Energy diagrams of the Dirac equation (2) for (a) a sharp and (b) a smooth potential step of height . The shaded regions are the forbidden energies that separate the electron and positron states, the dotted horizontal line is the energy of the incoming wave packet, and the dashed curve is the shape of the potential step . (c) Schematic of the grating structure that realizes the optical analogue of relativistic tunneling across a potential step. The grating comprises five sections, denoted by roman numbers (from to ) that are defined by the amplitude and phase gradient grating profiles.
