensor can be approximately expressed as S� ≈ 1/

√n2

p − n2s , in which np

and ns represent the refractive index of the prism and the analyte respectively [25]. In this case, the sensitivity of the SPRsensor with a high-index analyte (water) should be larger than the one with a low-index analyte (air).

In order to explain the origin of the sensitivity decreasing with the increasing silver thickness in the bimetallic film, theelectric field inside three SPR sensors in Table 1 were calculated by using the method described by Shalabney and Abdulhalim[23], and the maximal electric field of three SPR sensors for the analyte of water with the changing incident angle were plottedin Fig. 6. The integrals of the maximal electric field (S) for single gold layer, bimetallic film and single silver layer SPR sensorsare 44.14, 43.34 and 38.69, respectively. This result confirms the statement of correlation between the sensitivity and theelectric field integral as described by Benkabou and Chikhi [26].

To study the performances of FWHM of bimetallic film SPR sensors, the values of FWHM of the bimetallic film SPR sensorswith optimum thicknesses are presented in Fig. 7. With the increasing of the thickness of silver layer, the values of FWHMdecreases for both two analytes and the smallest value of FWHM shows at the single silver SPR sensor configuration.

To investigate the origin of FWHM decreasing with increasing silver thickness, we plotted the electric field distributionof three SPR sensors in Table 1 for the analyte of water, as shown in Fig. 8. The maximal electric field at the analyte-metalinterface for single silver layer, bimetallic film and single gold layer SPR sensor are 7.882, 4.329 and 3.356, respectively. Theresult indicates the inverse relation between FWHM of the SPR sensor and the maximal electric field at the analyte-metalinterface inside the sensor.

4. Concluding remarks

In conclusion, the optimized thicknesses of silver and gold layers for achieving minimal reflectivity of the bimetallic filmsensor with the analyte of air and water were found in this paper. The sensitivity and FWHM of the bimetallic film SPRsensors with optimized thicknesses were studied, and their underlying physics were investigated by analyzing the electricfield inside the bimetallic SPR sensor. The results indicate that it is hard to obtain the highest sensitivity and smallest FWHM

simultaneously for a bimetallic SPR sensor, because the highest sensitivity is shown in the SPR with single gold layer, but thesmallest FWHM is present in the one with single silver layer. However, using bimetallic film SPR sensors, the sensitivity andFWHM of the SPR sensor can be flexibly adjusted to meet the actual requirements, by changing the thicknesses of bimetalliclayers.

S. Chen, C. Lin / Optik 127 (2016) 7514–7519 7519

A

tP

R

[

[

[

[

[

[

[

[[

[

[

[

[

[

[

[

[

Fig. 8. The electric field for the single gold layer, bimetallic film and single silver layer SPR sensors.

cknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11547241 and 11547183),he Fundamental Research Funds for the Central Universities, China (Grant Nos. 2652014012 and JD1517) and the Openingroject of Shanghai Key Laboratory of All Solid-state Laser and Applied Techniques (ADL-2014003).

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