Introduction to Metallic Colloids

We have seen that introducing geometrical structure into a dielectric produces interesting consequences for the dispersion of light. Combine a complex geometric structure, with a dielectric function that has strong frequency dependence, and you have a recipe for some extremely interesting behaviour.

Our findings for colloids came as a complete surprise to us. Encouraged to look into the problem by Howie and Walsh who were doing experiments at the Cavendish with an energy resolving electron microscope, we modelled the random colloid with an ordered array of spheres, all of the same diameter. Surprisingly this seems to work. Even more surprising were the dispersion relationships presented in one of the panels below.

Some References:

Where Do We Find Metallic Colloids?

The Metallic Dielectric Function

epsilon(omega)
= 1 - omega sub p squared / omega squared
In metals the dielectric response is dominated by the plasma resonance at
omega = omega sub p
As a result the dielectric function is negative at low frequencies which gives rise to some interesting properties. This is especially true when the metal is highly structured as in a colloid.


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