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Sensors and Actuators B 122 (2007) 381–388

Tuning and sensitivity enhancement of surface plasmon resonance sensor

Gaurav Gupta a,∗, Jun Kondoh a,b

a Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japanb Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan

Received 13 February 2006; received in revised form 16 May 2006; accepted 2 June 2006Available online 17 July 2006

bstract

The performance of a surface plasmon resonance (SPR) sensor depends on its design properties. In this paper, we described, both theoreticallynd experimentally, the role of the prism material in sensor design. The analyses were carried out by employing three prisms of different refractivendices and the reflection spectra were studied under the angular interrogation mode. The results proved that the choice of prism material can tailorhe resonance condition and thus can effectively tune the working region of the sensor. Also, the experimental value of angular sensitivity increasedrom 94.46◦/RIU to 204.41◦/RIU on changing the refractive index of the prism from 1.597 to 1.456, without changing the other parameters. The

ariation of the angular sensitivity was in accordance with the analytical and numerical results. The dynamic range decreased and the full widtht half maximum (FWHM) of the reflection spectra increased as the refractive index of the prism was lowered, thereby affecting the sensor’serformance.

2006 Elsevier B.V. All rights reserved.

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eywords: Surface plasmon resonance; Angular sensitivity; Refractive index; G

. Introduction

An SPR sensor based on the attenuated total reflection (ATR)ethodology is a sensitive tool utilized for sensing applications.he phenomenon of SPR is exploited for numerous applica-

ions mainly in the fields of chemical [1,2] and biochemicalciences [3–6]. The remarkable results obtained in these fieldsake the SPR sensor one of the good techniques in compar-

son with ellipsometry, spectroscopy and interferometry. Withoods [7] explaining the concept of surface plasma excitation

n gratings in the early period of the 20th century, it was not until968 that Kretschmann et al. [8] proposed the practical means ofxcitation of SPR at the metal–dielectric interface in the prism-ased configuration. The first major application work utilizingPR was reported in 1983 by Liedberg et al. [9]. Since then

he work has been revolutionized, both in terms of applicationsnd modifications in system design to enhance its performance.

he utilization of optical fibers in sensor systems [10–12] hasttracted the attention as it enables remote sensing.

∗ Corresponding author. Tel.: +81 53 478 1221; fax: +81 53 478 1221.E-mail address: n2441012@ipc.shizuoka.ac.jp (G. Gupta).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.06.005

prism

A conventional SPR sensor either works in the angular inter-ogation mode or the wavelength interrogation mode. At a par-icular angle, termed the resonance angle (or wavelength termedhe resonance wavelength), the dispersion relation of the inci-ent light matches with that of the surface plasmon (surfaceode), at which the reflectance shows a dip. The reflectance

ip is attributed to the transfer of energy possessed by the pho-ons incident to the surface plasmon and is most sensitive to thehanges in the properties of the dielectric medium (refractivendex and thickness) adjacent to the thin metal film [13]. Thisroperty of the reflection spectrum is exploited to study the bio-hemical interactions including the estimation of concentration,eaction kinetics and structural analysis.

The performance of an SPR sensor is an important issue forts applications; hence, it must be investigated with the sensoresign parameters. The most significant performance parameters its sensitivity, which has been investigated extensively in thengular interrogation mode and wavelength interrogation mode14–16], for various metals [17]. Recently, a sensitivity analysisor various SPR modes has also been reported [18]. To achieve a

igh sensitivity, the design parameters of the sensor system areelected to optimize the resonance condition. The resonance con-ition depends on the refractive index of the prism, the dielectriconstant of the metal, and the wavelength of the incident light.

3 nd Actuators B 122 (2007) 381–388

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82 G. Gupta, J. Kondoh / Sensors a

revious report [14] has shown that the choice of wavelengthhould be towards the lower value as it leads to a higher angularensitivity. They also reported among silver and gold the SPReflections dips are narrower for silver, while gold metal filmhows a higher angular sensitivity. Using these results, we fur-her investigated the variation of angular sensitivity for differentrism materials, which was not discussed at length previously. Inhis study, we present both theoretical and experimental findingsn tailoring the dispersion relation by the variation of the refrac-ive index of the prism and study the performance of a chemicalPR sensor operating in the angular interrogation mode. Exper-

ments with three different prisms at a constant wavelength werearried out to estimate the refractive indices of ethanol solutionsf various concentrations. Experimental data was found to ben agreement with the theory and showed a stable tailoring ofhe dispersion relation to tune the working region while achiev-ng a high angular sensitivity for an appropriate choice of prism

aterial.

. Theory

In a three-layer Kretschmann configuration as shown inig. 1, a thin metal layer of thickness d is sandwiched betweenglass substrate and a comparatively less dense dielectricedium. The glass substrate and the dielectric medium are con-

idered to be semi-infinite. When the wave vector of the incidentM polarized electromagnetic wave parallel to the prism–metal

nterface matches with that of the surface plasmon, a strong sur-ace plasmon wave (SPW) is generated at the metal–dielectricnterface. This resonance phenomenon can be mathematicallyxpressed by equating the x-component of the input wave vec-or, kx, with the real part of the wave vector of the SPW, ksp, ashown in the following equations.

x = k0np sin θ1 (1)

sp = k0

√εmn2

s

εm + n2s

(2)

Fig. 1. Three-layer Kretschmann configuration for SPR excitation.

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3

plS

Fig. 2. Typical SPR reflection curves for two different dielectric media.

t the resonance,

x = k0np sin θsp = Re

{k0

√εmn2

s

εm + n2s

}. (3)

ere, k0 = 2�/λ0 is the wave vector in free space, λ0 the free-pace wavelength, np the refractive index of the prism, ns theefractive index of the dielectric sample, and εm the complexielectric constant of the metal. At a particular angle of inci-ence, θ1, when its value is equal to the resonance angle, θsp, theesonance condition of Eq. (3) is satisfied and SPR is observed.his results in a sharp dip in the reflected light in the prisms shown in Fig. 2. The resonance condition is sensitive to nsnd is projected as a shift in the position of the reflection dip,θsp, which is the working principle of the sensor system. This

hift �θsp is measurable even for a change in ns as small as0−6–10−7 RIU [19].

For an SPR sensor based on angular interrogation, the sensi-ivity is defined as the ratio of the shift in the resonant angle to thenit change in the RI of the sample. The analytical expressionor the angular sensitivity, S�, can be obtained by differentiatingq. (3) for θsp with respect to ns [14]. We get

θ = dθsp

dns=

(εmr

εmr+n2s

)3/2

√n2

p − εmrn2s

εmr+n2s

, (4)

here εmr = Re(εm). The above equation shows that the angularensitivity depends on the prism’s refractive index, the choice ofetal which determines εmr, and the wavelength of the incident

ight because the media are dispersive. The results on the varia-ion of the refractive index of the prism are shown and discussedn the later sections.

. Experiment

Fig. 3 shows the experimental arrangement used for therism-based SPR analysis. The optics in the setup included aaser diode (LD) at a wavelength of, � ∼670 nm (ILEE AG,witzerland) and a polarizer (P) in the left rotating arm, and a

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ftrsand 7.39◦, respectively. So the maximum shift was found fornp = 1.456. To show this more clearly, we numerically generatedthe reflection spectra with ns increasing in steps of 0.005 startingfrom 1.33 to 1.36 for each prism material. The resonance angle

ig. 3. Schematic of prism-based SPR setup working in angular interrogationode. LD: laser diode; P: polarizer; PD: photodiode; ADC: analog-to-digital

onverter.

hotodiode (PD) in the right rotating arm. The rotation of bothrms was synchronous, making the same angle with the verti-al. The unpolarized light from the LD was passed through the, which made it TM polarized, before hitting the glass prism.he reflected light intensity was measured by the PD, which was

nterfaced to a computer through an analog-to-digital converterADC) to record the reflectance as a function of incident anglet the prism–metal interface.

We acquired glass prisms and glass plates (Natsume Opti-al Corporation, Nagano, Japan), and index-matching liquidsMoritex Corporation, Tokyo, Japan) of three different refrac-ive indices, with values of 1.597 (S-TIM5), 1.51391 (BK7)nd 1.456 (synthesized quartz) at �∼670 nm. In the media con-guration, a thin gold layer of 50 nm thickness was thermallyvaporated on the glass plates of each material. The depositionas carried out at a pressure of about 3 × 10−6 Torr. Each sensor

hip was then optically attached to the respective prism usinghe corresponding index-matching liquid to obtain the sensoronfiguration. Ethanol solutions of various concentrations (5,0, 15, 20, and 25% by wt.) were prepared and SPR reflectionurves were recorded for each solution starting with water forach prism material. All the experiments were carried out atoom temperature (23 ± 2 ◦C) and the data were obtained forve sensor chips of each glass material.

. Results

.1. Numerical analysis

The propagation of electromagnetic waves in a multilayeredonfiguration is explained by the Fresnel formulae [20]. Since

nly the TM-polarized waves are active for SPR, the Fresneleflection formula for a p-polarized wave was used for the anal-sis. Three prism materials with refractive indices (np) of 1.597,.51391 and 1.456, a thin gold film (d = 50 nm) with a dielec-

FFmE

tuators B 122 (2007) 381–388 383

ric constant of −14.7671 + 1.1824i at λ = 670 nm [21] and twoielectric materials with refractive index values (ns) of 1.33 and.36 were considered. The SPR reflection spectra for three valuesf np with each value of ns were obtained as a function of incidentngle at the prism–metal interface. The reason for selecting thesearticular values of refractive indices for the numerical analysisas that they were close to the parameters used in the experi-ents, which aids in drawing a comparison between theory and

xperiments (The refractive indices of the ethanol solutions lieetween the two chosen values of ns).

Fig. 4 shows the SPR reflection curves obtained by numericalimulations for both values of ns and with each np value. Eachurve showed a well-defined sharp corner at the critical angle (asarked for curve A at θc ∼56.40◦) describing the total internal

eflection (TIR). The value of θc for each curve is decided byhe corresponding values of np and ns. Beyond this angle, totaleflectance was expected but it dipped at the resonance angle.

ith np = 1.597, the largest value considered, the resonance wasbtained at an angle of, θsp = 62.68o for ns = 1.33 (Fig. 4(a),urve A) and θsp = 65.60o for ns = 1.36 (Fig. 4(a), curve B).ith np = 1.51391, θsp = 69.40o for ns = 1.33 (Fig. 4(b), curve) and θsp = 73.52o for ns = 1.36 (Fig. 4(b), curve D). And withp = 1.456, θsp = 76.27◦ for ns = 1.33 (Fig. 4(c), curve E) andsp = 83.66◦ for ns = 1.36 (Fig. 4(c), curve F). One can observehat the resonance angle for the same dielectric material changesith the change in the prism material (see curves A, C and E).his is in accordance with the resonance condition.

It is important to note the shift in the resonance angle obtainedor the two values of ns with each value of np, as it correspondso the angular sensitivity of the sensor. The shift between theesonance angles (�θsp) of curves A and B was 2.92◦. Thehifts between curves C and D, and curves E and F were 4.12◦

ig. 4. SPR reflection curves as functions of incident angle obtained usingresnel formulae for two different dielectric media and three different prismaterials: (a) np = 1.597, (b) np = 1.51391 and (c) np = 1.456. Curves A, C, andare for ns = 1.33, and curves B, D, and F are for ns = 1.36.

384 G. Gupta, J. Kondoh / Sensors and Ac

Fis

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tcthe refractive index. The angular sensitivities obtained from theslopes of the lines in Fig. 5 for the three prisms were also plot-ted in Fig. 6 (filled triangles), which were close to the analyticalcurve.

ig. 5. Plot of numerically generated resonance angle as function of refractivendex of dielectric medium obtained for three different prism materials. Thelope of each line indicates the angular sensitivity.

f each curve was obtained and plotted as a function of ns ashown in Fig. 5. With response being almost linear in this rangeor each prism, the slopes of the straight lines denoted the angu-ar sensitivity. The slope of the lines were found to be 98.63,37.51 and 209.70◦/RIU for the prisms with refractive indexalues of 1.597, 1.51391 and 1.456, respectively, indicating anncrement in the angular sensitivity with the decrease in np.

Another observable feature was the increase in the FWHMf the SPR spectra with different np values. The FWHM valuesor curves A, C and E (Fig. 4) were found to be 2.64◦, 3.40◦ and.40◦, respectively.

.2. Angular sensitivity

The analytical expression of the angular sensitivity (S�) isiven by Eq. (4). Fig. 6 shows the plot of S� as a function of

ig. 6. Variation of angular sensitivity obtained using analytical expression asunction of refractive index of prism (solid curve). The numerical values ofngular sensitivity obtained from Fig. 5 (filled triangles) are also plotted foromparison.

Fc(wfiw

tuators B 122 (2007) 381–388

he refractive index of the prism (solid curve). The plot indi-ated that the angular sensitivity was higher for lower values of

ig. 7. Experimental SPR reflection spectra for water and ethanol solutions withoncentrations of 5, 10, 15, 20 and 25 wt.% obtained using three prism materials:a) np = 1.597, (b) np = 1.51391 and (c) np = 1.456. Curves A′, C′ and E′ are forater, and curves B′, D′ and F′ are for 25% ethanol solution. The numericallytted curves for each spectrum are also plotted (solid curves), which overlapell with the experimental data.

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G. Gupta, J. Kondoh / Sensors a

.3. Experimental

Fig. 7 shows the SPR reflection spectra obtained experimen-ally with water and ethanol solutions, along with the numeri-ally fitted curves (solid curves) for each of the prism materialsonsidered. With np = 1.597, spectrum A′ and B′ were obtainedor the water and 25% ethanol solution, respectively (Fig. 7(a)).he shifting of spectra was the result of a continuous increase

n the refractive index of the ethanol solution with concentra-ion. With np = 1.51391, spectra C′ and D′ were obtained for theater and 25% ethanol solution, respectively (Fig. 7(b)), andith np = 1.456, spectra E′′ and F′ were obtained for the water

nd 25% ethanol solution, respectively. The resonance positionor the same sample liquid was different for different valuesf np. The values and range of resonance angles for the chosenielectric samples and the prism were in agreement with the the-retical spectra (Fig. 4). The range broadened with decreasingp, proving that the shift between the SPR spectra increases.

Since the SPR spectrum profile critically depends on theielectric constant of the metal (εm) and its thickness (d) whosealues were not known exactly under the experimental condi-ions, it was not wise to compare the experimental spectra withhe numerically generated spectra in Fig. 4. Instead, the experi-

ental data was fitted with the numerical model. A simple andeliable method of curve fitting was adopted to check the agree-

ent between the theory and the experiment and to find the

alue of the unknown parameters. A program was written onhe basis of Fresnel coefficients for three layers, which fitted

Tte

able 1omparison of curve fit data with expected/theoretical values

rism material Conc. of ethanol (wt.%) Curve fi

ns (RIU

-TIM5

5 1.335510 1.338715 1.342220 1.345525 1.3491

K7

5 1.335210 1.338615 1.341620 1.344825 1.3483

ynthesized quartz

5 1.335110 1.338015 1.341120 1.344225 1.3476

= 50.00 nma εm = −14.77+1.18ia

onc. of ethanol (wt.%)

50505

a Expected/theoretical data.

tuators B 122 (2007) 381–388 385

he experimental data. The unknown parameters were εm, d andhe refractive index values of the ethanol solutions of differentoncentrations.

At first, the experimental reflection curve of water as a sampleolution with known refractive index (ns = 1.33166) was fitted tobtain εm and d. Then, by fixing εm and d, the reflection curvesor ethanol solutions were fitted to obtain the refractive indicesor each concentration. Fig. 7 shows plots of the fitted curvessolid curves) along with the experimental spectra and Table 1hows a comparison of the expected or theoretical values [21,22]ith the fitting data for the three prisms. The fit values of d for

ll the sensor chips fabricated simultaneously were close to thexpected value of 50.00 nm with the maximum deviated valuebtained as 55.04 nm. The error in the gold film thickness within5 nm from the central value of 50 nm is acceptable and does

ot affect the sensor’s response [23]. The dielectric constant ofetal is generally difficult to predict, as many factors (thickness

f the film [24], experimental conditions, surface morphology ofhe film, and deposition technique [25,26]) lead to the variationn its value. Hence, we can say that its fit value agrees well withhe literature value. The fit values of the refractive indices ofarious ethanol solutions also agreed well with their literaturealues [22] (reported at λ = 589 nm and at temperature of 20 ◦C).

The resonance angle for each dielectric sample was obtainedrom the numerically fitted curve. The values are listed in

able 1 for each prism material. They were plotted against

he corresponding fit values of the refractive indices of thethanol solution as shown in Fig. 8. The plots were fairly

t data

) θsp (◦) d (nm) εm

62.88 53.73 −15.42 + 1.27i63.1563.4563.8264.18

69.93 55.04 −14.95 + 1.22i70.4370.7971.1671.69

76.36 51.97 −15.84 + 1.30i76.9877.5178.2178.94

nsa

1.33601.33951.34321.34691.3506

386 G. Gupta, J. Kondoh / Sensors and Ac

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ig. 8. Plot between resonance angles and refractive indices of ethanol solutionsrom Table 1.

traight line for each prism, confirming that the sensor wasinear in the range of chosen dielectric samples. The slope ofach line denoted the angular sensitivity. Fig. 9 shows plotsf the experimental angular sensitivity with each prism. Thewo horizontal lines mark the range of sensitivities obtained forve sensor chips and the filled circles mark their mean values.or np = 1.597, the mean experimental angular sensitivity was,θ = 94.46◦/RIU. For np = 1.51391, Sθ = 134.40◦/RIU, and forp = 1.456, Sθ = 204.41◦/RIU. These experimental results agreedell with the analytical curve (dashed curve) and the numericalalues of the sensitivity (filled triangles), as shown in Fig. 9 foromparison.

The experimentally obtained SPR spectra also showed

ncrease in the FWHM as predicted by the numerically generatedig. 4. The FWHM for curves A‘, C‘and E‘(Fig. 7) were 2.24◦,.00◦ and 4.36◦, respectively. The reason for this behaviour isiscussed in the next section.

ig. 9. Variation of angular sensitivity as function of refractive index of prism:xperimental values (filled circles), analytical values (dashed curve), and numer-cal values (filled triangles). The two horizontal lines correspond to the range ofalues obtained for five sensor chips.

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tuators B 122 (2007) 381–388

. Discussion

The satisfaction of the resonance condition is the most criticalactor in deciding the working characteristics of the SPR sensor.n examining this factor, one can see that the design parametersetermine the resonance angle for a given refractive index ofhe dielectric sample to be sensed. For a fixed choice of metalroperties (dielectric constant and thickness) and wavelength,he position of reflection dip for the same sample can be con-rolled by the choice of prism material. This was illustrated inumerically generated Fig. 4 (see curves A, C and E), and thenn experimentally verified Fig. 7 (see curves A′, C′ and E′). Theesonance was observed in the low-angle region for an opticallyense prism and observed in the high-angle region for a compar-tively less dense prism. The results confirmed the possibility ofstable tuning of the working region, i.e., the resonance angleith the variation in the prism material.The features of Figs. 4 and 7 suggested that the sensor’s

erformance varies according to the working region. The mostmportant feature was the resonance shift between the SPR spec-ra for the change in the dielectric material, which describes thengular sensitivity of the sensor. The resonance shift was smalln the low-angle region and the shift was large in the high-angleegion. This fact was appreciated in Fig. 9, which elucidatedhat the angular sensitivity was high for a prism with low refrac-ive index. The experimental results were in excellent agreementith the theory, thus making the choice of the prism material is

n important consideration during sensor design.Another important feature was the dynamic range of the sen-

or. The range of dielectrics that can be sensed is limited by theesonance condition, which must be satisfied under the particularesign parameters. The maximum value of the refractive indexf the sample, which can be sensed, is the one for which sin θsp isust less than 1 in Eq. (3). For a prism with np = 1.597, the ns forhich θsp is 90◦ was calculated to be 1.4747, which set the limit,

ns)max. Thus, with this prism, all the dielectrics with refractivendices less than 1.4747 are detectable. A similar calculationor the prism with np = 1.456 gave the value of (ns)max = 1.3615.his shows that the dynamic range of the sensor system gets

educed for the prism with a low refractive index. Consideringhis fact, it will not be practically helpful to reduce the refractivendex of a prism for a broad-range detection sensor. The benefitsf a low-refractive-index prism can be appreciated only if theange of samples is fixed and known, and then the lowest np cane chosen for which the denominator of Eq. (4) is real for all nsalues.

The next feature in Figs. 4 and 7, was the variation in theWHM of the spectra. The numerical SPR curves in Fig. 4howed that the FWHM was small in the low-angle regionnd increased uniformly on moving towards the high-angleegion. This property was well confirmed by the experimentallybtained spectra in Fig. 7. The main cause of the broadening ofhe SPR curves is the complex part of the dielectric constant of

he metal. To understand the variation of FWHM with differ-nt prism materials, we checked the electric field strength at theetal–dielectric interface in accordance with the electromag-

etic field distribution described in a recent paper [13]. Fig. 10

G. Gupta, J. Kondoh / Sensors and Ac

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ig. 10. Mean-square electric fields of TM-polarized waves at metal–dielectricnterface as functions of incident angle for three different prism materials.

hows plots of the mean-square electric field of the TM-polarizedave at the metal–dielectric interface, 〈E2

||〉z=d, as functions

f incident angle for three prisms, keeping other parametersonstant. The incident angle for which the field strength wasaximum closely related the resonance angle and the width of

he field strength profile related the FWHM of the SPR spec-rum. The profile was narrower for np = 1.597 than for np = 1.456,hich proved that the FWHM is larger for a low-refractive-indexrism. Since the time-averaged mean-square field is proportionalo the energy of the electromagnetic wave, it is evident that thenergy is transferred across the metal layer over a broader rangef angles for the prism with a low refractive index; as a result,he sharpness of the resonance profile is reduced.

. Conclusion

Dispersion relation tailoring by the variation of a prism’sefractive index to determine the working characteristics ofhe sensor were studied in the paper. The effects of low- andigh-refractive-index prisms on the performance of the sensorere discussed, and it is concluded from the investigations that

he choice of prism material can enhance the sensor perfor-ance depending on the working requirements. In order to sense

he properties of chemicals or biochemicals having refractivendices in a narrow (and known) range, the resonance posi-ion can be tuned by varying the prism material. The angularensitivity can be increased using a low refractive index prism.he experimental results obtained using three prism materialshowed that the angular sensitivity became double by decreas-ng the refractive index of the prism from 1.597 to 1.456. Theariation followed the analytical expression given by Eq. (4)nd also matched well with the numerical values. The adverseffects of working with low-refractive-index prisms are a rela-ively low dynamic range of the sensor and a large FWHM of

he SPR reflection spectra, which must also be considered beforeelecting the prism material. Overall, the choice of prism mate-ial proves to be a vital design parameter and is significant foruning and optimizing the sensor’s performance.

[

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tuators B 122 (2007) 381–388 387

cknowledgements

The authors thank Dr. Mitsunori Sugimoto for valuable dis-ussions and Mr. Yoshikazu Matsui and Mr. Kengo Nakamuraor help in the experimental work. This work was partiallyupported by research grants of International Human Frontiercience Program Organization.

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JDU1I1aSScience and Technology (AIST). His current research interest is surface-wave-based sensors and actuators, such as SAW and surface plasmons. He is a memberof the Acoustic Society of Japan, Japan Society of Applied Physics, the Institute

88 G. Gupta, J. Kondoh / Sensors a

Pd, Pt, Ag, Ti, and W in the infrared and far infrared, Appl. Opt. 22 (1993)1099–1119.

22] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 86th ed., CRCPress, Boca Raton, Florida, 2005–2006.

23] S. Ekgasit, C. Thammachareon, F. Yu, W. Knoll, Influence of the metalfilm thickness on the sensitivity of surface plasmon resonance biosensors,Appl. Spectrosc. 59 (2005) 661–667.

24] A. Hoffmann, N. Kroo, Z. Lenkefi, Z. Szentirmay, A high precision ATRstudy of surface plasmon mediated reflectance in noble metal films, Surf.Sci. 352/354 (1996) 1043–1046.

25] T.J. Watson, J.R. Sambles, Optical characterization of palladium using sur-face plasmon-polaritons, Phil. Mag. B 65 (1992) 1141–1148.

26] D.E. Aspnes, E. Kinsbron, D.D. Bacon, Optical properties of Au: sampleeffects, Phys. Rev. B 21 (1980) 3290–3299.

iographies

aurav Gupta received his BSc degree in physics in 2002 from Delhi Uni-ersity, Delhi and MSc degree in physics in 2004 from Indian Institute of

oC

tuators B 122 (2007) 381–388

echnology Delhi, New Delhi (India). He is currently a doctor course student athe Graduate School of Electronic Science and Technology, Shizuoka University,hizuoka (Japan).

un Kondoh was born in Shiga, Japan in 1967. He received his BE, MS, andr. Eng. degrees in opto-electric and mechanical engineering from Shizuokaniversity in 1990, 1992, and 1995, respectively. From April 1993 to March997, he was a research fellow of the Japan Society for the Promotion of Science.n 1996, he was a guest scientist at Karlsruhe Research Center, Germany. In April997, he joined the Department of Systems Engineering, Shizuoka Universitys a research associate and was promoted to an associate professor in 2003.ince 2001, he is visiting scientist of National Institute of Advanced Industrial

f Electrical Engineers of Japan, the Institute of Electronics, Information andommunication Engineers, and the Electrochemical Society of Japan.