Ultrahigh sensitivity of Surface Plasmon

[kaps_nit]

Hi All,
Surface plasmon sensors show very high refractive index sensitivity. Can anybody give an insight how it achieves such sensitivity?

Thanks
Kapil

 

[Old Nick]

I'm not a physicist so I can't give you too much detail, but I've worked on several projects designing SPR sensors, so can give you my understanding of how SPR sensors work.
If a thin layer of metal, usually gold, is put at the interface between 2 media of differing refractive indices (Kretchmann-Rather configuration), polarised light (s polarised I think) will be coupled into the metal, the energy of the photon is then disipated as an evanescent wave (the plasmon) along the surface of the metal instead of being reflected. The angle at which plasmons are created is very sensitive to the refractive indices of the 2 media. If you plot reflected light intensity vs angle of incidence you see a sharp dip at the plasmon angle
I'm not a physicist so I can't give you too much detail, but I've worked on several projects designing SPR sensors, so can give you my understanding of how SPR sensors work. If a thin layer of metal, usually gold, is put at the interface between 2 media of differing refractive indices (Kretchmann-Rather configuration), polarised light (s polarised I think) will be coupled into the metal, the energy of the photon is then disipated as an evanescent wave (the plasmon) along the surface of the metal instead of being reflected. The angle at which plasmons are created is very sensitive to the refractive indices of the 2 media. If you plot reflected light intensity vs angle of incidence you see a sharp dip at the plasmon angle - the following will hopefully be a picture of this.
[I]**broken link removed**[/I]
Sensors sytems work at either the bottom of the dip, or on the falling side (working on the side gives information on whether the angle increases or decreases). The phase of the reflected light flips very-very sharply (much sharper than the falling edge of the plasmon dip) at the plasmon angle, and some super-sensitive SPR sensor schemes measure the phase instead of the intensity.
If you want better explanations I can point you in the direction of a few papers.