Photonic Crystal Fiber Based Plasmonic Sensors

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refractive index was smaller than that of the prism. Otto configuration is followed the attenuated total reflection (ATR) method. When the p-polarization light is ...

Photonic Crystal Fiber Based Plasmonic Sensors Ahmmed A. Rifat,1 Rajib Ahmed,2 Ali K. Yetisen,3,4 Haider Butt,2 Aydin Sabouri,2 G. Amouzad Mahdiraji,1 Seok Hyun Yun,3,4 and F. R. Mahamd Adikan1,* 1

Integrated Lightwave Research Group, Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur-50603, Malaysia 2

Nanotechnology Laboratory, School of Engineering Sciences, University of Birmingham, Birmingham B15 2TT, UK

3

4

Harvard Medical School and Wellman Center for Photomedicine, Massachusetts General Hospital, 65 Landsdowne Street, Cambridge, MA 02139, USA

Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

*Corresponding author: [email protected]

1 Principle of Conventional SPR Sensors Conventional SPR sensors are utilized by using prism, which is used to concentrate the incident light. When the p-polarized or TM light hits prism-metal dielectric interface, a sharp dip is obtained with respect to resonance angle (considering the angle of incident). By following the angular interrogation method, unknown analyte concentration could be detected by measuring the shift of resonance angle. Based on this sensing mechanism, Otto and Kretschmann configurations were developed. In 1968, Otto developed a prism coupling technique where the prism and metal were placed in a gap, which was filled with the sample liquid (Figure S1a) [1]. The sample liquid refractive index was smaller than that of the prism. Otto configuration is followed the attenuated total reflection (ATR) method. When the p-polarization light is incident on the prism-dielectric interface it produces evanescent wave (EW), which excites the SPW on the metal-dielectric interface at a particular angle.

1

(b)

(a)

θ

Prism Sensing medium Metal layer

Reflected light

Reflected light Incident light (p-polarized)

Incident light (p-polarized)

εg εs εm

θ

Prism EW SPW

Metal layer Sensing medium

εg εm εs

SPW

(c) kev = kgsinθ

Frequency, ω

kinc

kev = kg

ksp (metal-dielectric)

ω0

0

ksp (metal-prism)

kev = ksp Propagation constant, k

Figure S1. SPR sensors using (a) Otto (dielectric constants of the prism, metal and sensing medium are εg, εm and εs, respectively; and θ is the incident angle), (b) Kretschmann configuration and (c) incident light wave in dielectric medium (kinc), evanescent wave (ksp) and the dispersion curve for metal-prism interface and metal-dielectric interface. At a particular angle wave-vector of evanescent wave and SPR are matched together, where a minimum of reflected wave intensity is observed. At this condition, energy transfers from the EW to SPW. At the prism-dielectric interface, wave vector Kev of the evanescent field is, k ev 

 c

 g sin 

2

1

where ω is the frequency of incident light, c is the speed of light, εg is the dielectric constant of the material of the prism, and the incident angle is θ. Otto configuration has been used for studying single crystal metal surfaces and their absorption. In prism coupling, metal layer and prism is placed with a finite gap, which is a drawback. Kretschmann configuration has been developed to overcome this challenge. In 1968, Kretschmann configuration was introduced where the prism and metal layer were bound without a gap in between; sample liquids were placed outside the metal layer (Figure S1b) [2]. In this method, surface plasmons also excite by the evanescent wave as analogous to Otto configuration. At a specific angle, when the wave vector of evanescent wave and the surface plasmon wave matches and resonance occurs, a minimum reflected light intensity is observed. Incident light wave vector travelling along the prism surface kg and the evanescent field wave vector kev are correlated as follows: k ev  k g sin  

 c

 g sin 

2

The main drawback of Kretschmann configuration is that metal layer should be parallel to prism surface, limiting applications in curved or bend surfaces such as a metal cylinder or metal sphere [3, 4]. In prism based SPR sensor resonance is occur when the propagation constant of evanescent wave and surface plasmon wave are same. According to the Kretschmann setup, propagation constant of evanescent wave equation is shown in equation 2. Based on the Maxwell’s equation propagation constant, ksp of surface plasmon waves which propagate through the metal-dielectric interface can be expressed as [5]:

3

k sp 

 c

 m s m  s

3

where εs is the dielectric constant of dielectric or sensing medium, and εm is the dielectric constant of the metal. At the resonance condition, equation (3) is kev=kg. The resonance occurs at the following condition [6]:

 c

 g sin res 



 m s c m  s

4

Figure S1c describes the resonance condition by showing the incident light wave in dielectric medium (kinc), evanescent wave (ksp) and the dispersion curve for metal-prism and metal-dielectric interface. At the resonance angle kev=kg and the propagation constant of evanescent wave (kev) coincides with the SPW of metal-dielectric interface (Figure S1c). However, kev never coincides with the SPW of metal-prism interface. References [1]

A. Otto, "Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection," Zeitschrift für Physik, vol. 216, pp. 398-410, 1968.

[2]

R. KretschmannE, "Radiative decay of non-radiative surface plasmons excited by light," Z Naturforsch, vol. 23, pp. 2135–2136 1968.

[3]

W. Knoll, "Interfaces and thin films as seen by bound electromagnetic waves," Annual Review of Physical Chemistry, vol. 49, pp. 569-638, 1998.

[4]

B. Gupta and R. Verma, "Surface plasmon resonance-based fiber optic sensors: principle, probe designs, and some applications," Journal of Sensors, vol. 2009, 2009.

[5]

Z. Jie, L. Dakai, and Z. Zhenwu, "Reflective optical fiber surface plasma wave resonance sensor," Acta Optica Sinica, vol. 27, p. 404, 2007.

[6]

Y. Zhao, Z.-q. Deng, and J. Li, "Photonic crystal fiber based surface plasmon resonance chemical sensors," Sensors and Actuators B: Chemical, vol. 202, pp. 557-567, 2014. 4

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