biological sensor in integrated optics

Sensors and Actuators B 51 (1998) 316 – 320

Surface plasmon chemical/biological sensor in integrated optics O. Hugon a,*, P. Benech a, H. Gagnaire b a

LEMO, INPG-UJF-CNRS UMR 5530, Ecole Nationale Supe´rieure d’Electronique et de Radioe´lectricite´ 23 A6enue des Martyrs, BP 257, F-38016 Grenoble Cedex, France b LTSI, CNRS UMR 5516, 23 rue du Docteur Paul Michelon, 42023 Saint-Etienne, France Received 30 March 1998; received in revised form 18 May 1998; accepted 19 May 1998

Abstract A small and efficient chemical/biological sensor can be realized by the use of surface plasmon resonance in integrated optics. The attenuation coefficient of the device is modified by the presence of a sensitive material whose index and/or thickness depends on the concentration in certain molecules that are characteristic of it. A modal analysis of the sensor structure is performed and a very simple model is proposed to assist in the structure design and optimization. Finally, some experimental results for gas detection are presented. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Chemical/biological sensor; Integrated optics; Surface plasmon

1. Introduction Currently, chemical and biological sensors are needed in many various fields for a domestic, professional or industrial use. Gas leak detection, fire detector, environmental monitoring or medical diagnostics are some of the potential applications. Optical techniques can reach those needs with some interesting advantages such as immunity to electrical or electromagnetic interferences or possibility to work in hostile (explosive, inflammable,…) environment. However, existing solutions often suffer from several drawbacks. Bulk optics systems can offer a good sensitivity but they are generally difficult to use by a non-specialist, cumbersome and very expensive. On the other hand, integrated optics devices are simple and compact but their efficiency is still limited, particularly in the case of thin sensitive layers. The aim of this paper is to present a new kind of integrated component with an enhanced sensitivity thanks to the use of surface plasmon waves. In order to perform a chemical or biological detection with the integrated optics technology, a common solution consists in using the evanescent wave interac-

tion phenomena [1–3]. Usually, a sensitive material in contact with the evanescent field of a guided mode alters the mode phase velocity which can be highlighted for example with interferometric techniques. The efficiency of such a sensor is directly linked to the interaction length (limited by the size of the component) and to the intensity in the sensitive material. If the sensitive layer is very thin (compared with the optical penetration depth), only a very small proportion (about 1%) of the light takes part in the detection. This is why surface plasmon sensors are an interesting solution because the light intensity reaches its maximum at the interface between the metal and the sensitive material. Kreuwell et al. already proposed a metallized slab waveguide configuration [4] but for integration purpose a channel waveguide geometry is far more attractive because, with an appropriated design, it allows multianalyte sensing and/or signal processing on the same chip. In that way, various devices and modeling methods have recently been reported [5–7] showing the growth of interest in this research field.

2. Surface plasmon resonance * Corresponding 476856080.

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A metallic layer enlightened with a monochromatic plane wave will usually behave like a mirror but under

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Fig. 1. Transversal magnetic field distribution associated with the bounded Fano modes (SP and AP) supported by a thin metal film.

some particular conditions, a surface wave can be excited at one of the metal/dielectric interfaces and the reflection is attenuated [8]. Such a wave is called surface plasmon and results from the resonant coupling between a TM-polarized electromagnetic wave and the conduction electrons of the metal. In this case, the wave is not bound because of the excitation field but we can show that this structure can support a bound solution with an evanescent field on both sides of the interface [9]. If the metal layer is sufficiently thin, the plasmons from both surfaces can be coupled, leading to two possible bound field configurations known as Fano modes (Fig. 1). The first (SP) has a globally symmetrical look while the other (AP) is globally antisymmetrical. Note that those waves are quickly attenuated because of the absorbing nature of the metal. This situation has been widely described by many authors [10–12] but the realized devices generally based on prism coupling are too complex for a lowcost sensor. To get round this difficulty, we decided to excite those waves by the means of a waveguide just against the metal film. This is possible if the effective index of one of the Fano modes is close enough to the effective index of a ‘classical’ guided mode (typically between 1.5 and 1.51 with the technology employed here). Fig. 2 suggests that the phase matching should happen with the SP mode around a superstrate index of 1.455. Yet we are in a strong coupling situation and the whole structure must be considered by the model to rigorously study its behavior.

layer except over a length about 1 mm where it is covered by a thin silver film (Fig. 3). This length has been chosen because it corresponds to the maximal attenuation we were able to measure (about − 30 dB). The guide was realized by K + –Na + ion exchange in Menzel glass during 4 h 30 min in a pure KNO3 melt at 350°C [13]. The isolating layer was fabricated by cathodic sputtering of pyrex and its thickness is approximately 2 mm. The silver layer has a thickness of 60 nm and was settled by thermal evaporation. The central section (B) is the interaction zone while the adjacent zones (A and C) are here to guide the light from the source (a 822 nm laser diode) to the interaction zone and from the latter to a photodetector. The component was then coated with a sensitive material whose index is a function of the variable to measure. The eigenmodes of each section have been calculated with the transfer matrix method [14] and the Newton–Raphson algorithm has been employed to solve for the complex propagation constants. It appears that the interaction zone can support up to two different TM polarized guided modes (Fig. 4).

3. Description and modeling of the sensor The complete device consists in a straight, singlemode channel waveguide covered by an isolating

Fig. 2. Effective index evolution of a classical guided mode (GM) and the Fano modes (SP and AP).

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Fig. 3. Structure of the sensor (not to scale). A and C are passive guiding sections while B is the metallized interaction section.

We call them hybrid modes H1 and H2 because they are some kind of a mixture between a classical guided mode and a surface plasmon (Fig. 5). Depending on the substrate index, the ‘guide’ component or the ‘surface’ component of the hybrid modes is outstanding with propagation loss respectively small (about 10 dB cm − 1) or high (a few 100 dB cm − 1). The resonance index is the particular value at which the surface field (thus the losses) reaches its maximum. This structure is bimode in a small index range around the resonance. When one goes away from it, only one hybrid mode subsists and its surface component vanishes. So, the absorption coefficient of the interaction zone closely depends on the sensitive material index. The coupling coefficients between the zones have also being studied. Computations revealed that the coupling losses goes the same way as the absorption in the interaction zone, i.e. they are the greatest at the resonance (Fig. 6). This is not surprising because the field shape in the interaction zone is deeply altered by the resonance, leading to a small coupling coefficient with the adjacent zones. The global attenuation curve of the sensor is plotted on Fig. 7 in the case of a thick superstrate (larger than the optical penetration depth). The plain line corresponds to the simulation while the dots are measured values. The parameters for the simulation are: semi-infinite substrate, index 1.5; 2.81 mm waveguide layer, index 1.508; 60 nm of silver, index 0.0917–5.63i; and semi-infinite superstrate, variable index. The agreement between experiment and simulation is rather good taking into account the simplicity of the used model. Indeed, only one dimension of the problem was considered (slab waveguide approximation) and the graded index of the guide was re-

placed by a step index. For a maximum efficiency of the sensor, the variation range of the sensitive material index must follow one side of the attenuation peak, the sensitivity being directly linked with the slope.

4. Molecule detection The presented results concern the chemical species detection. The sensitive material we used is the squalane (C30H62), a compound mainly used in chromatography. It is a highly ramified hydrocarbonated molecule having special affinity with the alkanes family. Its index of 1.452 at 822 nm decreases appreciably in presence of those molecules. According to Fig. 7, the output optical power will then rise in respect with the concentration. This behavior has been experimentally verified. Fig. 8 shows the calibration curve obtained for a concentration range from 0.3 to

Fig. 4. Effective index evolution of the hybrid modes (H1 and H2).

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Fig. 5. Field intensity associated with the hybrid modes as a function of the superstrate index. The guide runs from 0 to − 2.8 mm and the silver layer from −2.8 to − 2.86 mm.

1% vol. in hexane vapors diluted in synthetic air. The interaction length was 1 mm. We notice that for the higher concentrations, the sensitivity of the sensor de-

creases progressively because the squalane index goes away from the resonance towards a region with smaller slope. The detection limit for this first device is esti-

Fig. 6. Coupling of the classical guided mode (GM) with the hybrid modes (H1 and H2).

Fig. 7. Attenuation curve of the sensor, taking into account the propagation and the coupling losses. The vertical plain line corresponds to the squalane index.

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[5]

[6]

[7] [8] [9] Fig. 8. Example of calibration curve (here for hexane vapors). [10]

mated to be about a few 10 ppm but it could be reduced with a correct optimization of the structure parameters. With regard to biological species detection, the sensor must be slightly modified to tune the resonance peak on the adequate index range which is typically from 1.34 to 1.41 for biological media. For that purpose, an additional dielectric layer can be inserted in the structure. We currently experiment a component with a pyrex layer on top of the silver. This configuration presents many advantages, it prevents the fragile silver layer from damages and oxidation and the biological species grafting is easier on pyrex than on silver.

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[14]

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5. Conclusion

Biographies

These first results show the interest of this simple sensor, it is easy to make and the performances for gas detection are encouraging if we consider that it is not optimized. Moreover, quick simulations can predict its behavior in a satisfying way. On the other hand, this configuration is particularly well adapted to perform biological detection with very thin sensitive layers because the field is the strongest near the interface. To shift the resonance peak, one can simply add a dielectric tuning layer in the structure of the sensor. Besides, the small size of the device (a few mm by 20 mm) allows to integrate many sensors on one chip in order to perform multidetection.

Oli6ier Hugon was born in Mons, Belgium, in 1971. He received the master’ degree of physic at the Universite´ Libre de Bruxelles in 1994 and the Diploˆme d’Etudes Approfondies from the Institut National Polytechnique de Grenoble (INPG), France, in 1995. He is currently working towards the Ph.D. degree in optics, optolelectronics and microwave at the INPG. His research interests include the modeling, realization and characterization of integrated optical sensors.

References [1] R.G. Heideman, R.P.H. Kooyman, J. Greve, Performance of a highly sensitive optical waveguide Mach–Zehnder interferometer immunosensor, Sensors and Actuators B 10 (1993) 209– 217. [2] J. Ingenhoff, B. Drapp, G. Gauglitz, Biosensors using integrated optical devices, Fresenius J. Anal. Chem. 346 (1993) 580 – 583. [3] I. Shanen Duport, H. Helmers, P. Greco, R. Wagner, R. Rimet, Integrated optics interferometric sensors on glass for chemical applications, Sensors and Materials 8 (4) (1996) 193–198. [4] H.J.M. Kreuwell, P.V. Lambeck, J.M.N. Beltamn, T.J.A. Popma, Mode coupling in multilayered structures applied to a chemical sensor and a wavelength selective directional coupler, Proceedings .

Pierre Benech was born in 1964. He received the Diploma of Electrical Engineering de Grenoble, France, in 1987 and the Ph.D. degree in optoelectronics from the Institut National Polytechnique de Grenoble, France, in 1990. Since then he has been working in the fabrication of glass integrated optic components and optoelectronics sensors. He teaches guided wave optics and electromagnetism at the Institut National Polytechnique de Grenoble. Henri Gagnaire received the degree of Doctor 3° cycle in chemistry in 1975 from the Ecole Nationale Supe´rieure des Mines de Saint-Etienne (France) and the degree of Doctor in Physics in 1985 from the University of SaintEtienne. As a professor at Saint-Etienne University, his research interest is in optics and in optical-fibre sensors.