We report on spectroscopy results from the mid- to far-infrared on wafer-scale graphene, grown either epitaxially on silicon carbide, or by chemical vapor deposition. The free carrier absorption (Drude peak) is simultaneously obtained with the universal optical conductivity (due to interband transitions), and the wavelength at which Pauli blocking occurs due to band filling. From these the graphene layer number, doping level, sheet resistivity, carrier mobility, and scattering rate can be inferred. The mid-IR absorption of epitaxial two-layer graphene shows a less pronounced peak at 0.37pm0.02 eV compared to that in exfoliated bilayer graphene. In heavily chemically-doped single layer graphene, a record high transmission reduction due to free carriers approaching 40% at 250 mum (40 cm-1) is measured in this atomically thin material, supporting the great potential of graphene in far-infrared and terahertz optoelectronics.
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