Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene

Graphene is a nonlinearmaterial in the terahertz (THz)frequencyrange, with & chi;((3)) & SIM; 10(-9) m(2)/V-2 & SIM; 15 orders of magnitude higher thanthat of other materials used in the THz range, such as GaAs or lithiumniobate. This nonlinear behavior, combined with ultrafast dynamicfor excited carriers, proved to be essential for third harmonic generationin the sub-THz and low (<2.5 THz) THz range, using moderate (60kV/cm) fields and at room temperature. Here, we show that, for monochromatichigh peak power (1.8 W) input THz signals, emitted by a quantum cascadelaser, the nonlinearity can be controlled using an ionic liquid gatethat tunes the graphene Fermi energy up to >1.2 eV. Pump and probeexperiments reveal an intense absorption nonlinearity at 3.2 THz,with a dominant 3rd-order contribution at E (F) > 0.7 eV, hence opening intriguing perspectives per engineeringnovel architectures for light generation at frequencies > 9 THz.