Surface Plasmon Resonance Characteristics of Optical Fiber
Incorporated with Au Nano-particles in Cladding Region
Seongmin Ju, Seongmook Jeong, Youngwoong Kim, Sang-Hyun Lee, and Won-Taek Han*
School of Information and Communications/Department of Physics and Photon Science,
Gwangju Institute of Science and Technology, Gwangju, 500-712, Korea
*Corresponding author Email:wthan@gist.ac.kr
A novel surface plasmon resonance (SPR) sensor based on specialty optical fiber having its
cladding doped with Au nano-particles (NPs) was developed by modified chemical vapor
deposition process. To optimize the SPR absorption and sensitivity of the fiber SPR sensor, effect
of the fiber length (20 cm - 90 cm) on sensing capability of refractive index (n = 1.418 - 1.448)
was investigated. Absorption peaks appearing at 392 and 790 nm were due to SPR from Au NPs
in the cladding region of the optical fiber. The SPR was found to occur at particular wavelengths
around 390 nm for the corresponding refractive indices regardless of the length of the fiber,
increased with the increase of the index. The measured SPR sensitivities (wavelength/RIU) of
the fiber were estimated to be 407 nm/RIU, 217 nm/RIU, and 54 nm/RIU with the fiber lengths
of 20 cm, 45 cm, and 90 cm, respectively. The SPR absorption intensity and FWHM decreased
with the increase of the fiber length because the propagation loss of the signal through the fiber
cladding region increased.
Keywords: Surface plasmon resonance, Nano-particles, Cladding-doped optical fiber,
Absorption
1. Introduction
The optical fiber sensors based on surface plasmon resonance (SPR) from metals nanoparticles (NPs) such as Au, Ag, and Cu have drawn much attention due to its all-optical remote
sensing capability, low cost fabrication, and compactness.1-14 Recently, we have developed a
novel optical fiber incorporated with Au nano-particles (NPs) in cladding region for surface
plasmon resonance (SPR) fiber sensor applications by eliminating a process of metal thin film
coatings.1 In the case of the novel optical fiber incorporated with metal NPs in cladding region,
the SPR is originated from the confined conduction electrons oscillating in resonance with the
electromagnetic field surrounding the metal NPs, resonantly excited when the wavelength of
incident light is equal to the characteristic wavelength of metal NPs.4-8,13-16 Therefore, metal NPs
exhibit selective photon absorption because the conduction electrons oscillate collectively only
to specific wavelengths of light. The selective absorption is observed at resonance angle because
1
of reduction in the energy of the reflected light due to its energy transfer to surface plasmons.
This resonance angle is very sensitive to variation in the refractive index of the sensing layer.47,13-16
In this work, the optical fiber sensor based on the Au NPs(cladding)-doped fiber was
demonstrated and in particular, the effect of refractive index change on the SPR characteristics
with various lengths of the Au NPs(cladding)-doped fiber was investigated to optimize the SPR
absorption and sensitivity of the fiber SPR sensor.
2. Experimental Details
The Au NPs(cladding)-doped fiber was fabricated by using the modified chemical vapor
deposition (MCVD) and fiber drawing processes. The doping solution was prepared by dissolving
0.025 mole of reagent grade Au(OH)3 powders (Aldrich Chem. Co. Inc., 99.9 %) in nitric acid
solution (Junsei Co., 70 %). After the deposition of porous germano-silicate layers onto the inner
surface of a silica glass tube by the MCVD process, the porous deposition layers were soaked in
the Au doping solution for two hours and the tube was dried and sintered to incorporate Au NPs.
Then a pure silica glass rod (refractive index, n = 1.4571 @ 633 nm) was inserted into the tube
with the deposited and doped layers and consolidated into a jacked rod to obtain a fiber preform.
The outer part of the glass rod, which was the original silica glass tube, was etched off using
hydrofluoric acid solution (J. T. Baker, 49%) to expose the doped layers to become a new surface
of the rod as a cladding. Thus the final preform consisted of the germano-silicate glass cladding
doped with Au NPs and the pure silica glass core. Finally, the fiber preform was drawn into a
fiber with 124.3 µm in diameter using the draw tower at 2150 °C. During the drawing process,
the fiber was coated with lower refractive index polymer (EFIRON UVF PC-375, n = 1.3820)
than that of the germano-silicate glass of the cladding to induce total internal reflection for light
transmission. The refractive index difference between the core and cladding was about 0.00125,
enabling a light to propagate into the cladding region not into the core. The cladding width and
total diameter of the optical fiber were 2.6 μm and 124.3 μm, respectively. A detail description of
the fiber fabrication process was described in reference 1.
To confirm the formation of Au NPs in the cladding, the optical fiber was examined by the
transmission electron microscope (TEM; Technai, G2 S-Twin 300 KeV). Optical absorption of
the fiber was measured by the cut-back method using the Optical Spectrum Analyzer (Ando AQ
6315B) and white light source (Ando AQ 4305) to assure the propagation of light through the
glass cladding and to confirm the existence of Au NPs. Then, to characterize SPR sensing
property, optical absorption of the fiber was measured by putting small drops of the refractive
index matching oil with various refractive indices (n = 1.418 - 1.448) on the surface of the
stripped portion of the fiber of 3 cm. The SPR sensitivity was optimized by using the Au
NPs(cladding)-doped fibers with various lengths of 20 cm, 45 cm, and 90 cm were used.
2
3. Results and Discussion
The existence and size distribution of Au NPs in the cladding region of the fabricated fiber
were verified by TEM morphology as shown in Fig. 1. The average diameter of Au NPs in the
cladding region of the fiber was 3.6 nm (size distribution: 2.6 nm ~ 5.2 nm), which was crystalline,
to be roughly spherical and homogeneous without agglomeration. The existence of Au NPs of
the fiber is the evidence of the preserved Au NPs in the cladding region of the fiber even after
drawing the preform at high temperature about 2150 °C. The existence of Au NPs was verified
again by optical absorption spectra of the fiber as shown in Fig. 2. The absorption bands due to
Au NPs in the cladding region of the fiber were found to appear peaking at 392 and 790 nm
depending on the particle size of Au NPs.1,17-19 The absorption band at 392 nm is known to be
due to Au NPs dispersed uniformly in the fiber cladding, while the rather broad absorption band
peaking at 790 nm may be due to the aggregates of the Au NPs.20-22
Effect of the length of the Au NPs(cladding)-doped fiber on SPR sensitivity was investigated
by measuring the SPR spectra after dropping the index matching oils onto the stripped portions
(3 cm) of the three fibers of 20 cm, 45cm, and 90 cm. Fig. 3 shows the absorption spectra of the
Au NPs(cladding)-doped fiber as a function of refractive index of the matching oils with various
fiber lengths. The SPR peak was found to occur at particular wavelengths around 390 nm for the
corresponding refractive indices regardless of the length of the fiber. The SPR peak wavelength
shifted towards longer wavelength with the increase of the refractive index. It is noted that the
SPR peak appearing at 390 nm after dropping the index matching oils (n = 1.418 - 1.448) was
related to the absorption peak of the fiber coated with low-index polymer (n = 1.382) observed
at 392 nm due to surface plasmon resonance. From the results shown in Fig. 3, the variation of
the SPR sensitivity and the SPR absorption intensity with full-width at half maximum (FWHM)
were estimated and shown in Fig. 4 and 5, respectively. Note that the SPR absorption intensity
and the FWHM were calculated after the baseline correction, which was carried out by fitting a
curve to the experimental measurements around the peak absorption wavelength.
The SPR peak wavelengths increased with the increase of the corresponding refractive
indices regardless of the length of the fiber as shown in Fig. 4(a). The observed red-shift of the
SPR peak with the increase of the refractive index is related to resonance condition, which is
satisfied at some higher value of the wavelength of the incident light due to the increase of the
wave vector of the surface Plasmon mode.1,4-7,12-15,23 The SPR sensitivities (wavelength/RIU),
which are the slopes of Fig. 4(a), were estimated to be 407 nm/RIU, 217 nm/RIU, and 54 nm/RIU
with the fiber lengths of 20 cm, 45 cm, and 90 cm, respectively, as shown in Fig. 4(b). The SPR
sensitivity was found to decrease with the increase of the fiber length, it may be due to the
increased coupling of the SPR absorption and the optical absorption from Au NPs with the
increase of the fiber length. With the change in SPR sensitivity, the shift of the SPR peak
3
wavelength with the fiber length change was also found. The SPR peak wavelength decreased
with the increase of the fiber length regardless of the refractive index. The extent of red-shift of
the SPR peak by changing corresponding refractive indices decreased with the increase of fiber
length due to the increase of the propagation loss. The variation of the absorption intensity and
the FWHM of the SPR spectra of the fibers with various fiber lengths of 20 cm, 45 cm, and 90
cm as a function of refractive index of the matching oils are shown in Fig 5(a) and (b), respectively.
The SPR absorption intensity and the FWHM decreased with the increase of the fiber length
because of the propagation loss of the light through the fiber cladding region. Note that no
significant increase of the SPR absorption intensity was found, maybe due to the amplification
of SPR peak with the increase of the fiber length.
Also, as the refractive indices of the oils increased, the SPR absorption intensity increased
regardless of the length of the fiber due to a more divergent light beam leaking out from the
cladding of the fiber.13,24-25 In the case of the FWHM shown in Fig. 5(b), on the other hand, the
broadening of the SPR was found with the increase of the refractive index, maybe due to the
spatial spreading and scattering of the conduction electrons.26 As the fiber length increased,
however, the change of the FWHM as well as the SPR absorption intensity was found to decrease
with the increase of the refractive index of oils, due to the signal distortion or overlap by
increasing the propagation length as mentioned before. As the fiber length increased from 20 cm
to 90 cm, the SPR sensitivity decreased from 407 nm/RIU to 54 nm/RIU. The average absorption
intensity and average FWHM were changed from 2.1 dB to 0.7 dB and from 60.6 nm to 27.5 nm,
respectively. From the results of length dependence of the SPR peaks from the Au NPs(cladding)doped fiber, the optimum length of the fiber SPR sensor was 20 cm. The SPR sensitivity, the
average absorption intensity, and the average FWHM with various fiber lengths are listed in Table.
1.
4. Conclusions
The surface plasmon resonance (SPR) optical fiber sensor based on specialty optical fiber
incorporated with Au NPs in cladding region has been developed. The average diameter of Au
NPs in the cladding region of the fabricated optical fiber was 3.6 nm. The absorption peaks
appearing at 392 and 790 nm were attributed to the SPR of the incorporated Au NPs in the
cladding region, which were due to the single Au NPs and the coupling effect of the Au NPs
according to the dipole–dipole interactions, respectively. The SPR peak was found to appear
around 390 nm for the corresponding refractive indices (n=1.418-1.448), increased with the
increase of the index by putting a refractive index matching oil directly on the surface of the fiber
after removing the coating polymer regardless of the length of the fiber.
As for the effect of the fiber length, the SPR peak wavelengths decreased with the increase
of the fiber length due to the increased coupling of the SPR absorption and the optical absorption
4
from Au NPs regardless of the refractive index. Also, as the fiber length increased, the red-shift
ratio of the SPR peak wavelength decreased with the increase of the refractive index of the oils
due to the increase of the propagation loss resulting in the decreased SPR sensitivity. The
measured SPR sensitivities were estimated to be 407 nm/RIU, 217 nm/RIU, and 54 nm/RIU with
the fiber lengths of 20 cm, 45 cm, and 90 cm, respectively. As the fiber length increased from 20
cm to 90 cm, the average SPR absorption intensity and average FWHM decreased from 2.1 dB
to 0.7 dB and from 60.6 nm to 27.5 nm, respectively, because the propagation loss of the light
through the fiber cladding region increased.
Acknowledgments:
This work was partially supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education (No.
2013R1A1A2063250), the Korea government (MSIP) (No. 2011-0031840), the New Growth
Engine Industry Project of the Ministry of Trade, Industry and Energy, the Brain Korea-21 Plus
Information Technology Project through a grant provided by the Gwangju Institute of Science
and Technology, South Korea.
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7
List of table captions:
Table 1. The SPR sensitivity, the average absorption intensity, and the average FWHM of the optical fiber
incorporated with Au NPs in cladding region with various fiber lengths.
Figure captions
Figure 1. (a) TEM images and (b) size distribution of the optical fiber incorporated with Au NPs in cladding
region.
Figure 2. Absorption spectrum of the optical fiber incorporated with Au NPs in cladding region.
Figure 3. SPR spectra as a function of refractive index of the matching oils with the fiber lengths of (a) 20
cm, (b) 45 cm, and (c) 90 cm.
Figure 4. Variation of (a) the SPR peak wavelength as a function of refractive index of the matching oils
and (b) the SPR sensitivity of the SPR spectrum with the fiber lengths of 20 cm, 45 cm, and 90
cm [Second order polynomial fit: Peak wavelength(■) = 7260x2 – 20390.840x + 14698.159 (R2
= 0.986), Peak wavelength(●) = 6240x2 – 17666.960x + 12881.634 (R2 = 0.999), Peak
wavelength(▲) = 1190x2 – 3353.400x + 2734.859 (R2 = 0.943), Sensitivity= 0.057x2 - 11.311x
+ 609.980].
Figure 5. Variation of (a) the absorption intensity and (b) the FWHM of the SPR spectrum as a function of
refractive index of the matching oils with the fiber lengths of 20 cm, 45 cm, and 90 cm [Second
order polynomial fit: Abs. intensity(■) = 4.406 x 10-4x2 – 0.049x + 1.614, Abs. intensity (●) =
5.396 x 10-4 x2 – 0.069x + 2.313, Abs. intensity (▲) = 0.001x2 – 0.136x + 4.371, Abs. intensity
(▼) = 0.002x2 – 0.255x + 8.686, FWHM(■) = -0.004x2 + 0.150x + 47.699, FWHM (●) = 2.984
x 10-4x2 – 0.442x + 64.399, FWHM (▲) = 0.00298x2 – 0.888x + 82.758, and FWHM (▼) =
0.004x2 – 1.029x + 900.374].
8
Table 1. The SPR sensitivity, the average absorption intensity, and the average FWHM of the optical
fiber incorporated with Au NPs in cladding region with various fiber lengths.
Average
Total fiber length
Sensitivity
absorption
intensity
Unit
Optical fiber SPR sensor based on
the Au NPs(cladding)-doped fiber
(Detector length of the fiber: 3 cm)
Average
FWHM
[cm]
[nm/RIU]
[dB]
[nm]
20
407
2.1
60.6
45
217
0.5
47.9
90
54
0.7
27.5
9
Au NPs(cladding)-doped fiber
(a)
Frequency [counts]
12
10
8
6
4
2
0
2
3
4
5
6
Particles Diameter [nm]
(b)
Figure 1. Seongmin Ju et al.
10
-1
Absorption coefficient,  [cm ]
0.10
Au NPs(Cladding)-doped Fiber
Abs. from one mode of
single Au NPs @ 392 nm
0.08
Abs. from coupled mode of
Au NPs @ 790 nm
0.06
0.04
0.02
400
600
800
1000
1200
1400
1600
Wavelength,  [nm]
Figure 2. Seongmin Ju et al.
11
Absorbance [dB]
10
Fiber Length = 20 cm
n = 1.418
n = 1.428
n = 1.438
n = 1.448
8
6
4
2
0
400
500
600
700
800
900 1000 1100 1200
Wavelength,  [nm]
(a)
8
6
4
Fiber Length = 45 cm
n = 1.418
n = 1.428
n = 1.438
n = 1.448
2.5
Absorbance [dB]
Absorbance [dB]
10
2.0
1.5
1.0
0.5
0.0
350
2
0
400
500
400
450
500
700
800
Wavelength,  [nm]
600
550
900 1000 1100 1200
Wavelength,  [nm]
(b)
10
6
4
2.5
Absorbance [dB]
Absorbance [dB]
8
Fiber Length = 90 cm
n = 1.418
n = 1.428
n = 1.438
n = 1.448
2.0
1.5
1.0
0.5
0.0
350
2
0
400
500
400
450
500
550
800
900 1000 1100 1200
Wavelength,  [nm]
600
700
Wavelength,  [nm]
(c)
Figure 3. Seongmin Ju et al.
12
SPR Peak Wavelength,  [nm]
400
Fiber Length = 20 cm
Fiber Length = 45 cm
Fiber Length = 90 cm
Polynomial Fit (Second order)
395
390
385
380
375
370
1.415 1.420 1.425 1.430 1.435 1.440 1.445 1.450
Refractive Index [n]
(a)
SPR Sensitivity [nm/RIU]
500
Polynomial Fit (Second order)
400
300
200
100
0
10
20
30
40
50
60
70
80
90
100
Fiber Length [cm]
(b)
Figure 4. Seongmin Ju et al.
13
SPR Peak Intensity [dB]
5.0
n = 1.418
n = 1.428
n = 1.438
n = 1.448
Polynomial Fit (Second order)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10
20
30
40
50
60
70
80
90
100
Fiber Length [cm]
Full Width Half Maximum [nm]
(a)
80
n = 1.418
n = 1.428
n = 1.438
n = 1.448
Polynomial Fit (Second order)
70
60
50
40
30
20
10
20
30
40
50
60
70
80
90
100
Fiber Length [cm]
(b)
Figure 5. Seongmin Ju et al.
14