Surface Plasmons

Au nanorods of increasing aspect ratio (width/length ≤ 1).

Optical properties of metal nanoparticles depend strongly on particle size and shape and the distance to neighboring particles. For Au, Ag, and Cu the collective oscillation of surface conduction electrons in response to the alternating electric field of incident light induces a quantized charge density wave (plasmon resonance) that absorbs some visible wavelengths.

“Nanometals: Formation and Color,” Materials Today, Feb 2004, pages 26-31.
M. A. El-Sayed, "Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes", Acc. Chem. Res. 34, 257-264 (2001).
M. Faraday, Experimental Relations of Gold (and Other Metals) to Light, Philos.Trans. R. Soc. Lond. 147, 145-181 (1857).