Fig. 3

OFE behavior revealed by TDDFT calculation. a The calculated I–F curve of (6,6) metallic CNT model. b The calculated I–F curve of (10,0) semiconducting CNT model. The simulated laser pulse is centered around 800 nm (ℏv = 1.55 eV) with a pulse width of 7 fs. MPP regime is indicated by a dashed line. Gray area indicates the curve bending up. c–e Excitation states of the semiconducting model (10,0) tube at three points marked by arrows in b. c At F = 1 V nm⁻¹, the number (∆n) of excited electrons clearly show a peak at −1.5 eV (in gray area). Photon-driven electrons are represented by blue square dots, while field-driven electrons are represented by yellow circular dots. d At F = 3 V nm⁻¹, the peak is unchanged, which also indicates a photon-driven dominated regime. e At F = 5 V nm⁻¹, the peak moves to −0.9 eV, and the excitation number decreases rapidly as the energy level goes deeper, which demonstrates field-driven tunneling behavior. The DOS data are plotted as a gray solid line in (c–e). VBM is marked by the dashed line