- Email: [email protected]
A waveguide-coupled surface-plasmon sensor for an aqueous environment
T
CHEMICAL
ELSEVIER
Sensors and Actuators B 22 (1994) 75-81
A waveguide-coupled
surface-plasmon sensor for an aqueous environment C.R. Lavers, J.S. Wilkinson
Optoelectronics Research Received
Cenhe, 7’he Univemi@
21 September
of
Southampton,
Southampton
1993; revised 9 May 1994; accepted
SO17 lB.J, UK
25 May 1994
Abstract
Planar optical waveguides integrated with thin-film dielectric and metallic overlayers may be configured as transducers to probe the optical properties of a broad range of sensing films. We present a simple sensor consisting of a potassium ionexchanged glass waveguide coated with a dielectric buffer layer and a thin silver film to demonstrate optical waveguide-coupled surface-plasmon resonance in an aqueous environment. For a given operating wavelength, the use of an intermediate buffer layer allows tuning of the coupling to a surface plasma wave in water. Theoretical and experimental results confirm that a magnesium fluoride buffer layer may be used to allow operation in an aqueous environment at visible wavelengths. In conjunction with thin sensing films, this waveguide probe is expected to find application in immunoscnsors for monitoring water pollution. Keywords.’ Surface-plasmonsensor; Waveguidecoupling
1. Intmductioll
Planar optical waveguides show great promise in the realization of novel chemical and biochemical sensors that use evanescent fields to probe specifically sensitized films on the waveguide surface. Several optical phenomena have been used to detect chemical reactions at waveguide surfaces [l-7], among which changes in the refractive index or thickness of surface layers monitored through surface-plasmon resonance (SPR) [4,5], waveguide interferometry [6] and the use of grating couplers [7] have provoked much interest. This is principally due to their inherent simplicity, and the lack of need for chemical labels, such as fluorophores, which require additional sample preparation. Recently laboratory instruments based upon disposable SPR devices, among other techniques, have become commercially available [8]. In realizing sensors based upon these techniques, waveguide-coupled devices have the potential advantages that they may be conveniently coupled to optical fibres for connection to instrumentation, and that multiple sensors may readily be integrated on a single optical ‘chip’ using microelectronics fabrication technology. A comparative evaluation of SPR, grating couplers [9], and waveguide Mach-Zehnder interferometers (MZI) [lo] has recently been published, showing that in the configurations considered, the SPR technique
has lower sensitivity when compared with the other techniques [ll]. However, the comparison was made between truly waveguiding geometries in which the interaction may be built up over the length of a relatively long device such as an MZI, on the one hand, and a conventional Kretschmann-Raether SPR configuration [12] where coupling is from a ‘ray’ at a point at the base of a prism on the other. A waveguide-coupled SPR configuration in which a waveguide mode and a surface plasma wave (SPW) are coupled through evanescent fields in a distributed manner allows greater control over the interaction and may lead to greater sensitivity. Such a sensor, used to monitor relative humidity, has been demonstrated previously using a multilayer slab waveguide on a silicon substrate [l]. We have recently demonstrated the sensitive detection of the electrochemically controlled adsorption and desorption of a monolayer of tetrabutyl ammonium ions on a thin silver electrode deposited on a potassium ion-exchanged channel waveguide, using waveguidecoupled SPR [13]. Potassium ion-exchanged waveguides have the advantages that they are inexpensive, low-loss and readily connected to optical fibres. However, one drawback of these simple devices is that, for a given wavelength and metal fihn, the resonance occurs around a fixed analyte index that depends principally upon the waveguide indices and dimensions. In our first waveguide-coupled SPW sensor [13], this problem was ad-
76
C.R Laws,
J.S. WVtion
/ Serwots and Actuators B 22 (1994) 75-81
dressed by ‘biasing’ the bulk analyte to an appropriate refractive index on the resonance through the use of a mixture of ethylene glycol and water. In real applications, such as the monitoring of water quality, this degree of freedom is not available. In this paper, theoretical and experimental results are presented, which show that a single low-index buffer layer, inserted between the silver film and the waveguide, allows coupling of a waveguide mode to a surface plasma wave in an aqueous environment at visible wavelengths where the optical properties of the metallic film are optimum for sensitive SPR experiments [14] and where convenient light sources are available. A multilayer waveguide model is used, in conjunction with published data on the indices of silver at various wavelengths [15], to design a waveguide-coupled SPR sensor that is sensitive to changes in surface index. Theoretically generated waveguide absorption spectra are compared with experimental absorption spectra in air and in water, and show good agreement. Comparison with experimental data for a device having no buffer layer clearly demonstrates the improvement in sensitivity to index changes caused by introduction of the buffer layer. Once optimized for sensitivity, and with appropriate sensing films attached for specificity, this device will find application in exploiting a successful bulkoptical laboratory technique in the form of a miniature sensor for water-quality monitoring.
2. Tbeeretieal
modelling
2.1. Description of the model The excitation of SPWs has been studied for many decades [16], particularly using the prism-coupling technique in both the Otto [17] and the KretschmannRaether [12] configurations. Kreuwel et al. have analysed and demonstrated a waveguide-coupled configuration for use in sensing applications, based upon ‘slab’ waveguides on silicon [1,18], while a similar device based upon multimode optical fibres has been demonstrated by Jorgenson and Yee [19]. Using a straightforward multilayer waveguide model, we show that by inserting an appropriate dielectric buffer layer in a simple planar waveguide configuration it is possible to adjust the superstrate index at which strong coupling between a waveguide mode and the SPW occurs to be that of water. Excitation of the SPW is observed as a dramatic fall in waveguide output intensity due to the lossy nature of the surface plasma wave. The theoretical model used to analyse these devices determines the absorption of the normal modes of a planar waveguide with surface overlayers having, in general, a complex refractive index. The waveguide structure analysed consists of the bulk substrate, a
MEDIUM
h +lk)
n4icKNEss
supersbate
(1.001, JO.0)
Bulk
1)1.333,JO.0)
Bulk
Silverlayer
(0.066.44.0)
40nm
Magnesium tluorlde
(1.378, JO.0)
300& te
WaveguIde
(1.521. JO.0)
2pm
Substrate
(1.512. JO.0)
Bulk
1
I
1
I
II
Fig. 1. Section through waveguide-coupled surface plasmon sensor. Refractive index values (n, +jk) are given for each layer at a wavelength of 632.8 nm.
homogeneous waveguiding layer, a magnesium fluoride buffer layer, a silver layer and a bulk super&ate medium, as shown in Fig. 1. The refractive index used for each medium is also shown. The index of the superstrate is taken to be either 1.001 (for air) or 1.333 (for water); the index of silver is a strong function of wavelength, and is taken from published data [15]. All layers are assumed to extend infinitely in the plane of the substrate. Although the experimental work concerns channel waveguides, which allow fibre input and output coupling, no attempt has been made to model channel waveguides. In this model only the complex refractive index of the silver film is varied with wavelength. It is found that the effects of material dispersion in the other media are negligible when compared with the strong variation of complex refractive index with wavelength for the silver film. A matrix method [20] has been employed to obtain the complex propagation coefficients of the modes of this structure. First, 2x2 matrices are derived from the boundary conditions for the fields at each dielectric interface, and are then multiplied together. A guidedmode solution is found when a coefficient of the resultant matrix is zero, representing a guided mode where evanescent fields decay to zero an infmite distance away from the guide, normal to the dielectric interfaces. This matrix coefficient is a function of wavelength and the film thickness and complex refractive index of each layer. As the equation involved is complex, Miillers’ method [21] is used to find the solution. The modal absorption is deduced from the complex propagation coefficient for the TM modes of the structure, as a function of wavelength. 2.2. Theoretical results for Ag-coated waveguides with MgF, buffer layer Insertion of a buffer layer having appropriate index and thickness allows phase matching between the wave-
C.R. Lava,
guide mode and surface plasma wave in the presence of a superstrate medium having an index near that of water (1.333). A single buffer layer of magnesium fluoride, a convenient dielectric used for multilayer interference filters, allowed mode coupling to be achieved. As the film is only weakly birefringent and its optical properties do not vary significantly across the wavelength range of interest (500-750 mn), a value for the refractive index of n = 1.378 was taken [22]. Fig. 2 shows the loss of the fundamental TM mode, with a superstrate index of 1.333, as a function of wavelength for various MgFz buffer layer thicknesses, for a 2.5 mm length of silver-coated waveguide. It is found that in the presence of a sufficiently thick buffer layer (>280 nm of magnesium fluoride for a waveguide thickness of 2 pm), only one solution to the eigenvalue equation having significant modal overlap with the input waveguide mode exists, such that only one TM mode need be considered in modelling the device, thereby considerably simplifying the theoretical analysis of the system. This composite mode exhibits intensity maxima at the silver/analyte interface, at the silver/MgF, interface and in the dielectric waveguide region. The relative magnitudes of these maxima vary strongly with analyte index, resulting in the sensing behaviour. It is not valid to consider the ‘individual SPW modes’ supported at each metal dielectric interface separately, as they are very strongly coupled in this structure. Fig. 3 shows similar curves for the case in which the water superstrate is replaced by air. The curves show broadly the same dependence as those in Fig. 2, with the position of the maximum absorption shifted by about 55 nm. Comparison of these two sets of curves clearly shows the strong dependence of the waveguide mode absorption upon superstrate index.
ii! s ‘G I 0 “, B ‘5 0”
150
100
50
0 500
550
600
17
J.S. Wilkinson I Sensors and Achutors B 22 (1994) 75-81
650
700
750
Wavelength (nm)
Fig. 2. Theoretical modal absorption as a function of wavelength for various discrete values (0) of intermediate MgFz buffer layer thickness, for water supers&ate.
300nm
a-
600
650
700
750
Wavelength (nm)
Fig. 3. Theoretical modal absorption as a function of wavelength for various discrete values (0) of intermediate MgF2 buffer layer thickness, for air superstrate.
3. Fabrication 3.1. Waveguidefabrication Ion-exchanged glass waveguides are inexpensive, compatible with optical fibres and both mechanically and chemically robust. Many different cation pairs and glasses have been used [23], and K+-Na+ ion exchange, first demonstrated by Giallorenzi et al. [24], is particularly favoured for monomode systems because of the low waveguide losses obtained in conventional sodalime silicate and borosilicate glasses, the low index change (which leads to convenient waveguide dimensions), and because this process is characterized by relatively low diffusion rates, which translates into improved control of the diffusion depth. In this work, the ion source used was a molten salt bath of pure KNO, where the diffusion depth of the K+ ions is controlled by varying the temperature of the melt and the duration of immersion. The substrates used were soda-lime silicate glass cover slides (ROWI), which were initially cleaned in l,l,l-trichloroethane, acetone, and fuming nitric acid, rinsed in deionized water obtained from an ELGA water-purification system and baked at 120°C for 30 min. Approximately 200 nm of aluminium was deposited on the substrates by thermal evaporation and channels varying in width from 4 to 6 pm were opened in this fllm by conventional photolithography, resulting in a diffusion mask. Ionexchanged waveguides were fabricated by immersing the masked substrates in the potassium nitrate melt for 62 min. Throughout the ion-exchange process the potassium nitrate bath was maintained at a temperature of 396&2”C. The aluminium mask was then removed and both ends of the waveguides polished for efficient fibre or lens coupling of incident light into the wave-
78
C.R. Lavem, J.S. F4Minson I Sensors and Actuators B 22 (2994) 75-81
guides. All waveguides were found to be single mode at an excitation wavelength of 632.8 nm. 3.2. Overlayerdeposition A magnesium fluoride layer was deposited on the waveguide samples by vacuum evaporation to achieve phase matching between the waveguide mode and the SPW as described. A secondary overlayer of silver was evaporated across the channel waveguides in a narrow strip, covering a 2.5 mm length of the waveguides. Optical waveguide measurements described below were taken before and after silver coating, on the same sample. The thickness of both films was measured using a profilometer (Alphastep, Tencor Corporation), which evaluates the vertical displacement of a mechanical stylus and averages multiple readings. The magnesium fluoride layer was found to have a thickness of 350f5 mn and the silver film was found to have a thickness of 40*1 nm.
4. Waveguide attenuation spectrum measurements 4.1. Egmimental procedure Waveguide attenuation spectra were measured over the wavelength range 500-750 nm using the apparatus shown in Fig. 4. Preliminary alignment was achieved by coupling light from a He-Ne laser, operating at 632.8 mn, into a single-mode optical fibre and buttcoupling the input fibre to the channel waveguide. Once input coupling was optimized, light from a white-light source, passed through a Jarrell Ash optical monochromator, was launched into the single-mode fibre, which remained butted up to the channel waveguide under investigation. The light emerging from the waveguide was focused onto a silicon detector through a dichroic sheet polarizer and the TM polarization selected. Spectral attenuation measurements were obtained by dividing the signal obtained in this way by that taken directly from the input fibre with the device removed, allowing the device attenuation, including input coupling loss, to be deduced. Attenuation spectra
were recorded for devices in air and when covered with water. To obtain the latter spectra, deionized water at room temperature was pipetted onto the surface of the silver film to cover the entire film. The deionized water used in these experiments was produced using an ELGA water purification system (0.2 pm filtered with resistance in excess of 14 Ma) and the refractive index was measured using an Abbe refractometer to have a value n = 1.331 at the wavelength of the sodiumD line (589.3 nm) under sodium lamp illumination. 4.2. Waveguidescoated with MgF, The experimental attenuation spectra of a waveguide 4.9 f 0.2 pm wide coated only with magnesium fluoride, including fibre input coupling losses, for both an air and a water superstrate are shown in Fig. 5. Data corresponding to the air and water superstrates are given by the closed and open circles, respectively, in Figs. 5-7. They are featureless across the wavelength range of study, with approximately 10 dB total loss observed over a 2.5 cm long waveguide substrate. This rather high baseline attenuation is principally due to fibre-guide coupling loss, as no attempt was made to design the waveguide for high coupling efficiency. Comparison of the spectra before and after coverage of the waveguide with deionized water shows that no significant change is detected in the attenuation spectrum. 4.3. Waveguidescoated with silver The experimentally recorded attenuation spectra of such a potassium ion-exchange waveguide coated with a 40& 1 nm layer of silver (deposited by thermal evaporation) for both an air and a water superstrate are shown in Fig. 6. There is a slight loading effect when water replaces air as the bulk super&ate. No significant
Monodromator
600 650 700 Wavelength (nm) Fig. 4. Apparatus
for waveguide spectral attenuation
measurements.
750
Fig. 5. Measured spectral attenuation of waveguide/MgF,/superrate system for both air and water superstrates: air, e, water, 0.
C.R.
Lavem,
IS.
tWkinson
I Sensors
and
Actuators
B 22 (1994)
19
75-N
Above 680 nm the transmission spectrum is virtually unaffected by the SPR and has approximately the same value as before the thin layer of silver was evaporated onto the buffer layer. In the case of a water superstrate, a marked shift of the resonance is observed toward a higher wavelength, with maximum attenuation of 25 dB occurring at a wavelength of 650 nm, and the resonance is observed to have become broader. The wavelength of peak attenuation has been shifted by approximately 50 nm on the addition of water.
01 500
1
I
550
I
I
700
750
I
600
650
5. Discussion
Wavelength (nm)
Fig. 6. Measured spectral attenuation of waveguide/silver/superstrate system for both air and water superstrates: air, 0; water, 0.
l*. . 0
.
0.
o
.
.
co 0 0
0
500
550
600
650
700
750
Wavelength bm)
Fig. 7. Measured spectral attenuation of waveguide/MgF2/silver/ superstrate system for both air and water superstrates: air, 0; water, 0.
To achieve a comparison between theoretical predictions and experimental data, the experimentally determined values for the thicknesses of both the magnesium fluoride layer and the silver layer were included in the model described in Section 2. Standard values for the optical dielectric constants of silver taken at discrete wavelengths and interpolated were used [15] as before, and the resultant theoretical curves are shown with the experimental data in Figs. 8 and 9. The theoretically predicted curves are given by continuous lines. It can be seen that there is broadly good agreement between theory and experimental results for both air and water bulk superstrates. The experimental determination of device attenuation is limited by the performance of the polarizing optics and by the low optical power levels being used, resulting in divergence of the theoretical and experimental curves at high attenuations. There is a slight broadening of the experimentally observed resonance in Fig. 9 for the device with a water superstrate. Similar broadening has been observed previously with both silver- and gold-coated systems by both Cowen [25] and Pollard et al. [26]. Pollard and co-workers explain their results in terms of a mixed
change is noted in the spectral response and therefore there is little sensitivity to changes in superstrate index.
30
4.4. Waveguides coated with MgF’, and Ag
25
The attenuation spectra of the same magnesium fluoride-coated waveguide, when coated with a further 40f 1 nm thin film of silver are shown in Fig. 7 for the cases of air and water superstrates . A large change is now observed in the attenuation spectrum of the waveguide near the wavelength of interest, corresponding to wavelength- and superstrate-dependent excitation of the SPR, as predicted theoretically. The resonance for the case of an air superstrate shows a minimum in transmission through the waveguide centred at a wavelength of approximately 600 nm, corresponding to a loss of 24 dB. The minimum power measurement is limited by noise and the quality of the polarizing optics.
600
650
700
750
Wavelength (nm)
Fig. 8. Spectral attenuation of waveguide/MgFy’silver/air experiment, l ; theory, solid curve.
system:
C.R. Lovers, J.S. Wdkimon / Semm
80
5
01
500
I
I
I
I
I
550
500
650
700
750
Wavelength
(nm)
Fig. 9. Spectral attenuation of waveguideiMgF&lverhvater experiment, 0; theory, solid curve.
system:
dielectric/metal layer existing at the metal/liquid interface caused by the presence of asperities and voids in the metal film. Cowen, however, suggests that the most plausible conclusion is that solvent molecules enter the metal film when it is placed against a liquid, which changes the apparent real and imaginary parts of e. This sensing device has been characterized using somewhat complex equipment, to allow a full comparison of the theoretical model with experimental results over a broad wavelength range. It is expected that, in operation as a sensor, a modulated semiconductor light source and simple detection circuitry would be employed, as full spectral monitoring would not be required.
and Achutors B 22 (1994) 75-81
as the superstrate medium and subsequently replacing the air with water, showing a wavelength shift of about 50 nm in the position of the absorption maximum. Earlier work [13] has shown that in the case of a device without a buffer layer, where phase matching is achieved by adjusting the superstrate index, changes in the index of an adsorbed monolayer strongly affect the coupling to the surface plasma wave, resulting in large changes in output power. Work is now in progress to investigate the adsorption of monolayers in aqueous media using the present device. Further modelling and experimental work will investigate optimization of the sensitivity to thin sensing films attached at the metal surface, and allow full comparison with interferometric and grating sensors. This paper has demonstrated a method of achieving strong coupling between a surface plasma wave and a mode in an underlying potassium ion-exchanged channel waveguide, in an aqueous environment and at visible wavelengths. The exploitation of this technique, using inexpensive fibre-compatrble ion-exchanged glass waveguides and simple overlayer configurations, is expected to lead to integrated sensors for water-quality monitoring.
Acknowledgements The authors acknowledge the support of the UK Science and Engineering Research Council through the Optoelectronics Research Centre. The authors would also like to thank D.J. SchSrin and R.D. Harris for helpful discussions.
6. Conclusions A simple waveguide-coupled surface-plasmon sensor that utilizes coupling between a waveguide mode and a surface plasma wave has been demonstrated. A buffer layer of magnesium fluoride inserted between a potassium ion-exchanged waveguide and a 40 nm silver film deposited on the structure allows excitation of a surface plasma wave in an aqueous environment at convenient optical wavelengths. A numerical model has been used to predict the modal absorption of a multilayer waveguide device as a function of wavelength and superstrate index. In particular, the use of a thin intermediate dielectric layer to achieve phase matching to a surface plasma wave in an aqueous environment has been investigated. A device was designed for maximum sensitivity at a wavelength of 632.8 nm in aqueous media, and experimental results show the surface plasmon absorption edge to be close to this wavelength in water. The sensitivity of the device to changes in superstrate index has been demonstrated by comparing results with air
References [ll H.J.M. Kreuwell, P.V. Lambeck, J.M.N. Beltamn and T.J.A.
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Popma, Mode coupling in multilayered structures applied to a chemical sensor and a wavelength-selective directional coupler, Froc. 4th European ConJI Integmted Opt., Ghgmv, Swhnd, May 11-13, 1987, pp. 217-220. A.N. Sloper, J.K. Deacon and M.T. Flanagan, A planar indium phosphate monomode waveguide evanescent field immunosensor, Sensors and Actuutcm, BI (1990) 589-591. C. Piraud, E.K. Mwarania, J. Yao, K. O’Dwyer, D.J. SchiRrin and J.S. Wilkinson, Optoelectronic transduction on planar optical waveguides, 1. Lightwave Technol., 10 (1992) 693. B. Liedberg, C. Nylander and I. Lundstrom, Surface plasmon resonance for gasdetection and biosensing, Sen.rorsamfAcfnu~ors B, 4 (1983) 299-304. J. Van Gent, P.V. Lambeck, H.J.M. Kreuwel, G.J. Gerritsma, E.J.R. Sudholter. D.N. Reinhoudt and T.J.A. Popma, Optimisation of a chemo-optical surface plasmon resonance based sensor, A&. Opt., 29 (1990) 2843-2849. N. Fabricius, G. Gauglitz and J. Ingenhoff, A gas sensor based on an integrated optical Mach-Zehnder interferometer, Sensors and Actuators B, 7 (1992) 672-676.
CR. Laws, [71 D. Clerc and W. Lukosx,
PI PI WI
WI WI
[I31
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Integrated optical output grating coupler as refractometer and (bio-)chemicaI sensor, Stussors and Achutors B, 11 (1993) 461-465. J. Hodgson, Light, angles, action: instruments for label-free, real time monitoring of intermolecular interactions, BiolTechnology, 8 (1994) 31-35. K. TiefenthaIer, Integrated optical couplers as chemical waveguide sensors, Adv. Bk~~enrors, 2 (1992) 261-289. R.G. Heidemann, R.P. Kcoyman and J. Greve, Performance of a highIy sensitive optical waveguide Mach-Zehnder interferometer hmnunosensor, Senrors and Achutors 8, 10 (1993) 209-217. W. Lukosx, Principles and sensitivities of integrated optical and surface plasmon sensors for direct affinity sensing and immunosensmg, Biosenrors Bioelectrun., 6 (1991) 215-225. E. Kretschmamr and H. Raether, Radiative &cay of nonradiative surface plasmons excited by light, Z. Nohuforsch. A, 23 (1968) 2135. CR. Lavers, C. Piraud, M. Brust, K. O’Dwyer, J.S. Wilkinson and D.J. Schilfrin, Eleetmchemicaily-controlled optical waveguide sensors, Rot. 9th Optical Fiber Sensors Conf (OFS 9), Florence, Italy, May 4-6, 1993, pp. 193-196. H.E. De Bruijn, R.P.H. Kooyman and I. Greve, Choice of metal and wavelength for surface plasmon sensors: some considerations, Appl. Optics, 31 (1992) 440-442. H. Raether, Su&ce Plasmons on Smooth and Rough Surfaces and on Gratings, Springer, Berlin, 1988, p. 125. R.W. Wood, The echelette grating for the infrared, Phi&x Mug., 2 (1910) 770-778. A. Otto, Excitation of non-radiative surface plasma waves in silver by the method of frustrated total reflection, Z. Phys., 216 (1968) 398. H.J.M. Kreuwell, Planar waveguide sensors for the chemical domain, Ph.D. Thesis, Ch. 4, University of ‘IXvente, The Netherlands, 1988. R.C. Jorgenson and S.S. Yee, A fiber-optic chemical sensor based upon surface plasmon resonance, Sensors and Actwton B, I.2 (1993) 213-220. P. Yeh, Optical Waves in Layered Media, Wiley, New York, 1988. S.D. Conte, Elementary Numerical Analysis an Algotithmic Appmach, McGraw-Hill, New York, 1972, p. 120.
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and R.W.H. Stevenson, Some properties of magnesium fluoride crystahii from the melt, Pmt. Phys. Sot. (#London), 72 (1958) 1001-1006. WI R. Ramaswamy and R. Srivastava, Ion-exchanged glass waveguides: a review, J. Lighhvuvc TechnoL, 6 (1988) 984-1002. [241 G. Giallorenxi, E. West, R. Kirk, R. Ginther and R. Andrews, Optical waveguides formed by thermal migration of ions in glass, Apple OWL, 12 (1973) 1240-1245. 1x1 S. Cmven, Optical studies of surface-modified electrodes, Ph.D. Ihesis, University of Exeter, UK, 1990. WI J.D. Pollard, G.W. Bradberry and J.R. Sambles, A study of the thin metal film/fluid interface using surface plasmen-polaritons, Solid State Common, 63 (1987) 803-806.
Biographies James S. Wilkinson received a B.Sc. (En& in electronics in 1977 and a Ph.D. in the field of integrated optics in 1985, both from University College London. From 1977 to 1979 he was with the GEC Hirst Research Centre working on optical-fibre telecommunications systems. From 1983 to 1985 he was with the Department of Nephrology of St. Bartholomew’s Hospital, London, investigating sensing and control techniques for dialysis procedures. He is now senior lecturer in optoelectronics in the Department of Electronics and Computer Science, University of Southampton, UK, partially seconded to the Optoelectronics Research Centre at Southampton University. His research interests include integrated optical lasers and amplifiers, and biological and chemical sensors. ChnktopherR. Lavers received a B.Sc. in physics in 1987, and a Ph.D. in the research field of optical probing within liquid crystalline systems (1987-1990), both from the Department of Physics, University of Exeter, UK. Since 1998 he has been a research fellow at the Optoelectronics Research Centre at Southampton University, UK His research interests include integrated optical biological and chemical sensors.
CHEMICAL
ELSEVIER
Sensors and Actuators B 22 (1994) 75-81
A waveguide-coupled
surface-plasmon sensor for an aqueous environment C.R. Lavers, J.S. Wilkinson
Optoelectronics Research Received
Cenhe, 7’he Univemi@
21 September
of
Southampton,
Southampton
1993; revised 9 May 1994; accepted
SO17 lB.J, UK
25 May 1994
Abstract
Planar optical waveguides integrated with thin-film dielectric and metallic overlayers may be configured as transducers to probe the optical properties of a broad range of sensing films. We present a simple sensor consisting of a potassium ionexchanged glass waveguide coated with a dielectric buffer layer and a thin silver film to demonstrate optical waveguide-coupled surface-plasmon resonance in an aqueous environment. For a given operating wavelength, the use of an intermediate buffer layer allows tuning of the coupling to a surface plasma wave in water. Theoretical and experimental results confirm that a magnesium fluoride buffer layer may be used to allow operation in an aqueous environment at visible wavelengths. In conjunction with thin sensing films, this waveguide probe is expected to find application in immunoscnsors for monitoring water pollution. Keywords.’ Surface-plasmonsensor; Waveguidecoupling
1. Intmductioll
Planar optical waveguides show great promise in the realization of novel chemical and biochemical sensors that use evanescent fields to probe specifically sensitized films on the waveguide surface. Several optical phenomena have been used to detect chemical reactions at waveguide surfaces [l-7], among which changes in the refractive index or thickness of surface layers monitored through surface-plasmon resonance (SPR) [4,5], waveguide interferometry [6] and the use of grating couplers [7] have provoked much interest. This is principally due to their inherent simplicity, and the lack of need for chemical labels, such as fluorophores, which require additional sample preparation. Recently laboratory instruments based upon disposable SPR devices, among other techniques, have become commercially available [8]. In realizing sensors based upon these techniques, waveguide-coupled devices have the potential advantages that they may be conveniently coupled to optical fibres for connection to instrumentation, and that multiple sensors may readily be integrated on a single optical ‘chip’ using microelectronics fabrication technology. A comparative evaluation of SPR, grating couplers [9], and waveguide Mach-Zehnder interferometers (MZI) [lo] has recently been published, showing that in the configurations considered, the SPR technique
has lower sensitivity when compared with the other techniques [ll]. However, the comparison was made between truly waveguiding geometries in which the interaction may be built up over the length of a relatively long device such as an MZI, on the one hand, and a conventional Kretschmann-Raether SPR configuration [12] where coupling is from a ‘ray’ at a point at the base of a prism on the other. A waveguide-coupled SPR configuration in which a waveguide mode and a surface plasma wave (SPW) are coupled through evanescent fields in a distributed manner allows greater control over the interaction and may lead to greater sensitivity. Such a sensor, used to monitor relative humidity, has been demonstrated previously using a multilayer slab waveguide on a silicon substrate [l]. We have recently demonstrated the sensitive detection of the electrochemically controlled adsorption and desorption of a monolayer of tetrabutyl ammonium ions on a thin silver electrode deposited on a potassium ion-exchanged channel waveguide, using waveguidecoupled SPR [13]. Potassium ion-exchanged waveguides have the advantages that they are inexpensive, low-loss and readily connected to optical fibres. However, one drawback of these simple devices is that, for a given wavelength and metal fihn, the resonance occurs around a fixed analyte index that depends principally upon the waveguide indices and dimensions. In our first waveguide-coupled SPW sensor [13], this problem was ad-
76
C.R Laws,
J.S. WVtion
/ Serwots and Actuators B 22 (1994) 75-81
dressed by ‘biasing’ the bulk analyte to an appropriate refractive index on the resonance through the use of a mixture of ethylene glycol and water. In real applications, such as the monitoring of water quality, this degree of freedom is not available. In this paper, theoretical and experimental results are presented, which show that a single low-index buffer layer, inserted between the silver film and the waveguide, allows coupling of a waveguide mode to a surface plasma wave in an aqueous environment at visible wavelengths where the optical properties of the metallic film are optimum for sensitive SPR experiments [14] and where convenient light sources are available. A multilayer waveguide model is used, in conjunction with published data on the indices of silver at various wavelengths [15], to design a waveguide-coupled SPR sensor that is sensitive to changes in surface index. Theoretically generated waveguide absorption spectra are compared with experimental absorption spectra in air and in water, and show good agreement. Comparison with experimental data for a device having no buffer layer clearly demonstrates the improvement in sensitivity to index changes caused by introduction of the buffer layer. Once optimized for sensitivity, and with appropriate sensing films attached for specificity, this device will find application in exploiting a successful bulkoptical laboratory technique in the form of a miniature sensor for water-quality monitoring.
2. Tbeeretieal
modelling
2.1. Description of the model The excitation of SPWs has been studied for many decades [16], particularly using the prism-coupling technique in both the Otto [17] and the KretschmannRaether [12] configurations. Kreuwel et al. have analysed and demonstrated a waveguide-coupled configuration for use in sensing applications, based upon ‘slab’ waveguides on silicon [1,18], while a similar device based upon multimode optical fibres has been demonstrated by Jorgenson and Yee [19]. Using a straightforward multilayer waveguide model, we show that by inserting an appropriate dielectric buffer layer in a simple planar waveguide configuration it is possible to adjust the superstrate index at which strong coupling between a waveguide mode and the SPW occurs to be that of water. Excitation of the SPW is observed as a dramatic fall in waveguide output intensity due to the lossy nature of the surface plasma wave. The theoretical model used to analyse these devices determines the absorption of the normal modes of a planar waveguide with surface overlayers having, in general, a complex refractive index. The waveguide structure analysed consists of the bulk substrate, a
MEDIUM
h +lk)
n4icKNEss
supersbate
(1.001, JO.0)
Bulk
1)1.333,JO.0)
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Silverlayer
(0.066.44.0)
40nm
Magnesium tluorlde
(1.378, JO.0)
300& te
WaveguIde
(1.521. JO.0)
2pm
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(1.512. JO.0)
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1
I
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I
II
Fig. 1. Section through waveguide-coupled surface plasmon sensor. Refractive index values (n, +jk) are given for each layer at a wavelength of 632.8 nm.
homogeneous waveguiding layer, a magnesium fluoride buffer layer, a silver layer and a bulk super&ate medium, as shown in Fig. 1. The refractive index used for each medium is also shown. The index of the superstrate is taken to be either 1.001 (for air) or 1.333 (for water); the index of silver is a strong function of wavelength, and is taken from published data [15]. All layers are assumed to extend infinitely in the plane of the substrate. Although the experimental work concerns channel waveguides, which allow fibre input and output coupling, no attempt has been made to model channel waveguides. In this model only the complex refractive index of the silver film is varied with wavelength. It is found that the effects of material dispersion in the other media are negligible when compared with the strong variation of complex refractive index with wavelength for the silver film. A matrix method [20] has been employed to obtain the complex propagation coefficients of the modes of this structure. First, 2x2 matrices are derived from the boundary conditions for the fields at each dielectric interface, and are then multiplied together. A guidedmode solution is found when a coefficient of the resultant matrix is zero, representing a guided mode where evanescent fields decay to zero an infmite distance away from the guide, normal to the dielectric interfaces. This matrix coefficient is a function of wavelength and the film thickness and complex refractive index of each layer. As the equation involved is complex, Miillers’ method [21] is used to find the solution. The modal absorption is deduced from the complex propagation coefficient for the TM modes of the structure, as a function of wavelength. 2.2. Theoretical results for Ag-coated waveguides with MgF, buffer layer Insertion of a buffer layer having appropriate index and thickness allows phase matching between the wave-
C.R. Lava,
guide mode and surface plasma wave in the presence of a superstrate medium having an index near that of water (1.333). A single buffer layer of magnesium fluoride, a convenient dielectric used for multilayer interference filters, allowed mode coupling to be achieved. As the film is only weakly birefringent and its optical properties do not vary significantly across the wavelength range of interest (500-750 mn), a value for the refractive index of n = 1.378 was taken [22]. Fig. 2 shows the loss of the fundamental TM mode, with a superstrate index of 1.333, as a function of wavelength for various MgFz buffer layer thicknesses, for a 2.5 mm length of silver-coated waveguide. It is found that in the presence of a sufficiently thick buffer layer (>280 nm of magnesium fluoride for a waveguide thickness of 2 pm), only one solution to the eigenvalue equation having significant modal overlap with the input waveguide mode exists, such that only one TM mode need be considered in modelling the device, thereby considerably simplifying the theoretical analysis of the system. This composite mode exhibits intensity maxima at the silver/analyte interface, at the silver/MgF, interface and in the dielectric waveguide region. The relative magnitudes of these maxima vary strongly with analyte index, resulting in the sensing behaviour. It is not valid to consider the ‘individual SPW modes’ supported at each metal dielectric interface separately, as they are very strongly coupled in this structure. Fig. 3 shows similar curves for the case in which the water superstrate is replaced by air. The curves show broadly the same dependence as those in Fig. 2, with the position of the maximum absorption shifted by about 55 nm. Comparison of these two sets of curves clearly shows the strong dependence of the waveguide mode absorption upon superstrate index.
ii! s ‘G I 0 “, B ‘5 0”
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100
50
0 500
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600
17
J.S. Wilkinson I Sensors and Achutors B 22 (1994) 75-81
650
700
750
Wavelength (nm)
Fig. 2. Theoretical modal absorption as a function of wavelength for various discrete values (0) of intermediate MgFz buffer layer thickness, for water supers&ate.
300nm
a-
600
650
700
750
Wavelength (nm)
Fig. 3. Theoretical modal absorption as a function of wavelength for various discrete values (0) of intermediate MgF2 buffer layer thickness, for air superstrate.
3. Fabrication 3.1. Waveguidefabrication Ion-exchanged glass waveguides are inexpensive, compatible with optical fibres and both mechanically and chemically robust. Many different cation pairs and glasses have been used [23], and K+-Na+ ion exchange, first demonstrated by Giallorenzi et al. [24], is particularly favoured for monomode systems because of the low waveguide losses obtained in conventional sodalime silicate and borosilicate glasses, the low index change (which leads to convenient waveguide dimensions), and because this process is characterized by relatively low diffusion rates, which translates into improved control of the diffusion depth. In this work, the ion source used was a molten salt bath of pure KNO, where the diffusion depth of the K+ ions is controlled by varying the temperature of the melt and the duration of immersion. The substrates used were soda-lime silicate glass cover slides (ROWI), which were initially cleaned in l,l,l-trichloroethane, acetone, and fuming nitric acid, rinsed in deionized water obtained from an ELGA water-purification system and baked at 120°C for 30 min. Approximately 200 nm of aluminium was deposited on the substrates by thermal evaporation and channels varying in width from 4 to 6 pm were opened in this fllm by conventional photolithography, resulting in a diffusion mask. Ionexchanged waveguides were fabricated by immersing the masked substrates in the potassium nitrate melt for 62 min. Throughout the ion-exchange process the potassium nitrate bath was maintained at a temperature of 396&2”C. The aluminium mask was then removed and both ends of the waveguides polished for efficient fibre or lens coupling of incident light into the wave-
78
C.R. Lavem, J.S. F4Minson I Sensors and Actuators B 22 (2994) 75-81
guides. All waveguides were found to be single mode at an excitation wavelength of 632.8 nm. 3.2. Overlayerdeposition A magnesium fluoride layer was deposited on the waveguide samples by vacuum evaporation to achieve phase matching between the waveguide mode and the SPW as described. A secondary overlayer of silver was evaporated across the channel waveguides in a narrow strip, covering a 2.5 mm length of the waveguides. Optical waveguide measurements described below were taken before and after silver coating, on the same sample. The thickness of both films was measured using a profilometer (Alphastep, Tencor Corporation), which evaluates the vertical displacement of a mechanical stylus and averages multiple readings. The magnesium fluoride layer was found to have a thickness of 350f5 mn and the silver film was found to have a thickness of 40*1 nm.
4. Waveguide attenuation spectrum measurements 4.1. Egmimental procedure Waveguide attenuation spectra were measured over the wavelength range 500-750 nm using the apparatus shown in Fig. 4. Preliminary alignment was achieved by coupling light from a He-Ne laser, operating at 632.8 mn, into a single-mode optical fibre and buttcoupling the input fibre to the channel waveguide. Once input coupling was optimized, light from a white-light source, passed through a Jarrell Ash optical monochromator, was launched into the single-mode fibre, which remained butted up to the channel waveguide under investigation. The light emerging from the waveguide was focused onto a silicon detector through a dichroic sheet polarizer and the TM polarization selected. Spectral attenuation measurements were obtained by dividing the signal obtained in this way by that taken directly from the input fibre with the device removed, allowing the device attenuation, including input coupling loss, to be deduced. Attenuation spectra
were recorded for devices in air and when covered with water. To obtain the latter spectra, deionized water at room temperature was pipetted onto the surface of the silver film to cover the entire film. The deionized water used in these experiments was produced using an ELGA water purification system (0.2 pm filtered with resistance in excess of 14 Ma) and the refractive index was measured using an Abbe refractometer to have a value n = 1.331 at the wavelength of the sodiumD line (589.3 nm) under sodium lamp illumination. 4.2. Waveguidescoated with MgF, The experimental attenuation spectra of a waveguide 4.9 f 0.2 pm wide coated only with magnesium fluoride, including fibre input coupling losses, for both an air and a water superstrate are shown in Fig. 5. Data corresponding to the air and water superstrates are given by the closed and open circles, respectively, in Figs. 5-7. They are featureless across the wavelength range of study, with approximately 10 dB total loss observed over a 2.5 cm long waveguide substrate. This rather high baseline attenuation is principally due to fibre-guide coupling loss, as no attempt was made to design the waveguide for high coupling efficiency. Comparison of the spectra before and after coverage of the waveguide with deionized water shows that no significant change is detected in the attenuation spectrum. 4.3. Waveguidescoated with silver The experimentally recorded attenuation spectra of such a potassium ion-exchange waveguide coated with a 40& 1 nm layer of silver (deposited by thermal evaporation) for both an air and a water superstrate are shown in Fig. 6. There is a slight loading effect when water replaces air as the bulk super&ate. No significant
Monodromator
600 650 700 Wavelength (nm) Fig. 4. Apparatus
for waveguide spectral attenuation
measurements.
750
Fig. 5. Measured spectral attenuation of waveguide/MgF,/superrate system for both air and water superstrates: air, e, water, 0.
C.R.
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Above 680 nm the transmission spectrum is virtually unaffected by the SPR and has approximately the same value as before the thin layer of silver was evaporated onto the buffer layer. In the case of a water superstrate, a marked shift of the resonance is observed toward a higher wavelength, with maximum attenuation of 25 dB occurring at a wavelength of 650 nm, and the resonance is observed to have become broader. The wavelength of peak attenuation has been shifted by approximately 50 nm on the addition of water.
01 500
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5. Discussion
Wavelength (nm)
Fig. 6. Measured spectral attenuation of waveguide/silver/superstrate system for both air and water superstrates: air, 0; water, 0.
l*. . 0
.
0.
o
.
.
co 0 0
0
500
550
600
650
700
750
Wavelength bm)
Fig. 7. Measured spectral attenuation of waveguide/MgF2/silver/ superstrate system for both air and water superstrates: air, 0; water, 0.
To achieve a comparison between theoretical predictions and experimental data, the experimentally determined values for the thicknesses of both the magnesium fluoride layer and the silver layer were included in the model described in Section 2. Standard values for the optical dielectric constants of silver taken at discrete wavelengths and interpolated were used [15] as before, and the resultant theoretical curves are shown with the experimental data in Figs. 8 and 9. The theoretically predicted curves are given by continuous lines. It can be seen that there is broadly good agreement between theory and experimental results for both air and water bulk superstrates. The experimental determination of device attenuation is limited by the performance of the polarizing optics and by the low optical power levels being used, resulting in divergence of the theoretical and experimental curves at high attenuations. There is a slight broadening of the experimentally observed resonance in Fig. 9 for the device with a water superstrate. Similar broadening has been observed previously with both silver- and gold-coated systems by both Cowen [25] and Pollard et al. [26]. Pollard and co-workers explain their results in terms of a mixed
change is noted in the spectral response and therefore there is little sensitivity to changes in superstrate index.
30
4.4. Waveguides coated with MgF’, and Ag
25
The attenuation spectra of the same magnesium fluoride-coated waveguide, when coated with a further 40f 1 nm thin film of silver are shown in Fig. 7 for the cases of air and water superstrates . A large change is now observed in the attenuation spectrum of the waveguide near the wavelength of interest, corresponding to wavelength- and superstrate-dependent excitation of the SPR, as predicted theoretically. The resonance for the case of an air superstrate shows a minimum in transmission through the waveguide centred at a wavelength of approximately 600 nm, corresponding to a loss of 24 dB. The minimum power measurement is limited by noise and the quality of the polarizing optics.
600
650
700
750
Wavelength (nm)
Fig. 8. Spectral attenuation of waveguide/MgFy’silver/air experiment, l ; theory, solid curve.
system:
C.R. Lovers, J.S. Wdkimon / Semm
80
5
01
500
I
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(nm)
Fig. 9. Spectral attenuation of waveguideiMgF&lverhvater experiment, 0; theory, solid curve.
system:
dielectric/metal layer existing at the metal/liquid interface caused by the presence of asperities and voids in the metal film. Cowen, however, suggests that the most plausible conclusion is that solvent molecules enter the metal film when it is placed against a liquid, which changes the apparent real and imaginary parts of e. This sensing device has been characterized using somewhat complex equipment, to allow a full comparison of the theoretical model with experimental results over a broad wavelength range. It is expected that, in operation as a sensor, a modulated semiconductor light source and simple detection circuitry would be employed, as full spectral monitoring would not be required.
and Achutors B 22 (1994) 75-81
as the superstrate medium and subsequently replacing the air with water, showing a wavelength shift of about 50 nm in the position of the absorption maximum. Earlier work [13] has shown that in the case of a device without a buffer layer, where phase matching is achieved by adjusting the superstrate index, changes in the index of an adsorbed monolayer strongly affect the coupling to the surface plasma wave, resulting in large changes in output power. Work is now in progress to investigate the adsorption of monolayers in aqueous media using the present device. Further modelling and experimental work will investigate optimization of the sensitivity to thin sensing films attached at the metal surface, and allow full comparison with interferometric and grating sensors. This paper has demonstrated a method of achieving strong coupling between a surface plasma wave and a mode in an underlying potassium ion-exchanged channel waveguide, in an aqueous environment and at visible wavelengths. The exploitation of this technique, using inexpensive fibre-compatrble ion-exchanged glass waveguides and simple overlayer configurations, is expected to lead to integrated sensors for water-quality monitoring.
Acknowledgements The authors acknowledge the support of the UK Science and Engineering Research Council through the Optoelectronics Research Centre. The authors would also like to thank D.J. SchSrin and R.D. Harris for helpful discussions.
6. Conclusions A simple waveguide-coupled surface-plasmon sensor that utilizes coupling between a waveguide mode and a surface plasma wave has been demonstrated. A buffer layer of magnesium fluoride inserted between a potassium ion-exchanged waveguide and a 40 nm silver film deposited on the structure allows excitation of a surface plasma wave in an aqueous environment at convenient optical wavelengths. A numerical model has been used to predict the modal absorption of a multilayer waveguide device as a function of wavelength and superstrate index. In particular, the use of a thin intermediate dielectric layer to achieve phase matching to a surface plasma wave in an aqueous environment has been investigated. A device was designed for maximum sensitivity at a wavelength of 632.8 nm in aqueous media, and experimental results show the surface plasmon absorption edge to be close to this wavelength in water. The sensitivity of the device to changes in superstrate index has been demonstrated by comparing results with air
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Biographies James S. Wilkinson received a B.Sc. (En& in electronics in 1977 and a Ph.D. in the field of integrated optics in 1985, both from University College London. From 1977 to 1979 he was with the GEC Hirst Research Centre working on optical-fibre telecommunications systems. From 1983 to 1985 he was with the Department of Nephrology of St. Bartholomew’s Hospital, London, investigating sensing and control techniques for dialysis procedures. He is now senior lecturer in optoelectronics in the Department of Electronics and Computer Science, University of Southampton, UK, partially seconded to the Optoelectronics Research Centre at Southampton University. His research interests include integrated optical lasers and amplifiers, and biological and chemical sensors. ChnktopherR. Lavers received a B.Sc. in physics in 1987, and a Ph.D. in the research field of optical probing within liquid crystalline systems (1987-1990), both from the Department of Physics, University of Exeter, UK. Since 1998 he has been a research fellow at the Optoelectronics Research Centre at Southampton University, UK His research interests include integrated optical biological and chemical sensors.