▎ 摘　　要

We theoretically investigate one of the third-order nonlinear optical (NLO) effects, namely, the quadratic electro-optic Kerr effect (EOKE), in doped graphene. To avoid the screening of an in-plane external dc electric field by the graphene's electrons, we propose to use 'low-amplitude' (less than or similar to 10 kV cm(-1)) terahertz radiation pulses focused onto the graphene sample collinearly with a normally incident optical beam of frequency.. Using the Dirac cone approximation for the pi-electron energy bands of graphene, we calculate the real part of the effective third-order NLO susceptibilityc chi((3))(-omega; 0, 0, omega), describing the EOKE in doped graphene under the above conditions. The results obtained show that a large electro-optic modulation of the graphene's refractive index n (up to Delta n approximate to 0.1) can be achieved by proper tuning the Fermi level E-F of charge carriers in the graphene sample via electrostatic gating. Furthemore, a change of sign of the electro-optic Kerr coefficient of doped graphene can occur in the spectral range below the photon energy threshold value of 2E(F), corresponding to the onset of the fundamental (single-photon) interband absorption in the graphene. These theoretical findings open up new opportunities for practical exploitation of the EOKE in graphene-based NLO devices.