▎ 摘　　要

We present an analytical investigation of the exciton optical absorption in a narrow armchair graphene nanoribbon (AGNR) in the presence of a longitudinal external electric field directed parallel to the ribbon axis. The two-body two-dimensional Dirac equation for the massless electron and hole subject to the ribbon confinement, Coulomb interaction, and electric field is employed. The ribbon confinement is assumed to be much stronger than the internal exciton electric field which in turn considerably exceeds the external electric field. In the single subband approximation of the isolated size-quantized subbands induced by the ribbon confinement, the exciton electroabsorption coefficient is determined in an explicit form. The pronounced dependencies of the exciton peak positions, widths, and intensities on the ribbon width and electric field strength are traced. The electron-hole exciton attraction enhances considerably the Franz-Keldysh electroabsorption in the frequency region below and significantly modifies it above the edges determined by the size-quantized energy levels. The ionization by increasing the ribbon width is shown to occur. In the double-subband approximation of the interacting ground and first excited subbands the total peak widths associated with the exciton electro- and autoionization caused by the electric field and intersubband coupling, respectively, are determined analytically. Our analytical results are in agreement with those obtained by numerical methods. Estimates of the expected experimental values for the typically employed AGNR show that for a weak electric field the exciton quasidiscrete states remain sufficiently stable to be observed in optical experiments, while relatively strong fields free the captured carriers to further restore their contribution to the transport.