Analysis of Phase Interrogated SPR Fiber Optic Sensors with

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Analysis of Phase Interrogated SPR Fiber Optic Sensors with Different
Bimetallic Combinations
H. Moayyedb,a*, I. T. Leite b,a, L. Coelhob,a, J. L. Santosb, a, A. Guerreirob,a, D. Viegasa,c
a
INESC TEC (coordinated by INESC Porto), Rua do Campo Alegre 687, 4169-007 Porto, Portugal
Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
c
INL- International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga s/n, 4715-330 Braga Portugal.
b
ABSTRACT
Optical fiber sensors based on the phenomenon of plasmonic resonance can be interrogated applying different methods,
the most common one being the spectral approach where the measurand information is derived from the reading of the
wavelength resonance dip. In principle, a far better performance can be achieved considering the reading of the phase of
the light at a specific wavelength located within the spectral plasmonic resonance. This approach is investigated in this
work for surface plasmon based fiber optic sensors with overlays which are combinations of bimetallic layers, permitting
not only to tune the wavelength of the plasmon resonance but also the sensitivity associated with the phase interrogation
of the sensors. The metals considered for the present analysis are silver, gold, copper, and aluminum.
Keywords: Plasmonics, optical fibers, sensors, sensor interrogation, phase reading, sensitivity, bimetallic combinations.
1. INTRODUCTION
The effect of surface plasmon resonance (SPR) has been widely used for highly sensitive and selective detection of
several physical, chemical, and biochemical parameters.1 Indeed, ever since the introduction of various optical methods
in the excitation of the SPR at a metal-dielectric interface,2 it has been widely recognized that such an excitation can be
used to achieve sensing or monitoring of various interfacial phenomena with ultrahigh sensitivity. These include, for
example, chemical and biological sensing,3 film-thickness sensing,4 temperature sensing,5 and angular measurement.6
Recently, it has been demonstrated that the SPR technique applied to chemical and biological sensing can achieve
resolutions down to 10-7 refractive index units (RIUs), values not accessible to others optical sensing techniques.7
Phase interrogation has not yet been widely explored in the context of SPR sensors. Surely, this methodology is
somewhat more complex to implement than most traditional ones, requiring deeper know-how on optical components,
modulation instruments and signal processing techniques, but eventually the main reason has to be with the scientific
domains that historically were the driving force for this sensing technology, mostly situated in the chemical and
biochemical fields where the application of spectroscopic characterization techniques is the standard. This is changing
since plasmonics is moving to areas other than sensing, such as imaging and data storage, attracting to the subject
researchers with more diverse backgrounds. A consequence of this dynamics is a clear improvement of the resolution
values obtained when considering phase interrogation.
Most of these works were performed with classical sensing heads based on a prism coupling system in Kretschmann’s
configuration. Under appropriate conditions the SPR induced phase change would be greatly amplified if the light
undergoes multiple attenuated total reflections, as happens when the sensing platform is an optical fiber. Therefore, the
investigation of SPR based optical fiber sensors with phase interrogation has attracted interest in recent years.8
During the years, the SPR technique has been widely used for a quick and accurate detection of several physical,
chemical, and biochemical parameters. The metal-dielectric interface supports an electromagnetic wave with
characteristics highly sensitive to the outer medium and, in general, silver and gold are the two main metals that are used
for SPR sensor applications.9 Silver-based sensors are known for their narrow spectral width, but are chemically very
*[email protected]; phone +351 22 60 82 601; fax +351 22 60 82 679
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unstable and are highly vulnerable to oxidation when used in liquid or gaseous environments. On the other hand Goldbased sensors are lesser accurate than Silver-based ones but, in contrast, are chemically very stable. It was shown that the
bimetallic layers based sensors not only displayed a high shift of resonance angle as a Au-based sensor, but also showed
narrower resonance curve as Ag-based sensors, plus the additional advantage of protecting silver against oxidation. 9
Such appealing characteristics of the bimetallic arrangement for SPR based sensing motivated us to study it further,
which is the focus of this work. The properties of different bimetallic combinations to be used in a fiber optic sensor
based on the SPR technique, particularly when phase interrogation is considered, are studied here. The metals chosen for
the present analysis are silver, gold, copper, and aluminum. The performance of these sensing structures is assessed,
particularly when considering phase interrogation.
2. SENSING STRUCTURE AND ANALYSIS
The structure of the sensing head under analysis is schematically illustrated in Fig. 1. It consists of a fiber layout with
two metal layers.
Figure 1 – Schematic of the SPR-based optical fibre sensing probe under analysis.
The length of the sensing region is assumed to be L, ρ0 the radius of the core and θ0 the angle of incidence of the optical
ray with respect to the normal to the core-cladding interface. The normalized transmitted power through the sensing
region of the fiber is given by 8
⁄
trans
ref
=
,
⁄
(1)
where Rp is the reflectivity of the multilayer structure and θcr = sin-1(n2/n1) is the critical angle for the light confinement
inside the fiber (n1 and n2 are respectively the refractive indices of the core and the cladding of the fiber). Nref ( ) is the
number of reflections the optical ray with θ0 undergoes in the sensing region, and is given by
=
.
(2)
ref
tan
Moreover,
sin
=
cos
(3)
cos
is the intensity distribution between the continuum of guided modes. In order to access the performance of phase
interrogation of SPR based sensors supported by fiber structures, it is convenient to express the amplitude reflection
coefficients rs and rp in the polar form as
=| |
, =
.
(4)
Then the phase difference variation δϕp,s resulting from a single reflection at a given incident angle between the p and the
s polarization components is
=
− ,
(5)
,
and the total phase difference variation considering all the reflections inside the sensing region is given by:
∆
,
=
ref
,
.
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(6)
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Figure 2 – (a) Phase difference (between s and p polarizations) and (b) phase sensitivity as function of the refractive index
of the surrounding medium, for a sensing structure comprised a 50 nm thickness metal layer. The considered interrogation
wavelength is 632.8 nm.
At this stage, it is useful to define the sensitivity of the sensing structure to the refractive index ns of the surrounding
medium, which can be regarded as the basic parameter quantifying the performance of the sensor. A change in the
refractive index ns = (εs)1/2 of the surrounding dielectric produces a significant variation in the propagation constant of
the surface plasmon, resulting in the modification of the SPR coupling condition. This can be observed as a change in
one or more of the properties of the light transmitted through the sensing structure. The sensitivity can be defined as:
S
,
=
(7)
where δξ is the change in a given property of light (e.g. intensity, resonance wavelength or phase) due to a variation of
δns in the refractive index of the surrounding medium. We will be particularly concerned with the case of phase
interrogation, where the property of light of interest is the phase difference Δ p,s between the p and s polarization
components. In this case, the phase sensitivity to refractive index variations is then defined as Sn,ϕ = δ p,s/δns.
3. RESULTS AND DISCUSSION
In our analysis, we have considered a step-index multimode silica fiber, with core diameter of 100 µm and 0.24
numerical aperture, and a 1 cm long sensing region. The metals considered for the present analysis are Au, Ag, Cu, and
Al. The refractive indices for these metals were obtained from experimental data available in the literature.10 The study
was performed considering an incident light with wavelength of 632 nm. Figure 2(a) shows phase difference between s
and p polarizations and Fig. 2(b) shows the corresponding phase sensitivities for different single metal layers with
thickness of 50 nm. From these figures, we can see Au and Ag have better phase sensitivity and maximum phase
sensitivity for Cu is close to the refractive index of water (ns = 1.333 at 632.8 nm). On the other hand a single layer of Al
does not show resonance in phase interrogation.
In Fig. 3 we show phase difference between s and p polarizations and maximum phase sensitivity as function of different
bimetallic combinations. The configuration contains a combination of two metallic layers with the thickness of 40 nm
and 10 nm for inner and outer layers, respectively. Figure 3(a) shows phase difference and sensitivity detection when
inner layer of metal combination is Ag. Regarding to the Fig. 2, the point of maximum phase sensitivity for a single
metal layer of Ag is in 1.354 RIU, therefore by coating with a layer of Au and Cu this maximum shifts to 1.333 RIU and
1.347 RIU, respectively. Maximum phase sensitivity for the single layer of Au is -3.5×104 degrees/RIU that happens on
refractive index of 1.32 RIU. Figure 3(b) shows by coating with a layer of Ag to the single layer of Au, maximum phase
sensitivity reaches the value of -6.7×104 degrees/RIU, besides the tuning of this maximum to the refractive index of
1.34.
The inclusion of bimetallic combination has also the benefit of shifting and increasing the maximum phase sensitivity for
Cu, as illustrated in Fig. 3(c). It can be seen that with coating of Au and Ag phase sensitivity rises and shifts to newer
refractive index values. It can also be observed that combination of metals with Al does not show relevant results when
addressing phase interrogation.
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Figure 3 - Phase difference (between s and p polarizations) and phase sensitivity as function of the refractive index of
the surrounding medium, for a sensing structure comprised of 40 nm and 10 nm for inner and outer metallic layers,
respectively. The considered interrogation wavelength is 632.8 nm. Figures report three different combinations when (a)
Ag (b) Au and (c) Al are the inner layers.
4. CONCLUSION
In this work we studied the characteristics of fiber optic SPR sensors incorporating the bimetallic combination in what
concerns the sensitivity to variations of the surrounding refractive index when considering phase interrogation. These
results suggest that the bimetallic sensing structure can be particularly advantageous in the context of phase interrogation
since it substantially increases the phase sensitivity to refractive index variations.
ACKNOWLEDGMENTS
This research was performed in the framework of project PTDC/FIS/119027/2010 (Plasmonics Based Fiber Optic Sensing with
Enhanced Performance) funded by National Funds through FCT – Fundação para a Ciência e a Tecnologia. L. Coelho
acknowledges the support from FCT grant SFRH/BD/78149/2011.
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