Two-Photon Absorption in Graphene Enhanced by the Excitonic Fano Resonance

We report our observation of two-photon absorption (2PA) in graphene enhanced by the excitonic Fano resonance at the saddle point with femtosecond laser pulses in the spectral range from 1.8 to 2.9 eV (or, from 435 to 700 nm). On the basis of the second-order, time-dependent perturbation theory on interband transitions among three states near the saddle point of two-dimensional systems (used in the Bassani–Hassan theoretical model), we develop a semiempirical model to take excitons in graphene into consideration. We find that the model is in agreement with the photon-energy dependence of the observed 2PA spectrum with a scaling factor of <i>B</i> = (1–5) × 10² cm/MW/eV<sup>5