Fig. 1
From: Extreme nonlinear strong-field photoemission from carbon nanotubes
Operation principle of extreme nonlinear OFE. a Diagram of OFE from CNT. Electrons (blue balls) are emitted from semiconducting CNT, driven by a negative half-cycled (red line) strong electromagnetic field of a femtosecond laser. b Diagram of CB-dominated OFE (CB-OFE) that occurs at a relatively low optical-field strength. The electron-filling states (orange) in allowed bands (purple lines) follow the Fermi–Dirac distribution near Fermi level (EF). Red and blue lines represent the periodical vacuum level driven by a strong optical-field in negative and positive cycles, respectively, with a circular frequency of ω. Purple balls present the tunneling electrons. c Diagram of transition into VB-dominated OFE (VB-OFE) that occurs when the optical-field strength is increasing. d Illustration of the nonlinear photoemission I–F curve in log–log plot. The upward bending of the curve is a result of transition from CB-OFE (green line) to VB-OFE (red line). Both CB-OFE and VB-OFE follow the FN form (γ < 1). The difference in emission rates is induced by different electron amounts in CB and VB. Multiphoton photoemission occurs at a low driving field limit (γ > 1)
