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:

• Valence Loss Spectra from Composite Media A. Howie and C.A. Walsh, Micros. Microanal. Microstruc. 2 171 (1991).
• Energy Loss by Charged Particles in Complex Media JB Pendry and L Martin-Moreno, Phys. Rev. B50 5062 (1994).

Where Do We Find Metallic Colloids?

• Photography provides the most common occurrence of colloids: silver salts are reduced in the process to form finely divided metallic silver of black appearance
• Ruby glass, popular in Victorian times, is coloured by a dilute suspension of colloidal gold.
• Colloidal platinum, commonly known as platinum black, is used as a very strong absorber of infra-red radiation, for the reasons discussed in this poster.
• Colloidal transition metals are commonly used as catalysts in the oil industry. As yet there is no connection with their optical properties, but we are working on it!

The Metallic Dielectric Function

In metals the dielectric response is dominated by the plasma resonance at

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.

last updated Wed Apr 08 16:11 bst 1998 by RAE