All the colors of noise

In the frequency domain electrical noise of a two terminal device under steady state is characterized by its spectrum, Sx(f), with X indicating voltage or current fluctuations depending upon the boundary conditions of the experimental/theoretical set-up. The measured spectrum covers the typical range from micro to Tera Hz, corresponding to colors in the dark region for human eyes but clearly detectable by experimental apparata and/or predictable by theoretical modeling. Overall, essential information on the microscopic time scale of charge transport is available from the knowledge of this “dark” noise-spectroscopy. This paper reviews some interesting features of noise spectra that have been challenged during my research activity and which I believe can still be sources of discussion and open problems, as remarked in the text.