OAM & Surface Plasmon Resonance

The ultimate goal of the Surface Plasmon Resonance (SPR) experiment is to understand angular momentum states of surface plasmons (SPs). This wiki article assumes that the reader has general knowledge of optics and light fields.

Surface Plasmon Resonance

A surface plasmon is very similar, conceptually, to a photon confined to a 2-dimensional surface. The coupling of light to a conductor generates charge density waves that propagate based on the dielectric constant and thickness of the conductor. These two conditions generally dictate the momentum of a surface plasmon (and therefore the associated wave number, k).

When coherent light undergoes total internal reflection in at an interface in a dialectric, an evanescent field propagating parallel to the component of the incident light in the plane of the dielectric interface is generated. When a conducting film as described above, whose associated plasmon wave number matches the wave number of the evanescent field, a surface plasmon resonance is is generated. In order to tune the resonance, the incident angle of the light can be adjusted, thereby adjusting the wave number of the evanescent field.

In practice, an approximately 43nm thick gold film applied to the back of a glass prism makes for an excellent plasmon setup. Gold is an excellent conductor. Better conduction increases the wave number associated with surface plasmons in a material, allowing for the plasmon resonant angle to be above the critical angle. While silver is an even better conductor, its corrosion in atmosphere makes it less practical.