Gate-Tunable Ultrafast Optical Response of Single-Layer Graphene

Summary form only given. The ultrafast relaxation dynamics in single-layer graphene (SLG) is of key importance for its applications in optoelectronic devices which rely on the dynamic response of charge carriers, such as photodetectors, saturable absorbers and modulators[1]. The absorption via interband transitions of a photon of energy in ħω SLG promotes an electron from an energy -ħω/2 in the valence band to an energy ħω/2 in the conduction band. This strongly non-equilibrium distribution thermalizes by electron-electron scattering (t ~ 10-20 fs) to a hot Fermi-Dirac (FD) distribution, which in turn equilibrates with the cold lattice via interaction with strongly-coupled optical phonons (SCOP, t~200-300 fs) and, on a longer time scale, via anharmonic coupling with acoustic phonons (t ~ 1-2 ps) [2]. In this work we study the dependence of the carrier relaxation dynamics on the Fermi level in SLG by performing femtosecond pump -probe spectroscopy on a large area sample grown by chemical vapour deposition (p -doped, Fermi energy EF = -0.2 eV), see Fig. la. The SLG is gated with an ionic liquid which allows to induce carrier densities as high as 10" cm' changing EF from -0.8 to 0.5 eV with low gate voltages (VG up to ± 1.2V).