Nonlinear optics of graphene in the presence of Rabi oscillation

When graphene is subjected to a strong light field, its nonlinear optical response allows only odd harmonic emissions because of a centrosymmetric structure. We demonstrate that this limitation no longer holds when the generation of an electric current density is considered within a quantum theory because of the involvement of the Rabi oscillation in the nonlinear optical response. By transforming the time-dependent Dirac equation into the length gauge, the Rabi frequency in graphene can be defined, allowing the identification of the Rabi coupling effect on the current-induced harmonic spectra. In particular, a peak at the spectral position of the second harmonic of the incident light can be generated in graphene when the intensity of the incident light causes the Rabi frequency to become comparable with the frequency of the incident light, which was demonstrated in ZnO with a measurement of carrier-envelope-offset frequency [Phys. Rev. Lett.90, 217404 (2003)10.1103/PhysRevLett.90.217404PRLTAO0031-9007]. Moreover, as the light intensity increases, we show that a broadband harmonic spectrum is generated and that a compact electron distribution caused by multiphoton absorptions is produced because of highly nonlinear effects associated with the Rabi oscillation.