Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

No abstract

Phase-sensitive sum-frequency spectroscopy provides correct characterization of vibrational resonances of water-vapor interfaces and allows better identification of interfacial water species contributing to different parts of the spectra. Iodine ions emerging at an interface create a surface field that tends to reorient the more loosely bonded water molecules below the topmost layer.