Controlling The Ultrafast Optical Properties of Graphene with Ionic Liquid Gating

Single-layer graphene is endowed with broadband optical absorption, largely dependent on the Fermi energy. Ionic liquid gating allows to tune the Fermi energy to hundreds of eV and control its optical response in the near-infrared and THz ranges. Transient absorption measurements in the near-infrared reveal that also the out-of-equilibrium optical properties can be largely tuned using ionic gating. By varying the Fermi energy via electrostatic gating, the sign of the transient optical response of graphene is switched while the recovery dynamics significantly slows down due to the quenching of the relaxation of the hot electrons through the emission of optical phonons. The electrostatic control of the recovery dynamics and of the sign of differential transmission, opens intriguing perspectives for applications of graphene as a tunable saturable absorber for generation of tunable near-infrared pulses or alternatively as optical limiter or logic gate. Moreover, ionic gating is exploited to conceive compact optoelectronic devices acting in the THz range as amplitude modulators, saturable absorber mirrors and frequency tuners. The large control of the THz optical response is used to compensate the cavity dispersion of a multimode THz quantum cascade laser and induced stable operation as frequency comb for metrological applications.