Description:

Gold, silver, and copper particles exhibit optical activity in the visible range; meaning they strongly absorb and scatter light of different wavelengths. This has been known experimentally for a very long time: gold colloid (spherical shaped particles, mostly) produce the brilliant red color of many medieval windows. Silver colloid imparts a yellow color and copper a greenish one, but they have been produced with efficiency only recently, thus aren't as prominent in historical items. Advanced sensor technologies are being developed now which utilize these spectral properties.

Metal nanoparticles also play a starring role in the mystery of surface enhanced spectroscopies, a topic related to proposed optical technologies and single (small) molecule detection. Molecules behave differently when they are near a rough noble metal surface (such as a nanoparticle), and often times their spectral signal is enhanced by many orders of magnitude. The primary mechanism of this enhancement is electromagnetic, where the particle enhances the light's electric field near the surface.  The image above shows how new methods of calculation are gaining understanding of these important experimental principles.

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