Ultrafast dynamics of an unoccupied surface resonance state in B i2 T e2Se

Electronic structure and electron dynamics in the ternary topological insulator Bi2Te2Se are studied with time- and angle-resolved photoemission spectroscopy using optical pumping. An unoccupied surface resonance split off from the bulk conduction band previously indirectly observed in scanning tunneling measurements is spectroscopically identified. Furthermore, an unoccupied topological surface state (TSS) is found, which is serendipitously located at about 1.5 eV above the occupied TSS, thereby facilitating direct optical transitions between the two surface states at ω=1.5eV in an n-type topological insulator. An appreciable nonequilibrium population of the bottom of the bulk conduction band is observed for longer than 15 ps after the pump pulse. This leads to a long recovery time of the lower TSS, which is constantly populated by the electrons coming from the bulk conduction band. Our results demonstrate Bi2Te2Se to be an ideal platform for designing future optoelectronic devices based on topological insulators.