Theory and modelling of optical waveguide sensors utilising surface plasmon resonance

Abstract

The use of optical waveguides in optical sensors offers numerous advantageous features such as small size, ruggedness, potential for realising various optical functions on a single chip (integration with other optical components), multichannel sensing etc. The first integrated optical sensors based on the principle of a surface plasmon resonance (SPR) have been described in late eighties. Since then, integrated optical SPR sensors have been intensively studied and SPR sensing devices using slab waveguides, channel waveguides and even more complex channel waveguide structures have been developed.

As the excitation of surface plasmons in guided-wave structures is more complicated than in bulk-optic configurations such as attenuated total reflection method, more sophisticated modelling tools are required for the design and optimisation of SPR integrated-optical sensing structures. For modelling properties of integrated-optical waveguide SPR sensing devices, various methods based on the analysis of properties of waveguide modes have been recently used which differ mainly in the assumed complexity of the interaction among surface plasmon waves (SPW) and waveguide modes. This is the purpose of this paper to analyse the phenomenon of excitation of SPWs in a simple integrated-optical waveguide structure, and also to compare the performance of existing modelling approaches.

We compare here methods developed independently at four laboratories actively working in this field: MESA Research Institute of the University of Twente (UT), the Optical Research Centre of the University of Southampton (ORC), the Institute of Radio Engineering and Electronics of the Academy of Sciences of the Czech Republic, (IREE), and the General Physics Institute of the Russian Academy of Sciences (GPI). In the first stage, the process of excitation of a SPW in a waveguide structure is described qualitatively, based on the analysis of modal properties of waveguide modes and SPWs. Then, a rigorous approach to analysis of light propagation through a waveguide structure with a metal overlayer supporting SPW's is formulated, as an application of a bi-directional mode expansion propagation method. As the back-reflections in the structure are found to be very weak and most of optical power is transmitted by only a limited number of modes of the sensing structure, the method can be considerably simplified.

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Identifiers

ORCID for J.S. Wilkinson: orcid.org/0000-0003-4712-1697

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