Broadband ultrafast spectroscopy of bilayer graphene

Cooling of electrons is a fundamental problem in high speed electronic devices, where transport is dominated by hot carriers. This process is achieved by energy transfer from the electronic system to phonons. The dynamics of carrier cooling becomes of particular interest in carbon based low dimensional structures, such as carbon nanotubes and graphene, because of their extraordinary mobilities and current densities , in the presence of strong electron phonon interactions. This makes such material systems unique, when compared to other semiconductors and semi-metals. Hot carriers are expected to emit optical phonons as long as their energy with respect to the bottom of the band is larger than the phonon ones. At this point cooling via acoustic phonons is the dominant mechanism for equilibrating the electronic system with the lattice. This paper considers the dynamics of carrier cooling in bilayer graphene by femtosecond transient absorption spectroscopy probing the energy range 0.25-1.3 eV, thus below two times the optical phonon.