First Principles dielectric spectroscopy of graphene in a tera-Hz frequency region*

The complex dielectric function ε₁+iε₂ of graphene were calculated from its electronic structure by using a first principles broadband dielectric spectrometer UVSOR (Universal Virtual Spectrometer for Optoelectronic Research) [1-3]. The ε₂ was calculated by considering the direct inter-band transitions of electrons of graphene between its conduction and valence bands including those between the two-hold degenerated bands at its Dirac point. Calculations showed that the ε₂ of graphene is proportional to 1/ω (ω is incident photon frequency) in a low frequency region where the electron transitions in the Dirac cone occurs and that it positively diverges as w →0eV. The optical spectra of graphene in a frequency region from 1 to 30 tera (T) Hz were calculated. Calculated optical absorption coefficients showed that graphene is a weak absorber of THz electro-magnetic waves. Details of calculations and calculation results will be given in the presentation.