Phase properties of a surface-plasmon resonance from the viewpoint of sensor applications

The physical picture of the behaviour of the phase of a light wave, reflected from a metal film under surface-plasmon resonance conditions, was determined theoretically and experimentally for various values of a whole series of parameters. It was found in particular that for a p-polarised wave the dependence of the phase on the angle of incidence and on the radiation wavelength, and also on the refractive index of a medium or on the thickness of a (bio)chemical receptor layer on the metal surface, had a section with an abrupt jump-like change. The steepness of this section, i.e. the sensitivity of the phase to a change in the relevant parameter, can exceed by several orders of magnitude the steepness of the intensity resonance profile and in theory it can tend to infinity as the intensity at the reflection minimum approaches zero. This is the basis of a discussion of promising (bio)chemical sensor methods ensuring a significantly higher sensitivity and yet the same wide dynamic range as the traditional detection of an intensity minimum.