plasmonic effect which appear mainly in metals such as gold and silver
and due to veselgo chart for materials as metal have negative permitivity and complex refractive index
but for dielectric
epislon is positive
so why they encounter plasmonic effect
Dear Hadeer,
The answer to your question is yes, but with many subtleties to do with the relationships that you suggest to exist.
One example is the boron nitride BN are ceramics with have a honeycomb structure as the graphene, thus they are not metallic, but they have a real permittivity which is negative (the imaginary part always is positive under an AC field) and there are some other non metals which share this property.
The pure silicon material at low temperature doesn't exhibit plasmonic behaviour and only doping it as it happens in boron-doped silicon nanocrystals can you have a strong contribution at its surface. But the plasmonic property of these materials is not directly related with having an electronic plasmonic phase, which mainly depends of the free electrons density. If that density is high enough as, in metals, then the real permittivity is this negative and it is got when the frequency of the AC field is higher than the plasma frequency. Thus you have one extra condition due to the external field applied.
Even for dielectrics the real part of the dielectric function can be negative, e.g. around phonons. So you can have a "phononic" effect with e.g. also large field enhancements but also with strong absorption. See e.g.
http://onlinelibrary.wiley.com/book/10.1002/9783527618156
Many polar dielectrics exhibits resonance. Near the resonance the permittivity may be negative.
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