Optical spectroscopy of bilayer graphene

Abstract

Infrared spectroscopy reveals unusual tunable electronic structure and optical behaviour in electrically gated bilayer graphene. In a dual-gate bilayer graphene device, we were able to control the carrier doping and a semiconductor bandgap independently by using different combinations of the top and bottom gate voltages. The field-induced bandgap can be probed directly through the emerging interband transitions in infrared absorption spectra. A tunable bandgap up to 250 meV has been observed in our dual-gate bilayer graphene devices. This unique tunable bandgap can lead to many new physical phenomena. One example is an unusual phonon-exciton Fano resonance when the electronic bandgap is tuned to match the phonon vibration energy. Here (continuous) electron-hole transitions and (discrete) phonon vibrations form a coupled system described by the Fano resonance, and the infrared absorption spectra exhibit characteristic quantum interference between the phonon and exciton transitions. Remarkably, this coupled phonon-exciton Fano resonance can be continuously tuned through electrical gating in bilayer graphene, and its behaviour is described quantitatively by theory. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.