Understanding terahertz optical properties of amorphous carbon thin films

The nanostructure of amorphous carbon thin film is described in terms of a disordered conductive sp2 phase embedded in an electrically insulating sp3 matrix. The conductive sp2 phase is dominated by Drude type free-carries response, and the carrier conductive between sp2 clusters is hopping. The two conductivities are in a serial configuration under a percolation approximation. In view of this, we extend Shimakawa's model for compact nanoparticles [14] to amorphous carbon thin films in order to provide understanding on its terahertz (THz) optical complex permittivity. Furthermore, the contribution of phonons for THz optical complex permittivity is included in the scope of Debye theorem phenomenologically. Considering the condition of ω<ωTO and the narrow frequency range, we interpret the complex phonon permittivity experimentally as ε̃phonon=Aωeiθ for amorphous carbon. Thus, the real and imaginary permittivity can be simultaneously explained by the above considerations. It is the first qualified analysis on the THz optical properties of amorphous carbon thin films experimentally.