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

TL;DR

This paper uncovers a second unoccupied Dirac surface state in Bi$_2$Se$_3$ that can be directly excited by ultrafast laser pulses, opening new avenues for optical control of topological surface states.

Contribution

It reports the discovery of an unoccupied Dirac surface state in Bi$_2$Se$_3$ that is optically accessible, which was not previously known.

Findings

Identification of a second unoccupied Dirac surface state at 1.5 eV above the conduction band.

Demonstration of direct optical excitation of this state using a Ti:Sapphire laser.

Confirmation of the state’s topological and spin-textured nature.

Abstract

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi$_2$Se$_3$ 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.