Direct optical coupling to an unoccupied Dirac surface state in the topological insulator Bi₂Se₃

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi₂Se₃ by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.