Band structure asymmetry of bilayer graphene revealed by infrared spectroscopy

We report on infrared spectroscopy of bilayer graphene integrated in gated structures. The dominant feature of the optical conductivity is a resonance peak due to interband transitions between the two conduction bands or two valence bands. Both the frequency and the voltage dependence of the peak show a significant asymmetry upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.