SPPs result from collective excitations of electrons at the
boundary between a metal and a dielectric. The strong sensitivity
of the plasmon-propagation constant to the permittivity of a nanoscale
layer of material (bioreceptors) deposited on the metallic
surface has led to the monitoring of surface plasmon resonance
(SPR) to detect (bio)chemical changes occurring between molecules.
As a result, biochemical reactions are measured with SPR by
monitoring the evolution of its effective refractive index. In practice,
the so-called Kretschmann–Raether approach realizes this by
launching light beams from a high refractive index prism to a thin
metallic interface at an angle such that light is totally reflected [1].
In doing so, an evanescent wave extends in the metal overlay.
When the component of the propagation constant of the light
along the interface matches the one of a plasmon wave at the
other side of the metal layer, part of the light couples to the
plasmon, which decreases the reflection. The interrogation is
made either by varying the wavelength and keeping the incidence
angle constant or by using monochromatic light and modifying the
angle. In both cases the polarization state of the light has to be
parallel to the incidence plane so that the plasmon wave is normally
polarized to the interface. Biochemical reactions occurring
above the metal surface affect the effective refractive index of the
plasmon wave, which is detected through a shift of the SPR. The
sensitivity to the surrounding refractive index often ranges in the order of 106
–107