Interference of excitons and surface plasmons in the optical absorption spectra of monolayer and bilayer graphene

By using spectroscopy ellipsometry, we investigated the optical absorption spectra of monolayer and bilayer graphene in the deep-ultraviolet region up to 6.42 eV as a function of temperature. The optical absorption spectra of monolayer and bilayer graphene can be fitted by two Breit-Wigner-Fano (BWF) spectral functions, in which each BWF is originated by the interference of discrete excitonic spectra and continuous surface plasmon spectra. Each BWF peak shows narrower linewidth and larger peak intensity with increasing temperature. The unusual temperature dependence of the BWF peaks is understood by a shorter lifetime of the surface plasmon at a higher temperature, which gives a smaller BWF asymmetric parameter. The optical absorption by the surface plasmon is usually observed only at the surface of the three-dimensional crystal or two-dimensional material. The contribution of the surface plasmon becomes relatively larger in the case of monolayer or bilayer graphene than that of single-crystal graphite because of the two-dimensional nature of electrons. The observed optical absorption spectra are confirmed by calculating the complex dielectric function using the first-principles calculations.