  The optical conductivities of graphene layers are strongly dependent on their
stacking orders. Our first-principle calculations show that while the optical
conductivities of single layer graphene (SLG) and bilayer graphene (BLG) with
Bernal stacking are almost frequency independent in the visible region, the
optical conductivity of twisted bilayer graphene (TBG) is frequency dependent,
giving rise to additional absorption features due to the band folding effect.
Experimentally, we obtain from contrast spectra the optical conductivity
profiles of BLG with different stacking geometries. Some TBG samples show
additional features in their conductivity spectra in full agreement with our
calculation results, while a few samples give universal conductivity values
similar to that of SLG. We propose those variations of optical conductivity
spectra of TBG samples originate from the difference between the commensurate
and incommensurate stackings. Our results reveal that the optical conductivity
measurements of graphene layers indeed provide an efficient way to select
graphene films with desirable electronic and optical properties, which would
great help the future application of those large scale misoriented graphene
films in photonic devices.
