▎ 摘　　要

The second-order nonlinear optical susceptibility chi((2))( - omega(3); omega(1), omega(2)) corresponding to three-wave mixing of coherent radiation of the form omega(3) = omega(1) - omega(2) is calculated for epitaxial graphene on a SiC substrate inducing the sublattice (inversion) asymmetry of the graphene and opening up a gap of about 0.26 eV in its pi-electron-energy spectrum. The analytical treatment of the chi((2)) is based on the tight-binding approximation for pi electrons and the original Genkin-Mednis nonlinear-conductivity-theory formalism including mixed intra- and interband terms. It is found that throughout the transparency region of the graphene, the absolute magnitude of the chi((2)) may be as large as 10(-5) esu, which opens up new opportunities to generate terahertz (THz) coherent output from the graphene excited by two collinear mid-infrared omega(1) and omega(2) laser beams normally incident on its surface. The output power density produced at the difference frequency omega(1) - omega(2) of 10 THz is estimated to be 0.1 mu W/cm(2) for 10 MW/cm(2) pump peak intensities, and conditions are discussed under which a few orders of magnitude enhancement of the output power could be achieved in future experiments. (C) 2015 Elsevier B.V. All rights reserved.