Coupled long range surface plasmons for the investigation of thin films and interfaces

We report a new approach for the investigation of thin films and interfaces based on the spectroscopy of coupled long range surface plasmons (cLRSPs). These modes are supported by a symmetrical layer structure with two metallic waveguides along which long range surface plasmons (LRSPs) propagate. For sufficiently small gap between the metallic waveguides, coupling between these LRSPs is established which gives rise to two new cLRSP modes. In surface plasmon resonance (SPR) sensors, cLRSPs can simultaneously probe events occurring on the sensor surface that are accompanied with refractive index changes. As the evanescent field of the two cLRSPs exhibits different penetration depths into the sensed medium on the top of the sensor surface, the spectroscopy of cLRSPst allows for the interrogating of the distribution of refractive index changes perpendicular to the surface. This feature advances the performance of current SPR sensors. We demonstrate the potential of the spectroscopy of cLRSP in an application in which we investigated swelling of thin hydrogel films and the diffusion of protein molecules into these gels.