לוח שנה משולב

Optical spectroscopy provides an excellent means of understanding the distinctive prop-erties of electrons in the two-dimensional system of graphene. Within the simplest picture, one has a (zero-gap) semiconductor with direct transitions between the well-known conical bands. This picture gives rise to a predicted frequency-dependent absorption of  = 2.3%, where  is the fine-structure constant. We will demonstrate that this relation is indeed satisfied in an appropriate spectral range in the near infrared, but that at higher photon energies electron-hole interactions significantly modify this result through the formation of saddle-point excitons. Optical spectroscopy also permits a detailed analysis of how the linear bands of graphene, corresponding to massless Dirac Fermions, are modified to yield massive electrons through interlayer interactions in bilayer and few-layer graphene sheets. The observation of a tunable band gap in bilayer and trilayer graphene will be discussed. We will also present recent results on monolayers of the transition metal dichalcogenide MoS2. Because of the lowered structural symmetry, this material exhibits a significant band gap, as well as distinctive properties associated with the strong spin-orbit effects, such as the possibility of optical generation of valley polarization.