Design and theoretical analysis of aphotonic crystal fiber based onsurface plasmon resonance sensing

Chao LiuFamei WangJingwei LvTao SunQiang LiuHaiwei MuPaul K. Chu

Design and theoretical analysis of a photonic crystalfiber based on surface plasmon resonance sensing

Chao Liu,a,* Famei Wang,a Jingwei Lv,a Tao Sun,b Qiang Liu,a,c

Haiwei Mu,a and Paul K. Chud

aNortheast Petroleum University, School of Electronics Science, Daqing 163318, ChinabInstitute of Microelectronics, Agency for Science, Technology and Research, 117685,

SingaporecSchool of Science, Harbin Engineering, Harbin 150001, China

dCity University of Hong Kong, Department of Physics and Materials Science,Tat Chee Avenue, Kowloon, Hong Kong, China

Abstract. A photonic crystal fiber based on surface plasmon resonance (PCF-SPR) sensing of asilver layer is investigated. The sensor has two advantages: polarization independence and lessnoble metal consumption. The coupling characteristics and sensing performance of the sensorare numerically investigated by the finite-element method employing the COMSOLMultiphysics® software. The optical loss spectrum of the PCF-SPR sensor can be easilytuned by varying the parameters of the structure. The defined amplitude sensitivity is4398 dB∕RIU and spectral sensitivity is 4500 nm∕RIU corresponding to resolution of 2.27 ×10−6 RIU and 2.22 × 10−5 RIU, respectively. © 2015 Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI: 10.1117/1.JNP.9.093050]

Keywords: photonic crystal fiber; surface plasmon resonance sensor; finite element method;COMSOL Multiphysics®; sensitivity.

Paper 15036 received May 18, 2015; accepted for publication Aug. 18, 2015; published onlineSep. 21, 2015.

1 Introduction

Rapid and accurate diagnostics is demanded by modern medicine. Conventional biosensorsbased on enzyme-linked immunosorbent assays or polymerase chain reaction require a numberof steps, including the use of a labeling agent to elicit a signal, and can be time consuming andexpensive.1 Much research has been devoted to the development of fast, accurate, and label-freebiosensors to enhance medical diagnostics, biochemical detection, and organic chemical detec-tion.2–6 Several photonic biosensing techniques are available, for instance, microring resonatorsensors,7 photonic crystal microcavities,8,9 and surface plasmon resonance (SPR) sensors.2,4 Inparticular, SPR sensors offer many advantages such as high sensitivity as well as label-free, highsensitivity, and rapid real-time detection.6 Because of the high sensitivity to small variations inthe refractive index of the probed medium, small concentrations of biomolecules can be detected.

SPR involves the optic excitation of the charge-density oscillations localized at the interfacebetween a metallic film and dielectric materials under polarized light radiation.10 Most of thereported SPR sensors utilize a prism-based configuration to excite surface plasmons. In thisconfiguration, a coupling prism coated with a metal-thin film is used, and when irradiatedby p-polarized light at a specific angle SPR occurs at the metal–dielectric interface.11

However, the prism-based SPR device has a number of shortcomings such as heavy size, movingmechanical parts, and so on. Driven by the need for miniaturized SPR sensors, optical fiber SPRsensors offer many advantages over traditional prism-based SPR sensors including the simplifiedand flexible optical design, remote sensing capability, continuous analysis, and in situ monitor-ing.12,13 So far, several types of optical fiber SPR sensors with diverse structures such as D-shape,

*Address all correspondence to: Chao Liu, Email: msm-liu@126.com

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cladding-off, FBG-based SPR have been experimentally and theoretically demonstrated.14–17

Recently, a new optical fiber, photonic crystal fiber (PCF), has attracted a great deal of attentiondue to its unique optical properties.18,19 The microstructured air channels along the PCF clad-ding, enhanced evanescent field, and light–analyte interactions bode well for biosensing. PCF-based SPR (PCF-SPR) sensors combine photonics technology, plasmonic science, and coatingtechnology to solve issues such as phase matching and packing.20–23 Theoretical assessmentindicates that the refractive-index resolution of PCF-SPR for aqueous analytes can be10−4 RIU,14,15 and a multicore holey fiber sensor based on SPR possesses an average sensitivityof 2929.39 nm∕RIU for refractive indexes between 1.33 and 1.42.17 The performance of PCF-SPR sensors can be optimized by adjusting the microstructure such as the metal-layer thickness,and pitch and diameter of the air holes, and numerical investigation is a necessary step for thedesign of high-sensitivity PCF-SPR sensors.

Herein, a multicore PCF-SPR sensor with a selective analyte channel and silver depositedcore is investigated numerically by the finite element method (FEM). Single resonance andhighly sensitive properties of the fundamental mode are used and our results reveal an amplitudesensitivity of 4398 dB∕RIU and spectral sensitivity of 4500 nm∕RIU corresponding to resolu-tion of 2.27 × 10−6 RIU and 2.22 × 10−5 RIU, respectively.

2 Principle and Numerical Modeling of Photonic Crystal Fiber Basedon Surface Plasmon Resonance Sensor

A multicore PCF-SPR sensor with a silver film as the sensitive layer is investigated by usingthe COMSOL Multiphysics® software (COMSOL, Inc., Burlington, Massachusetts). Figure 1displays the schematic diagram of the sensor. The cross section of the PCF comprises a hole inthe center surrounded by six identical silica rods and three-layered air holes with a hexagonallattice structure. There is only one analyte channel in the center hole to effectively suppress theinterference from neighboring analyte channels.22 In order to obtain SPR, silver layers withvarious thicknesses are grown on the wall of the analyte channel and the dielectric constant ofsilver is determined by the Drude model.15 The sensing mechanism of PCF-SPR sensor isthrough coupling the leaky core mode to the plasmon to achieve resonance sensing.14 Theleaky core mode can efficiently excited the surface plasmons on the surface of the silver

cr

ar

Λ

PML

Cladding

Analyte

Silver

Air

Fig. 1 Schematic design of the multicore photonic crystal fiber based on surface plasmon reso-nance sensor.

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layer, as effective refractive index of such a mode can be tuned to be well below the value of acore index. The use of the PCF makes it easy to equate the effective refractive index of the coremode to that of the material under test. Thus, phase matching condition between the core modeand the plasmon is satisfied at the required wavelength and then resonance occurs.21

As shown in Fig. 1, Λ ¼ 2 μm, ra ¼ 0.4 μm, and rc ¼ 0.9 μm representing the pitch of theair holes, radius of the air holes, and radius of the channel, respectively; and nclad ¼ 1.45 andna ¼ 1.0 represent the refractive indexes of the silica glass and air holes, respectively. The refrac-tive index of the silica glass is given by the Sellmeier dispersion relationship.23

The refractive index (nc) of the analyte flowing in the channel varies between 1.33 and 1.42.This study focuses on the two-dimensional simulation of the multicore PCF-SPR sensor and

the propagation modes of the electromagnetic wave analyzed by the FEM with the COMSOLsoftware.24,25 A quarter of the cross section of the PCF is modeled to save calculation time andthe perfect magnetic conductor boundary and perfect electric conductor boundary are used. ThePCF-SPR sensor is subjected to transmission loss of electromagnetic waves and when the trans-mission loss of the core mode is utilized to evaluate the SPR properties, a perfectly matched layerboundary condition is considered at the numerical calculation zone edges. According to Ref. 17,the attenuation constant αloss is proportional to the imaginary part of the effective index as shownin the following:

EQ-TARGET;temp:intralink-;e001;116;520αloss ¼40π

λ ln 10ImðneffÞ × 106ðdB∕mÞ; (1)

where λ is the wavelength of the incident light in vacuum and neff represents the effective refrac-tive index of the guide mode.

3 Results

Figure 2 shows the optical field distribution of the fundamental mode and supermode of the PCF-SPR sensor and the arrows represent the direction of the electric field. The parameters areΛ ¼ 2 μm, ra ¼ 0.4 μm, rc ¼ 0.9 μm, tAg ¼ 30 nm, na ¼ 1.0, nc ¼ 1.42, and λ ¼ 850 nm.The real part (1.438982) of the effective refractive index of the fundamental mode is largerthan that of the supermode (1.43873), and these two modes have different electric field directionsfor an analyte with a refractive index of 1.42 at 850 nm. The electric fields in the six cores havethe same direction for the fundamental mode, but the electric field distributions in the neighbor-ing cores for the supermode mode have different directions.

Figure 3(a) shows the dispersion relationship of the fundamental mode and plasmonic mode.The blue line represents the imaginary part of the effective refractive index of the fundamentalmode, whereas the real part of the effective index of the fundamental mode and the plasmonicmode are represented by the black and green lines, respectively. The real part of the effectiverefractive index decreases gradually with increasing wavelength, while the dispersion curve ofthe imaginary part of the effective refractive index exhibits a distinct peak at 770 nm. In addition,there is obvious electromagnetic field overlapping between the fundamental mode and plas-monic mode at the resonance wavelength. The phase matching coupling phenomenon is con-firmed by the coincidence of the loss peak and intersection between the dispersion relations ofthe fundamental mode and plasmonic mode, as shown by point (m) in Fig. 3(a). Inset (a) displaysthe electric field distributions of the plasmonic mode and light is confined on the surface of thesilver layer. Inset (b) presents the electric field distributions of the fundamental mode and light isconfined in all the cores outside the resonant wavelength. Similarly, inset (c) shows the electricfield distributions of the core mode and light exists on the metal surface and inside all the cores.The transmission loss is proportional to the imaginary part of the effective refractive index basedon Eq. (1), and the loss spectrum of the fundamental mode is displayed in Fig. 3(b) which showsan obvious resonance peak at 770 nm indicative of SPR. Resonance peak/resonance wavelengthcan be used for sensing due to its sensitivity to the changes of environmental conditions such asconcentration, pH, and temperature.

The thickness of the metal-thin film affects the half-width and amplitude of the resonantpeak. Figure 4 shows the loss spectra of the fundamental mode for different silver layer

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thicknesses. The resonance wavelength moves toward longer wavelengths and the intensity ofresonance peaks diminishes gradually as the silver layer thickness increases from 30 to 45 nm asconsistent with Refs. 26 and 27. The resonance depth decreases, while the half-width of resonantpeaks and transmission loss outside the resonance wavelength increases with silver layer thick-ness in the calculated range.

Fig. 2 Optical field distributions: (a) fundamental mode and (b) supermode in which the arrowsrepresent the direction of the electric field.

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Figure 5 shows the dependence of the loss spectra of the core mode on the size of core. Theresonance peak shifts to longer wavelengths and the resonance depth increases when rc is variedfrom 0.7 to 0.9. It can be concluded that the size of the center hole (rc) influences the resonancewavelength and by adjusting the size, the resonance wavelength can be tuned to thedesired value.

Figure 6 displays the modal loss spectra for several air holes. The depth of the resonance peakdecreases rapidly when ra is varied between 0.4 and 0.6. Nonetheless, the resonance wavelengthdisplays a weak dependence on the size of the air holes because the resonance peaks hardly shiftswhen ra is varied between 0.4 and 0.6.

Based on the earlier results, we selected tAg ¼ 30 nm, ra ¼ 0.4 μm, and rc ¼ 0.9 μm as themain structure parameters for further study. Figure 7 depicts the loss spectra of the fundamentalmode with the pitches of the air holes varying from 1.8 to 2.2. The resonance peaks shift to thelonger wavelengths with increasing pitch of the air holes. However, the depth of the resonancepeaks is about the same, indicating that the resonance depth is not affected by the pitch of theair holes.

The loss spectra of the fundamental mode for different analytes are shown in Fig. 8. Theresonance peaks shift to longer wavelengths when the refractive indexes of the analytes vary

660 680 700 720 740 760 780 800 820 840 860

1.439

1.440

1.441

1.442

1.443

1.444

Wavelength (nm)

Re

(nef

f)

0

5

10

15

20

25

30

35

40

(c)

(b)

(a)

Im (

n eff)*

106

(m)

660 680 700 720 740 760 780 800 820 840

0

5

10

15

20

25

30

Loss

(dB

/cm

)

Wavelength (nm)

(a)

(b)

Fig. 3 (a) Distribution of the refractive index of the sensor and (b) loss spectrum of the fundamen-tal mode (Λ ¼ 2 μm, r c ¼ 0.9 μm, r a ¼ 0.4 μm, tAg ¼ 30 nm, na ¼ 1.0, nc ¼ 1.42).

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from 1.40 to 1.42 and the depth of the resonance peaks increases gradually with increasingrefractive indexes.

Since absorption by the silver layer depends on the refractive index of the surroundingmedium, the shift of the resonance wavelength can be detected even when the refractiveindex of the analyte changes only slightly. Sensitivity is one of the most important parameterswhich can be improved by varying the refractive index of the analyte. There are two main meth-ods to define the sensitivity. The first one is by the amplitude method in which all of measure-ments are conducted at a constant wavelength. Based on Eq. (1), the amplitude sensitivity S isdefined as 20

EQ-TARGET;temp:intralink-;e002;116;361SðλÞ ¼ 1

PðL; λ; nÞjPðL; λ; nþ dnÞ − PðL; λ; nÞj

dnðRIU−1Þ: (2)

600 625 650 675 700 725 507 775 800 825 850 875 900

0

3

6

9

12

15

18

21

24

Loss

(dB

/cm

)

Wavelength (nm)

rc

= 0.7 μm

rc

= 0.8 μm

rc

= 0.9 μm

Fig. 5 Dependence of loss spectra of the core mode on the core size.

680 700 720 740 760 780 800 820 840 860 880 900 920 940

0

5

10

15

20

25

Loss

(dB

/cm

)

Wavelength (nm)

tAg = 30 nm

tAg = 35 nm

tAg = 40 nm

tAg = 45 nm

Fig. 4 Simulated transmission loss for different silver layer thicknesses.

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Furthermore, Eq. (2) is often expressed as

EQ-TARGET;temp:intralink-;e003;116;460SðλÞ ¼ 1

PðL; λ; nÞjPðL; λ; nþ dnÞ − PðL; λ; nÞj

dn10

lnð10Þ ðdB∕RIUÞ; (3)

where L is the propagation length and is limited by the modal transmission loss. The relationshipbetween the propagation length L and transmission loss αðλ; nÞ can be expressed as

EQ-TARGET;temp:intralink-;e004;116;392L ¼ 1∕αðλ; nÞ: (4)

Based on Eqs. (3) and (4), the amplitude sensitivity S is given by22,24

660 680 700 720 740 760 780 800 820 840 860 880 900

0

4

8

12

16

20

24

Loss

(dB

/cm

)

Wavelength (nm)

Λ = 1.8 μmΛ = 2.0 μmΛ = 2.2 μm

Fig. 7 Fundamental mode loss spectra for the different pitches of air holes.

660 680 700 720 740 760 780 800 820 840 860 880

0

3

6

9

12

15

18

21

24

Loss

(dB

/cm

)

Wavelength (nm)

ra

= 0.4 μm

ra

= 0.5 μm

ra

= 0.6 μm

Fig. 6 Loss spectra for the different sizes of the air holes r a.

Liu et al.: Design and theoretical analysis of a photonic crystal fiber. . .

Journal of Nanophotonics 093050-7 Vol. 9, 2015

EQ-TARGET;temp:intralink-;e005;116;477SðλÞ ¼ 1

αðλ; nÞ∂αðλ; nÞ

∂n10

lnð10Þ ðdB∕RIUÞ: (5)

Figure 9 shows the amplitude sensitivity curve of the sensor. The maximum sensitivity can beachieved at 800 nm and is equal to 1013 RIU−1, implying that the amplitude sensitivity isapproximately 4398 dB∕RIU. Assuming that a transmitted intensity of 1% can be detected reli-ably, the sensor resolution is 2.27 × 10−6 RIU.

The second method to define the sensitivity is based on wavelength scanning. In the wave-length interrogation mode, variations in the refractive index of the analyte can be obtained bymeasuring the spectral peak displacement and the spectral sensitivity S is expressed as 24

EQ-TARGET;temp:intralink-;e006;116;356SðλÞ ¼ ΔλpeakΔna

ðnm∕RIUÞ: (6)

660 680 700 720 740 760 780 800 820 840-200

-100

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

(725 nm,660/RIU)

The

am

plitu

de s

ensi

tivity

(1/

RIU

)

Wavelength (nm)

(800 nm,1013/RIU)

Fig. 9 Amplitude sensitivity curve of the sensor.

580 600 620 640 660 680 700 720 740 760 780 800 820 840 860-2

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Loss

(dB

/cm

)

Wavelength (nm)

Loss1.40 Loss1.41 Loss1.42

Fig. 8 Fundamental mode loss spectra for different analytes.

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Journal of Nanophotonics 093050-8 Vol. 9, 2015

The wavelength resolution is assumed to be Δλmin ¼ 0.1 nm. The refractive index resolution ofthe proposed sensor can be defined as16

EQ-TARGET;temp:intralink-;e007;116;552R ¼ ΔnaΔλmin∕Δλpeak: (7)

Here, the peak shift is about Δλpeak ¼ 45 nm estimated from Fig. 9. When the variation in theanalyte refractive index isΔna ¼ 0.01, the sensitivity is approximately 4500 nm∕RIU for refrac-tive indexes between 1.33 and 1.42, leading to a sensing resolution of 2.22 × 10−5 RIU.

Table 1 lists the key parameters for the sensing performance of PCF-SPR sensors with differ-ent structures. The multicores PCF-SPR sensor here shows a much higher sensitivity and a wideroperation wavelength range compared to those described in Refs. 17 and 20–22 for the samedynamic detection RI range. By means of a detailed survey on these sensors, it can be concludedthat the difference in the sensing sensitivity stems from the various arrays of holes in the PCF.The higher average sensitivity of 4500 nm∕RIU is attributed to the stronger energy couplingbetween the plasmonic mode and core-guided mode in this work and hence, it is importantto optimize the hole configuration.

4 Conclusion

Amulticore PCF-SPR sensor with a silver layer as the sensing layer is numerically assessed. Thesensor which has six identical solid cores surrounding one silver coated analyte channel has thetwo advantages of polarization independence and less noble metal consumption. The loss spectraanalysis demonstrates that the structure of the sensor influences the resonance wavelength andpeak intensity. The spectral and amplitude sensitivity are up to 4500 nm∕RIU and4398 dB∕RIU, respectively, and the resolutions of the sensor are 2.22 × 10−5 RIU and2.27 × 10−6 RIU, respectively. The average sensitivity of the sensor is higher than SPR sensorscontaining other holey fibers. Our results show that the sensing performance of multicore PCF-SPR sensors can be enhanced by optimizing the structure.

Acknowledgments

This work was jointly supported by the National Natural Science Foundation of China(51474069, 51374072), Program for New Century Excellent Talents in HeilongjiangProvincial University (1253-NCET-002), Research Project of Science and Technology ofHeilongjiang Province (12541053, 12541096), and City University of Hong Kong AppliedResearch Grant (ARG) No. 9667104.

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Table 1 Key parameters associated with the sensing performance of various photonic crystalfiber based on surface plasmon resonance sensors.

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Operation wavelengthrange (nm)

Average sensitivity(nm∕RIU) Ref.

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Biographies for the authors are not available.

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Journal of Nanophotonics 093050-10 Vol. 9, 2015