Four-Wave Mixing in Landau-Quantized Graphene

For Landau-quantized graphene, featuring an energy spectrum consisting of a series of nonequidistant Landau levels, theory predicts a giant resonantly-enhanced optical nonlinearity. We verify the nonlinearity in a time-integrated degenerate four-wave mixing (FWM) experiment in the mid-infrared spectral range, involving the Landau LL_{-1}, LL_{0} and LL_{1}. A rapid dephasing of the optically induced microscopic polarization on a timescale shorter than the pulse duration (~4 ps) is observed, while a complementary pump-probe experiment under the same experimental conditions reveals a much longer lifetime of the induced population. The FWM signal shows the expected field dependence with respect to lowest order perturbation theory for low fields. Saturation sets in for fields above ~6 kV/cm. Furthermore, the resonant behavior and the order of magnitude of the third-order susceptibility are in agreement with our theoretical calculations.