▎ 摘　　要

The enhanced absorption probability of gigahertz and terahertz electromagnetic wave in monolayer graphene, sandwiched by the dielectric materials, up to 100% has been shown in our previous works by solving the classical Maxwell equation with transfer matrix method. In this paper, we show by a quantum description of the phenomena that the origin of the enhanced optical absorption is equivalent to the excitation of surface plasmon. The interaction between a photon with a surface plasmon is calculated, and the excitation probability of surface plasmon can be obtained by the Fermi golden rule, which can be compared with the optical spectrum that is calculated by the Maxwell equation. The calculated results show that both the intraband single-particle excitation of electron and collective excitation or surface plasmon contribute to optical absorption.