  The optical properties of graphene are strongly affected by electron-electron
(e-e) and electron-hole (e-h) interactions. Here we tune these many-body
interactions through varying the density of free charge carriers. Measurements
from the infrared to the ultraviolet reveal significant changes in the optical
conductivity of graphene for both electron and hole doping. The shift,
broadening, and modification in shape of the saddle-point exciton resonance
reflect strong screening of the many-body interactions by the carriers, as well
as changes in quasi-particle lifetimes. Ab initio calculations by the GW
Bethe-Salpeter equation (GW-BSE), which take into account modification of both
the repulsive e-e and the attractive e-h interactions, provide excellent
agreement with experiment. Understanding the optical properties and high-energy
carrier dynamics of graphene over a wide range of doping is crucial for both
fundamental graphene physics and for emerging applications of graphene in
photonics.
