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Since the introduction of optical fiber technology in the field of sensor based on the technique of surface plasmon resonance (SPR), fiber-optic SPR sensors have witnessed a lot of advancements. This paper reports on the past, present, and future scope of fiber-optic SPR sensors in the field of sensing of different chemical, physical, and biochemical parameters. A detailed mechanism of the SPR technique for sensing purposes has been discussed. Different new techniques and models in this area that have been introduced are discussed in quite a detail. We have tried to put the different advancements in the order of their chronological evolution. The content of the review article may be of great importance for the research community who are to take the field of fiber-optic SPR sensors as its research endeavors.

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1118 IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

Fiber-Optic Sensors Based on Surface Plasmon

Resonance: A Comprehensive Review

Anuj K. Sharma, Rajan Jha, and B. D. Gupta

Abstract—Since the introduction of optical fiber technology in

the field of sensor based on the technique of surface plasmon res-

onance (SPR), fiber-optic SPR sensors have witnessed a lot of ad-

vancements. This paper reports on the past, present, and future

scope of fiber-optic SPR sensors in the field of sensing of different

chemical, physical, and biochemical parameters. A detailed mech-

anism of the SPR technique for sensing purposes has been dis-

cussed. Different new techniques and models in this area that have

been introduced are discussed in quite a detail. We have tried to

put the different advancements in the order of their chronological

evolution. The content of the review article may be of great im-

portance for the research community who are to take the field of

fiber-optic SPR sensors as its research endeavors.

Index Terms—Optical fiber, sensitivity, sensor, signal-to-noise

ratio (SNR), surface plasmon resonance (SPR).

I. INTRODUCTION

T

HE ORIGIN OF the phenomenon of surface plasmon res-

onance (SPR) is almost a century old. In 1907, Zenneck

formulated a special surface wave solution to Maxwell's equa-

tions and demonstrated, theoretically, that radio frequency sur-

face EM waves occur at the boundary of two media when one

medium is either a "lossy" dielectric, or a metal, and the other

is a loss-free medium [1]. Zenneck also suggested that it is the

"lossy" (imaginary) part of the dielectric function that is respon-

sible for binding the EM wave to the interface. In 1909, Som-

merfeld found that the field amplitudes of surface waves pos-

tulated by Zenneck varied inversely as the square root of the

horizontal distance from the source dipole [2]. Furthermore, it

was a fast wave and it decayed exponentially with height above

the interface. However, the real progress to the phenomenon of

SPR was made in 1957 when Ritchie theoretically demonstrated

the existence of surface plasma excitations at a metal surface

[3]. In 1960, Powell and Swan observed the excitation of sur-

face plasmons at metal interfaces using electrons [4]. Soon after,

Stern and Ferrell showed that surface electromagnetic waves at

a metallic surface involved electromagnetic radiation coupled

to surface plasmons [5]. They also derived the dispersion rela-

tions for electromagnetic surface waves at metal surfaces. Fur-

ther, in 1968, Otto devised the attenuated total reflection (ATR)

prism coupling method to enable the coupling of bulk electro-

magnetic light wave with surface electromagnetic waves [6].

The Otto configuration, due to a finite gap between prism base

Manuscript received March 1, 2007; accepted March 4, 2007. This work was

supported in part by the CSIR under Grant 03(1025)/05/EMR-II. The associate

editor coordinating the review of this paper and approving it for publication was

Prof. Krishna Persaud.

The authors are with the Indian Institute of Technology, New Delhi 110016,

India (e-mail: anujsharma@gmail.com).

Digital Object Identifier 10.1109/JSEN.2007.897946

and metal layer, was more suited to the surfaces, which would

not be damaged or touched by the prism and is important for

the study of single-crystal surfaces. Kretschmann [7] modified

the Otto configuration and is the most famous configuration for

the excitation of surface plasmons till date. In the Kretschmann

configuration, a thin metal layer with a thickness of the order of

10–100 nm contacts the prism base.

While presenting this review, we already have a few important

reviews in the related areas. For instance, one of them focusing

only on SPR sensors, outlined main application areas and pre-

sented important examples of applications of SPR sensor tech-

nology [8]. Another review concerns the analysis of the perfor-

mance of other techniques (interferometry and luminescence)

along with SPR in chemical and biological sensors [9]. Perfor-

mance parameters were compared for the sensing techniques of

interferometry, SPR, and luminescence. A detailed explanation

of the physical and chemical/biological properties required for

optical sensors was included along with the principle of oper-

ation of the sensors. Another review highlighted the status of

different fiber-optic chemical sensors and biosensors [10]. How-

ever, a separate review for optical fiber-based SPR sensors is ab-

sent. In the present paper, we review SPR-based fiber-optic sen-

sors with two main objectives. First, we are inclined to provide

a systematic and comprehensive introduction to the technique

of SPR for researchers (particularly, new students) who plan to

carry out research in the field of SPR-based sensors. Second, we

wish to highlight the role of optical fibers in efficient and flexible

SPR sensors because fiber-optic-based SPR sensors have exces-

sively more advantages over the conventional prism-based SPR

sensors. In the end we provide a detailed discussion on emerging

techniques and fields in the area of fiber-optic SPR sensors.

The organization of review is as follows. A brief but neces-

sary history of SPR is given in Section I. The phenomenon of

SPR is described in great detail along with the explanations of

very fine technical points. The description of SPR is followed

by its application for sensing purposes. The main performance

parameters of SPR sensors are discussed along with their de-

finitive points. The feasibility of optical fibers in SPR sensors

is discussed, which is then followed by the general configura-

tion of fiber-optic SPR sensors. Finally, past and present of the

fiber-optic SPR sensors is discussed in sufficient detail until the

end of the year 2006. In the end, we provide the future scope of

research and development of fiber-optic SPR sensors.

II. S

URFACE PLASMON RESONANCE

A. Plasmons or Plasma Oscillations

There is a dense assembly of negatively charged free electrons

inside a conductor (free electron charge density is

cm

and, therefore, the group of free electrons can be compared with

1530-437X/$25.00 © 2007 IEEE

SHARMA et al.: FIBER-OPTIC SENSORS BASED ON SURFACE PLASMON RESONANCE: A COMPREHENSIVE REVIEW 1119

Fig. 1. Exponential decay of field intensity of surface plasmon mode in a metal

and dielectric system.

a plasma of particles), and also an equally charged positive ion

lattice. Since, positive ions have an infinitely large mass com-

pared with these free electrons, therefore, according to the jel-

lium model, these ions can be replaced by a positive constant

background. However, the total charge density inside the con-

ductor still remains to be zero. If the density of free electrons

is locally reduced by applying an external field on the con-

ductor so that the movement of free electrons may take place,

the negative free electrons are no longer screened by the back-

ground and they begin to get attracted by the positive ion back-

ground. This attraction acts as a driving force for free electrons

and they move to positive region and accumulate with a den-

sity greater than necessary to obtain charge neutrality. Now, at

this point, the Coulomb repulsion among the moving free elec-

trons acts as a restoring force and produces motion in opposite

direction. The resultant of the two forces (i.e., attractive driving

force and repulsive restoring force) set up the longitudinal oscil-

lations among the free electrons. These oscillations are known

as plasma oscillations. A plasmon is a quantum of the plasma os-

cillation. The existence of plasma oscillations has been demon-

strated in electron energy-loss experiments [3], [4].

B. Surface Plasmons

A metal-dielectric interface supports plasma oscillations.

These charge density oscillations along the metal-dielectric in-

terface are known as surface plasma oscillations. The quantum

of these oscillations is referred to as surface plasmon (also

a surface plasmon wave or a surface plasmon mode). These

surface plasmons are accompanied by a longitudinal (TM-

or p-polarized) electric field, which decays exponentially in

metal as well as dielectric (Fig. 1). Due to this exponential

decay of field intensity, the field has its maximum at metal-di-

electric interface itself. Both of these crucial properties of

surface plasmons being TM-polarized and exponential decay

of electric field are found by solving the Maxwell's equation

for metal-dielectric kind of refractive index distribution. By the

solution of Maxwell's equation, one can also show that the sur-

face plasmon wave propagation constant

is continuous

through the metal-dielectric interface and is given by

(1)

Fig. 2. Dispersion curves for surface plasmon wave and the direct light

incident through the dielectric medium

. is the plasma frequency of

metal layer.

Fig. 3. Illustration of setting up of an evanescent wave at prism-metal interface

at

.

where

and represent the dielectric constants of metal layer

and the dielectric medium;

represents the frequency of inci-

dent light, and

is the velocity of light. The above equation im-

plies that the properties of surface plasmon wave vector depend

on both media, i.e., metal and dielectric.

C. Excitation of Surface Plasmons by Light

The maximum propagation constant of the light wave at fre-

quency

propagating through the dielectric medium is given by

(2)

Since

(i.e., for metal) and (i.e., for dielectric), for

a given frequency, the propagation constant of surface plasmon

is greater than that of the light wave (of same polarization state

as that of the surface plasmon wave, i.e., p-polarized) in dielec-

tric medium (Fig. 2). Hence, the direct light cannot excite sur-

face plasmons at a metal-dielectric interface and is referred to as

nonradiative surface plasmon. Therefore, to excite surface plas-

mons, the momentum and hence the wave vector of the exciting

light in dielectric medium should be increased. In other words,

an extra momentum (and energy) must be imparted to light wave

in order to get the surface plasmons excited at a metal-dielectric

interface.

D. Otto Configuration

The general idea behind this configuration was the coupling

of surface plasmon wave with the evanescent wave, which is set

up due to ATR at the base of a coupling prism when a light beam

is incident at an angle greater than the critical angle

at

prism-air interface [6] (Fig. 3). The nature of evanescent wave

is known to have the propagation constant along the interface

and to decay exponentially in the dielectric medium adjacent

to metal layer. Both of these characteristics of evanescent wave

1120 IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

Fig. 4. Otto configuration for the excitation of surface plasmons at metal-di-

electric interface.

Fig. 5. Kretschmann configuration for the excitation of surface plasmons at

metal-dielectric interface.

are similar to those of a surface plasmon wave, therefore, there

is a strong possibility of interaction between these waves. The

x-component of the wave propagation constant of the evanes-

cent wave at prism-air interface is given by

(3)

If a metal surface is now brought in contact of this decaying

evanescent field in such a way that an air gap remains between

the prism base and metal layer, then the evanescent field at

prism-air interface can excite the surface plasmons at the air-

metal interface (Fig. 4). However, this configuration is diffi-

cult to realize practically as the metal has to be brought within

around 200 nm of the prism surface. This approach has been

found to be very useful in studying the single-crystal metal sur-

faces and adsorption on them.

E. Kretschmann–Reather ATR Method

As a significant improvement to Otto configuration,

Kretschmann and Reather realized that the metal layer could

be used as the spacing layer, i.e., evanescent wave (an expo-

nentially decaying wave propagating along the interface of

two media due to the occurrence of total internal reflection)

generated at the prism-metal layer interface can excite surface

plasmons at the metal-air interface so long as the metal layer

thickness is not too large. They devised a new configuration [7],

given in Fig. 5. In this configuration as well, surface plasmons

are excited by an evanescent wave from a high refractive index

glass prism at ATR condition. However, unlike Otto configura-

tion, the base of the glass prism is coated with a thin metal film

(typically around 50 nm thick) and is kept in direct contact with

Fig. 6. Dispersion curves for direct light wave in dielectric , evanescent

wave

for and , surface plasmon wave at metal-

dielectric interface (M/D) and at metal-prism interface (M/P).

the dielectric medium of lower refractive index (such as air or

some other dielectric sample). When a p-polarized light beam

is incident through the prism on the prism-metal layer interface

at an angle

equal to or greater than the angle required for ATR

, the evanescent wave is generated at the prism-metal

layer interface. Fig. 6 shows the dispersion curves of the sur-

face plasmon along with those of the direct light and the light

incident through a glass prism of higher refractive index. The

wave vector

of the evanescent wave, corresponding to

incident angle

, is equal to the lateral component of the wave

vector of the incident light in the prism, as given in (3). The

excitation of surface plasmon occurs when the wave vector of

the propagation constant of evanescent wave exactly matches

with that of the surface plasmon of similar frequency and state

of polarization. This occurs at a particular angle of incidence

and the corresponding resonance condition for surface

plasmons is written as

(4)

Fig. 6 clearly shows that the propagation constant curves corre-

sponding to surface plasmon wave and evanescent wave cross

each other at many positions lying between

and

[i.e., for different sets of angle of incidence and

frequency

]. This implies that the propagation constant

of evanescent wave

may match with that of the surface

plasmon wave (at the metal-dielectric interface M/D) depending

on the frequency and angle of incidence of light beam. As a

very important observation, the surface plasmon wave propa-

gation constant for metal-prism interface (M/P) lies to right of

the maximum propagation constant of evanescent wave

, and the two curves never cross each other. This suggests

that there is no excitation of surface plasmons at metal-prism

interface.

F. Minimum of Reflectance at Resonance

The excitation of surface plasmons at metal-dielectric inter-

face results in the transfer of energy from incident photons to

surface plasmons, which reduces the energy of the reflected

light. If the normalized reflected intensity (R), which is basi-

cally the output signal, is measured as a function of incident

angle

by keeping other parameters and components (such as

frequency, metal layer, and dielectric layer) unchanged, then a

SHARMA et al.: FIBER-OPTIC SENSORS BASED ON SURFACE PLASMON RESONANCE: A COMPREHENSIVE REVIEW 1121

Fig. 7. Reflectance (R) as a function of angle of incidence at the prism-

metal interface (angular interrogation). A sharp drop in reflected signal is ob-

served at angle

.

sharp dip is observed at resonance angle due to an efficient

transfer of energy to surface plasmons (Fig. 7).

The minimum of the normalized reflected intensity (R) can be

quantitatively described with the help of Fresnel's equations for

the three-layer system p/m/s (see the appendix). Here "

" stands

for high refractive index material prism, e.g., quartz prism, "

"

stands for metal film of thickness

, and " " stands for low

index dielectric medium, e.g., air, water, etc.

The light wave is incident at an angle greater than the corre-

sponding ATR angle. At this point, one has to remind that the

energy conservation requires that

, i.e., the sum of

relative reflection, absorption, and transmission is unity. Since

at ATR, hence we are left with in the present

case.

The light wave having passed the glass prism

, is re-

flected partially at prism-metal interface. A part of the incident

light wave energy traverses the metal film (of thickness

)as

an exponentially decaying evanescent wave. At the metal-di-

electric (m/s) interface, it induces the surface plasmon excita-

tions, which radiate light back into the metal film. If the metal

layer thickness

is small, the backscattered field tends to in-

crease. Since, this backscattered wave is out-of-phase with the

incoming wave, the two interfere destructively and cause R to

reduce. For minimum value of

, they compensate each other

and R becomes equal to zero. Thus, the absorption A becomes

equal to 1, i.e., whole radiation field is captured in the metal film.

On the other hand, if the metal layer thickness is large enough,

then the backscattered field disappears and R approaches to 1. It

means that no absorption of incident light wave is taking place.

As a conclusive statement, one can say that the value of R

depends on the combination of incident light frequency, angle

of incidence, and the thickness of the metal layer.

G. Sensing Principle of SPR: Performance Parameters

The sensing principle of SPR sensors is based on (4). For a

given frequency of the light source and the dielectric constant of

metal film one can determine the dielectric constant

of the

sensing layer adjacent to metal layer by knowing the value of

the resonance angle

. The resonance angle is determined

by using angular interrogation method as discussed above. The

resonance angle is very sensitive to variation in the refractive

index (or, dielectric constant) of the sensing layer. Increase in

refractive index of the dielectric sensing layer increases the res-

onance angle.

Fig. 8. The shift in resonance angle with a change in refractive index

of the sensing layer

by

. is the angular width of the curve at half

reflectance for sensing layer refractive index

.

The performance of a SPR sensor is analyzed with the help of

two parameters: sensitivity and detection accuracy or signal-to-

noise ratio (SNR). For the best performance of the sensor, both

parameters should be as high as possible to attain a perfect

sensing procedure. Sensitivity of a SPR sensor depends on how

much the resonance angle shifts with a change in refractive

index of the sensing layer. If the shift is large, the sensitivity is

large. Fig. 8 shows a plot of reflectance as a function of angle of

the incident light beam for sensing layers with refractive indices

and . Increase in refractive index by shifts the

resonance curve by

angle. The sensitivity of a SPR sensor

with angular interrogation is defined as

(5)

The detection accuracy or the SNR of a SPR sensor depends

on how accurately and precisely the sensor can detect the reso-

nance angle and, hence, the refractive index of the sensing layer.

Prior to the evaluation of SNR, a commonly more natural and

practical parameter

) related to the SNR in terms of the re-

flectivity (R) and resonance angle

can be defined as

(6a)

This simply represents the slope of the reflectivity curve. The

above parameter provides primary information regarding the de-

tection sensitivity for SPR sensing system. The SNR is then

derived soon, knowing the properties of the signals in the in-

strumentation chain. In other words, it is to be emphasized that

the SNR is realized only at the end of the instruments chain,

when one finally measures the resonance angle and the reflec-

tivity with a real instrument. For instance, minimum angular res-

olution and minimum sensitivity of the optical detector used to

measure the reflectivity are the important and critical parameters

of a real instrument. Only if these above instrument parameters

are known, one can make a statement about SNR.

Apart from the limitations of a real instrument, the accuracy

of detection of resonance angle further depends on the width of

the SPR curve. Narrower the SPR curve, higher is the detection

accuracy. Therefore, if

is the angular width of the SPR

1122 IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

Fig. 9. Illustration of a typical fiber-optic SPR sensor.

response curve corresponding to 50% reflectance, the detection

accuracy of the sensor can be assumed to be inversely propor-

tional to

(Fig. 8). The SNR of the SPR sensor with angular

interrogation is, thus, defined as [11]

(6b)

Actual SNR of the real SPR sensing system critically depends

on how well one measures the signals with real instruments.

III. F

IBER-OPTIC

SPR SENSOR

Introduction of optical fibers in the SPR sensing system is

based on a very logical reason that guidance of light in optical

fibers is also based on total internal reflection (TIR). Since, a

prism is used in SPR sensing system in order to create TIR

at the prism-metal interface; therefore, coupling prism used in

the basic SPR theory can be conveniently replaced by the core

of an optical fiber to design a fiber-optic SPR sensor. Among

other important reasons are the advantages of optical fiber over

coupling prism such as simple and flexible design, miniaturized

sensor system, and capability of remote sensing. In general, the

silicon cladding from a certain small portion (middle portion

for most of the cases) of the fiber core is removed and is coated

with a metal layer, which is further surrounded by a dielectric

sensing layer (Fig. 9). The light from a polychromatic source,

if spectral interrogation method is used, is launched into one of

the ends of the optical fiber. The TIR takes place for the rays

propagating with an angle in the range varying from the crit-

ical angle (depending upon the numerical aperture of the fiber

and the light wavelength) to approximately 90

. Consequently,

the evanescent field is generated, which excites the surface plas-

mons at the fiber core-metal layer interface. This coupling of the

evanescent field with surface plasmons strongly depends upon

light wavelength, fiber parameters, fiber geometry, and metal

layer properties. For instance, coupling mechanism will be dif-

ferent for single-moded and multimoded optical fibers due to

having different mode transmission properties depending upon

the number of modes a fiber will support. Similarly, a straight

fiber and a tapered fiber will show different strengths of light

coupling because these fibers will show different penetration

depths of the evanescent field due to having different geomet-

rical configurations. A tapered fiber shows a substantial varia-

tion in evanescent field penetration along the tapered sensing

region length whereas an untapered fiber exhibits uniform pen-

etration of the evanescent field along the sensing region. Fur-

ther, penetration of the evanescent field and, therefore, strength

of light coupling with surface plasmons depends on an impor-

tant fiber parameter known as numerical aperture, which is re-

lated to light acceptance limit of the fiber. Furthermore, unlike

prism-based SPR geometry, the number of reflections for most

of the angles is more than one for fiber-based SPR sensor geom-

etry. Besides its angle, the number of reflections for any ray de-

pends on other fiber parameters, namely, sensing region length,

and fiber core diameter. The number of reflections directly af-

fects the SPR curve width, therefore, performance parameters

(SNR and sensitivity) of the sensor depends upon fiber proper-

ties in this way also. These different aspects related to fiber's

optical and geometrical properties along with their advantages

and disadvantages will be discussed in more detail at appropriate

spaces in Section III of the review.

Finally, the spectrum of the light transmitted after passing

through the SPR sensing region is detected at the other end.

The sensing is accomplished by observing the wavelength corre-

sponding to the dip in the spectrum. This wavelength is called as

the resonance wavelength. A plot of resonance wavelength with

the refractive index of the sensing layer gives the calibration

curve of the fiber-optic SPR sensor. Unlike prism-based SPR

sensor where angular interrogation method is used, the spectral

interrogation method is generally used in the fiber-optic SPR

senor because in the fiber-optic sensor all the guided modes are

launched in the fiber.

IV. E

VOLUTION OF THE

FIBER-OPTIC

SPR SENSORS

The development of fiber-optic SPR sensors began in early

nineties of last century. Among the first reports on fiber-optic

SPR sensors was the one proposed by Villuendas and Palayo

[12]. They presented the experimental results for sensitivity and

dynamic range in the measurement of sucrose concentration

in aqueous solution. Their work was followed by a four-layer

fiber-optic SPR sensor with improved dynamic range and sen-

sitivity compared with the three-layer sensor [13]. Soon after, a

sensor based on excitation of SPR on the tip of a single-mode

fiber was reported, which was based on the analysis of the state

of polarization of the reflected beam [14]. At the same time,

Jorgenson and Yee reported the theoretical as well as experi-

mental work on a fiber-optic chemical sensor based on SPR by

fabricating the probe on a highly multimoded optical fiber and

using a white light source instead of a monochromatic one [15].

The sensor proposed by them was able to detect the changes

in the parameters like bulk refractive index, film thickness, and

film refractive index. Slightly later, an "in-line" optical fiber

SPR sensor configuration was proposed for a large range of

sensing applications [16]. As another significant application, the

fiber-optic SPR sensor was used the first time to monitor the

deposition of a multilayered cadmium arachidate Langmuir–

Blodgett film [17]. Experiments showed that there were con-

stant shifts in resonance wavelength as the number of mono-

layers was increased. This provided the method of calculating

the film thickness by measuring the changes in SPR spectra.

The sensitivity and operating (or dynamic) range are the

two important parameters of a practical sensor. Controlling

of these two parameters is another important issue. It was

addressed in the beginning years itself [18]. The sensitivity to

refractive index was reported to be of the order of

RIU,

while the dynamic range was found to be between 1.25 and

1.40 RIU. The dynamic range of the sensor can be tuned 1.00

SHARMA et al.: FIBER-OPTIC SENSORS BASED ON SURFACE PLASMON RESONANCE: A COMPREHENSIVE REVIEW 1123

Fig. 10. Side polished single-mode optical fiber SPR sensor.

to 1.40 RIU by using a thin high refractive index overlay film.

The above range includes gaseous samples as well. Moreover,

the upper limit of the above range was extendible to 1.70

by the use of a sapphire core fiber. Soon after, a fiber-optic

SPR remote sensor for the detection of tetrachloroethane was

proposed by using a gas sensitive polymer film on the metal

layer [19]. The sensor showed good response time (of 2 s), and

reproducibility apart from a long-term stability (of more than

three months). Around the same time, a fiber-optic SPR sensor

with monochromatic excitation and angle of incidence was

reported for the detection of refractive index [20]. The sensor

was shown to have a resolution of the order of

RIU with

dynamic range between 1.33 and 1.40. Later, the modeling of

sensing signal was reported by the same group [21]. Homola

and Slavik [22] reported a SPR sensor using side polished

single-mode optical fiber and a thin metal overlayer (Fig. 10).

The configuration of the probe is different from that shown in

Fig. 9. The guided mode propagates in the optical fiber and

excites a surface plasmon wave at the interface between the

metal and a sensing medium, if the two modes are closely phase

matched. As the surface plasmon wave is lossy, the coupling

results in damping of the fiber mode. Because the coupling

strength depends dramatically on the refractive index of the

dielectric adjacent to the metal film (sensed medium), even

small variations in the refractive index of the sensed medium

may produce large changes in the attenuation of the fiber mode.

Since surface plasmons are inherently TM-polarized waves,

only fiber modes of the corresponding polarization state may

be involved in this interaction (TM-polarized mode), while the

modes with the orthogonal polarization state (TE-polarized

mode) are attenuated only due to ohmic loss in the metal layer.

The TE-polarized mode, the attenuation of which is not sensi-

tive to changes in optical properties of the sensed medium, may

be used as a reference. Such single-moded fiber-based SPR

sensors are considered to be more sensitive, more accurate, and

containing less noise in comparison to those with multimoded

fibers. However, their fabrication is much more complex and

sophisticated compared with those for multimoded fibers. The

sensor was highly sensitive (sensitivity around

dB/RIU

for a refractive index range of 1.41–1.42 RIU) and a very small

amount of sample was required for measuring the refractive

index.

Study of self-assembled monolayers (SAMs) such as n-alka-

nethiol for the protection of silver film due to oxidation on fiber

core was carried out [23]. Such thiol monolayers were found to

be capable of protecting silver layer from oxidation. Further,

the sensor with SAM was shown to have no ageing problem

and had high stability. The SAM was later used in a fiber-optic

SPR sensor for gas detection [24]. For detecting gases and

vapors, the dielectric medium consisted of polyfluoroalkyl-

siloxane, was deposited over SAM. Halogenated hydrocarbons

such as trichloroethylene, carbon tetrachloride, chloroform,

and methylene chloride were tested with detection limits of

0.3%, 0.7%, 1%, and 2%, respectively. Response time of

the sensor was less than 2 min. A side-polished single-mode

fiber-optic SPR sensor discussed above was further modified

[25]. In the modified design, the output end of the fiber was

made reflecting. Therefore, in the modified sensor, instead of

measuring transmitted light, the back reflected light from the

mirrored end face of the fiber was detected. The operation range

of the sensor was tuned toward aqueous media by using a thin

tantalum pentaoxide overlayer. The resolution of the sensor was

of the order of

RIU. Another sensor based on different

configuration and reported around the same time showed the

resolution of the order of

RIU using a single-mode fiber

[26]. In this sensor, the fiber has one end polished at an angle

relative to the longitudinal axis and coated with a thin gold film.

The spatial SPR was observed by allowing the guided light to

diffract out of the fiber probe. The resonance effect depends on

the wavelength and the refractive index changes in the medium

next to the metallic film at the fiber tip. Another single-mode

fiber SPR sensor was reported for the measurement of refractive

index [27]. The sensor can be used as a spectral, as well as an

amplitude sensor. The theoretical analysis presented was based

on the equivalent planar waveguide approach along with mode

propagation and expansion method. In the mean time, chemical

sensing with gold-coated SPR fiber-optic sensor was proposed

[28]. Theoretical simulations showed that the proposed con-

figuration allowed reaching a thirtyfold increase in sensitivity

in comparison to the previous SPR-based sensors. Similarly,

a simple quasi-geometric model to analyze the behavior of

compound waveguide structures used as fiber-optic SPR sen-

sors was reported [29]. The model took the nonmonochromatic

nature of the light source.

The effect of polarization of the incident light in a multimode

fiber is another critical factor in context of an SPR sensor. An

important research work was carried out in this direction [30].

A three-dimensional skew ray model was developed to com-

pletely explain the experimental phenomenon for an intrinsic

SPR multimode fiber sensor. The polarization of light source

does not have much influence, while the SPR probe is far from

the input end of the fiber. The model was important to explain

the variations of the refractive index of the bulk medium and

thickness of the thin dielectric layer. As another crucial devel-

opment, gold island fiber-optic SPR sensor was proposed [31].

The fundamental optical absorbance at around 535 nm for gold

particles was shown to shift if the fiber was immersed in dif-

ferent media.

The other configuration studied for SPR sensor was metal-

coated tapered fiber for refractive index sensing [32]. Quasi-cir-

cular polarization insensitive devices and asymmetric polariza-

tion sensitive devices were fabricated and their characteristics as

refractive index sensors were measured. High sensitivity of both

1124 IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

and their feasibility of using as wavelength—output or ampli-

tude—output sensors were demonstrated. A novel probe based

on monochromatic skew ray excitation of SPR was shown as a

whole surface probe [33]. It was able to monitor the SAM for-

mation and the immunoassay. A fiber-optic SPR sensor based

on spectral interrogation in a side polished single-mode fiber

using depolarized light was reported [34]. The sensor was able

to measure refractive index variation as small as

. Suit-

ability of the sensor for biosensing was demonstrated by de-

tecting IgG through respective monoclonal antibodies immo-

bilized on the SPR sensor surface. Lin

et al. [35] developed a

fiber-optic sensor based on silver for chemical and biological

applications. The range of refractive index was shifted down by

coating an overlay of zirconium acetate on the silver surface by

sol-gel method. The SPR sensor was reported to operate in the

aqueous media with the detectable range of refractive indices of

1.33 to 1.36. A SAM of long chain thiol was introduced to cover

the surface of silver in order to prevent it from deterioration.

A novel theoretical approach was applied to provide a better

understanding of surface plasmon excitation in fiber-optic sen-

sors [36]. The model was based on the calculation of the pro-

posed fields in the waveguide structures and that enabled to

evaluate the optical power loss from energy conservation con-

siderations. The agreement of theoretical results with experi-

mental data of real sensors was reported to be very good. Fur-

ther, another improvement was made in context of fiber-optic

SPR biosensor when the fast detection of Staphylococcal en-

terotoxin B (SEB) was reported [37]. The biosensor was based

on side polished single-mode fiber and was able to detect ng/ml

concentrations of SEB in less than 10 min. As another impor-

tant development, an SPR sensor using an optical fiber with an

inverted Graded-Index profile was proposed [38]. Both the sim-

ulation and experiments showed that the sensor exhibited high

sensitivity for changes of the surrounding media in a RI range

from 1.33 to 1.39. A fiber-optic SPR sensor for the detection of

hydrogen was reported [39]. A thin palladium layer deposited

on the metal coated core of a multimode fiber was used as the

transducer. The modification of the SPR was due to variation in

the complex permittivity of palladium in contact with gaseous

hydrogen. This effect was enhanced by using selective injection

of high-order modes in the fiber through a collimated beam on

the input end of the fiber. Measurements of concentrations as

low as 0.8% of hydrogen in pure nitrogen were reported. The

response time varied between 3 s for pure hydrogen and 300 s

for the lowest concentrations.

The use of single-mode polarization maintaining fiber (PMF)

for SPR sensors was another milestone [40]. The above struc-

ture utilized both polarization-separation and broadband radia-

tion depolarization in PMFs to enhance sensor's stability. The

effect of polarization cross coupling was also analyzed. The ex-

perimental resolution of the sensors was of the order of

RIU. A fiber-optic sensor with nanocomposite multilay-

ered polymer and nanoparticle ultrathin films was reported for

biosensing [41].

A new approach of hetero-core structure for a fiber-optic SPR

sensor was introduced [42]. The hetero-core structured fiber-

optic SPR probe consists of two fibers with different core di-

ameter connected by thermal fusion splicing (Fig. 11). This

Fig. 11. Hetero-core structured optical fiber sensor, with the silver layer of 50

nm thickness on the cladding surface of the single-mode fiber.

Fig. 12. (a) Single-mode tapered fiber with uniform waist. (b) Cross section of

the waist after metallic coating.

was done, deliberately, to leak the transmitted power into the

cladding layer of small core diameter fiber so that the leaked

light may induce an optical evanescent wave required for SPR

excitation. Silver was deposited around the cladding surface for

SPR excitation. The spectral interrogation method was used to

sense the refractive index. The most beneficial finding with this

new sensing structure is its simplicity in the sensor fabrication.

Additionally, because this structure has no need to eliminate

the cladding layer of fiber, the fabricated fiber sensor probe is

able to provide the characteristic advantage of optical fibers for

remote monitoring. Next, a fiber-optic SPR sensor with asym-

metric metal coating on a uniform waist single-mode tapered

fiber, as shown in Fig. 12, was reported [43]. Due to the varying

film thickness, the sensor transmission spectrum exhibited mul-

tiple resonance dips. The multiple dips increase the dynamic

range of the sensor. A fiber-optic conical microsensor for SPR

using chemically etched single-mode fiber was reported [44].

The probe was prepared by coating a gold-metallic film on the

etched portion of the fiber containing conical core (Fig. 13).

When the SAMs were applied on the metallic surface in order

to avoid any adsorption or contamination, the sensor responded

to the refractive index range of 1.33–1.40 with a sensitivity

of 0.008 RIU. The issues related to the calibration of fiber-

optic SPR sensors in aqueous systems were also addressed [45].

Among the biosensors, a real-time fiber-optic SPR sensor to de-

tect biologically relevant concentrations of myglobin and car-

diac troponin I in less than 10 min was proposed [46]. Fur-

ther, work on robust SPR fiber-optic sensor [47], and fiber-optic

SHARMA et al.: FIBER-OPTIC SENSORS BASED ON SURFACE PLASMON RESONANCE: A COMPREHENSIVE REVIEW 1125

Fig. 13. Fiber-optic SPR microsensor.

microsensor based on white-light SPR excitation [48] were re-

ported. A comprehensive model of on absorption-based fiber-

optic SPR sensor for the detection of concentration of chemi-

cals was proposed [49].

Fast responding fiber-optic SPR biosensor [50], analysis of

a fiber-optic SPR sensor based on a crossing point of the two

SPR spectra obtained from the sample fluid and the deionized

water [51], the application of single-crystal sapphire-fiber-optic

SPR sensor in the extreme environment [52] are some of the

other advancements in this area. The use of tapered fiber-optic

SPR sensor for vapor and liquid phase detection [53] is an-

other important advancement. This technique of tapered-fiber

employs a fiber-optic SPR probe with a modified geometry to

tune the SPR coupling wavelength-angle pair. The observed

composite spectrum includes two distinct SPR dips associated

with surface plasmons excited in the gas and liquid active re-

gions. This sensor is able to detect refractive index changes

in both vapor and liquid phases individually by simultaneous

monitoring SPR coupling wavelengths from the two sensing

surfaces. The most important advantage of such dual-tapered

and tetra-tapered fiber-optic SPR sensors lies in the simulta-

neous detection of liquid and vapor phases. However, as a big

advantage, detection accuracy (i.e., SNR) is bound to decrease

due to an increase in the number of reflections followed by the

SPR curve broadening for tapered fiber geometry. Application

of D-type optical fiber sensor based on heterodyne interferom-

etry has also been a very interesting addition to the fiber-optic

SPR sensor technology [54], [55]. The above design of sensor

(Fig. 14) is valuable as it may reach the sensitivity of the order of

RIU. Further, a high resolution, RIU, refractive

index sensing has been achieved by means of a multiple-peak

SPR fiber-optic sensor [56]. A detailed sensitivity analysis along

with performance optimization for multilayered fiber-optic SPR

concentration sensor has been reported [57]. The sensitivity and

SNR analysis of a fiber-optic SPR refractive index sensor has

been carried out for different conditions related to metal layer,

optical fiber, and light launching conditions along with an ex-

tension to remote sensing [58]. Development of a SPR-based

fiber-optic sensor for bittering component (Naringin) has been

recently reported [59].

Fig. 14. ID-type optical fiber SPR sensor probe.

Fig. 15. A fiber-optic SPR sensor with metal nanoparticle layers.

Among the most recent developments is the application of

nanoparticle films in fiber-optic SPR sensors. The analyses

of the fiber-optic SPR sensor with metal-host nanoparticle

layers (Fig. 15) [60] and silver-gold alloy layers [61] have been

reported. The sensor with metal nanoparticle layer was found to

have better performance than a similar sensor with bulk metal

layer. Also, Ag-Au nanoparticle alloy-based sensor was found

to be more sensitive and accurate in comparison to that with

metal-host nanoparticle layers. Furthermore, the application of

localized surface plasmon resonance (LSPR) in optical fiber

sensors [62] and a colloidal gold-modified long-period fiber

grating for chemical and biochemical sensing [63] have been a

crucial advancement. Long-period fiber gratings (LPFG) offer

a variety of applications in sensors owing to their low insertion

losses, low back-reflection, polarization independence, and

relatively simple fabrication. Among the main advantages of

the LPFG-based SPR sensors is their simple construction and

ease of use. Moreover, the sensor has the potential capability

for on-site and remote sensing, can be easily multiplexed to

enable high-throughout screening of bimolecular interactions,

and has the potential use for disposable sensors. The above

sensor with long-period fiber grating has shown a detection

limit for anti DNP (dinitrophenyl compound) of

M.

Among the other techniques, fiber Bragg gratings (FBGs)

have also found applications in SPR sensing [64]. A novel tech-

nique of FBG assisted surface plasmon polariton (SPP) sensor

has been proposed [65]. As another advance in this direction,

the theoretical model of a new hollow core fiber sensor based

on the specific properties of the SPP excited with a FBG is pro-

posed [66]. The main principle of operation of this new device

is based on the ef ficient energy transfer between the fiber wave-

guide mode (FWM) and the surface plasmon-polariton (SPP)

provided by a properly designed short-period FBG imprinted

1126 IEEE SENSORS JOURNAL, VOL. 7, NO. 8, AUGUST 2007

into a waveguide fiber layer of a specially designed hollow core

optical fiber. The waveguide fiber layer is the dielectric layer of

the fiber with the highest refractive index. The FWM is a fiber

mode oscillating in this layer and exponentially decaying in all

other fiber layers. The simulations are done with the help of cou-

pled mode theory and performed for well-developed telecom

wavelength ranges. These new models based on FBG may be

able to design a highly flexible sensor system.

Recently, a new waveguide model of LSPR-based planar mul-

tireflection sensing system with gold nanospheres has been pro-

posed [67]. In this sensing system, a strong enhancement of

LSPR was observed with striking linearity and reproducibility

by using a metallic layer of 20–30 nm gold nanoparticles. The

feasibility of a fiber-optic SPR sensor for the determination of

salinity has also been reported [68]. The effect of temperature

on the performance of a fiber-optic SPR sensor has been theoret-

ically studied in great detail [69] by incorporating the thermo-

optic effects is metal layer, fiber core, and sensing layer. In a

similar fashion, a model for fiber-optic SPR sensor for temper-

ature detection has also been proposed [70]. Recently, a com-

parative study for the properties and surface characterization of

a SPR-based fiber-optic sensor has been reported for different

metals such as Au, Ag, Cu, and Al [71].

In recent times, SPR sensors based on photonic crystal wave-

guide [72] has been proposed. In the photonic crystal waveguide-

based SPR sensor, plasmons on a surface of a thin metal film

are excited by a Gaussian-like leaky mode of an effectively

single-mode photonic crystal waveguide. It has been demon-

strated that effective refractive index of a waveguide core mode

can be designed to be considerably smaller than that of a core

material, enabling efficient phase matching with plasmons at any

wavelength of choice, while retaining highly sensitive response

to changes in the refractive index of an analyte layer. This is quite

an ideal technique for the development of portable SPR bio-

chemical sensors. As another crucial advancement, the concept

of a microstructured optical fiber-based SPR sensor with opti-

mized microfluidics is proposed [73]. In this design, plasmons

on the inner surface of large metallized channels containing

analyte can be excited by a fundamental mode of a single-mode

microstructured fiber. Phase matching between plasmon and a

core mode is enforced by introducing air filled microstructure

into the fiber core, thus allowing tuning of the modal refractive

index and its matching with that of a plasmon. Sensitivity studies

show that refractive index changes of

RIU leads to easily

detectable 1% change in the transmitted light intensity.

V. F

UTURE SCOPE OF WORK IN

FIBER-OPTIC SPR SENSORS

Fiber-optic SPR sensors are bound to encounter more

advancement in future, especially in biosensors, nanosen-

sors, and photonic crystal-assisted sensors. All these fields

have a vast opened area of research in context of fiber-optic

SPR sensors.

The future of SPR-based fiber-optic biosensors will be driven

by the need of the consumers, and hence it is important that the

sensor should be made more consumers friendly. The areas in

SPR-based fiber-optic biosensor, which really needs to be ad-

dressedpresently,andwhichthrowsachallengetotheresearchers

working in this field is its specificity, i.e., how can a sensor be

used to detect specific molecules from groups of molecules. Bi-

molecular recognition molecules may exhibit affinity to similar

types of other unwanted molecules present in a given system.

Theseunwantedmolecules willreactwith the biosensorsandwill

alter the refractive index, hence, affecting the different important

parametersuch as sensitivity, detectionaccuracy,reproducibility,

etc., of the biosensor. If the concentration of unwantedmolecules

is high, then sensor response is more affected by it rather then by

the molecule we want to detect. The other challenge lies in the

nonspecific interactions between sensor surface with unwanted

molecules and background refractive index variations. These

variations can be because of temperature, humidity, and compo-

sitional fluctuations. The important issue, which eagerly waits

the commercialization of SPR-based fiber-optic biosensor, is its

use in the out of laboratory environment. To use the biosensors

in field, mobile analytical systems need to be developed, which

should enable rapid detection of given biological entity. Future

development of these systems requires significant advances

in miniaturization of biosensing platform, high specificity,

robustness, and user friendliness.

On a similar note, nanoparticle-based fiber-optic SPR sensors

are also due to get attention in the future. The technique of LSPR

with metal nanoparticle layers has shown a lot of promise. Fur-

ther optimization of the crucial factors and parameters to better

the sensor's performance is required. New combinations such

as metal-semiconductor nanocomposite, different bimetallic al-

loys (with nanoparticles of different metals such as Cu, Au, Ag,

and Al, etc.) are to be used in SPR-based fiber-optic sensors.

The added phenomena like Surface Enhanced Raman Scattering

(SERS) are also there to havea look into. The scope of fiber-optic

SPR sensors based on metal nanoparticle layers has to be ex-

tended for the detection of other parameters such as tempera-

ture, humidity, etc. The collaboration of fiber gratings and LSPR

technique is another candidate for further work in this area.

Finally, photonic crystal fiber-based SPR sensors are bound

to find new heights in the coming future due to their unique

optical properties such as omnidirectionality, negative refractive

indices, and gapless guidance.

VI. C

ONCLUSION

The present paper is devoted to a comprehensive review of

the SPR-based fiber-optic sensors. The collaboration of SPR

technique and optical fiber technology has brought a lot of ad-

vancements in sensing of various physical, chemical, and bio-

chemical parameters. We have tried to put forward a chrono-

logically collective and systematic evolution of fiber-optic SPR

sensors reported in the last 20 years or so. We believe that the

present review will provide the researchers valuable informa-

tion regarding fiber-optic SPR sensors and encourage them to

take this area for further research and development.

APPENDIX

The three-layer Fresnel equation for the reflected light inten-

sity (R) is given by (Fig. 5)

(A1)

SHARMA et al.: FIBER-OPTIC SENSORS BASED ON SURFACE PLASMON RESONANCE: A COMPREHENSIVE REVIEW 1127

For p-polarization

(A2)

(A3)

Further

(A4)

with

(A5)

In the above expressions (A1)–(A5),

and , respectively,

are the amplitude reflection coefficients for the prism-metal

layer and metal layer-sensing layer interfaces;

is the dielec-

tric constant of

th medium; is the wave propagation vector

in

-direction, i.e., perpendicular to the interface in the medium

; is the evanescent wave propagation constant parallel to

the metal layer-sensing layer interface;

is the thickness of

the metal layer;

is the angular frequency of the incident light,

and

is the speed of light.

In general, prism is considered to be nondispersive. However,

for the sake of completeness, the wavelength dependence of the

refractive index

of the fused silica prism is given

by Sellmeier dispersion relation

(A6)

where

, and are Sellmeier coef ficients.

The value of coefficients are

[74].

For the dispersion in metal layer, one may use the Drude

model, given as [75]

(A7)

where

and denote the plasma wavelength and collision

wavelength, respectively. For instance, the following values of

the plasma wavelength and collision wavelength for gold are

used

m and m.

According to the Kretschmann theory, for p-polarized light,

the reflected light intensity given by (A1) may be transformed

to [76]

(A8)

with

(A9)

(A10)

and

(A11)

where

is the complex wave vector of the surface plasmon

wave generated under the Kretschmann ATR configuration;

is the complex wave vector of the surface plasmon wave in

vacuum i.e., in the absence of the prism;

is the perturba-

tion to

in the presence of prism. The imaginary part of

is known as the intrinsic damping and it represents the Joule

loss in the metal layer. Similarly, the imaginary part of

is

radiative damping

and represents the leakage loss of the

SPW back into the prism.

Taking all the above expressions into consideration, (A1) for

R can be approximated in the region of the resonance by another

Lorentzian type of expression, which is essential in revealing its

physical meaning, given by

(A12)

Equation (A12) demonstrates that R passes through a minimum

that becomes zero for

(A13)

The radiative damping,

, which is the imaginary part of

, depends on the thickness of the metal layer [according to

(A11)] such that it is large for small thickness and vice versa.

Thus, above expression suggests that the exact matching con-

dition also depends on the thickness of the metal layer. There

is always a certain thickness

of metal layer at a certain

frequency for which R becomes zero. Further, this matching

condition depends on frequency in two ways: first, a direct de-

pendence as frequency

is present in the expression for ,

and second, an extrinsic dependence is also apparent due to fre-

quency dependent metal dielectric function

. At a certain

wavelength (or frequency), internal damping

remains con-

stant for any metal layer thickness, whereas internal damping

varies with thickness. Therefore, resonance point is different for

different values of metal layer thickness.

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a D-type optical fiber sensor based on the Kretschmann's configura-

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ered surface plasmon resonance-based fiber optic sensor: A theoretical

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ratio of a step-index fiber optic surface plasmon resonance sensor with

bimetallic layers, " Opt. Commun., vol. 45, pp. 159 –169, 2005.

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of a surface plasmon resonance based fiber-optic sensor for bittering

component—Naringin, " Sens. Actuators, vol. 115, pp. 344 –348, 2006.

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tures: Fundamentals and Appl., vol. 3, pp. 30–37, 2005.

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plasmon resonance with Ag-Au alloy nanoparticle films, " Nanotech-

nology, vol. 17, pp. 124–131, 2006.

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ical and biochemical probes based on localized surface plasmon reso-

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period fiber grating, " Sens. Actuators B, vol. 119, pp. 105 –109, 2006.

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gratings based on long-range surface plasmon-polariton waveguides, "

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tive-index hollow core fiber sensors assisted by a fiber Bragg grating, "

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Anuj K. Sharma received the M.Sc. degree in

physics from the Indian Institute of Technology,

Roorkee, India, in 2001 and the Ph.D. degree in

physics from the Indian Institute of Technology,

New Delhi, India, in 2006.

Currently, he is a Research Associate at the Indian

Institute of Technology, New Delhi. He has been a

Research Fellow at the University Grants Commis-

sion, India, from 2002 to 2006. He has published

more than ten research articles in international jour-

nals of repute. His areas of research are fiber-optic

sensors, surface plasmon resonance (SPR), and metal nanoparticles.

Dr. Sharma has been a member of the Optical Society of America (OSA).

Rajan Jha received the M.Sc. degree in physics from

the Indian Institute of Technology, New Delhi, India,

in 2001. He is currently working towards the Ph.D.

degree at the Indian Institute of Technology.

B. D. Gupta received his M.Sc. degree in physics

from the Aligarh Muslim University, Aligarh, India,

in 1975 and the Ph.D. degree in physics from the

Indian Institute of Technology, New Delhi, India, in

1979.

In 1978, he joined the Indian Institute of Tech-

nology, where he is currently an Associate Professor

of Physics. He has also worked at the University of

Guelph (Canada) from 1982–1983, the University of

Toronto (Canada) in 1985, and Florida State Univer-

sity in 1988. In 1993, he visited the Department of

Electronic and Electrical Engineerin, University of Strathclyde (UK), to work

on fiber-optic chemical sensors under the Indo-British Fiber Optics Project. In

1992, he was awarded the ICTP Associateship by the International Centre for

Theoretical Physics, Trieste, Italy, which he held for eight consecutive years. In

this capacity, he visited ICTP (Italy) in both 1994 and 1996. He is the authored

of Fiber Optic Sensors: Principles and Applications (NIPA New Delhi, 2006)

and is the Co-Editor of the Asian Journal of Physics, Proceedings of SPIE

(USA) Vol. 3666 (1998), and Advances in Contemporary Physics and Energy

(Supplement Volume) (Allied Publishers, New Delhi). He has published more

than 60 research papers including four review articles in international journals

of repute. His current area of interest is fiber-optic sensors.

Dr. Gupta is a member of the Optical Society of America and Life Member

of the Optical Society of India and the Indian Chapter of ICTP. He is a recip-

ient of the 1991 Gowri Memorial Award of the Institution of Electronics and

Telecommunication Engineers (India).

... If p-polarized with the matching wave vector, these waves hit the metal boundary, these surface plasmon waves become excited and the light energy gets coupled into them causing the reflectivity of the metal surface to have a dip. The coupled light needs to match the wave vector of the polaritons and the way to achieve that is the use of a prism, fiber gratings, or D-shaped fiber evanescent mode [15][16][17]. ...

... The prism is a vital part of the sensor system and for this simulation, SF11 or SF10 were chosen. A very thin layer of gold (with a thickness ranging from 45 to 60 nm) is deposited on the prism surface [14,15,37,38]. After that, the thiol-tethered DNA of the analyte (the virus) will be immobilized on the surface of that gold layer. ...

... Surface plasmon polariton (SPP) is the resonance mode resulting from coherent coupling between the electromagnetic fields of transverse magnetic (TM) polarization and collective oscillation of surface electrons at a metal-dielectric interface. SPP can be excited by injecting right photon momenta in parallel to the surface via the evanescent coupling of an incident light into the interface, typically in a Kretschmann-Raether configuration [4][5][6][7], in optical fibers [8][9][10][11] or via grating couplers that modulate the surface parallel photon momenta [12][13][14]. In a wavelength interrogation method, the optical reflectance from the metal-dielectric interface reduces significantly at a narrow spectral band due to the SPP excitation at the surface, the so-called attenuated total reflection (ATR). ...

• Heongkyu Ju

The applicability of the Kramers–Kronig relation for attenuated total reflection (ATR) from a metal–dielectric interface that can excite surface plasmon polaritons (SPP) is theoretically investigated. The plasmon-induced attenuation of reflected light can be taken as the resonant absorption of light through a virtual absorptive medium. The optical phase shift of light reflected from the SPP-generating interface is calculated using the KK relation, for which the spectral dependence of ATR is used at around the plasmonic resonance. The KK relation-calculated phase shift shows good agreement with that directly obtained from the reflection coefficient, calculated by a field transfer matrix formula at around the resonance. This indicates that physical causality also produces the spectral dependence of the phase of the leakage field radiated by surface plasmons that would interfere with the reflected part of light incident to the interface. This is analogous with optical dispersion in an absorptive medium where the phase of the secondary field induced by a medium polarization, which interferes with a polarization-stimulating incident field, has a spectral dependence that stems from physical causality.

... In the literature, several sensor configurations that take advantage of the SPR phenomenon have been reported [1][2][3]. Furthermore, when using an optical fiber as the SPR medium, additional advantages, such as immunity to electromagnetic interference, remote sensing capabilities, a light weight, etc., can be obtained [4][5][6][7][8][9]. In this case, both silica and polymeric optical fibers (POF) can be used to develop groundbreaking sensors in several application fields [10][11][12][13]. ...

In this work, we experimentally analyzed the effect of tapering in light-diffusing optical fibers (LDFs) when employed as surface plasmon resonance (SPR)-based sensors. Although tapering is commonly adopted to enhance the performance of plasmonic optical fiber sensors, we have demonstrated that in the case of plasmonic sensors based on LDFs, the tapering produces a significant worsening of the bulk sensitivity (roughly 60% in the worst case), against a slight decrease in the full width at half maximum (FWHM) of the SPR spectra. Furthermore, we have demonstrated that these aspects become more pronounced when the taper ratio increases. Secondly, we have established that a possible alternative exists in using the tapered LDF as a modal filter after the sensible region. In such a case, we have determined that a good trade-off between the loss in sensitivity and the FWHM decrease could be reached.

• Yogendra Prajapati
• Jitendra B. Maurya
• Anuj K. Sharma

In this work, we propose a graphene-assisted plasmonic structure with photonic spin Hall effect (PSHE) for sensing applications in near infrared (NIR) with an emphasis on tunable and spin control aspects leading to enhanced performance. We comprehensively investigate PSHE in view of variable chemical doping of graphene monolayer in the structure and manipulation of the spin dependent splitting by considering single and cross polarization states. There is observed a considerable variation in spin shift due to increase in chemical potential or Pauli blocking, which fundamentally controls the light absorption by graphene. Our simulation results reveal that the amplified spin dependent shift is 1.13×104 times higher than the conventional spin dependent shift at 0.436 eV of graphene chemical potential. Further, this structure is utilised for sensing application, and it is observed that graphene-assisted plasmonic based structure possesses significantly greater spin dependent sensitivity (5.53 times), figure of merit (8.56 × 105 times), and extremely finer limit of detection (by a factor of 18.10) are achieved compared to the structure without graphene. The results indicate that chosing the proposed graphene-assisted plasmonic structure with variable chemical potential and light polarization components, an extremely enhanced sensing performance can be achieved. The results are consistent with the physical rationale and are particularly important for potential biosensing applications.

Here, we present a portable, selective and cost-effective fiber-optic surface plasmon resonance (SPR) based platform for early detection of Dengue virus. NS1 protein was targeted as the biomarker of dengue. Antibody-antigen specific binding was exploited for NS1 antigen detection. The binding of antibody was assisted by a self-assembled monolayer of alkanethiols on the surface of silver-coated unclad fiber. A wavelength interrogation mode of SPR was utilized to detect NS1 antigen in the dynamic range of 0.2–2.0 μg/ml. The 40 nm thick silver coated optical fiber exhibited resonance wavelength around 500 nm and change in resonance wavelength was monitored for each attachment step on the fiber. The sensitivity at the lowest concentration of NS1 antigen is found to be 54.7 nm/(μg/ml). The limit of detection of the sensor was found to be 0.06 μg/ml, which lies in the physiological range of NS1 protein present in the infected blood, hence the present technique may provide a very early detection advantage. Real blood serum samples were also successfully tested on the set-up, confirming compatibility with the conventional methods. The presented field-deployable platform has wide applications in mass monitoring of dengue, such as during outbreaks and epidemics.

Optical fiber sensing systems have been widely developed for several fields such as biomedical diagnosis, food technology, military and industrial applications and civil engineering. Nowadays, the growth and advances of optical fiber sensors (OFS) are focused on the development of novel sensing concepts and transducers as well as sensor cost reduction. This review provides an overview of the state-of-the-art of OFS based on sol-gel materials for diverse applications with particular emphasis on OFS for structural health monitoring of concrete structures. The types of precursors used in the development of sol-gel materials for OFS functionalization to monitor a wide range of analytes are debated. The main advantages of OFS compared to other sensing systems such as electrochemical sensors are also considered. An interdisciplinary review to a broad audience of engineers and materials scientists is provided and the relationship between the chemistry of sol-gel material synthesis and the development of OFS is considered. To the best of the authors' knowledge, no review manuscripts were found in which the fields of sol-gel chemistry and OFS are correlated. The authors consider that this review will serve as a reference as well as provide insights for experts into the application of sol-gel chemistry and OFS in the civil engineering field.

At the present time, there are major concerns regarding global warming and the possible catastrophic influence of greenhouse gases on climate change has spurred the research community to investigate and develop new gas-sensing methods and devices for remote and continuous sensing. Furthermore, there are a myriad of workplaces, such as petrochemical and pharmacological industries, where reliable remote gas tests are needed so that operatives have a safe working environment. The authors have concentrated their efforts on optical fibre sensing of gases, as we became aware of their increasing range of applications. Optical fibre gas sensors are capable of remote sensing, working in various environments, and have the potential to outperform conventional metal oxide semiconductor (MOS) gas sensors. Researchers are studying a number of configurations and mechanisms to detect specific gases and ways to enhance their performances. Evidence is growing that optical fibre gas sensors are superior in a number of ways, and are likely to replace MOS gas sensors in some application areas. All sensors use a transducer to produce chemical selectivity by means of an overlay coating material that yields a binding reaction. A number of different structural designs have been, and are, under investigation. Examples include tilted Bragg gratings and long period gratings embedded in optical fibres, as well as surface plasmon resonance and intra-cavity absorption. The authors believe that a review of optical fibre gas sensing is now timely and appropriate, as it will assist current researchers and encourage research into new photonic methods and techniques.

The ease of controlling waveguide properties through unparalleled design flexibility has made the photonic crystal fiber (PCF) an attractive platform for plasmonic structures. In this work, a dual analyte channel's highly sensitive PCF bio-sensor is proposed based on surface plasmon resonance (SPR). In the proposed design, surface plasmons (SPs) are excited in the inner flat portion of two rectangular analyte channels where gold (Au) strip is deposited. Thus, the surface roughness that might be generated during metal deposition on circular surface could be effectively reduced. Considering the refractive index (RI) change in the analyte channels, the proposed sensor is designed and fully characterized by the finite element method based COMSOL Multiphysics software. Improved sensing characteristics including wavelength sensitivity (WS) of 186,000 nm/RIU and amplitude sensitivity (AS) of 2,792.97 RIU-1 in the wide RI range of 1.30 to 1.43 is obtained. In addition, the proposed sensor exhibits excellent resolution of 5.38 × 10-7, signal to noise ration (SNR) of 13.44 dB, figure of merits (FOM) of 2188.23, detection limit (DL) of 101.05 nm, and detection accuracy (DA) of 0.0204 nm-1. Outcomes of the analysis indicate that the proposed sensor could be suited for accurate detection of organic chemicals, bio-molecules, and biological analytes.

In this paper, both sides flat photonic crystal fiber-based surface plasmon resonance (PCF – SPR) sensor has been proposed. In this external sensing mechanism based sensor, we have used gold as plasmonic material with an adhesive TiO2 layer over it, enhancing the interaction between analyte channel and gold metal. Using the wavelength interrogation and amplitude interrogation method, the maximum wavelength and amplitude sensitivities are obtained 22,800 nm/RIU and 947 RIU⁻¹, respectively, for analyte range 1.30 - 1.40 with a maximum wavelength resolution of 4.38 × 10⁻⁶. With Full-width half maxima (FWHM) around 45 nm, the sensor provides a high figure of merit (FOM) 507 RIU⁻¹. The proposed sensor can be fabricated from newly available technologies such as the sol-gel method, stack and draw method for several practical applications in the bio-medical field, bio - chemical field to provide higher and accurate sensing results. The presented sensor has also played a vital role in cancer cell detection with higher sensing performance.

• Otto S Wolfbeis

This biannual review covers the time period from January 2004 to December 2005 and is written in continuation of previous reviews. Priority was given to fiber-optic sensors (FOS) of defined chemical, environmental, and biochemical significance and to new schemes and materials. The review does not include the following: (a) FOS that obviously have been rediscovered; (b) FOS for nonchemical species such as temperature, current and voltage, stress, strain, displacement, structural integrity (e.g., of constructions), liquid level, and radiation; and (c) FOS for monitoring purely technical processes such as injection molding, extrusion, or oil drilling, even though these are important applications of optical fiber technology. Unfortunately, certain journals publish articles that represent but marginal modifications of prior art, and it is mentioned here explicitely that the (non-peer-reviewed) Proceedings of the SPIE are particularly uncritical in that respect.

A novel design of surface plasmon resonance (SPR) sensor is reported which leads to a highly miniaturized optical fiber sensing element with high sensitivity. A surface plasmon wave is excited on a thin metal film on a side-polished single-mode optical fiber and variations in the refractive index of analyte are detected by measuring changes in the intensity of the light back-reflected from a mirrored end face of the fiber. The operation range of the sensor is tuned toward aqueous media by using a thin tantalum pentoxide overlayer. It is demonstrated that the sensor is capable of detecting changes in the refractive index below 4×10−5.

• Maksim Skorobogatiy
• A. V. Kabashin

Resonant excitation of a plasmon by the Gaussian-like leaky core mode of a metal covered 1D photonic crystal waveguide is presented. Applications in sensing and major advantages over the existing waveguide-based schemes are discussed.

• E. A. Stern
• R. A. Ferrell

Following Ritchie, the anomalous characteristic energy losses of energy lower than the plasmon energy, exhibited by some metals, are attributed to quantized surface waves of the degenerate electron gas. Although Ritchie's theory has been verified for an ideal pure metal surface by Powell and Swan by reflection of high-energy electrons, the transmission experiments show a lower energy loss generally. This is accounted for by taking into account the relaxation produced by the oxide coating on the surface of the metal. In this way, the experimental data is completely accounted for without the assumption of any anomalous bulk dielectric properties of the metal. The present paper studies the dependence on thickness of the oxide coating, and it is found that a surprisingly thin coating, say only 20 angstroms thick, can produce a significant effect. It is established that a measurement of the dispersion of the energy loss versus angle of scattering in the transmission experiment would yield a measurement of the oxide film thickness. A further check on the theory is suggested by a measurement of the angular dependence of the intensity of the lowlying characteristic energy loss. A special effect is predicted for non-normally incident fast electrons. It should be found that the intensity pattern should flare away from the plane of incidence. Besides these special angular effects it is predicted that because of the sensitivity of the surface plasma oscillations to any surface coating the value of the surface characteristic energy loss can be varied between wide limits by choosing the appropriate coating. In particular, making double films of two different metals should produce surface characteristic energy losses in between the bulk characteristic energy losses of the two separate metals.

• C. J. Powell
• J. B. Swan

Measurements of the characteristic electron energy loss spectra of aluminum and magnesium were made (in a reflection experiment) during oxidation of a fresh evaporated layer of either metal. It was found that surface oxidation results in the rapid disappearance of the low-lying energy losses (10.3 ev in aluminum and 7.1 ev in magnesium) and the appearance of modified low-lying losses of 7.1 ev in aluminum and 4.9 ev in magnesium. The general changes in the loss spectra and the particular changes in the spectrum of aluminum were in good agreement with the predictions of Ferrell and Stern.

A fibre optic surface plasmon resonance (SPR) sensor with a four-layer configuration is presented. Three-layer SPR sensors have been proposed before, but in many applications their dynamic range is not enough. Theoretical studies are carried out to achieve different dynamic ranges using the fourth layer which also gives toughness to the sensor configuration. Concentration data in different sensitivity and dynamic range situations are measured with four sensors in a fully automatized process where their operation is tested.

Source: https://www.researchgate.net/publication/3431883_Fiber-Optic_Sensors_Based_on_Surface_Plasmon_Resonance_A_Comprehensive_Review

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