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We characterize the occupied and unoccupied electronic structure of thetopological insulator Bi$_2$Se$_3$ by one-photon and two-photon angle-resolvedphotoemission spectroscopy and slab band structure calculations. We reveal asecond, unoccupied Dirac surface state with similar electronic structure andphysical origin to the well-known topological surface state. This state isenergetically located 1.5 eV above the conduction band, which permits it to bedirectly excited by the output of a Ti:Sapphire laser. This discoverydemonstrates the feasibility of direct ultrafast optical coupling to atopologically protected, spin-textured surface state.

Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi$_2$Se$_3$